The AGC-80/30

A Dual-Band Regenerative Receiver
for 80 Meters and 30 Meters,
with Audio-Derived AGC

March 13, 2007 by Rick Andersen, KE3IJ

NOTE: This article is in its raw, unfinished state. I will be polishing it over the last week of March 2007; in the meantime, it's up at the website so that anyone who has either built the original AGC-80 receiver, or is interested in a generic-transistor approach to a Regenerodyne radio [a tuneable Regen detector preceded by a switched Crystal Converter for multiband reception], can see my first attempt at converting the AGC-80 into a Regenerodyne. Please bear with me as I flesh-out the article over the next few weeks!

One thing that I knew had to be dealt with was the need for some LC filtering up-front. The first embodiment of the AGC-80 boasted that there was no need for ANY inductor or filter other than the toroid being used in the detector; very soon afterward I realized that my resonant antenna tuner was doing some selective filtering for me. If I took the ATU out of the circuit and connected the AGC-80 directly to the outside antenna (a horizontal loop, self-resonant at 2.6 MHz), there was some out-of-band bleedthrough in the form of many tweeting carriers, but not in the form of "AM blanketing" that often plagues simple receivers [where Radio Havana Cuba is heard in the background, no matter where you tune on the dial]. So I added a simple series LC (series because I designed the front-end around a 50-ohm impedance, which requires that LC configuration) of 12 uH and 150 pF, and immediately had a useable 80 meter Regen receiver with no noticeable out-of-band interference.

After a while I began to think about eventually turning my AGC-80 prototype into a "Regenerodyne", and realized that there would be potential problems with "images" and also cross-talk (better, "leakage" or "bleed-through") between the 80 and 30 meter bands. So I began designing fancier filters, some of which made their way into my AGC-80 prototype several months before I finally decided to sit down and design a front-end crystal converter. That's why Filter #2, in the schematic below, can be seen in my AGC-80 article as revised February 2007. But whereas the fancier filters really weren't necessary in the original AGC-80, they're essential in this new AGC-80/30 dual-bander. I used an old version of MicroCap III to design and perform an AC analysis (Amplitude vs. Frequency plot) on different filter configurations I came up with, finally settling on the #3 form shown in the diagram below. The advantage to this circuit is that I don't need variable tuning caps; I just put larger-than-expected values of capacitance in the filters, with easily-obtainable values such as .001 uF [1000 pF], and am therefore able to wind inductances with much lower turns than normal. The Q of these filters seems to be high enough to provide good rejection of out-of-band signals. The capacitors I use are the green "chiclet" rectangular mylars that I originally got from Electronix Express several years back.

Some of you may find my use of McDonalds straws a little silly-- frankly I have gotten too many emails from people who seem to be hung up on those darn little Amidon red-gray T50-2 cores that I used in earlier projects. I also did not want to go with 1 1/2 " toilet paper cardboard rolls, or 1" plastic prescription bottles as coil forms. So I opted for small, 1/4" diameter plastic forms on which to wind my coils....which just happened to be widely available after each "McMeal." Sure, you need a few more turns than you would if you were using a larger diameter coil form, but I compensate by using bigger-than-normal values of capacitance in my filters, which cuts the inductances' turns down a bit and work just as well. So that's my McDonalds alibi.

I finally settled on filter #3 as my basic tuned circuit used in the front-end of the AGC-80/30. Sideways Pi.... Combo series LC and parallel tank LC.........with better selectivity (Q, sharpness) than either individually CT gives step-up then caps give step-down through AC voltage divider action.... Need one at the receive frequency (10.1 MHz / 30 meter band) to reject surrounding frequencies and especially the IMAGE at (RF + 2 IF) = (10.1 + 2 (3.8))= 18 MHz roughly.... Need another at the "down-converted" or ("I.F.") of 3.8 - 4 MHz or so of the original 80 meter receiver (the AGC-80). Later, found the need to add an 80 meter "Wavetrap" up front, to stop the bleed-though of 80 meter signals into the receiver while listening to the 30 meter band. Wavetrap is a parallel tank circuit whose high impedance at the resonant frequency (approx. 3.8 MHz) effectively blocks the 80 meter signals, reflecting it back to the antenna. At 10.1 MHz this 80m wavetrap looks like a short-circuit (a clear path through) to 30m signals.

The AGC-80/30 Front End

The new schematic is shown below. In brief, it's a 3-transistor add-on to the front of the original AGC-80's "AGC" circuit. To avoid having to renumber all the transistors, I've left the original AGC transistor as "Q1", etc., but I've given the new transistors slightly more descriptive reference designators: The Local (crystal) Oscillator transistor is named "Qosc", the Mixer/Converter is called "Qmix", and the post-mixer RF amplifier is called "Qamp". That last stage (Qamp) was found to be a necessary addition since the reception of 30 meter signals seemed weaker than the straight-through 80 meter signals. You may consider it to be a 'first I.F stage' if you want. Its [PNP] collector load is the LC tank circuit that serves as an 80 meter preselector at the input of the original AGC circuit, where the antenna-protection diodes used to be. (I took them out because there is a direct DC path to circuit ground through the centertapped inductor(s), now. They would be superfluous, whereas in the original circuit, there was a blocking cap and the antenna itself had no way to bleed any accumulated charge to ground.

[Much more detailed description of the new front-end soon to come.....please be patient!

The Regenerative Detector -- the heart of the receiver -- is shown in the schematic below. It is the same circuit as that used in the AGC-80 receiver, and a detailed analysis of it can be found in the AGC-80 article. The only difference between the original AGC-80 version and this AGC-80/30 version, is the inclusion of one additional 1N4001 rectifier diode in parallel with the two that were in the original circuit. The addition of this 3rd diode stretches the tuning range to approximately 200 KHz, which works out well for the 30m band, since it falls between 10.1 and 10.15 MHz (150 KHz wide). The 200 KHz allows reception of the 30m Ham band, and also time station WWV at 10 MHz, whose familiar ticks and alternating tones provide a benchmark telling me where I am in an uncalibrated homebrew world without any Frequency Counter to point the way. Listening to WWV is also very helpful when you are peaking the 14-turn centertapped coil on the McDonalds straw -- squeezing and spreading the turns on this coil cause a very sensitive detuning of the 30 meter preselector filter -- and you need to find WWV and peak it, then drip some melted candle wax on the coil to hold the turns in place where you set them. If you've peaked WWV at 10 MHz (which, by the way, MAY mean you will have to wait for nighttime to do this, depending on where you live), you can be sure that you will be able to pick up the CW stations between 10.1 and 10.15 MHz (30 meters). If you mis-tune that preselector, you may hear lots of carriers and/or digital communications 'hash', but you won't be listening to the 30 meter band! You may actually be hearing somewhere around 9 MHz or 11 MHz. So be sure to peak your coil with WWV as your test station.

There are no changes at all, from the original AGC-80 receiver, in the 2-stage audio amplifier circuit shown in the schematic below, except that readers who remember the "ORIGINAL original" (first published in March 2006) will note that I have added a quick-n-dirty passive audio filter between the Volume control pot and the Audio Output jack which I feed into an external Amplified Speaker. With no "single-signal" capability (unlike a Superhet receiver or even the "R2" Direct Conversion design with its phasing circuits for opposite sideband nulling), the chirps and squeaks get to be annoying after several hours of listening. I find that even a mediocre low-pass filter helps mellow the audio and makes copying CW less distracting when there are several high-pitched carriers dit-dah-ing away several KiloHertz off.


I hope you will consider putting one of these together and letting me know how yours performs. Feel free to email me at rick@ke3ij.com


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