[time-nuts] Modified Extron DA [WAS: Rb video]
csteinmetz at yandex.com
Fri Aug 9 18:51:59 EDT 2013
>I still think that a distribution amp based on logic ic's is cheaper
>/ simpler / lower power / higher performance. A pair of NC7SZ125's
>will dump 20 dbm into 50 ohms all day long running at 5.5 volts.
>Good isolation as well. Do the lowpass filter right and the
>harmonics are not an issue. Two coils / one cap plus dc blocking
>does it quite nicely.
The filters add a potential source of close-in phase noise due to the
temperature coefficients of the parts. Granted, this may be academic
in the case of a simple reference distribution, and you could always
do it with harmonic traps instead of BP or LP filters to minimize the
problem. Also, I'd rather not generate fast edges in the first place
if I don't need them -- they can leak out and be pesky. But these
are not religious positions for me. In some cases the digital
solution may be preferable.
However, I think the large interest in video DAs has more to do with
the fact that they are cheap and often need very little
modification. When I first wanted to distribute 10 MHz, before I
built my own DA, I grabbed an Extron out of the junk pile and
modified it in a couple of hours. Indeed, if you can tolerate the
mismatch and the overall 2 dB loss, video DAs can often be pressed
into service with no modification (not my preferred solution, but
it's been done). Digital DAs, OTOH, are more for the person who is
rolling his or her own from the ground up.
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