[Premium-Rx] Prescaler / Frequency Divider

[Brooke Clarke]

Hi Mike:

I too have given this some thought.  The part of your proposal that's missing 
is all the filtering needed.  If you look at some of the military premium 
receivers you'll see that the band switch covers segments that are typically 
and octave or less.  This is because you can not filter out the second harmonic 
if the coverage is more than an octave.  For those receivers that do work with 
octave or wider bands they either suffer from second harmonics or are very 
linear so as to not generate them.

Then you need filters to keep adjacent out of band signals from getting into 
the mixer.  And filters on the output to limit what goes into the final device, 
like the SDR-IQ.  Receivers that have all this filtering also have many metal 
cans to isolate the LO signals from receive path signals and tend to be heavy 
and expensive.

Some down converters leave the RF port wide open and use a harmonic type mixer. 
  Here the idea is that you are only interested in the strongest signal and it 
gets mixed down to the output band.  I think this is how the Opto counter works 
that displays the frequency of the strongest signal.

Have Fun,

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



mikea wrote:
> On Tue, Jun 12, 2007 at 11:15:20AM -0700, Brooke Clarke wrote:
> 
>>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
> 
> 
> Which recalls my project of making or buying a block downconverter for 
> my SDR-IQ.
> 
> I'd like to be able to use it on 30 to at least 450 MHz, which means 
> that I need a downconverter. It seems to me that a suitable down-
> converter would let me set a switch to choose which 30 MHz band got 
> mixed down to the roughly 0-30 MHz baseband.
> 
> I'd like to have this work up through the 70 cm. ham band for sure,
> and as much higher as I can get. I know that construction techniques
> change as one gets above HF, and change _radically_ at UHF and above.
> How hard is this going to be, overall?
> 
> The general setup I envision is something rather like
> 
>                                LO
>                               maybe 
>                               n*30MHz
>                  maybe variable in 5 or 10 MHz steps
> (n to                          |
> (n+1))                         |
> * 30 MHz    maybe some         V        0 to 30 MHz
> input --> preselection    --> mixer --> bandpass filter --> to SDR-IQ 
>             or RF preamp                                   
> 
> Some questions for the (perhaps not older: I'm 60) wiser heads with
> more design-and-construction experience: is this at all a reasonable
> thing to do? Does anyone have any ideas on suitable mixers? Am I
> looking at a disparate-enough set of frequency bands that I will wind
> up having to use multiple mixers, possibly one per 30 MHz band, or
> per octave? It is trivially obvious to me that I'll need different
> antennas.
> 
> Is anything like this available as a kit or commercially? I've looked
> at Down East, and nothing there quite suits. 
>