The Twelve: List of Rigs

In my last post, I introduced my foolhardy campaign to homebrew a rig (receiver, transmitter, or transceiver) each month from November, 2020 to November 2021. 

I’m already deep into the first one for November 2020. I don’t want to say too much about it in advance of its completion—it has a seasonal application—but here are a few pictures.  [Move cursor onto image & click arrow to scroll]

 

What follows below is a list of the other projected members of the Twelve, along with a few comments on my current thinking regarding their features and design ideas. [Hover your cursor over underlined words or terms for a brief definition]

December 2020: WARC-SSB

This will be an single-sideband and CW transceiver for the 12 and 17 meters WARC bands. It will be my own design1 using my UDVBM-1 universal digital VFO/BFO based on an Arduino and an Si5351 clock generator. It will use termination-insensitive (TIA) IF amplifiers following the design by Wes Hayward W7ZOI and Bob Kopski K3NHI.2

Unlike many SSB designs such as the BITX family pioneered by Ashar Farhan VU2ESE3, the WARC-SSB will not use a bidirectional architecture. The balanced modulator, product detector, IF amps, and crystal filters will be built as separate modules, each with a shield can and SMA interconnections.

As of this writing, I am still noodling over the choice of IF frequency. Since I’ll be using an Si5351 for both VFO and BFO, and also using separate filters, I don’t even need to use the same IF. I probably will, though, since different IFs would require four crystal filters instead of just two.

It might seem like I’m really building two transceivers here and just putting them in a single enclosure, but the 12 and 17 meter sections will share a power supply—12VDC for the exciter/receiver circuitry and 24VDC for the finals—the audio sections (microphone and audio output), a switched-capacitor pre-selector/band-pass filter (on receive), and the transmit low-pass filter with a cut-off frequency of 26MHz. They’ll also share the unidirectional broadband TIA amps.

January 2021:  6U8A Regen

One purpose of the Twelve is to use many different technologies and construction methods, so at least one needs to use “thermionic valves” or, in American usage, vacuum tubes. This little tube-based regenerative receiver will be based on Forrest Cook W0RIO‘s Piglet Regen. I decided to name my version after its detector/amp tube to avoid confusion with Dieter “Diz” Gentzow W8DIZ‘s Pig Rig CW transceiver4 I build several years ago. 

The W0RIO Piglet circuit shares an important feature with Charles Kitchen N1TEV‘s well-known high-performance design5: it uses a buffer stage between the antenna and the detector to minimize radiation back out the antenna—a common problem with regens.6 Kitchen‘s buffer was a grounded-base 2N2907 BJT transistor. Cook‘s Piglet (so named because it squeals so readily) used a grounded-grid buffer made from the triode section of the 6U8A. The pentode section is the “beating heart” of the receiver—its oscillator, its detector, and its audio amplifier rolled into one. RF from the buffer is fed into the control grid of the pentode, and receiver is tuned in the grid-leak circuit. “Throttling” is done by varying the positive bias on the pentode’s screen grid. 

The Piglet was built for 5 to 10MHz reception. As such, its tuning rate is pretty fast. W0RIO‘s answer to that was to switch capacitors in parallel with the main tuning cap to re-scale the tuning range to a narrower segment. He also experimented with plug-in crystals to lock the tuning in even narrower. This had the additional effect of significantly increasing the overall gain of the receiver. I look forward to trying this myself.

February 2021: BITX-60

Unlike the WARC-SSB, this rig will pretty-much have a straight BITX bidirectional architecture except that it will use a TIA design for the bidirectional IF amps instead of the single-stage ones Farhan used. The audio circuits, balanced modulator/product detector, filters, and mixer will be straight BITX. Like the WARC-SSB, though, I’ll use my UDVBM for the VFO and BFO. I’ll also experiment with front-panel knobs to adjust both the frequency of the BFO and its phase relative to the suppressed carrier of the incoming sideband signal. This may improve the fidelity and clarity of voice reproduction. The BITX-60 will use 24VDC on the IRF510 final which will be mounted on a large heat sink with a 50mm cooling fan.

Since 60 meters is channelized, I’ll use a rotary switch to select the channel, and I’ll include a smaller knob on an encoder to vary the center frequency on channel 3 to prepare for prospective changes in the FCC allocation for that channel. Pete Juliano N6QW built this feature into his DifX transceiver in 20177.

Why not use a single knob for the VFO, the BFO, and the BFO phase and just use a menu and the encoder button to step through the functions? The answer is: because I don’t want to, and I don’t need to, either. Sorry, but I hate multi-function controls like that. I can tolerate them when front-panel space must be conserved for a very-good reason (as on a small field rig), but this BitX60 will stay in my shack at my operating position and there is enough space for a few more knobs (for crying out loud!). By the way, I put rotary encoders, manually-operated switches, and the LCD display on the same I2C bus as the Si5351 anyway (using PCF8574-based 8-bit expanders), so I’m not “wasting” Arduino pins, either. In fact, unless I want an analog input or PWM output, I use just two I/O pins on the Arduino (SDA1 and SCL1) for everything.

March 2021: Beacon 6

As the name implies, this will be a 2-channel, 6-Meter CW automatic-beacon transmitter. It will be built using nothing but discrete components. Its RF sources will be home-brewed ovenized crystal oscillators running at 50.060 or 50.080 MHz, the two FCC-mandated beacon frequencies for 6-meters. No means known to me personally to maintain oscillator accuracy and stability will go unused. In the end, this will come down to thermal stability. Ovenizing the crystal will be a big part of that, but even maintaining the stability of those is not a trivial matter. Since I’m determined to use only discrete components, I can’t use op-amp ICs for some kind of PID feedback mechanism, and I’m not going to homebrew op amps! It may be within reach to work-up a simple differential amp circuit that would do the trick using a thermistor, thermocouple, or diode as a sensor.8

I’ll be operating the beacon on a half-wave end-fed antenna oriented vertically. It will automatically transmit my call sign, grid square, power level (5W barefoot), and Zulu time every minute at 13 wpm.

Hold on, you say, how will I do that without using something digital to automate those transmissions? Well, here’s my loophole: I’ll built the Beacon 6 as a CW-only transmitter, with a jack for plugging in a key in the usual way. It’s possible, in fact, that if I provided the rig with some frequency agility by switching in parallel capacitance (fixed or variable), I could use it as an ordinary VXO CW transmitter—and now that I think of it that’s a damn good idea! Naturally, though, I can also plug in an automated keyer in place of a manual one without changing the character of the rig at all. How’s that for fast talking?

April 2021: TAK-20

I’ve had a TAK-xx rig on my todo list for a long time. The original TAK-40 was a design by Jim Veatch WA2EUJ that won him first prize in the 2008 ARRL Homebrew Challenge contest.9 The challenge was to design and build a transceiver for both CW and SSB to operate on 40 meters with at least 5W PEP. That doesn’t sound so hard, does it? Here was the big catch: it had to be designed to use all-new-and-available parts (nothing from the junk box or hamfest), and it had to cost less than $50USD (not including the power supply or other supplies such as wire, screws, etc.). Veatch‘s primary means of accomplishing that was to go with integrated circuits and digital control as much a possible.

For my version for 20 meters, I’ll take the integration one step further:  WA2EUJ used a varactor-tuned VFO with the control voltage coming from a DAC driven by a PIC MCU and a rotary encoder. I will dispense with the VFO, the DAC, and the PIC and go straight to a three-channel Si5351A clock generator driven by an Arduino Nano. I’ll likewise not use his crystal-controlled and varactor-tuned BFO, but instead employ one of the Si5351 channels.

I’ll keep all the rest of the original TAK architecture intact: 74HC125 tri-state buffers for TX/RX signal steering, NE612 double-balanced mixers, a MC1350P for IF amp and AGC , a six-pole crystal filter, and TL084 op amps for various purposes, including audio. Lastly, I’ll keep the IRF510s he used for the driver and final amp (he used a 510 for the driver simply because it was the cheapest suitable device he could get at the time). I will have to come up with all-new software for the Arduino to take over the tasks Veatch assigned the PIC, but that’s just par for the course.

May 2021: Binaural-30

Here’s another name for a rig that is mostly self-explaining. It’ll be for 30 meters, and since there’s no phone (AM, SSB, or FM) allowed on that band, this will be a CW-only transceiver. The transmitter section will be a little unusual but hardly unknown: a super-VXO for frequency agility between 10.100 and 10.030MHz, a span of 20KHz.10 To maximize stability, I’ll use the ovenizing techniques to be developed for my Beacon-6, and I’ll use grounded-base and emitter-follower buffer stages after the VXO for a nice, chirp-free signal. The VXO will be left running whenever the rig is turned on, and keying will take place in the last buffer stage. 

The receiver section will be a little more unusual, though again not unknown: it will use a Tayloe detector for quadrature “I” and “Q” binaural audio output.11 By separating the audio into two streams, one ninety degrees off phase from the other, the human psychoacoustic system perceives sounds of different pitch (such as CW signals at different frequencies as heard within the passband of a receiver) as taking a physical location in three-dimensional space.12

Exploiting this characteristic of the “meat computer” we have in our heads can help us to separate one CW signal from another in a crowded band. There are other ways to separate the signals by narrowing the passband, by using switched-capacitor audio (SCAF) filters to process the audio, or using DSP. I want to try the binaural IQ method. Those who have say it’s an eerie experience hearing the signals moving around inside your head when listening with headphones.

June 2021: μBITX Fielder

This will be an almost-straight μBITX design13 for 160 through 10 meters, except that I’ll use my own Si5351/Arduino system (the UDVBM-1) and firmware, and I’ll build it in shielded modular form using circuit boards I lay out myself. As the rig’s name implies, it will be for use in the field rather than in the shack. I already have an enclosure for it: a U.S. Army field crystal case I’ve had kicking around for several years. Once used for FT243 crystals, it is built in two halves, with the bottom half serving as the lid for the top.

After removing all the felt padding, I’ll put the electronics in the top half (including a small antenna “tuner” and 49:1 balun) and use the bottom half for a LiPo battery pack and to store a code key, microphone, end-fed antenna wire, a few carefully-chosen tools, and spare parts such as fuses and replacement IRF510s. How will I replace the MOSFET finals in the field? They’ll be installed in a shielded module with their tabs screwed to the side of the field case and their leads clamped in three-pin terminal blocks. For replacement, I’ll just unscrew them. I’ll decide later if I need to do something similar with the 2N3904 drivers.

I have no illusions this rig will be useful for hiking or backpacking. The empty field case alone weights three pounds, and it’s about eleven-inches long and almost five-inches tall. As you’d expect of anything Army issue, it’s built like a brick . . . hamshack. With the electronics, the battery pack, and the accessories, the rig could approach ten pounds. It might be usable for a SOTA activation if the walk from the car to the summit isn’t too far.

July 2021: GS-1 SDR

This rig will be about as close to the bleeding edge of radio technology as I ever care to get. This will be an SDR “homebrew” in a denotational sense only, i.e., I’m “building” it at home. It’s really more of a “system integration” at home, though, since I’m putting systems together that are already available on the market and that have many other applications. The heart of the GS-1 will be the Great Scott HackRF unit.14

This device is capable of both receive and transmit on 160 meters through the 6cm (5650-5925GHz) bands, though for TX it serves only as an exciter. It will need a separate linear amplifier(s) and switchable low-pass filters. I have heard that the HackRF is slow switching between RX and TX, so I may want to use a separate RTL dongle for receive. I’ll use SDR# (SDR “Sharp”) for software running on an embedded  Raspberry Pi.

 

August 2021: AM-80/160

And now for something at almost the other end of the radio-technology timeline, a tube-based transceiver for 80 and 160 meters AM phone. The AM-80/160 will take up to about 100 Watts of input power into a pair of 807s, 811s, 813s, 6146s, or 6JS6Cs, whichever is available at a good price as new-old stock. I have some of those used in my collection already, and I might settle on a pair if they test strongly enough. Everything in this rig will be tube driven, except for the VFO: that will be my UDVBM (Si5351/Nano based). I’ll probably also use it for a BFO as that might come in handy if I want to use the rig for CW or LSB. The mic amp will use something like a 6U8A or 6EA8, the IF amps and mixers will use 6BA6s for similar, and the speaker output will be a single-ended 6L6. I’ll be drawing heavily on old ARRL Handbooks for design fragments . . . and maybe even a complete design. I can’t say I’d turn one down should it offer itself.

And what will I use for the heavy iron”,” you may ask? To an extent, that will depend on the tubes I use for the finals. I’m not too keen on exceeding around 750VDC by much for the plate supply, and even that is enough to scare me straight. I may look into a diode-capacitor voltage “multiplier” as an option, though I don’t know about current capacity or voltage regulation with one of those.15

If it turns out I need to settle for lower input power because of self-imposed limits (cost of the power transformer being the main one), that would be okay. 50 Watts instead of 100, for instance, would only be worth a half S-unit, and it’s not like this will be a primary or constantly-used rig anyway. I don’t really even know how many AMers there are in my neck of the woods in the first place. Maybe I should find out. 

September 2021: Kitchen Regen

Of course I already have a regen project on the list, but the coverage of the 6U8A Regen ends at 30 meters. The Kitchen Regen will pick up at 31 meters and go all the way to 10 meters.  And, no, this will not be a rig I’ll use while cooking. The name comes from Charles Kitchen N1TEV, widely esteemed as the foremost authority on this now-ancient but still very-cool receiver technology. I’ll be using his all-solid-state design for the High-Performance Regenerative Receiver I mentioned above in connection with W0RIO‘s Piglet Regen, and that I’ve cited below in Ref. 5.

Unlike the 6U8A Regen which controls (“throttles”) regeneration by changing the bias on the pentode screen grid (sometimes called “electron coupling”) , Kitchen‘s design uses a variable capacitor to adjust the degree of coupling between the detector/oscillator and the feedback signal from the “tickler” coil. Throttling by this method allows for greater stability in both the tuning and the level of regenerative feedback. By the way, Kitchen provides a concise historical overview of the development of the regen receiver—beginning with E. Howard Armstrong‘s invention in 1914—and receiver topologies in general (regens, D-Cs, and heterodynes). I recommend reading Ref. 5 to learn more.

To get the frequency agility I’m looking for, I’ll make up several plug-in coil units (each with the three front-end coils on a single PVC coil form) to plug into octal sockets. To maximize the “cool” factor, I’ll mount the coil and the variable capacitors (main tuning, fine tuning, and throttle) in the open, using clear plastic for the front panel. I’ll mount the capacitors as far back from the panel as possible and use long plastic shafts from the biggest-damn knobs I can find. This will prevent any hand-capacitance monkey business.

October 2021: 2M-SSB

The capstone for this year-long campaign will be a rig for which I’m least prepared in terms of knowledge or prior experience. But like many Hams, I’ve long-since become bored with 2-meter FM—at least the handy-talkie and repeater type. It’s handy and useful sometimes, but it’s no longer fun.

Sure, I can whip out my cheap-as-dirt Baofeng and talk to everyone listening along a more-than three-hundred-mile front between southwest Washington and northern California. Meh. I can also talk to anyone on Planet Earth by pulling out my cell phone—a state-of-the-art SDR radio and multi-core computer to boot.  Also meh. Don’t get me wrong, I do dutifully contribute funds to a few regional repeater systems, mainly because of amateur radio’s (and therefore my) public-service obligations. That’s just it: it’s an obligation, not fun.

So I’d like to do something different and fun with VHF and UHF, maybe as half-way houses for eventual microwave activity. It’s my impression that there’s not a lot of CW and SSB being done on 2 meters, and I’m just anti-social enough to find that attractive. I have visions of doing SOTA activations and making other mountain-top QSOs at five Watts using a light-weight but many-element Yagi ten-feet long when it’s assembled, or tossing a twin-lead J-Pole up in a tree while I lounge in a campsite and chatter away on CW with some other Ham up on a peak.  Doesn’t that sound like a blast?

By now you’ve noticed I haven’t said anything out the 2M-SSB rig itself, yet. Well, there’s not much in the way of specifics I can discuss. Here’s some things I only supposing at this point.

  • The first is that I’ll most-definitely be using digital means of signal generation and control. I think I can use an Si5351 for that, but I may need to do my own PCB layout for it if the ones currently available are not suitable for reliable VHF operation.
  • I don’t think I can use the filter method to suppress the carrier and opposite sideband. I think that’s okay because I ought to be able to use quadrature instead, made easier by the multi-channel Si5351.
  • I don’t think I can use any of the easy-going construction methods such as “ugly” or “Manhattan.” I do think I’ll need to use careful RF-layout methods using four-layer PCBs with impedance-controlled striplines and inner-layer ground and power planes.
  • I don’t think I can use or will even need toroids for filter inductors. Most of the inductors will be in nano-Henries and air-core coils will do the job. Actually, it’s possible there’s no stable core material for VHF anyway. 
  • I do think I may need to use helical resonators for band-pass and low-pass filtering, and maybe for LO coupling. If so, I don’t think I’ll buy them. I’ll try to make them instead. I have some machinist and metal- fabrication skills, which even includes electroplating silver on the helices and shield cans. By hell! If I can pull off homebrewing helical resonators then that ought to earn me a Bad Hombre patch for my ARRL ball cap!
  • I think I’ll limit the rig to those segments of the 2-meter band plan for CW and “weak-signal” use: 144.05-144.10 (general CW and weak signals), 144.10-144.20 (EME and weak-signal SSB), and 144.200-144.275MHz (general SSB operation).
  • Whether or not keeping the rig within that 225KHz segment will have any advantages is something I don’t think I can say at this time. At a minimum, that’ll allow for some very-tight front-end filtering, and when it’s cold outside, squeezing through a narrow portal is better than swinging open a 4MHz-wide barn door.

That’s about all I have so far on this long-shot of a homebrew rig. Fortunately, I have nearly a year for lots of research and noodling.

November 2021: Fabricante 13

October’s rig, the 2M-SSB, will be the twelfth one in this series, so the Fabricante (trans.: “maker”) 13 will be a bonus rig to be completed as the campaign began—during my birthday month. I don’t have many concrete ideas yet. I’ve thought about a receiver dedicated to the 5 and 10MHz WWV frequencies with ultra-tight front-end filtering. I’ve also toyed with the idea of going primordial with a artfully-designed and finely-wrought crystal set for the commercial AM broadcast band—that 1100KHz-wide wasteland filled with holy rollers, conspiracy theorists, and assorted kindred crackpots. But hey, that’s where broadcast radio began and where it reigned for decades until it moved its headquarters (and all its bad habits—high advertising-to-music ratios and bucket-mouthed DJs) to FM by the 1980s.

If you all have any ideas for the Fabricante 13, let me know. QSL?


Notes

1. Almost no one’s designs are completely their “own.” What they do is to put bits and pieces together they learned from someone else. How one does that and synthesizes those bits may be unique, advanced, and even revolutionary (mine are none of those things), but it is all built on “prior art” (https://en.wikipedia.org/wiki/Prior_art)

2. Wes Hayward & Bob Kopski, “A Termination Insensitive Amplifier for Bidirectional Transceivers” (2009). http://w7zoi.net/bidirectional_matched_amplifier.pdf

3. Ashar Farhan, “BITX–An easy to build 6 watts SSB transceiver for 14MHz” (2004). https://www.vu2ese.com/index.php/2020/06/18/bitx-an-easy-to-build-6-watts-ssb-transceiver-for-14mhz/. See also https://www.hfsignals.com/

4. https://www.kitsandparts.com/fpra-7030.7.php

5. Charles Kitchen, “High Performance Regenerative Receiver Design” QEX (1998). http://www.arrl.org/files/file/Technology/tis/info/pdf/9811qex026.pdf

6. Doug Adams, “Regenerative Radio Receivers” (2012). http://large.stanford.edu/courses/2012/ph250/adams1/; Eddie Insame, “Designing Super-Regenerative Receivers” (2002). http://www.eix.co.uk/Articles/Radio/Welcome.htm; and Rodney Champness, “Behind the Lines: A Short History of Spy Radios in WW II” (1998). http://messui.polygonal-moogle.com/valves/SC199809.pdf

7. ARRL News, http://www.arrl.org/news/view/arrl-asks-fcc-to-allocate-new-5-mhz-band-retain-channels-and-current-power-limit & Pete Juliano, “A New line of Transceivers: DifX” (2017). http://n6qw.blogspot.com/2017/03/a-new-line-of-transceivers-difx_23.html

8. For example, see Alan Wolke W2AEW, “#193: Back to Basics: the Differential Amplifier . . .” YouTube video, https://www.youtube.com/watch?v=mejPNuPAHBY.

9. Jim Veatch, “The TAK-40 SSB CW Transceiver” QST (May 2008). http://www.arrl.org/files/file/QST/Homebrew%20Challenge/HBC%201%20Winner-WA2EUJ.pdf

10. See Peter Parker VK3YE experiments with super-VXOs: https://www.youtube.com/playlist?list=PLIe6EGOjQefd00TojphLIxC-eaLOdkPq1

11. Dan Tayloe, “Ultra Low Noise, High Performance, Zero IF Quadrature Product Detector and Preamplifier” (2013). Handout for Northern California QRP Club presentation. http://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdf

12. https://en.wikipedia.org/wiki/Psychoacoustics

13. https://www.hfsignals.com/index.php/ubitx-v6/

14. https://greatscottgadgets.com/hackrf/

 

Sunspot Number, Recent Events, and a Classic American Song

Holy Cow! Get a load of this:

I hadn’t looked at any information on current “space weather” for some time now. So imagine my surprise and pleasure at seeing the figures for today: sunspot count (SN): 84, and solar-flux index (SFI): 115!!!!!

It really has been a crappy year, but with Solar Cycle 25 now clearly underway, and with that other felicitous development in November, I’ve been humming this song all day long. A real ear-worm:

Historically fitting, don’t you think?

The Twelve

twelve_romanNo, I’m not referring to the horror novel by Justin Cronin, nor do I mean the twelve days of Christmas, the twelve disciples, or the Reginald Rose film “Twelve Angry Men.” I’m not talking about the Twelve Steps of Alcoholics Anonymous, Darryl F. Zanuck’s “Twelve O’Clock High,” Shakespeare’s “Twelfth Night,” or Robert Aldrich’s “Dirty Dozen.” What I have in mind may be related obliquely to the twelve signs of the Zodiac because those represent the twelve months.

My “The Twelve” will be a year-long campaign to build a ham rig–a receiver, a transmitter, or a transceiver–each month beginning November (my birth month) 2020. I prefer not to think of this undertaking as particularly “ambitious,” “aspiring,” or “hopeful.” Perhaps “industrious” might be apt, inasmuch as I have long believed that humans are more industrious than they are intelligent.

My purposes here are manifold. One is to so fill the life of my mind with enough planning, designing, and “noodling,” that sloth, ennui, and disgust over the state of national affairs can find no roosting place. Another is to climb above the plateau I’ve rested on for some years now in order to turbocharge my knowledge and experience in the design and construction of RF electronics. In the process, I expect to become fluent in the use of the KiCad suite of design tools, Spice and other simulation packages, and the full use of my Rigol oscilloscope and other test gear. I also expect to resurrect some skills in metal work, PCB fabrication, and silk screening, to name a few atrophied faculties.

As I’ve already mentioned, some will be receivers, some transceivers, and at least one will be a stand-alone transmitter. Some will be CW, others SSB, and one will be a good ol’ AM phone. Some will be Manhattan-style construction, some will be built on home-designed and etched PC boards, some with all through-hole parts, and some with SMD components when needed. One of the receivers will be a regen, probably Charles Kitchin’s high-performance design. For reasons I can’t explain, I’ve also been hankering to build a super VXO something or other–probably for CW and maybe for a band that has a short CW segment such as 12 or 17 meters. One of the rigs will be entirely discrete components, including voltage regulation. One of the rigs will use as many ICs as possible, including regulation, audio, IF amps, PLL, etc. And lastly, one rig will use vacuum tubes as the active components.

I’ll be building one of the Twelve for 60 meters SSB. To ease what will be, by then, my overtaxed design burdens, I might just make it a BitX60, with an Si5351-based VFO/BFO, Hayward-Kopski TIA (termination-insensitive amps) IF amplifiers, and with recalculated values for the 5.5 MHz low-pass and band-pass filters. I might just use this rig to experiment with using the Si5351 and a second rotary encoder to allow for adjusting the phase of the BFO to better match that of the suppressed carrier of an incoming SSB signal. This may improve the fidelity and clarity of voice reproduction.

I’ll include a third encoder to allow for on-the-fly BFO frequency adjustments as well. Why not use a single encoder for the VFO, the BFO, and the BFO phase and just use a menu or the encoder button to step through the functions? The answer is: because I don’t want to, and I don’t need to, either. I hate multi-function controls like that. I can tolerate them when front-panel space must be conserved (as with a small field rig), but this BitX60 will stay in my shack and there will enough space for a few more knobs (for crying out loud!). I put rotary encoders on the I2C bus anyway, so I’m not “wasting” Arduino pins, either.

In November 2021, I’ll bring this break-neck and bust-ass campaign to a close with a real stretch: a QRP two-meter SSB transceiver. I’ve long been fascinated by VHF SSB, and I have in mind to use this rig mostly for mountain topping with many-element yagis. Except for some high-end all-band/all-mode commercial rigs, there’s very little out there for two-meter “weak signal” (VHF-speak for QRP) SSB. Rather than generate the USB at HF and then upconvert, I will attempt to make a phasing exciter native to two-meters and add a five-watt power amp. In addition to SSB, the rig will also operate in true CW mode (not MCW). If I’m reasonably successful with this, others might improve on what I do and build their own.

Sometime in the next few days, I’ll finalize the list of twelve rigs to build. Then will begin the intense research, design, and noodling as I bring together the information and intelligence to make all this happen. I’m not so optimistic to think I can come up with all the design details and logistics of each month’s build within that same month, so I’ll be weaving some of that in with each-month’s construction. They’ll be opportunity for doing so when I set the rig I’m working on aside (in lieu of drop-kicking it off my back deck) when I get stuck or stymied on some phase of the construction.

Of course I’ll post updates here about the steps along the way. Each post in the series will have “The Twelve” in the title. I’m also resurrecting the DIY RF YouTube channel and I’ll show some mileposts along the way. QSL?

Boredom, Vapor Lock, and JO2

Have you ever been bored out of your mind even when you had plenty of things to do and little time to do them? The child’s whine, “I’m bored. There’s nothing to do!” has never been my problem. My whine has always been, “Damn-it-to-hell-and-back, there’s too much to do!” It’s a much bigger problem when most of the things to do are perfect storms of complexity, uncertainty, frustration, struggle, disappointment, discouragement, bafflement, and unrealistic expectations. Sometimes, I freeze up, or experience a kind of mental vapor lock. Over the decades, I’ve found in such a funk that what I need is an injection of accomplishment feedback to my non-inverting inputs–my eyes, ears, tongue, and sense of touch (I’ll leave what my single inverting input is to your imagination). So I need to get something done and working (or cooked and eaten, written and mailed, laundered and folded, etc.), and I need it quickly.

So in such a mental state the other day, as I was rather aimlessly browsing YouTube hoping to find a video on yet-one-more piece of esoteric knowledge I couldn’t live without, Alan W2AEW’s video on the Michigan Mighty Mite (hereafter M3) came up–a one-transistor, crystal-controlled “transmitter” built up on a piece of copper-clad using five discrete components, hand-wound coils, and a variable capacitor. I knew I had all the needed parts on hand, and in a previous funk I had organized them in such a way that I could gather everything in just a few minutes. I had an already-cut piece of copper-clad board just the right size, and I had just purchased new soldering-iron tips which needed to be baptized in tin and lead. “By hell! The M3 is just the thing!,” I exclaimed. In addition to helping to break my vapor lock, I could once again experience what Bill N2CQR and Pete N6QW call the “Joy of Oscillation” (hereafter JO2). I found a circuit for the M3 from George KC6WDK’s website, and I proceeded to round up the parts.

I had a number of variable capacitors in my Junque Box I could use. From the M3s I’d seen, and from the value called out on the schematic, I supposed I would need either an air variable or a polyvaricon. I had several of each type I could use, though in the back of my head was the dim thought, “Why do I need a variable with such a wide range? To get the nominal frequency of the crystal, only a single as-yet-unknown capacitance is needed, and if pushing or pulling the frequency a bit is desired, putting in a 365pF air-variable or even a 200pF polyvaricon would be gross overkill.” As I said, though, it was a dim thought, and not in the forefront of my stream of consciousness.

I wound the coil primary and secondary using the instructions on the webpage. For the coil form, I used one of the dozens of pill bottles I’d been saving for years (along with any other plausible coil form of paper, plastic, or ceramic that fell into my hands). For wire, I used 20 AWG copper wire from–wait for it–Michael’s Crafts. They’ve got lots of different gauges of enameled copper and aluminum wire in their beading department. The enamel is not quite as durable as genuine magnet wire, but in ordinary handling it works well, and it’s almost as tough to scrape or sand off as the real stuff. As I usually dope my coils with two coats of clear nail polish, I don’t worry about durability.

The primary winding of the finished-and-doped coil measured 32μH from top to bottom, and the tap was 7.7μH up from the bottom. At this point, that dim thought I had earlier came to the front of my mind. Looking at the schematic, I confirmed that the variable capacitor sits across the entire 32μH. “It’s a tank,” I said to myself, pleased to be fluently conversant with RF slang. It has to be resonant at the crystal frequency (or, within narrow limits, the crystal can be made to resonate with it). Whipping out my laminated reference sheet, I found the formula for a parallel LC circuit:

f = 1 / (2π√LC)

with f  = 3.5795, L and C in Henries and Farads. I needed to rearrange the formula to solve for C and plug-in 0.000032H for L and then . . . instead I pulled out my cell phone, brought up my EverythingRF app, entered the L of 32μH, and the answer for C was 62pF. As it turned out, I had several old 60pF mica-ceramic compression caps on hand, so I wired one across the primary coil with a 33pF disk in parallel to give me 30pF up and down from the calculated resonant capacitance. So in reality, a variable cap isn’t needed at all if the purpose is simply to experience JO2. A combination of disk caps (NP0 type for S&Gs) in parallel and/or series would get the thing working, and there’d be no hand-wringing over the need for a hard-to-find variable (even though they’re not really hard to find at all).

Instead of the 2N3904 transistor usually called out for this circuit, I used a 2SC710. I have a few hundred of them, and they were developed for RF use while the 3904 was not (but works well anyway). This was actually my first use from the big bag I bought several years ago. The 2SC710 is a Japanese part number, and there have been several manufacturers, most of them not indicated on the part itself, and with some variety of pin-outs among the various versions. You don’t need to ask how I know, I will tell you.

Again pulling out my smartphone, I googled for a datasheet. Still not thinking at maximum gain, I clicked on the first promising link, and I found the pin-out for the 710’s TO-39 case is E-C-B, with the flat facing the viewer. Naturally, I wired it accordingly, but no JO2. Now a bit more alert, I confirmed all connections were correct and sound. I was about to lookup other versions of the schematic to make sure the fault wasn’t from that source, when I realized I hadn’t tested the transistor or the crystal. I don’t usually test passive components, but I think it’s a good idea to test active ones when practical. Pulling the 710 out of the circuit, my cheap-as-dirt (hereafter CAD) transistor tester confirmed the device was okay, with a β of 90, a Vbe of 754mV, and a pinout of B-C-E. “Okay, there’s nothing wrong with the transi . . . . wait, B-C-E?!” So I soldered it back in place, with the base lead on the left this time. The circuit oscillated, but still no joy. My CAD Sanjian frequency counter indicated a frequency at the third overtone of the colorburst crystal:

After tightening down the compression cap as far as I could, I got it to jump down to the second overtone:

It’s important to note the frequency was not by pulled continuously down to the second overtone. It actually pulled only a few-hundred Hz before it jumped down. Many crystals can operate (stably) only at their fundamental frequency. This one was stable at the second and third overtones. I wonder if it would be at the higher ones?

Okay, that was a very interesting diversion, but I still needed to get it to work at 3.579545 MHz. I wasn’t long finding the problem: I had left one end of the 33pF capacitor I wanted parallel to the trimmer unsoldered. A puff of rosin smoke later and proper JO2 was achieved! A little tweaking of the trimmer got it dead stable on the nominal frequency. All I could say at that point was SOSINTS! (“The Smell of Success is Never Too Sweet”–pronounced sō-sǐntz).

At the risk of splitting hairs (another of my hobbies), it’s worthwhile noting that overtone I isn’t exactly the same thing as harmonic. A harmonic is an exact integer multiple of the fundamental frequency. So for this particular crystal as trimmed to its nominal, that would be 7.159090 MHz for the second harmonic and 10.738635 MHz for the third. What the counter actually read, though, was 7.158627 (a difference of only 463Hz down) and 10.555258 MHz (a whopping 183.377 KHz greater!), respectively. The second overtone as counted was within the probable pulling range to the second-harmonic figure, but the thing is I couldn’t get it to trim to that before it jumped abruptly to the fundamental. That is, I could not get the oscillator to run at the second harmonic, only the second overtone. With regard to the third overtone, there’s no way any amount of trimming could account for a 183KHz increase in frequency. As soon as I tried to move it more than a few KHz downward, it jumped down to the second overtone. Apparently, the main cause of this behavior and the reason for a distinction between harmonic and overtone is that the motional capacitance (“Cm”)—the effective series capacitance of the crystal itself—is not the same when operated in overtone mode as in fundamental mode, and the Cm plays an unavoidable part in the resonance characteristics of the crystal. See http://hyperphysics.phy-astr.gsu.edu/hbase/Music/otone.html and https://www.electronics-notes.com/articles/electronic_components/quartz-crystal-xtal/overtone-operation.php.

You’ll see from the photos that since my purpose here was to get something done and working with a minimum of fuss, I didn’t spend a lot of time on non-essentials. You’ll also see I had to resort to some infelicitous wire routing. I ended up running the capacitor-side connection to the crystal across the board and under the capacitor itself. It’s only two inches (50mm) long, so it won’t matter at this frequency, but it looks a little goofy. Additional awkward connections were made unavoidable by the way I wound the coil, dressed its leads, and super-glued and coil-doped everything solid. I fed the lead from the top of the primary coil back down through the form, perpendicular to the windings, to emerge from a tiny hole at the foot of the form. I had started the winding by threading the wire through two closely-spaced holes, also at the foot, to keep the wire from pulling out as I wound. I left an inch sticking out for connections. Without thinking carefully enough, I made the lead from the top of the coil emerge to the right of the bottom lead when it would have made for much easier circuit connections had it emerged on the left. It proved to be less of a problem when I decided to abandon an air variable or polyvaricon in favor of a compression trimmer. For a while, though, I thought my version of the Mighty Mite was going to have to play a game of Twister.

In the end, it came out nicely enough. I put some memorial inscriptions on it because someday it will end up in the K7TFC museum (a battered U-Haul box in the attic). As pleased as I am with my M3, the real success here was that I managed to cure both by boredom and my vapor lock with a little accomplishment feedback that took only about an hour and a half to complete. Give it a try sometime if you find yourself in a funk.

QSL?

Still Not Heaven, but Not Hell, Either

Back in 2014, when I was living in Medford, Oregon, I visited Portland and wrote a post about sources for electronic components in the “big” city (http://mostlydiyrf.com/2014/10/not-exactly-heaven-but/). Compared to sources down-state where only Radio Shack was living out its last year of brick-and-mortar existence, Portland was awash in parts sources. Alas, six years later, most of what I found then is gone. Surely this has to do with consumer habits and the rapid rise in online buying, because the three-county Portland metro area is alive with high-tech and electronics activity.

I now live in Portland, the epicenter of the “Silicon Forest”–Oregon’s answer to Silicon Valley. Within a ten-mile radius of my QTH are the Tektronix headquarters, Intel (the largest private employer in the state), Lattice Semiconductor headquarters, FLIR Systems, a big Quorvo campus, a major Maxim Integrated plant, dozens of lesser-known tech firms, Linus Torvalds (the mercurial and irascible inventor of Linux), Jason Milldrum (NT7S, the developer of the widely-used Si5351 library), and all the authors and contributors of Experimental Methods of Radio Frequency Design. Every once in a while, I feel a tingling of RF in my lymph nodes coming from the energy of this technology vortex.

What is almost completely missing from all this now are retail sources for components, even popcorn types such as 2N2222 transistors, 741 op amps, and 4148 diodes. Back in 2014, Fry’s Electronics was the place to go for almost anything one might need except for special-purpose parts. It had the full NTE line, the complete line of Arduino, Raspberry Pi, and the sensors that are part of those ecosystems. There were aisles of Molex, Amphenol, and JST connectors, still more aisles of every sort of cable you’d ever want, another aisle for wire, including several gauges of magnet wire, an aisle of test equipment including digital oscilloscopes, half an aisle of soldering gear and supplies, a five-foot rack of shrink tubing of every size and color, and the largest supply of PC gaming and build-your-own computer components (motherboards, disk drives, power supplies, graphics cards, etc.) in the entire state of Oregon. Batteries? Every kind you can think of and many you didn’t know existed. There was a big long shelf of various sizes of gel cells and LIPO packs, and a charger suitable for each one. There was another few shelves of plastic and metal enclosures from Bud, Hammond, and others. There was lots more, but this was the best-stocked and largest array of electronic components, materials, and supplies I had ever seen and had the pleasure of patronizing.

And now? Gone. All gone. If you go into a Fry’s today, you’ll find mostly-empty shelves and displays. Imagine a Walmart with ninety percent of the merchandise missing, and that’s what a Fry’s looks like now. I have no idea how it’s possible they stay in business at all. They still have a few big-screen TVs for sale, but even their inventory of consumer-grade PCs and laptops is down to almost nothing. There’s some conjecture they’re just waiting out their leases before they close for good. Officially, the company says nothing about the transformation that’s taken place over the past year and a half. To my utter disgust, they’ve instructed their employees to lie when asked about it. “Oh, we’re having trouble with our vendors,” they’d say. “The boss says they’ll be pallets and pallets of stuff any day now.” I heard that line from several check-out clerks when I was still bothered to drive the ten miles just to leave empty handed. It’s possible the employees and even their boss believed that insulting twaddle coming from higher-ups, but it was the same well-practiced answer every time. Pobrecitos y pobrecitas! 

Also gone now is Oregon Electronics. They were a much-smaller outfit, but they still had ten or twenty times more than a typical Radio Shack would have. Like Fry’s, they also carried the full NTE line and had a respectable assortment of cables, connectors, hardware, supplies, and micro-controller (Arduino and RPi) paraphernalia. Their location was a bit out of the way in a business and light-industry park, and the company had the feel of one started and run by enthusiasts rather than experienced business people. In spite of a useful inventory, they may have been under-capitalized and couldn’t hold out long enough to reach profitability equilibrium. Just as likely, they couldn’t compete with eBay, Amazon, Digikey, Sparkfun, and Adafruit for the hobbyist dollar. But for cryin’ out loud! The population of the Portland metropolitan area is almost two-and-a-half million people. One would think it could support at least one establishment like Oregon Electronics.

That brings us to the one bright spot on the electronics-supply landscape: Surplus Gizmos. They have some of the NTE line and some Arduino stuff, but what they mostly have is about 8,800 square feet of customer-accessible space filled with nearly-every kind of surplus electronic parts you could want, some of them decades old and otherwise unobtainium. There’s at least three times that floor space in the back stacked with stuff to be sorted and put out for sale. Here is where the still-valuable offscourings of the Silicon Forest come to wait their turn to once-again have electrons pumped through them in the workshops, Ham shacks, and laboratories of techno-geeks and electronics enthusiasts.

Surplus Gizmos is located in Hillsboro on the western side of the Portland metro area, and home to Intel and several-dozen other tech firms. It’s about five or six miles from Tektronix in Beaverton, and not surprisingly much of the stock at Gizmos is from that source. Here are some highlights:

  • About ten feet of linear shelf space devoted to computer and RF crystals–most of them the older and larger HC-49 type rather than the squat computer-grade ones. This includes a large bin box filled with what must be a few thousand 3.579545MHz “colorburst” crystals with ground wires presoldered.
  • Nearly twenty feet of bin boxes filled with potentiometers, including about three feet of multi-turn precision types.
  • Perhaps forty feet of both large and small transformers of all kinds from “heavy iron” ones for high-voltage power supplies to tiny current and signal-isolation ones.
  • Maybe sixty linear feet of capacitors of all kinds: ceramic, polyethylene, polystyrene, polyester, mica and silver-mica, NP0 and COG, tantalum, and electrolytics with voltage ratings into the 1000s of VDC.
  • Their stock of resistors is smaller at “only” forty feet, but it includes all the types there are, including SMD. By the way, you can get a whole reel of SMDs for less than five bucks if you’re willing to sort through the somewhat-disorganized shelves of them.
  • Almost twenty feet of customer-accessible semiconductors and ICs sorted by type in bin boxes. If you can’t find what you need there, just ask at the counter. They’ve got what could be as much as a hundred feet of sorted ICs and other devices that they’ll dive into and most likely come back with what you asked for. You also need to ask for the new NTE devices they stock.
  • Last but not least, Surplus Gizmos has about a hundred linear feet of used test instruments and other such gear. Most have been tested and tagged with notes on their condition. At any given time, they’ve got five or six venerable and beloved Tektronix model 465 oscilloscopes. I think one of the Gizmos staff is an old Tek tech because these have been gone over, well tested, and repaired if needed. Again, there’s always notes on the condition attached, and they seem to have a standard price of $150 for a fully-functional unit.

That should give you an idea of just how useful Surplus Gizmos is to the electronics enthusiast or radio amateur. One of the things this means is that they know the value of their merchandise and it’s mostly priced accordingly. This is a surplus store, not a flea market. It’s mostly a place to go to get items you can’t get elsewhere, or that you need (or want) faster than it can be shipped to you. Frankly, it’s also a place to patronize frequently so that it remains profitable enough to stay in business. Here in northwest Oregon and southwest Washington, we’re pretty fortunate to have a source like Surplus Gizmos. The Silicon Forest isn’t exactly Heaven for DIY electronics and radio, but it’s a long way from Hell.

 

Postscript: I guess I should have acknowledged that there’s a Ham Radio Outlet “appliance” store in Tigard, on the south-central side of the Portland metro area. For me, their raison d’etre is that they have RF coax and antenna wire that you can’t count on Gizmos having. A few times a year, I get a bug up my cathode (conventional current) and I pay HRO a visit. I buy a CQ Magazine or two, and maybe some coax if I need it. I mean no offense, but I just don’t like factory-made, store-bought Ham gear. It’s not fun for me–unless it’s more than fifty-years old and features vacuum tubes.

Santa Baby!

rigol_ds1102Santa Claus brought me a very nice gift this year. He used his excellent connections with eBay and got me a Rigol DS1102E oscilloscope for Christmas! In my letter to him, I told him of the great reviews I’ve found on it; in particular several videos by Dave Jones of EEVblog and also the personal experience of Bill N2CQR as described in his SolderSmoke podcasts. Bill also has a video showing the DS1102 in action displaying a frequency sweep of a bandpass IF filter: Sweeping a Filter with a FeelTech Sig Gen and a Rigol ‘scope.”

In a few SolderSmoke podcasts, Bill also talked about using the FFT (“fast-fourier transform”) feature on the Rigol’s math menu as a “good-enough” spectrum analyzer. That’s what sold me on the DS1102E.

After a quick trip to the local Fry’s Electronics to pick up a 10x scope probe (Santa forgot that), I gave my Rigol a little test using the built-in 1KHz square-wave generator that’s used to adjust the compensation capacitor in the probe. After adjusting the probe for nice sharp corners on the square wave with no over or under shoot, I turned on the FFT function.

screenshot_squarewave_1khz_fftI was delighted to find just what I expected in the spectral content of the square wave: pronounced peaks on each of the odd harmonics (1st, 3rd, 5th, etc.) and only a gradual decline in the amplitude of each. Here’s the screenshot made with another nice feature of the DS1102E: screen capture as a bmp file and savable to a USB drive.

You can see Cursor A on the fundamental 1KHz wave, with Cursor B on the 7th harmonic at 7KHz. The vertical scale of the FFT portion of the screen is 20dB per division. Notice that by the 25th harmonic (!!) at the far right of the display that it’s down by only about 20dB (for more on the harmonic content of square waves see en.wikipedia.org/wiki/Square_wave and www.allaboutcircuits.com/textbook/alternating-current/chpt-7/square-wave-signals/).

Here’s an especially-cool simulation that shows a series of sine waves at odd harmonics adding up to a square wave: Sine Wave to Square Wave using Fourier Series. This one is even more cool: Fourier Series Animation (Square Wave).

Tomorrow, I hope to pump some RF into the Rigol and see how it likes it. I’m very eager to know how well the FFT function works on the HF-bands frequencies.

QSL?

Embrace the New, But Keep the Old, Too

I’ve recently jumped head first into the mysterious world of vacuum tubes. When I was a child, tubes were those little jewels my father pulled from the back of the old 1954-1000A_linear_RF_deck_build_by_K5LAD6 Zenith television–black and white, of course. I’d go with him to the dime store (remember those?) and he would test them on the cheap emission tester that was placed atop the steel cabinet that held new stock in their little elongated boxes. He’d put some back in his pocket, and some he would toss into the waste basket that the store owner hoped would be used more often than the tester.

Replacing them in the television and switching it on, there was that glow from the little curlicue on the top of some tubes, the bigger ones seemed to glow from down deep in their bowels. After ten seconds or so warm up, the picture emerged, this time no longer rolling from a faulty vertical-hold or whatever it was that was now fixed. And there was that smell–impossible to fully describe–a perfume mixture of warm phenolic, solder rosin, capacitor wax, coil and transformer dope, air ionized by high voltage, and dust.

Bigger tubes, 6L6’s of one subtype or another, made up the cityscape on the open-chassis monaural phono amp (a Heathkit I think) that was mounted on the wall inside the furnace closet. My grandfather had a similar setup, except his was homebrewed and it was distributed on three chassis inside a living-room cabinet. One chassis was for the power supply with its huge transformer and oil-filled capacitors, another was for the intermediate stages of the amplifier, and the third–the largest–mounted the final stage, its immense output transformer, and the also-huge chokes of the crossover network (he ran separate wires for the bass, mid-range, and tweeter to the speaker across the living room). He built a Heathkit tube preamp that I loved for it gold facade and knobs the size of silver dollars, but even his speaker (singular, since stereo was still rare)–the size of a small refrigerator–was home brewed.

The beginning of the end for vacuum tubes in our family came the day the preamp built into the turntable cabinet failed. I don’t know the specific reason, but I distinctly recall going with my father to a local parts store where he bought a kit for a transistorized preamp. The new whiz-bang. The wave of the future.

I guess it was as a kid, then, that electronics meant tubes to me, and the little TO-5 transistor can just didn’t have the same allure. At least it was metal. Now, plastic is the enclosure-of-choice, an army of TO-92s marching into the future. Not only were they tiny and featureless, but they didn’t glow, and they didn’t have the futuristic entrails that so fascinated me then (and now) with vacuum tubes. And without the heat and high voltage, the smell was gone.

Fast-forward more than half a century, and I’m rediscovering 307px-Williamson_home_constructed_amplifier,_c_1949._(9663806448)these little crystalline beauties, this time with some knowledge of how they work, and why. But aren’t they obsolete has-beens? Fragile, bulky, and prone to burn out? The twenty-pound power transformers, and filter caps the size of soda cans? Surely these are the dinosaurs of audio, video, and radio technology.

The thing is, they still work as well as they always did, and they have some characteristics that took a while for solid-state devices to emulate. According to electric-guitar players and hyper-sensitized audiophiles, nothing has yet replaced the sound of the vacuum tube, often described as warmer than modern transistorized and integrated audio circuits. They’re still necessary for high-power amplifiers, both radio and audio.

So why not at least keep a hand in the old tech? Why must new technology mean the total abandonment of the old? In some cases, of course, it doesn’t. Paper, pen, or pencil have not been obsoleted by the typewriter and now by the computer. Books have not gone away, and not likely to. Photography and now video did not mean the abandonment of oil or watercolor painting (or even photo-chemical photography), and I predict that 3D printers are not going to obsolete sculpture, either. I think there’s something healthy about maintaining (and passing on) competence in older ways. It means adding to the body of human knowledge rather than dumping some overboard to make room for the new. If the new truly represents progress, then it has to make the capacity for knowledge larger, and not just the same size with newer contents.

Naturally, this applies to modes of radio operation as well. Digital modes are really cool, but the simple straight key (for telegraph or radio) kept the world informed for a century and a half before it faded into a special skill some worry will eventually die. Using the hand and the trained ear to communicate in dits and dahs is pretty cool, too. Animals communicate with a few tones and clicks patterned in a recognizable manner, and they can do so in a way that can carry some distance and that cuts through the background noise. Surely anything a bird can do we can do better, right? Yes, we too can encode meaning in simple patterned sounds–the dit and the dah–and we can send those halfway around the world with no more energy than is given off by a struck match ( a few Watts).

If there are others of you who are hot on vacuum tubes, or CW for that matter, make yourself known. Share your latest exploits. We are fortunate there are still a large number of Hams who spent the better part of their lives with the old technology. We should beg them to teach us.

73,

Todd K7TFC

LTC6431 Low IMD IF Amp

Recently, there was a bit of traffic on the EMRFD* Yahoo group about a new IF amp chip from Linear Technology: the LTC6431 ( https://groups.yahoo.com/neo/groups/emrfd/conversations/topics/11168). The main interest in this device is its low intermodulation distortion (IM) as expressed by its third-order intercept point–in this case on the output (abbreviated as OIP3**): 47dBm OIP3 at 240MHz into a 50? Load. Its specified bandwidth is 20 to 1700MHz and it provides 15.5dB of power gain.

LTC6431_breakoutThomas Knutsen, LA3PNA, quickly designed up a breakout board for the LTC6431 and made it available at OSH Park for anyone to order (https://oshpark.com/shared_projects/5qq6sXUX). I ordered a set of three for a mere $10.30USD. They were so cheap because OSH Park waits to fill a pcb panel with enough other boards to share the production costs. Once a panel is full, they run it and then distribute the boards to those who ordered them. It’s a great idea, and in this case it only took about ten days for the finished pcbs to show up in my mail. Their quality is excellent. This was my first experience with OSH Park, and I’m pretty pleased.

I haven’t populated the boards yet with LTC6431s, but once I do and play around with them, I’ll post an update.

73!

*EMRFD stands for the title of Wes Hayward’s popular book, Experimental Methods for Radio Frequency Design, published by the ARRL. The Yahoo group is devoted to discussions based on the book and its design examples.

**See https://en.wikipedia.org/wiki/Third-order_intercept_point. For a not-too-difficult discussion of IM, see Doug Smith, KF6DX, “Improved Dynamic-Range Testing” QEX July/August 2002 (http://www.arrl.org/files/file/Technology/tis/info/pdf/020708qex046.pdf).

 

Vackar VFOs

Over on the QRP-Tech Yahoo group, Ed Edwards, AE7TE, posted a link to an article (http://www.w7ekb.com/glowbugs/VFOs/vackar_vfo.html) by Kenneth Gordon, W7EKB, on Vackar-design VFOs designed in the 1950s for vacuum-tube applications. The Czech engineer Jiri Vackar introduced the design in 1949*.  The schematic example Kenneth shows uses a 12AT7 dual triode in common-cathode mode with both plates at 255VDC. The driver is cathode-keyed. The output triode is arranged as a cathode follower. The VFO is series-tuned.

Ji?í Vacká?
Jiri Vackar

According to W7EKB, Vackar VFOs have three important design characteristics: they have a 2.5:1 tuning range, they have a constant signal-level output across that range, and they’re “rock solid” in their frequency output. It so happens I have a bag of 12AT7s just waiting for me to play with. I’ve been wanting to design and build a single-band tube transceiver just for the fun of watching the little buggers glow in the dark. Someday soon I hope to test this Vackar design as part of that longer-term project. I’ll let you know how it comes out.  73.

—————————————————-

*For more on Vackar and his design, see https://cs.wikipedia.org/wiki/Ji%C5%99%C3%AD_Vack%C3%A1%C5%99 (copy and paste text into Google Translate) and https://en.wikipedia.org/wiki/Vack%C3%A1%C5%99_oscillator. For a solid-state version of the Vackar VFO, see Floyd Carter, K6BSU, “Meet the Remarkable but Little-Known Vackar VFO,” QST September 1978: 15.

A Circuit Development Base

DSC00624I hate haywire. I can’t stand jumpers and cables and cords strewn everywhere when I’m working on a project. Psycho-optically, I’m driven nuts by the rat’s nest that results from power and test leads when the project is in full swing.

I’m especially put out by stiff cables knocking the usually-small items I’m working on all over the bench. An Arduino, being small and pretty light, is nothing to a springy USB cable. Coax, even the relatively-thin type such as RG-8x, will be happier straight than bent, and it will likewise toss the thing I’m working on back and forth. Arrrggg!

DSC00625My solution is to get as much anchored and tied down as I can and I’ve found what I’m calling a “development base” to be invaluable. Here you see an Arduino Uno with an Adafruit Proto Shield on which is mounted a small breadboard, an Adafruit breakout board for the Si5351 clock-generator chip, and a rotary encoder. It’s a project I’m working on for a simple antenna analyzer.

You also see a 2×16 LCD display, and both it and the Arduino assembly are mounted on standoffs. The white base is a piece of expanded PVC I had on hand, but it could just as well be anything of a suitable size and heft—including a real breadboard. You’ll also notice I’ve affixed (with repositionable glue stick) a chart of pinouts for easy reference. Other important notes can be placed on the base as well, perhaps even with fine-tipped dry-erase markerDSC00626s or a grease pencil.

 

 

 

Instead of haywire jumpers, I’ve used ribbon cable to the display, and even the jumpers I do use on the Arduino assembly are routed, folded, zip-tied, or even loosely knotted to keep them in check. Neat and tidy.

There’s plenty of real estate left on the base to mount other boards or circuit components, perhaps with nothing more permanent than double-stick tape (not the semi-permanent foam type!), Bostik’s Blu-Tack®, or Elmer’s Poster Tack®.

DSC00627I took the trouble to mount the Arduino and the LCD display with screwed-down standoffs since I’ll be using the same footprints for other development projects. They might even stay there more-or-less permanently, to be replaced by other ones in the finished projects.

The size of the development base is not important except that it be adequate. One other advantage to using a base of this sort is that it can be picked up and moved intact, to be replaced on the bench by another suddenly-more pressing project, or moved out of the way of dinner dishes if it’s on the kitchen table.DSC00628