Friday, 31 January 2014

Digital Overload

Playing with PSK31 earlier today, and after modifying my interface, ive realised that im badly over driving the transmitter at all but the lowest settings of the interface's level control. I seem to need a fair amount of initial attenuation in the tx audio path, if im to use the system without having to worry about the volume settings of the PC. To do this, I need to add another variable resistor, a preset pot, into the tx audio path. This will be set, with the level control in bypass and the output of the PC soundcard at maximum, to the point where the Tx power just stops getting any higher. Anything above this, and its over driving, and engaging the transmitters ALC very heavily. With this preset in place and set up, the level control should then act to set the effective transmit power without any distortion.

As soon as I can get the 1:1 transformers, I will build another interface unit, which will effectively be a 'universal' interface. This will have the same max drive and level controls as above, but also a volume control in the receive audio path. It will also use a tone operated PTT circuit, rather than the serial port driven PTT in the current interface. Control of the PTT will then be entirely by the datamode software sending a control tone at high level on the Right audio channel. To get the transformers I need a load of old modem cards! Ive put feelers out at work, im hoping someone in IT will dump some my way.

To try and get another audio channel available to dedicate to digimodes, I installed one of my old Soundblaster audio PCI cards. Windows took a look, and told me to naff off! No drivers available for it for Windows 7. So, ive decided to take a chance with an el-cheapo USB soundcard. For £1.50 if it doesnt work ive not lost out much. If it does work then all well and good.

The problems with the audio levels are annoying because it means I havent been able to play with my new PSK-20 transceiver yet. I have however, decided already that it needs some mods! Nothing drastic, but the addition of an on/off switch, and a Power LED, would seem prudent. Ive also found out about a simple Tuning indicator mod. It seems early versions of the PSK-20 (the ones that required serial port PTT lines) had a RF sniffer test point. This is missing on the later model I have, but consists of just a pair of 1N4148 diodes, a resistor and a capacitor. This then drives an LED and gives a visual indication of transmit status, and also of modulation and potential VSWR problems. I will add these, but I must take my time marking and drilling, as the case is very nice and I dont wish to spoil it.

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