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!

Links/Reference

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

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