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Re: Creating stable LO & control head idea



Larry,

I have done something similar by putting a diplexer at the high frequency
converter and sending the microwave LO reference up the coax. In my case I
sent the 100MHz reference up the coax and the PLL would lock to this
reference. The results were great because the stability was greatly
improved. The LO is also adjustable from the shack.
One problem I see with using a DDS to drive a Drake or Transystems is the LO
purity. The Local oscillator the way it is on these converters is already
poor from the stability and phase noise specs. If you use a DDS as the 8 MHz
reference and multiply it by 256 as in the drake and TS converters, the
spurs would also be multiplied. This would result in a very very very dirty
local oscillator. A local oscillator generated in this fashion would be even
worse than the one already in there. It would be totally unusable unless a
clever way was devised to clean the signal before multiplying it by 256
using a crystal filter or something similar. You can read more on the
problems with this approach here:

http://www.analog.com/techsupt/application_notes/AD9854_tech_note.pdf

If you are serious about this, I recommend that the LO be totally replaced
by a 282MHz*8 or 188MHz*12. That way the phase noise is greatly improved.
The 282 and 188 could be generated by a crystal LO in the shack.

Pieter Ibelings


----- Original Message -----
From: "Larry P. Thomas wa0gwa" <lpthomas@krell.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, May 21, 2001 11:37 AM
Subject: [amsat-bb] Creating stable LO & control head idea


> At 04:37 PM 5/18/2001 -0500, you wrote:
> >Paul Williamson wrote:
> >
> >I find that my downconverter, at least, is not stable enough for
> >this to work. Its drift is often greater than the Doppler shift!
>
> I have an idea to toss out in case is has merit and someone has time to
> work on it.  The down converter already send the IF frequency down the
> cable and the DC supply voltage up the cable.  Why not use some 10 MHz low
> pass filters (i.e. just a couple caps and couple coils) to allow the
> insertion of the required local oscillator crystal frequency into the
cable
> in the shack and its extraction at the convertor end.  After extraction at
> the down converter end it could be used in place of the crystal to drive
> the PLL which creates the LO signal.  In fact, why could the crystal
> frequency not be generated by one of the low cost DDS devices
> available.  Even the cheap one attached to the parallel port of a PC
should
> work.  Extrapolating on the idea a little, a PIC could control the DDS,
> thus knowing the LO frequency without measuring it, output control RS-232
> to control the serial interface on the radio serving as IF, and display
the
> resultant 2.4 GHz receive frequency.  Using a rotary encoder into the PIC
> would allow the PIC to act like the receiver.  A second rotary encoder
> could serve as a RIT/Doppler/DeltaF/etc control or a second RS-232 channel
> could be feed with the tracking program.
>
> The idea carried to its fullest could result in a nice sharp little
control
> head for A040 and others.  The idea in its minimum would still result in
> stable LO frequency and should be easy to implement. I am still working at
> getting my basic receive capability up and intend to pursue this when that
> is done.  I would love to hear from anybody that thinks the idea has merit
> and is willing to work on it.
>
> Later
> Larry P. Thomas wa0gwa
>
> - Imagine it / Achieve it - Dream it / Become it -
>   Larry P. Thomas    wa0gwa  voice : 1 913 888-0282
>   Krell Technologies           fax :
>   8960 Bond                  pager : 1 913 617-HELP
>   Overland Park, KS              www.krell.com
>   66214-1722    USA            lpthomas@krell.com
>
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