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Re: Prime Star Dishes etc.



A reply sent to me only, but I'm replying to the list in case my answer is
either useful or factually flawed ..

IIRC, the DirecTV and PrimeStar type dishes differ from a typical analog
TVRO in that they have narrowband LNB's with a passband about the size of
one downlink transponder and a 70 MHz IF instead of an broadband L-band
output, and instead of having a fixed LO they use a VCO circuit that is
tuned by the LNB supply voltage on the coax.  (Which is why you can only
get one channel per LNB and need a dual LNB and a second IRD to drive a
second TV..)  My guess is that the IRD uses some sort of feedback control
and maybe a pilot signal on the downlink to keep the signal on frequency.
The LNB on these residential grade units is actually the LO, mixer, and
first IF stage of a two-part Ku-band receiver -- the IRD is more of a
second IF stage with a microcontroller to manage the whole thing and
control the digital decoder machinery.

If my assumptions are correct, then there's probably considerable out of
band RX capability in the LNB, since it needs to be guaranteed plug'n'play
stable on DBS transponder freqs, and the 10GHz band is probably well within
reach from the 11-12GHz Ku-band digital downlinks.  You could certainly
test that by feeding the LNB a variable supply voltage and watching the
noise output to see where the limits of LO stability are, and if you can
find a known Ku-band signal like an analog downlink you may even be able to
tell if it's an inverting or non-inverting mixer.  (Probably non-inverting,
since analog TVRO LNB's on Ku-band are generally non-inverting -- lower LO
frequency and thus a cheaper LNB..)  Be sure not to overvoltage the LNB, or
you'll probably burn it out.

If anyone can verify this, or correct any errors I've made, please post to
the list .. ;-)

>One of the ARRL Microwave Experimenters' Manuals mentions it and using an LNB
>for 10 GHz, but I couldn't find any technical details.
>
>Bruce Bostwick wrote:
>
>> Curious about this myself as well .. they would make decent receivers,
>> since they have a Ku-band LNA and downconverter built in.  Pick up two and
>> put an upconverter on the other one, and you might be able to make some 10
>> GHz SSB contacts.  (Bonus points if you can design and build a good duplex
>> feed and combine TX/RX into one dish ..


"Orthodoxy is orthodoxy because it won, not because it is true." -- Bp.
John Spong
 "Power takes as ingratitude the writhing of its victims." -- Rabindranath
Tagore
               --... ...-- -.. . -. ..... ...- -...
         Bruce Bostwick  mailto:lihan@ccwf.cc.utexas.edu
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