[RTTY] RadioShack transformer IMD

[Kok Chen]

On May 15, 2012, at 1:57 PM, Joe Subich, W4TV wrote:

>> I was expecting a higher IMD reading with the transformer inserted
>> between the input and output of the soundcard as compared to the
>> straight-through cable. What am I missing?
> 
> You're not missing anything ... you will not see any increase in IMD
> (or harmonic generation) as long as the signal level stays in the
> linear area of the transformer's B-H curve (generally stay out of
> saturation) ... if the transformer is well designed.

Yep.  It could even mean that the transformer in there has a core which only saturates with a larger signal than where your sound card saturates (sound cards often saturate at a few dB above 0 dBu).  In that case, that Radioshack line isolator could be a keeper (especially if it is flat to above 8 kHz).  I won't mind bypassing the K3's transformers and hang three of the Radioshack transformers from the back of the rig :-).

For maximum dynamic range (at least we wideband guys use sound cards that way :-), you want to push the sound card near to saturation for the loudest signal so that the weakest signals are still a decent amount above the sound card's noise floor.

If you are using single signal narrow band IF filter stuff, you don't need to push the sound card (nor the transformer).  If don't see any IMD, just keep the level there :-).  Let the AGC bring up the weak signals.

Second harmonic distortion (just need a square law component in the nonlinearity for it to appear) is often a bigger problem for wideband users.  I have a crummy 20-bit sound card here whose IMD is in the -85 dBc range, but I get second harmonic of about -50 dBc from it.  That presents a problem, for example when you are trying to copy a PSK31 signal at 1800 Hz and a different strong PSK31 signal comes up at 900 Hz.  Only way to get around that is to attenuate the 900 Hz signal before it reaches the sound card, or nudge the VFO knob a little (the harmonic relationship changes with the VFO knob :-).

I am glad there are more people worried about "second order" effects of RTTY demodulation!

One way to test how far you can push your transformer is to apply an attenuator before the signal reaches the sound card.  This way, you can force the transformer to saturate (it will eventually happen :-) before your sound card clips.

73
Chen, W7AY