At 01:58 PM 10/15/06 -0700, John Geiger wrote:
>How much stock does everyone place in the Sherwood
>Engineering receiver test numbers (www.sherweng.com)?
>While I admit that the 2khz spacing is a much more
>useful measurement than the 20khz the ARRL has used
>for a long time, and I believe that Rob Sherwood knows
>what he is talking about when he tests receivers, I
>have a hard time believing some of his numbers.
I too would be interested in knowing his exact test setup. I know when we
were doing IP3 measurements on the Manpack radio we were designing, we
required at least 70 dB of isolation between generators and each generator
had to be low pass filtered to get the 2nd harmonic content down around -90
dBc. Since 3rd order is 2F1-F2 and 2F2-F1, you want the 2F component to be
generated by the Device Under Test, not the generators.
Another problem that is never addressed in measurements at HF is multiple
carriers although this is routinely tested in the CATV industry. During a
contest, multiple carriers that are strong in the frontend bandpass filter
can vectorally add in time to create a huge signal that drives the RF
amplifier into compression causing desense and cross modulation.
I believe Rob Sherwood still sells 5 KHz wide crystal filters for various
parts of the 20 and 40 meter band to alleviate this problem.
73 Hardy N7RT
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