[time-nuts] ADEV

[Magnus Danielson]

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