[hpsdr] Norton Amps

[FRANCIS CARCIA]

Bob,
  I suffer from a pair of 8640Bs, an HP141 and an RMS volt meter. It gets pretty hard to measure dynamic range above 100 db close in. I seem to remember the third order spur showing up with the input signals around -27 dBM. I was pretty sure the crystal filter after the amplifier was generating the spur.  I hate to quote this kind of performance without the right test equipment.  Also I was seeing a MDS of better than -140 dBM. This was with my homebrew second mixer into the Racal RA6830 IF using a 300 Hz filter at 455 KHz. The stock SD5000 first mixer / filter module is passive with an active limiter ahead of that. This means there is 10 dB of loss before there is any gain stage. The stock Racal has a U310 as the first gain stage and it also has an MDS (3db) of better than -140 dBM at 40.455 MHz. Yes I could say -143 to 145 but who knows how good the leakage is around the output attenuators. I had a 10 dB pad on the generator.
  The RA6830 has an (3dB MDS) of -130 to -134 DBM out to 30 MHz with this configuration. It does not need an RF amplifier when using a good antenna. The nice thing about the Norton is the low gain. This allows maximum dynamic range without saturating. 
  Ulrich mentioned CATV amplifiers. The performance is great but what do you do with all the extra gain. I have never been able to find a way to take advantage of the performance due to the high gain. I never found a way to get gain distribution right.
  They do look like a great LO amplifier for a high level DBM.
  I can say the push pull Norton easily outperformed the 40.455 MHz Racal crystal filter for dynamic range. Many filters are rated 30 dBM IP3 so the amplifier beat that.
  Dallas Lankford did a lot of testing in his article and he quotes IP3 performance above 40 dBM. I trust his numbers and know he is careful about the way he measures this kind of performance.  I can send you a copy of his article later if you don't have it. 
  Also around -25 dBM out of my combiner is about the point there seems to be some interaction between my generators. I use a pair of 6 dB pads at the output of the generators but the isolation does not seem good enough. I'm pushing the test system limits around the performance limit of the stuff I'm testing.   Frank
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