[hpsdr] Dynamic range and stuff

[FRANCIS CARCIA]

Racal DF receivers used 2 DB steps in the AGC. There was a 32 db step attenuator at the antenna input. .5 to 30 MHz then about 90 dB in the second IF. My TCI 8174 does 1 DB steps with analog switches and resistive dividers. 
  I have measured SDR close in dynamic range at 2 KHz spacing quite good with a stock Dell sound card of around 84 dB. This is pretty hard to do in an analog system,
  Wideband I measured only 95 DB 20 KHz. spacing. While close in the numbers are quite good but wider spacings I would like to see better performance. I don't know of a preselector that will give skirt selectivity for performance at 20 KHz. 
  The best front end I have seen was the H mode mixer into a pair of matched filters driven by a quad hybrid. This was published in QST some years back but I have not built it yet. I have collected 2 pairs of roofing filters 2 8 KHz Racal units and 2 Harris
  12 KHz parts from the HF350 for this project.
  The high performance mil tuned preselectors I have seen all have bandwidths at 10% that don't help much in traffic. Cubic made a nice 1U rack mounted unit and Harris has a few. 
  I suspect an RF amplifier with 10 dB of gain should be plenty under 50 MHz with a 10 dB pad switchable pad. Then use a mixer that has at least 40 dB IP3 followed by minimum gain. 
  Racal RA6830 uses a nice active limiter with gas discharge tube ahead of the front end to deal with ESD and RF power.  Cubic R3030 senses input power and pulls in a relay to open the antenna line.  frank WA1GFZ 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/attachments/20060817/7133cd47/attachment-0003.htm>