[Premium-Rx] Prescaler / Frequency Divider

[Brooke Clarke]

Hi Steve:

It's my understanding that 21.4 MHz was chosen since it's twice 10.7 MHz which 
is a very common IF used for FM radios.  The reason 10.7 was not used is that 
you can not get the needed bandwidth there.  Maybe the IF bandwidth is about 2% 
of it's center frequency. That is also why 10.7 MHz is used for FM radios which 
need about 200 kHz IF bandwidth instead of the also very common 455 kHz IF 
frequency used with AM radios that need about 10 kHz in the IF.

There are other applications where more bandwidth than can be had at 21.4 MHz 
is needed and they use 160 or 240 MHz IF frequencies for those.

So if you want to process a signal that fits into an IF centered at 10.7 MHz or 
455 kHz then down converting from 21.4 MHz to one of those frequencies would 
make sense.  But remember the output of the converter may have spectrum flipped 
depending on whether or not the LO is above or below the input 21.4 MHz signal.
You probably can get these standard converters as kits or ready built.  The 
simpler option is to feed the 21.4 MHz into a conventional HF receiver.  When I 
did this the problem was my HF receiver did not have a NBFM mode.
http://www.prc68.com/I/DR33.shtml

Another more interesting option would be to use something like the SDR-IQ as 
the demodulator.  It can do the classical narrow band modes and at the same 
time show you a real time spectrum display a little under 200 kHz wide.

W.J. and others made/make a lot of demodulators and spectrum display units that 
worked from 21.4 MHz.  The C.E.I. and other TEMPEST receivers also use 21.4 MHz 
IFs.  http://www.prc68.com/I/CEI.shtml


Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.precisionclock.com



stephous wrote:
> Has anyone played with down converters for use with the Watkins-Johnson 21.4
> MHz IF outputs?
> 
> This is what I'm trying to do: Down convert the 21.4 MHz IF output for use
> with demodulators and other devices.
> 
> Any off the shelf or design recommendations?
> 
> Best Regards,
> Steve Pappin
> 
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