Sunday, 2 February 2014

'All in One' Digimode Interface design

Whilst working out the design for my next digimode interface, which will be used with the Small Wonder Labs PSK-20, I looked online at problems and mods for these dirt cheap USB sound cards. One thing that seems to be a trouble with some of them, is a large DC offset on the headphone port. This it seems is due to tight arsed manufacturers saving a penny per unit (or probably per 100 units) by not including blocking capacitors in the audio lines, but instead just having the pads bridged.

So, knowing that one of the first things Im going to do with these sound cards is take them out their case and check them, it occurred to me that I could leave one out of its case, remove its USB plug and the jack sockets, and wire it directly to the interface isolation transformers and level controllers. By including the Tone Controlled PTT circuit as well, this would create an interface that requires just plugging into a computers USB port, and connecting up the radio.

The USB sound cards are fairly simple inside, everything being done by one VLSI chip

But a problem with including it in the box with the rest is that I'd need a USB socket, not plug. Luckily, somewhere in the loft I have a defunct BT Voyager 210 router, which has a USB B socket that I can salvage

Theres not many other bits on it that are likely to be of much use, as it used to run very hot, and I wouldnt trust many of its parts. Perhaps the DC socket and the reset switch could come in handy.

Had a better look at the HF antenna socket on my car roof today. I think the problem is a bit of tarnish on the threads, what I need is to give it a good go over with a wire brush!

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