[TenTec] SDR-1000 vs Orion vs SDR-X

[Stuart Rohre]

I don't know why they switch the sound cards, but it was my understanding 
that this limited the speed of transmit- receive "break-in", which is a 
parameter which made Ten Tec radios justly famous.  The switching was not 
something Flex was doing deliberately.

It may be that certain circuits are shared in the Sound card between audio 
in and audio out paths, and those must be switched.  Since the usual use of 
sound cards for music does not anticipate a fast change over from audio 
input to audio output, this might have been a reasonable thing to do, and 
something another user designing for the sound card would not have 
anticipated.  It has been about a year since I was being told this by a 
person working for Flex Radio.  I am sure once they realized the limitations 
of sound cards, they were eager for the project to replace the computer 
sound card with a device suited for a transceiver application.

To answer your question, my informant said that sound cards commonly had a 
slow change over from sound in to sound out which of course involves 
software, and that was before Flex used them for radios.  The subject of RF 
infiltration is a good thought and may be another issue entirely.

Remember Windows is not a Real Time operating system!  We run into this all 
the time with our A/D converters implementation.  We are presently fighting 
to see if we can fully use the 3 MHz speed of some new National Instruments 
A/D converters we bought for a project.  So far, Windows is winning.

FLEX was on a learning curve about RFI issues, mainly with spurious transmit 
responses.  There are so many variables when you have someone building 
something outside the manufacturers lab; and when you are sourcing 
components in smaller quantities than high volume consumer product 
manufacturers, you may get components built in various batches, which may be 
at differing ends of the spec.

The small manufacturer also faces issues of having printed circuit boards 
made outside, and all the issues that might entail.  Holes get drilled the 
wrong size, boards might need recleaning after being shipped in from a 
distant vendor, etc.

The implementation of Software Defined Radio is very much a non trivial 
computer application, to say nothing of the marriage of analog circuitry to 
digital circuitry.  As speeds go up, the problems multiply.  Digital signals 
really are RF, and with sharp edges compared to traditional radio signals, 
thus having many harmonics to get into places you do not wish to have them. 
The makers of such advanced DSP applications are to be commended for their 
pioneering.

But, we are only at the beginning.  There was easily a quarter century 
before the superhet was tamed and became the mass production wonder we knew 
as "The All American Five" receiver.  (For its minimal active device count 
of five tubes for all functions).

-Stuart
K5KVH