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Re: 1296 Transverter design advice

> To:            amsat-bb@AMSAT.Org
> From:          Anthony Monteiro <aa2tx@postoffice.worldnet.att.net>
> Subject:       Re: [amsat-bb] 1296 Transverter design advice
> Date:          Fri, 26 Feb 1999 01:42:24 +0000

> >So far I have learned that the DEM 
> >transverter leaves something to be desired.  L.O. problems mostly.
> For what it is worth, I purchased a DEM 432 MHz kit and the L.O
> did not work correctly. The problem is that the oscillator 
> design does not compensate for the xtal holder capacitance. 
> This makes the oscillator unreliable when tuned to the proper 
> frequency. However, it was exceedingly easy to fix, just a 
> coil of the proper inductance across the xtal to tune out the 
> capacitance. I haven't built the 1296 MHz kit but it wouldn't 
> suprise me if it had the same problem. The DEM kits are not 
> for a beginner but if you know what you are doing, they are 
> fairly cheap and can save you a lot of time looking for parts.
> 73,
> Tony AA2TX@amsat.org
> North Andover, MA

Dear Tony,

The problem you mention is inherent to the Butler oscillator.
It should be renamed the "Never on Frequency Oscillator."
A few years ago I designed and built a new two meter receiving
converter for my Oscar station.  Following everyones advice,
I used a Butler oscillator circuit.  I ended up sending the crystal 
back to the manufactuer three times for correction.  I even sent
the schematic of the oscillator all three times.  They never were
able to get the crystal even close!

I played with the holder capacidance, tried the coil, and damned
near everything else.  I finally gave up and used a conventional
oscillator circuit.  I went back to the first crystal and it worked perfect!

I have built a DEM kit, the 2.4Ghz receiving converter.  It worked, but
the L.O. drifted and naturally was not on frequency or even close.
I can't fault DEM (under the old management).  To keep the price
down, they make the L.O. as simple as possible.  No oven or TCXO,
something I think you need at that high multiplication.

For my 222Mhz transverter I purchased the crystal from DEM.  They 
assured me that their crystals had the correct correlation for the 
Butler circuit and they were right!  With very minor fidling the crystal 
moved right on frequency.  Note that I plan to add a simple oven to 
my L.O. to make sure the xtal stays put.  Someday I like to see someone
publish the specifications for Butler oscillator crystals in case we 
need one that DEM does not stock.  

Some day I plan to go back and overhaul the 2.4Ghz kit and make
it settle down.  Maybe a better xtal from DEM and a oven for sure.
--73-- Dave WA0AUQ

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