Re: [linrad] A simple front end

["John Harrison, NI1B" <jmh6@xxxxxxxxxxxxxx]

Hi all,

  Regards low noise. I have an I&Q with diode mixers running on 40/20m. I
finally wound up with a TI part IIRC 2227 p/n or so. It has 1.3uv/root
hertz noise. Was 10-20 db quieter than til082.

  Regards Bandwidth into audio card. This is a MUCH more serious problem
than it first appears.

  Older ISA audio cards, no problem. They all seem to have good
anti-aliasing filters. 

  On many PCI (3 or 4) that I have tested, there is *NO* anti-aliasing
filter on the board, *AT ALL*. Motherboard audio seems to be particularly
bad this way. 

  Given the difficulties of building discrete audio filters, finding a
board with real filters in whatever chip is probably the easiest approach. 

  That said, at the higher rates (96k and 192k) the chip filters have been
designed with *MUCH* too high a cutoff frequency for use in I&Q designs. 
This limits a 192k part to quite a bit less. 

  So far I have not found an easy workaround, though an additional digital
filter on the output of the ADC appears possible.

  warm regards,
  john, ni1b


On Sat, 9 Aug 2003, W6WO wrote:

> 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?
> 
> 
> >If you want only the lower 25 KHz then you don't need quadrature 
> >detection and can
> >get that with just an ordinary soundblaster type card running at 
> >44100 HZ sampling
> >rate.
> >
> >to play around just do this:
> >
> >Direct conversion:
> >
> >antenna to
> >
> >[RF amp if needed] to
> >
> >TUF-1 to
> >
> >single AD797 to sound card
> >
> >
> >or dual conversion like I did, but omitting the crystal filter to 
> >start.  As you
> >know, if you omit the filter you will then have images to worry 
> >about, twice the
> >noise, etc, but for quick and dirty it will work.  I know because I 
> >played with it.
> >
> >Try:
> >
> >antenna to
> >
> >[RF amp if needed] to
> >
> >TUF-1 to
> >
> >simple single transistor or ERA or MAR 1st IF amp to
> >
> >second TUF-1 to
> >
> >AD797 or other baseband amp to
> >
> >soundcard.
> >
> >You can make it as simple as you want and add the extra pieces 
> >later.  I had a lot of
> >fun doing this and seeing what the practical effects of adding / 
> >changing filters
> >were, etc., as I am not an electronics person and so it was all 'new 
> >territory' to
> >me.
> >
> >73,
> >
> >Roger
> >W3SZ
> >----- Original Message Follows -----
> >>  Thank you all for the suggestions regarding a simple front end. Many
> >>  of you are aiming much higher than I am. My operation is almost
> >>  exclusively in the low 25 kHz end of 20 meters so I am not seeking
> >>  wide bandwidths and given the inherent atmospheric noise I am not
> >>  seeking VHF noise performance.
> >
> >
> >73,
> >Roger Rehr
> >W3SZ
> >FN20ah
> >http://www.qsl.net/w3sz
> 



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