Re: QCX PTT Mod


Hans Summers
 

Hi Paul
 
In the paper Hans put out on the PTT mod to the QCX for use with the 50W PA, he describes a hardware mod
 by adding a 10K resistor (value entirely uncritical) between the RX signal and +5V. My question is this:
- my junk box has 100, 100k and 1k resistors.
 - so Hans says value uncritical- so would any of these I have work in this application?

Even the mod itself is not particularly critical. I did find it prevented a particular glitch startup condition with a particular power supply, which once in 6 months of operation, caused me to kill the IRF510s. 

What happens is that on power-up, the microcontroller resets itself. Then starts the program. Early on in the program, the processor pin 14 is set to high, which puts the QCX into RECEIVE mode. But until this happens, pin 14 is high impedance and IC3 pins 12 and 13 are left floating; the only things on this signal are IC3 pins 12 and 13, and the gate of Q5 (Tx/Rx switch) and C34, 0.1uF to ground. Regrettably C34 will cause the voltage to be - momentarily at least - pulled to ground; thereby putting QCX into TX state and activating the PTT. 

To avoid this, a pull-up resistor from pins 12 and 13 to +5V keeps everything in a determined state (Receive) during this short interval. Ok so it still has to charge up the 0.1uF capacitor. I chose 10K. 1K would be fine too. If you go too low, then during TX the microcontroller I/O pin will have to work too hard to pull current through it, which could exceed the rating of the I/O pin. 1K will draw 5mA which doesn't harm anything but is a bit of a waste. 10K, 0.5mA. If the resistor is too high, then it will be slow to charge up the 0.1uF capacitor and we will be back to a momentary activation of PTT. 

If your junkbox only has 1K and 100K resistors then 

1) Please stock up on some more junk ;-)

2) I'd tend to use the 1K resistor, accepting the 5mA higher current draw on TX, which won't be noticeable, in preference to the unknown quantity of any risk of lengthening the momentary PTT pulse.

73 Hans G0UPL
http://qrp-labs.com

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