[time-nuts] ADEV

[David I. Emery]

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.

	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.

	These ER types are more apt to be used for more exotic
specialized applications where very high frequency accuracy or some
degree of phase coherence  with other equipment or LNBs is useful.

	Obviously with the high multiplication factor, one needs a quiet
reference inside the PLL bandwidth (and that is pretty wide to ensure 
reliable lock) - one suspects that issues with degradation due to
mechanically induced noise and phase shift in the cables can be a
problem.


-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."