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.
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