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Re: Uninformed comments on AO-40 24 GHz RX

>From: "Murray Peterson  VK2KGM" <vk2kgm@ihug.com.au>
>Warning: These comments are from someone who has never built anything to
>operate above X band!
>    The obvious starting point for me in my thinking is to built the
>complementary receive circuit to the K band transmitter on AO-40. Some of
>the details are at http://www.amsat-on.org/ .
>    The 24.048 GHz is down converted to 2827.06 MHz then 469.177 MHz then
>76.197 MHz and finally 10.7 MHz which could go to a HF tranceiver (many HF
>transcievers like mine have a RX only input). The interesting thing about
>this collection if IFs is that they are all between 1/5 and 1/10 of the
>preceeding frequency and so filtering is not difficult and all the LO
>frequencies are derived from a single 65.496 MHz crystal oscillator.
>    Maybe it's just that I think it is quite a good logical approach to
>K-band and we know it works!


Your suggestion appears to be sound, especially if you are building
everything above HF.  My previous suggestions was trying to take in account
the equipment that those of us on mode-US already have.  In that case the
24,048-->2401-->145 approach only requires the addition of antenna, preamp
and 24,048/2401 converter.

I wonder if any one has noticed that 24,192 - 24,048 = 144 MHz.  I would
seem that 24,048 has been chosen for a convenience to 144 IF's.  A 24,192
LO is derived as the 168th multiple of 144, and many circuits have been
designed for producing RF on multiples of 144: i.e 432, 1296, 2304, 3456,
5760, 10,368.  A simple downconverter could be made from an
oscillator-multiplier chain starting at 144 to a mixer and using an 144 IF.
 Only problem would be LO feedthru birdie exactly on 24,048 [144].  Since
the AO-40 passband is 24,048.25-24,048.75, that is not a problem.

But as Charlie, G3WDG,  pointed out, image frequency filtering is much
harder with such a low IF.  Using 2401 as the first IF is much easier!

A clarification: easier I suppose for those experienced in microwave
design...that is not me.  Charlie, I'm guessing you are thinking of making
a kit?


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