[time-nuts] ADEV

[jimlux]

David I. Emery wrote:
> On Sat, Nov 13, 2010 at 07:01:26AM -0800, jimlux wrote:
>> 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)
> 
> 	There are three kinds of LNBs in common use in the VSAT world...
> 	
> 
> 	1.  Open loop unlocked DROs, often with around a MHz or two or
> more error due to temperature and calibration and drift over time.  More
> expensive higher grade ones are tighter spec'd, but rarely much less
> than 250-500 KHz over time and temp.   Most all DTH dishes use these,
> often with rather loose frequency specs since the DTH carriers are wide,
> fast signals.
> 
> 	2.  Closed loop DROs phase locked to a crystal or I believe
> about as common, a UHF or higher frequency oscillator phase locked to a
> crystal reference with some degree of multiplication to the final LO
> frequency (maybe not much these days with fast prescaler/divider chips).
> Crystal in this case is - depending on price - just a plain XO or either
> a TXCO or in certain cases a OXCO.   More expensive ones have better
> stability specs.    Generally these sell for 5-10 times what a cheap DRO
> LNB for the mass market might go for.  And can be as good as 1 PPM or
> so.


Bummer.. so much for making myself a radio camera with scavenged Ku-band 
  DBS dishes/feeds (when people around here subscribe, they just junk 
the existing, almost identical, assembly)..

I was willing to spend a few tens of dollars/element.. But $100/element 
is pushing it out of the "hmm, interesting project for not much money, 
can I accumulate some more junk in the garage" category.


> 
> 	PLL LNBs are mostly used for data or audio transmissions on
> narrower, lower  bit rate carriers than TV but also used for many critical
> professional TV broadcast and similar applications.
> 
> 	3. External reference LNBs with 10 MHz (pretty universal) going
> up the cable that also carries power and brings the L band signal down.
> I'm not entirely sure how many of these designs simply bandpass filter
> and then limit the 10 MHz and use that directly as a PLL reference and
> how many phase lock a VCXO to the 10 MHz coming in.  Otherwise similar
> (and often  derived from designs for) the internal reference PLL types
> in 2 above.

And, I'll bet those are fairly pricey.. (after all, it needs another 
connector, and that's a price sensitive application..)



Hmm.. I wonder if one could do the pilot tone technique.. I can radiate 
a weak Ku band signal, in band, and record what I see from the LNB, then 
post process to "close the loop" in software.  Have to think about that 
some more.. The Ku band beacon/pilot is easy (a comb), so what I'd need 
is a multichannel "softrock" type receiver (which has other uses) that 
can deal with the L-band IF from the cheap LNBs.  I wonder if the LO of 
the cheap LNBs are within, say, 50kHz of each other.. Then I could use a 
common L-band LO.. But based on the discussion above, probably not.