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Re: Mode-S Antennas

> From:          "Frederick M. Spinner" <fspinner@hotmail.com>
> To:            amsat-bb@AMSAT.Org
> Subject:       Re: [amsat-bb] Mode-S Antennas
> Date:          Tue, 13 Feb 2001 15:15:58 -0000

> The hard part is the oscillator.  I'm kicking around the idea of using
> a Maxim MAX2751 VCO ($3.50 in one-seys, $1 in Qty) and some sort of PLL 
> circuit-- the simple one in the Drake-- a fixed divider and a XOR phase 
> comparator-- or more complex like a National LMX2326 and a PIC to program it 
> (same parts count, and minimal difference in price-- and more flexible)...

A quick look through the Mini-Circuits catalog shows a lot of nice
parts.  A HP MGA MMIC as the front end, a MiniCircuits DBM as a
passive mixer followed by a diplexer and maybe a ERA MMIC
post amp.  Simple!

Then we get to  the hard part,  the oscillator!
Converters are easy, its the LO that is difficult.  There are some
advantages to using a conventional crystal LO as opposed to a
PLL.   For one thing, phase noise will be lower.  Also no PIC is 
required.  Keep in mind that there are many of us that are not
programers and most likely never will me.  Ordering a xtal from
ICM is trivial, getting a PIC programed is not (at least for me).

The idea of placing the LO (or at least the low frequency portion
of it) in the shack and piping the signal up to the converter is a good
one.  I considered it long ago and made that idea part of my plans.

In my shack, 2M will be used for the IF for the 1.2Ghz uplink transmitter.
This precludes using 2M as the 2.4Ghz receive IF.  My plan is to use
a UHF IF, say below 70cm, no point in cluttering up that band either.
Maybe 400Mhz, then using a second converter to move that down to
28Mhz.  This would allow me to use my "good" HF receiver as my
main sat receiver.  

> If I/we can do that, the filters, amps and mixers are commercially 
> available.  But, no it won't be a $25-$50 downconverter-- but it could be 
> smaller and higher performance than the Drake.  Right at the back
> of a Helix feed would be nice-- or in a dipole reflector feed for the 
> Conifers...  Maybe send a TXCO high-stability freq reference (nice thing 
> about PLLs) from the shack up one cable, and the signal down another.. lots 
> of cool ideas here....
> If I only had more time to work on things like this....:-)

You and me both!!!

I have only gotten as far as some developmental work on taming 
the conventional Butler oscillator circuit.  I have learned a few things.
LIke the inductor you always see strapped across the crystal should
not be there.  And that ICM can provide crystals that will oscillate on
frequency!  I've solved some drift problems as well and have just
started my next LO project.  One that will evolve into my 1.2Ghz
up converter.  
> Fred W0FMS

--73-- David WA0AUQ

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