[aprssig] FTM-350 cables

Stephen H. Smith wa8lmf2 at aol.com
Thu Mar 10 18:07:48 EST 2011

On 3/10/2011 12:45 PM, Julian, G4ILO wrote:
> I made something similar to the one you did using one of the
> ubiquitous USB sound devices costing a few bucks:
> http://www.g4ilo.com/usblink.html . It's based on a circuit by Skip,
> KH6TY.

I recently built a copy of KH6TY's latest design published in the latest (March 
2011) QST which is self-powered by rectifying the audio via a stepup 
transformer and a classic AC voltage doubler circuit.

It worked perfectly for modes based on a single tone at a time, or just two 
tones such as PSK31 where the average-to-peak value of the audio tone is 
relatively high .   However, it failed miserably on EasyPal "digital SSTV" 
which uses either 16 or 64 simultaneous QAM audio tones.   The average-to-peak 
value was just too low to trigger reliably.  I even tried higher transformer 
step-up ratios and adding an additional voltage-doubler section, but it just 
wouldn't work.

I went back to the LM158 op-amp design I came up with years ago, despite 
requiring external power.


This design keys reliably on any audio level output from a sound device ranging 
from 50 mV to over a volt RMS, and does it fast -- less than 3 mS which lets it 
work on packet with essentially no TXD.    I can vary the audio-out level to 
get the desired TX modulation without worrying about keying reliably.



Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      WA8LMF  or 14400    [Think bottom of the 2M band]
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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