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03-26-2008

AM Walkie-Talkie Experiments

Alan Yates

Michael Rainey (AA1TJ) has been acting as my muse again. He knows of my fondness for minimal component radios and sent me an old Italian article "Il Pigmeo", which features a one-transistor 27 MHz walkie talkie (apparently capable for 400 metres range). Naturally with my high school Italian long forgotten, I couldn't read a word, but the circuit diagram was quite clear.

The circuit uses a 3PDT switch to make one transistor (an OC171) behave as either a collector modulated Pierce oscillator (with a carbon microphone), or a super-regenerative detector feeding a dynamic earpiece through an impedance transformer. The power supply is a 9 Volt battery and the telescopic antenna contains a base loading coil.

The switching did intimidate me a bit. While not a complex circuit and having a bag of 4PDT switches in stock, the switches are unfortunately for rectangular mounting holes and I didn't feel up to drilling and nibbling a neat hole in a piece of PCB just to build a prototype. Instead I built the TX and RX circuits independently to see how well they'd work.

Transmitter

I started with the TX as the receiver looked "odd" and I wanted a nice stable signal source for experimenting with it later. Even just thrown together on a solderless breadboard the circuit worked well - that said, you can't really stuff up a Pierce oscillator.

Prototype TX Breadboarded

I didn't have a carbon microphone, so I put another transistor in the collector circuit as an emitter follower and biased it such that its emitter sat at mid-rail. Capacitive coupling my AF signal generator into its base gave me beautiful AM modulation.

First AM Modulation Waveform

Connecting up to the old VR-500 receiver tuned to 2BL gave me a chance to listen to the audio over the air. Listening on my FT-815 and RCI-2950 proved it was quite a reasonable fidelity TX. To make it a stand-alone test transmitter, I built a phase-shift oscillator to feed the modulator.

Prototype Self-Contained AM Modulated TX

This was quickly built into a more permanent version, as two modules, the AM TX itself and the AF oscillator signal source. Each is quite useful in its own right.

Permanent Boards for the Test TX

The collector modulation looks wonderfully linear on a trapezoid test.

Trapezoid Test of the AM TX

Receiver

With a good test signal now available, I started on the RX side. I imagine the circuit was meant to be super-regenerative, but it wouldn't super-regenerate for me. Perhaps I stripped out too much circuit when removing the TX bias and crystal, I've seen a base-quenched circuit before, but try as I might it wouldn't super-regenerate for me. Instead I replaced the emitter circuit with my more familar RFC to quench RC and it sprang to life.

Prototype RX Detector

I was fortunate enough to have a variable inductor in stock from an old eBay win that would cover the roughly 1.56 uH I needed to resonate with 22 pF I picked. I carefully dismantled the inductor (5 pin base, only a single untapped ~ 1.4-1.7 uH winding) and wound a two turn link onto its spare pins for the antenna connection.

With a small AF transformer in the collector feeding a sensitive telephone receiver, or a piezo earpiece across a 10K collector resistor I got audio of a similar amplitude to that of my single transistor transceiver. This is readable, but not very compatible with a noisy environment. To improve the circuit I built a two stage amplifier with some filtering that it capable of driving 32 Ohm headphones. The circuit is a simple two pole passive low-pass quench filter feeding a common emitter amplifier, followed by a Sallen-Key low-pass filter implemented as a emitter follower which can drive headphones directly to a reasonable volume.

Prototype RX AF Amplifier

Notes

This is as far as I've gotten with the general idea so far, but I will likely be building a complete transceiver with similar circuits soon. I have lots of 27.195 MHz crystals, so there is a certain desire to use that frequency, but digging through the junkbox I also have lots of 21.330 MHz (15 metres) crystals too which might make it a more HAM compatible project.

I think a noise squelch would be a nice addition to the deluxe version of the transceiver. I have a prototype circuit floating around on a solderless breadboard which could be easily integrated, (unfortunately it is 4 transistors, which is nearly as many as the whole transceiver will be).

Videos

Here is a video of the prototype TX broadcasting some audio to the lashed-up RX.

Audio Test
Audio Test
(6.118 Mb)

And a video demonstrating the "AGC effect" of the super-regenerative detector. Note how the recovered audio amplitude doesn't change much as I give the receiver more signal, but the SNR improves enormously as my finger approaches the antenna input pad.

AGC Demonstration
AGC Demonstration
(1.027 Mb)

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