USB Digimodes Interface follow up.

  Hi, I've had several emails regarding the USB digimodes interface article which appeared in the September 2010 Radcom. The article was originally written over two years ago and the circuit has been tweaked in minor ways since. One modification that has been implemented (not by me) is powering the circuit from the USB 5V supply and substituting a 5V coil relay.

Although the interface works with no problems on my IC735, obviously not all radios are the same. I received the following email from one builder who has amended the circuit to optimize the vox circuit for his radio.


          Many thanks for your article in Radcom that inspired me to build the interface for use with a couple of QRP sets with the intention of going PSK31/P.

  I thought you may be interested in my experience of building and testing the design you described. Please note that this is not intended as any kind of criticism, just my findings intended as feedback on your article.

  For simplicity's sake I omitted the USB soundcard (although I thought it was an ingenious idea) and also to make the interface as compact as possible for /P work.

  The changes to your design I found necessary are as follows:

       The VOX circuit was unstable, any changes or spikes in the voltage at pin 3 (due to an unstabalised supply) would cause the relay to 'chatter' or just operate randomly regardless of input. A 2.2uf and 10nf capacitor between pin 3 and ground seemed to cure this (see attached .pdf article regarding this, I used this as a guide).

  I had to put a 10nf  DC blocking capacitor between pin 1 and pin 5, I found that without this the circuit would latch on without any input signal.

    The input capacitor C1 had to be changed to 10nf, (the 10nf value was cut-and-try, and also because I have a bag of them !) it would seem that with the original 0.1uf a low source impedance, ie. the transformer secondary at the input caused the circuit to latch on. Not sure why this is, but my circuit was oddly sensitive to the value of C1.....

    The output level was way to high initially for connection to a Mic input, I put a 15k resistor in series with the VR 3 wiper to the output to the TX.

  I intend to use the interface with two sets, an Icom 703 (I have yet to build a connecting cable for the Icom and so cannot comment on it's performance with this set) and a Racal PRM-4051 HF manpack, I have tested it with the Racal and after the modifications outlined above the results are promising.

     Again, please do not regard this note as anything other than feedback, many thanks for taking the time to develop the interface and submit the article. I look forward to seeing more of your homebrew designs in Radcom in the coming months.

  Look forward to maybe working you on PSK31 using your interface.


                   Best 73s

                                   Ian Smith 



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