20 October 2013

Semi-Break-In Keying Control Circuits

After much experimentation and enduring very loud pops in my headphones on T/R switching of my homebrew field radio, I finally managed to develop workable semi-break-in keying control circuitry for this rig.  Albeit by deploying no less than 8 transistors to achieve the objective.

The Rx is basically as already described and is a W1FB NE602 based system with an audio preamp,  LM386 audio Amp and a high Q tuned audio filter between the Audio Pre-Amp and the Pre-Amp stage. This Rx generates tremendous headphone pops on any input disturbance. The Tx is a W7ZOI design and outputs 1.44Watts. This Tx was extremely simple to build and put on the air. The Tx is Xtal controlled to operate on 7020Khz and the Rx is VFO controlled with a very restricted tuning range from about 7017Khz to 7022Khz. A kind of RIT control.

Reference is made to the Control Circuit diagram below.

On KEY DOWN the following actions occur.

1) 12V is applied to the Sidetone Circuit.
2) The Pre-Amplifier is turned off
3) The input to the Audio Filter is muted.
4) The Transmitter Oscillator is keyed. The circuit includes a shaping function to reduce key clicks.
5) The T/R switch is switched to TX.

On KEY UP condition the Sidetone is stopped, the Pre-Amp is connected, and  the Tx oscillator is depowered. The T/R circuit will switch to the Rx after about 0.75 seconds and finally the Mute circuit will gracefully unmute the audio filter and audio amplifier amplifier after about 1 second. This has the effect of muting the pops associated with the change in state of all the other circuits BEFORE the audio recovers fully. Thus the pops are eliminated on Rx recovery. There is a slight thump on keydown but this is very much down in the noise and entirely tolerable from an operating perspective.

The tail delay of 1 second is about right although the first character of the transmitting station signal may be lost if the transmitting station is quick on the come back.

I am very pleased with the operation of the semi-break-in keying feature of this radio which makes it a pleasure to operate in the field.

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