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Re: Prime Star Dishes etc.



Bruce Bostwick wrote:
> 
> 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

Hello,

No that is is not correct.  You are talking about either a LNA or a LNC
setup that from my understanding was not used for Ku Band, only C band.
LNA was a straight low noise amplifier that you hooked to a separate convertor
through a thick, short (but still lossy) piece of coax.  The convertor
brought it down to 70 MHz and was tuned by a separate voltage wire.
The rare LNC basically combined the LNA and Converter.

LNB (Low Noise Block) does a block conversion.  Both the DirecTV and
Primestar use 950 to 1450 MHz as the IF output.  The reason for the dual
outputs? Two polarities are used and both receivers need to tune both
independently.  LNBs are powered up the coax from the reciever.  Usually
+18 volts is used (see below).

DirecTV (or Dish Network, new Expressvu, etc) use the US DBS band of 12.2
to 12.7 GHz.  They are circular polarized. Primestar (also StarChoice, old
Expressvu) uses the US Ku Band of 11.7 to 12.2 GHz.  They are linear
polarized.  There are two types of Primestar LNBs.  The single output one
uses the voltage sent up the coax to switch polarity.  13 volts is one
polarity and 18 volts is the other.  This is also how DirecTV LNBs work
(Dual DirecTV LNBs have two separate Low Noise amps and two polarity
switches).  The dual output Primestar LNBs bring out both polarities.  They 
have a 'V' near the vertical output and a 'H' marking near the horizontal
output.  It is powered by sending voltage up one of the outputs (forget
which one).

LNBF is the term used for LNBs that switch polarity via the 13/18 volts up
the coax.

European LNBs are more complex as they use a wider band of frequencies.
They have Universal LNBs that besides using the 13/18 volts for polarity
use a 22 KHz signaling protocol to change frequency bands.  As far as I
know they also use the 950 to 1450 range but some systems use 950 MHz to
2050 MHz for the IF.

All the LNBs would need modification for 10 GHz use.

73 Eric eac@shore.net  WB1HBU
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