RE: A simple front end

[Leif Asbrink <leif.asbrink@xxxxxxxxxxxxxxxxxxxx]

Hi Again,

> Thanks Roger, nothing so valuable as practical advice HI.  I was 
> pleased to find that there is a DIP version of the AD797 and from its 
> specs it it truly a very fine component. As it is such a wideband 
> device I would ask if there any merit in limiting the bandwidth into 
> the sound card?
I think I was on this question before. It is pretty important.

The very wideband AD797 has extremely low IM2 and IM3 when forced to
deliver 10V peak-to-peak into a Delta44. A very small RF signal on
the input can lead to several hundred millivolts output at say
14 MHz and that would destroy the intermodulation properties
for low frequency signals. A signal 1000 times higher in frequency
has to be well below 1000 times in amplitude at the output to be 
harmless. (I do not mean to be exact here, it just gives something
to remember the phenomenon by. I do not know the mechanism, it is just
what I found from experiment)

The RX2500 uses two cascaded LC filters to bring the RF leakage out
from the mixers to a harmless level. Layout is a little critical here
too. The mixer sends out LO overtones well into the VHF region.


73

Leif / SM5BSZ




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