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Old DEMI Mode-S converter stability



Yes, frequency stability.  The PTC thermistor helps quite a bit with
long term stability.  The choke and especially replacing the RSK-1 
transistor's with the MPS5179's helped short term stability.

The other stability issue, which I didn't bring up in the previous
post is to run the unit off of 13.8V and not 12V.  This is mentioned
in "Mode-S: The Book" and seems critical on my unit.  The alternative is to 
rebias all of the MMICs (not sure if just the osc, the converter board, or 
both) for say, 10V and use a low-dropout regulator.  I may
still do that someday, but for now, I run the higher voltage.
If the voltage drops, all sorts of wierd spurs and oscillations occur
on my converter.

My converter stabilizes fully in about 2-3 minutes now...

Fred W0FMS



>From: Douglas Braun & Nadia Papakonstantinou <dougnad@bellatlantic.net>
>To: "Frederick M. Spinner" <fspinner@hotmail.com>
>Subject: Re: [amsat-bb] S2 Received strong in Iowa  1345Z 3 Jan 01
>Date: Thu, 04 Jan 2001 09:43:21 -0500
>
>Hello,
>
>When you say "stable" below, to you mean frequency stability, or something 
>else?
>I have a stock DEM converter, and yesterday I ordered the PTC thermistor 
>for it.
>Apart from drift, the unit seems to perform OK for me.
>
>Doug
>NA1DB
>
>At 10:50 AM 1/3/01 , you wrote:
> >I must of finally fixed the old DEM Mode-S downconverter.  The suggestion 
>of adding a small inductance in parallel with the 94 MHz
> >XTAL, helped me net the osc to freq better, and the unit was MUCH
> >more stable.  My unit has the RSK-1's replaced with MPS5179's which
> >helped a lot, a 60*K PTC thermistor soldered to the xtal and now a
> >0.33 uf molded choke across the 94 MHz Xtal.  All three mods seemed
> >to help.
>

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