Xtaflex 40m VXO regenerative receiver - Michael J. Rainey, AA1TJ

This circuit is based on the Autoflex/Spontaflex receiver designed by Sir Douglas Hall. It turns out that Sir Douglas' clever circuit works well as a one-stage, crystal-controlled (VXO'd) regenerative receiver.

As he explains in his June, 1964 article (thanks Geoff!), the transistor functions as a common-collector radio-frequency (RF) amplifier in which the gain is augmented by regeneration. The high impedance looking into the base helps to reduce the input tank circuit loading. C8 places the collector at RF ground. Demodulation is provided by the Germanium diode at the emitter. The base of Q1 is at ground potential for AF. The parallel combination of C8, T4 and the headphones provide a 15k Ohm collector load resistance (15k + j0 Ohms) at 700Hz. Q1 operates as a common-base amplifier at AF. The circuit may thus be described as a crystal-controlled, regenerative, reflex receiver; or, Xtaflex, for short!

This crystal-controlled regenerative detector is virtually immune to frequency shifts due to hand-capacitance or antenna "swinging." Neither does the frequency pull when receiving strong signals at a low beat note. Here is an audio snippet sampled at the headphone terminals. Please notice how it's possible to tune through zero-beat with a strong, incoming signal without the slightest hint of synchronization (frequency "pulling"). In operation, this circuit "feels" more like a direct-conversion receiver than a straight-regenerative set (of course, a regenerative set is a direct-conversion receiver "at heart").  
The circuit shown in my schematic diagram is built for 40m. I've used it on the 30m band by changing the frequency-sensitive components. In fact, the rock-solid frequency stability of this regenerative receiver will shine progressively brighter as the frequency is raised. What's more, the degree of VXO frequency shift will increase along with the operating frequency. My 40m prototype exhibited a VXO shift of 3kHz. On 30m the shift was 5kHz.

Under crystal control, a stable regenerative receiver for 15, 10 or even 6m appears to be a practical proposition (an RF amplifier placed ahead of the detector will likely prove useful on these higher bands). On these these higher frequencies it may be possible to eliminate the bandpass filter and connect the signal source directly to the C4/C5 node.

Generally speaking, there are a few tricks for obtaining smooth regeneration using modern, high current-gain, transistors. Wes, W7ZOI, recently mentioned a friend of his that builds smooth-operating regenerative detectors from (modern) bipolar transistors by swapping their collector and emitter in order to reduce the current gain. I've been achieving the same results (without needing to swap the emitter - collector) using early, low current-gain, Germanium transistors. The Xtaflex, for example, uses a Philco, 2N504 MADT (Micro Alloy Diffusion Transistor); date-coded, September of 1959. Another example is my Talking Doll, which uses a 2N107 in the regenerative detector.

Charles, N1TEV's well-known bipolar regenerative detector achieves the same end with a 2N2222A, running at an unusually low collector voltage.

Lacking such methods the device transconductance will increase dramatically with collector current; as noted on page two of Ian Hickman's Imp (click-on "PW Imp: I. Hickman," third from the top) receiver article. Ian cleverly linearizes the regeneration control by using a differential-pair in his bipolar transistor-based detector.
I would like to express thanks to my friend, Jim Kearman, KR1S, for re-planting this idea for crystal-controlled regenerative receivers. I happened to be doodling with a Sponatflex receiver when Jim's message arrived, telling of his experiments with crystal-controlled regnerative detectors. Talk about serendipity!


http://qrp.kearman.com/html/vxoregen01.html  Jim, KR1S's, JFET, VXO regenerative detector

http://home.comcast.net/~phils_radio_designs/  Dee/Mitch-Dyne; the JFET diode is interesting

http://www.io.com/~nielw/3tube_xtal/3tube_xtal.htm  Quartz crystal inside regenerative FB loop

"An Ultra Simple W1AW Receiver," QST Magazine, May 1997, by N1TEV and WU2D

The old website was http://www.aa1tj.com/xtaflex.html

80m Passive Heterodyne Receiver - Michael J. Rainey, AA1TJ

Using a full-size antenna and a reasonably sensitive headphone, this simple switching mixer will produce an amazing abundance of signals on 80m.

I used a 74HC4052 CMOS multiplexer for the simple reason that I found one in my junkbox. Almost any similar CMOS switch capable of working at this frequency would be equally useful. Likewise, the input balun could be wound on virtually any low(ish)-loss, moderately high-permeability core. Aim for a minimum of 20uH, or so, of inductance per winding. I stole the square-wave BFO from my current transmitter which uses a VXO circuit similar to the one used in my Snowflake. Of course, any number of simple square-wave oscillators - built around CMOS or TTL logic gates - would do as well.


Please bear in mind the mixer output impedance is roughly the same as the source impedance; typically, 50 Ohms. Directly coupling a pair of 600 Ohm headphones to the mixer - as I show in the schematic - will result in an insertion power loss of roughly 5dB. There are a number of ways to improve the impedance match to the headphones you choose to use. An audio transfomer between the mixer output and the headphones is an obvious possibility. My solution was to alter the primary/seconday turns-ratio of the input balun transformer in order to raise the mixer working impedance to approximately 600 Ohms. For this, I used a turns-ratio of 1 to 3.5.

In the past ten days I've used this receiver to make a total of twenty contacts on 80m. I particularly enjoyed working Paul, VE1DY, who was running an Elecraft K2 at 4W, and Bill, W9VC, using his Drake 2NT with an output power of 66W.

The 80m square-wave BFO

Construction information

T1: 7 trifilar turns on FT37-43 or Minicircuits T4-1, etc.
IC1: 74HC4052; ground pins are 2,5,6,7,8,14,15
U1 (BFO): 7400 TTL NAND
X1 (BFO): 3.58MHz ceramic resonator
VC1 (BFO): 20-60pF variable capacitor

The old website was http://www.aa1tj.com/80mttldirectconversionreceiver.html

80m TTL ("One Chip") Direct Conversion Receiver - Michael J. Rainey, AA1TJ

This simple receiver is constructed around a single, 7400, Quad-NAND, TTL, integrated circuit. Please don't substitute 74S00, 74LS00, 74HC00, etc., for U1 in this circuit. At a minimum, the biasing would be incorrect for devices other than the "Plain Jane," 7400 IC.

Two gates create a VXO with a square-wave output of sufficient frequency-range to cover most of the 80m CW band.  

The third gate functions as a linear RF amplifier which is enabled or disabled by the VXO signal on U1-pin 9. This switching action creates a simple product detector. Resistors R3 and R4 set the proper bias for linear operation of the enabled amplifier.

L1, C5 and C6 comprise an impedance matching network having a "peaked" lowpass characteristic. This provides a measure of AF bandpass filtering.

The fourth gate acts as a linear AF amplifier. R5 and R6 produce the DC bias needed for linear operation. C7 rolls-off the amplifier response at high frequencies.

Of course, it's possible to use an input bandpass filter other than the one shown in the schematic. The mixer input impedance is approximately 1200 Ohms.

Some oscillator leakage is to be expected using this type of mixer. I measured 6uW of leakage into a 50 Ohm resistor connected to the antenna terminals. Also, I wouldn't advise using this circuit at 40m; I suspect the SWBC bleed-through would be objectionable.

On my first night (April 2, 2008) using this receiver I was pleased to have worked Michel, F5IN, as well as Pat, KØPC. Michel was my first transatlantic DX contact in over 30 years. A few days later I had a message from Pat, saying that he'd looked through his Novice logbook and discovered that we'd met before on the air. On January 3, 1971, Pat (then, WNØZSF) and I (then, WNØDEN) had worked each other on 40m. I'd been using my Ameco AC-1 and Realistic DX-120 station.

Construction information

T1: 10mm, shielded "IF can", 16-turn main winding, 3.5 to 6uH; 1-turn coupling
T2: Ditto above, except 6-turn coupling
U1: Basic 7400 TTL Quad NAND (not 74LS00, 74HC00, etc.)
Ground U1 Pin 7
Measured bias voltage; U1-pin 10 ~1.17Vdc, U1-pins 12/13 1.45Vdc
U1d output impedance ~100Ohms; use transformer matching for other headphone impedance

The old website was http://www.aa1tj.com/80mttldirectconversionreceiver.html

80m ceramic VXO - Alan Yates VK2ZAY

The following schematic has been taken from http://www.vk2zay.net/article/95 article about an VXO 80 metre receiver.
"I started by building a VXO circuit, to see just how far these things could be pulled. It was found to be quite practical to pull them about 100 kHz and still have a stable oscillator, meaning the two available devices could cover most of the interesting parts of 80 meters."

Construction information

The ceramic resonator are 3.58 or 3.68 MHz. The 2 transistors are BC549 used as oscillator and 2N3904 used as buffer.

40m receiver with Polyakov Detector - Michael J. Rainey, AA1TJ

T1,T2, T3: 4uH with adjustable slug, 10mm square, shielded; pry off shield to wind coupling-link over grounded end.
Headphones: 600 Ohm impedance or higher; change AF transformer (10k:500 Ohms), T4, to match phones used.

The old website was https://sites.google.com/site/mjrainey/

80m band pass filter and amplifier - Alan Yates VK2ZAY

The following schematic has been taken from http://www.vk2zay.net/article/95 article about an VXO 80 metre receiver.

Both band pass filter transformers are 455kHz IF transformers that are rewound with 12 turns and 2 turns on the primary and the original capacitor has been removed. The transistor used in the amplifier is a BC549.

Regenerative Receiver for 40 Meters - Rick Andersen KE3IJ (revised 2006)

[I have revised this article as of January 15th, 2006, showing an alternative to the toroid-core inductors of the original design. The revised version uses #26 or #28 enamelled copper 'magnet wire' (such as sold in 3-packs by Radio Shack) wound on McDonalds' plastic straws as coil forms, then covered with molten candle wax to hold the turns in place. See details below.]
I absolutely love regenerative detectors. They amaze me to no end. For those of you who don't know the basics: 99% of all commercially-made radios these days are based on the Superheterodyne design. It has proved itself worthy of universal praise and reliability many times over since it replaced earlier designs in the 1930's. It made Television practical. It will probably always be used as the core of any communications receiver ever built in the future.

But there were earlier beasties that worked well, too, at least for the era in which they lived. The...THE...most sensitive design was the Regenerative circuit. The idea, invented by Major Edwin Armstrong in the 1910's, was to 1) tune in a feeble radio station, 2) amplify it at RF [he used a vacuum tube; we use a transistor today], and, here's the punch-line: 3) feed a small fraction of the amplified signal back to the input, in phase with the incoming antenna signal. A snowball effect occurred, where the signal was reinforced by a boosted version of itself, over and over again -- the precise amount of positive feedback usually held in a delicate balance, right at the edge of the point where the tendency would be to break into a squealing oscillation (like the inevitable feedback howl that your high school principal would experience at the podium microphone when he was getting ready to bawl the student body out about some prank somebody pulled). It is at this threshold that the regenerative detector has unmatched sensitivity (well, the Superregen, covered elsewhere, is actually better, but not as selective).

It was found that this just-before-oscillation setting of the feedback or Regen control was best for listening to AM radio stations; advancing the regeneration any further would cause the station's carrier frequency to be heard as an audio beat note -- a loud squeal -- so your Grandpop would tune in a station with one hand, then set the regeneration to just below oscillation with the other hand. For CW (Morse Code transmissions) or SSB (Single Sideband transmissions) he would leave the regeneration in its oscillating region; the only way to hear the "beep, beep" of a Morse Code signal would be to purposely generate that audio beat note, by "beating" or heterodyning [subtracting the frequencies of] the on-and-off, inaudible CW carrier with an almost-the-same-frequency "fake" carrier generated inside the radio itself... which is what you get when the feedback is past that 'avalanche' point mentioned up above.

So why do we use Superhets nowadays, and why are Regens ancient relics?

Well, because Superhets are free of almost every problem that plagued the earlier types of radios. But we Radio Geeks don't care about the Good Life, do we? We would rather build something that takes a brain surgeon's steady hand and patience to operate, than be hypnotized by the wiles of Madison Avenue! Besides, We built it ourselves!

So be warned: Regens are not "user-friendly" in the convenience-store, modern-day, spoiled-brat, lazy-ass sense. (Yep, I'm soap-boxing again!) Instead, they're fussy little circuits that need re-tweaking and fidgeting as you tune up and down the band.

They're notorious for accidentally re-radiating (transmitting) their own internal RF oscillations back out into the world (when you use a single regen detector stage, directly connected to an antenna. You can solve that problem by putting an RF Amplifier stage ahead of the main detector, which then serves to isolate the antenna from the oscillating detector. The gain of the new stage need not be high, since the Regen stage already has fantastic sensitivity, and you want to avoid overloading it with too strong a signal).

They share, along with another kind of receiver design called the Direct Conversion receiver, an annoying susceptibility to front-end AM overload (you hear your normal stations normally, but at the same time some super-powered station like Radio Moscow or Radio Havana Cuba can be heard simultaneously, all over the dial, even though you're not tuned anywhere near its actual frequency) but, again, this can be improved by adding another stage with another tuned circuit, or else using a resonant bandpass antenna tuner to provide some extra rejection of out of band signals. (The Direct Conversion crowd does it by using what is called a Doubly-Balanced Mixer in the front end; I felt I'd better mention that before they start sending me nasty emails about my ignorance of the new and improved direct conversion designs out there.)

Without the front-end isolation mentioned above, regen receivers will also rock back and forth slightly in tuning frequency as the wind blows your antenna wire back and forth outside -- very annoying if you're listening to a single sideband voice signal or CW.

If you don't build 'em with at least some metal ground plane or shielding in the box, you'll notice that body capacitance can be a problem: the tuning shifts when you reach for the tuning knob! (A similar effect with nearby metallic objects led to the development of metal detectors; one man's problem is another man's solution!)

And, most annoying to me, but not mentioned too often in other articles about the regen, is that the tuning changes when you advance the regeneration into oscillation. So if you build your own Shortwave radio using a regenerative detector, and attempt to calibrate its tuning dial (I paste a piece of semicircular paper on the front panel and make pencil markings), you will find that the tuning is 'off' when you're listening to CW or sideband, from when you calibrated the dial for AM, just before the point of oscillation.

Well, not a true Electronics Geek / Radio Head. We love this little circuit because it's almost magic. It can dredge the weakest signals almost out of thin air due to the ingenious application of controlled positive feedback. We just need to be able to control it, that's all! If you're a Generation-Xer, but have been bitten by the Bug (why else would you still be reading this page, you sadist!?), know that your Grandpappy loved, cherished, and relied on his Homebrew "Genny"; your Dad (my age) at least built a few of these as "toy" kits sometime during the 1950's - 1970's, and a spate of articles in QST and other Amateur Radio publications appeared in the 1990's, especially by Charles Kitchin, that revived the old Regen for a new generation of tinkerers, with some new bells and whistles added to smooth out some of the bugs that Regens were known to have.

I guess I've waxed eloquent enough.

My 40 Meter Regenerative Receiver

Here's a schematic of my version of the trusty old Regen radio. The heart of it is Q2, where I use a common JFET (Field Effect Transistor), an MPF102, in a modified Armstrong "tickler coil" design where I supply feedback from the "bottom"-- the Source of the JFET -- rather than the usual top -- the Drain of the FET. Works just as well either way.
The tuned circuit is set up for the 40 meter Ham band -- approx. 7.1 - 7.35 MHz in this radio.

Simple regenerative Radio Receiver by Rich Bonkowski – W3HWJ

Charles Kitchin has written many articles about regen radios for QST and other hobby magazines. I like his approach to design and have had good luck with his circuits. The first attempt at a "Kitchin" used a printed circuit board that I bought from Far Circuits. This was an excellent learning tool and prompted me to continue experimenting and changing the design and the circuit board.

To house this project, I "re-purposed" a metal container that originally contained Christmas cookies. After drilling the mounting holes for all the controls, I used a bit of steel wool to create a matte finish, cleaned with mineral spirits, and then spray painted. The labels for the controls were made with a Brother P-touch labeler. Thanks to Dave Schmarder, N2DS, for getting me interested in the Brother. It makes a nice laminated label that is good-looking, sturdy, and solidly adherent.

By changing the plug-in coil, I can cover 3.5 to over 10 MHz. It's a bit difficult to copy SSB signals as the detector is not super stable, but it works well enough! I made a few mistakes in this version, such as mounting the speaker on the side of the case. It needs to be on the front or the top cover to direct sound toward the listener. Also, using a large value main tuning capacitor proved to be a problem, even though I used an 8:1 reduction dial. The vari-cap "fine tuning" is definitely needed to copy SSB successfully!

Things I learned:

  • For drilling large holes in thin sheet steel, a stepped drill bit (Uni-bit) is very helpful. The Harbor Freight Tools version is cheap but serviceable. 
  • Spraying a lacquer "clear coat" over a not-fully-cured enamel paint job can cause an un-intended "wrinkle" finish. You can't see the cover in these photos. 
  • Designing and making your own pc board is challenging and educational. I learned about ExpressPCB software, blue Press 'n Peel transfer film, and using peroxide and muriatic acid to etch boards. I also found that the toner used by Brother laser printers doesn't work as well as HP toner for making etching masks using Press 'n Peel film. 
  • PVC plumbing pipe and fittings make good coil forms when used in conjunction with bases salvaged from defective radio tubes. This design uses a 6-pin tube base. At most ham swap meets, I can buy dead 6-pin tubes for 25 cents.

The RF stage is using an 2N2222 bipolar transistor, the regenerative detector an N-Channel RF MPF102 transistor and the audio amplifier a LM386 IC.

This receiver has been taken from http://www.w3hwj.com/index_files/HBradioweb.html website.

Super-regenerative Air Band Radio 108MHz-136MHz - Pilar Ortega

Civil aviation still uses AM communications between 108MHz and 136MHz. It is fairly easy to build a receiver that operates on these frequencies, however, it will also be found easy to build a receiver that won’t tune high enough! The trick to getting the tuning range high enough is to use a high frequency transistor and VERY short leads. I used a BF199 transistor but a 2N2369 will probably work well too.
By far the most reliable construction method is ‘ugly’ construction on a 1 inch by 2 inch piece of copper clad board (I built this circuit on perforated board many years ago and it only just tuned high enough to receive my local airport signals). By using the copper foil as a ground plane we can mount components in mid air. This is more robust than it might seem because many components connect to decoupling capacitors that provide an anchor to the board. Keeping leads short reduces stray inductance and capacitance, and helps ensure stability and good tuning range. The circuit is based on an old one advertised in a hobby magazine many years ago, modified slightly.

The tuning coil "L" is simply four turns of 0.8mm diameter wire with a diameter of 5mm. There is no former. The coil is compressed or expanded to adjust the tuning range. We need to arrange that we can just tune the upper end of the FM broadcast band when the tuning potentiometer is at minimum setting (wiper at the ground end). This will bring the Air Band into the main tuning range of the receiver. The antenna is a short telescopic one and couples with a ¾ turn link turn. Note that, if the coupling is too tight (the link too close to the main coil) then the regenerative stage will be too loaded and won’t oscillate.
The varicap diode ‘D’ was an unmarked component in my receiver, however, the BV409 should work admirably here. All capacitors with the exception of the electrolytic are ceramic. The transistors are best mounted upside down with their leads bent sideways.
No switch was used in my receiver. Instead, a stereo 3.5mm socket was used and the third contact used to switch the supply. Note that this trick only works when a mono plug is inserted! I mounted the earphone socket such that it traps the battery in one corner of the case. I expect this apparently cool solution is going to come back to bite me when I drop the radio, and the socket gets smashed off by the momentum of the battery!
In operation the regeneration control is increased until a rushing sound is heard. This noise indicates that the circuit is oscillating but will diminish when a signal is received. At this time the tuning and regeneration potentiometers can be adjusted for best reception. The receiver will be found to be most sensitive when the regeneration is set so that the circuit is just oscillating.
Purists will add an RF gain stage prior to the regenerative section, as this minimises coupling of oscillations to the antenna as well as adding sensitivity. In practise the circuit is sensitive enough but one should be aware that RF radiation from the antenna can interfere with nearby equipment, in particular, televisions.

This receiver has been taken from http://geopodium.com/files/Pilar/index.htm website, which is currently unavailable. Apparently the new homepage is http://techlib.com, but this receiver is no longer posted on the new website.