Re: [linrad] A simple front end

["w3sz" <w3sz@xxxxxxxxxxxxxxx]

HI, 

Yes.  You can put a filter before the AD797 to protect it from the higher frequencies
also as they will degrade its performance.  Leif shows this in figure 3 at:

http://www.antennspecialisten.com/~sm5bsz/linuxdsp/hware/optiq.htm .  I did not do
this but then I had IF filters in place.

There is a discussion of the pre-AD797 filter and considerations pertaining to it
just above the figure at the URL just noted.

In the RX2500 Leif does use an antialias filter optimized for the sampling rate /
bandwidth of a Delta 44.  It is at:

http://www.antennspecialisten.com/~sm5bsz/linuxdsp/rxiq/antiali.htm

Again I did not use that, to keep it simple.

There are some good simple Direct Conversion Receiver schematics and circuit
descriptions at:

http://www.qrp.pops.net/popDC.htm .  I highly recommend this for getting started.

By the way this is part of the QRP Home Builder Page that has a lot of good stuff I
think you would be interested in!:
http://www.qrp.pops.net/default.htm

Just remember that when you use these front ends with Linrad, you do NOT want to
limit the bandwidth to 3000 Hz or whatever in hardware, as Linrad will do the 'final
filtering' in software, and wants to see as wide a bandwidth as your soundcard and
CPU speedwill permit ;)   Change the components so that you limit the bandwidth to 20
KHz or 25 KHz or whatever so that you reduce soundcard ailiasing.  The Home Builder
page has separate pages for RF filters, Diplexers, RC audio filters, etc that should
help with the calculations to change the filters.

These are just my thoughts.

I hope they help.  Please everyone keep everyone else posted on what works and
doesn't work.  Then we all learn!

73,

Roger
W3sZ

----- Original Message Follows -----
> 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?
> 



73,
Roger Rehr
W3SZ
FN20ah
http://www.qsl.net/w3sz




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