On 11/13/2010 04:01 PM, jimlux wrote:
Mike Feher wrote:
I sure do agree, that with very low data rate systems it is
significant. In
fact, when David Allan & Fred Walls came up with the proposal of using
this
measurement as an FOM for oscillators over 30 years ago, digital
communication rates were slow, and, the measurement was a good one.
Due to
the filtering process within ADEV by collecting and integrating a large
number of samples, has a filtering effect of its own. Therefore, it
can, and
will, miss the fact that there may be instantaneous phase transitions
that
could cause havoc with high data rates and higher order PSK modulation
schemes. So, again, I apologize, as I should have mentioned higher data
rates. However, you must admit that your application, while extremely
critical, is in the minority. I like to use the example of something like
DirecTV. Here, they use a down-converter that utilizes a free running
DRO,
that is ridiculously noisy, and, varies all over in frequency, especially
over the temperature ranges it subjected to. In spite of all of that, one
gets a perfect pictures. Regards - Mike
no apologies necessary.. After all, I spend a small, but significant,
amount of time explaining why we'd care about such things, since we are
in the distinct minority of the radio comm world (trying to write nice
comments on failed SBIR proposal evaluations to explain why they missed
the big picture)
And, on the one hand, it's frustrating being the orphan child of the RF
user community: you can't get off the shelf test equipment. On the other
hand, it's cool, because then you have to *build* your test equipment.
Hmm. Should do more of that.
To the Ku-band downconverters.. They're pretty crummy (but have a decent
SNR to work with).. however, I've seen that there are two kinds.. a
vanilla LNB and ones described as "crystal locked"... both are cheap
($20-30 for the former, maybe twice that for the latter)... what's the
difference? And, getting into time-nuts territory here, where's the
reference for the "locked" variety coming from? Up the coax? inside the
LNB? And, can it be retrofitted from a much quieter oscillator? I was
thinking that one could build a radio camera with a small array of
Ku-band dishes, if you could lock all the receivers together. They *are*
pretty low noise (20-30K)
The key seek-term to add is "external reference" and it seems that 10
MHz sine seems to be the standard external reference frequency for LNBs
with external reference. I know it will be a tricky frequency for you to
score, but the things you do for science.
Best of luck.
Interesting approach.
Cheers,
Magnus
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