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RE: Prime Star Dishes etc.


I can't speak for the PrimeStar LNBs, but the DirecTV units are actually
quite similar to analog LNBs.  The input frequency is the band 12.2 to 12.7
GHz.   This band is down converted to 950 to 1450 MHz.  The LO is at 11.25
GHz +/- a couple of MHz.

These units actually have two front ends, one for right hand circular
polarization, one for left.  The DC voltage on the coax powers the unit and
also switches either the left or right downconverted signal to output.  The
switch-over point is about 15 volts...  A DC lower than 15 gets you one
polarization, higher than 15 gets you the other.

If you do the math, you find the image freuqency band is 9.8 to 10.3 GHz ...
pretty close to 10.451 GHz!  The problem is, there is a microstrip band pass
filter which passes only the 12.2 to 12.7 band and provides pretty good
image rejection.  Someone who is very clever might find a way to cut that
filter out and make it work for the 10.4 GHz band.

The reason your IRD can only receive one channel at a time is that it only
has enough processing power to decode the bit stream from one transponder at
a time and generate one analog video and associated audio signals.

Jim AF9A

> ----------
> From: 	Bruce Bostwick
> Sent: 	Wednesday, January 3, 2001 1:14 PM
> To: 	amsat-bb@AMSAT.Org
> Subject: 	Re: [amsat-bb] 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
> >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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