[time-nuts] Modified Extron DA [WAS: Rb video]

Charles Steinmetz csteinmetz at yandex.com
Fri Aug 9 18:51:59 EDT 2013

Bob wrote:

>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.

Best regards,


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