Mike wrote:
>On Wed, 2012-08-01 at 16:19 -0400, Gudguyham@aol.com wrote:
>> Bill, I'm SURE with today's multitude of hams with SDR radios, you
>> should easily get dozens of hams to give you "on the air" results. It
>> seems there is a lot of IMD cops out there. Pick one.
>>
>
>Trouble is most of 'em don't know how to tell the difference between
>your IMD and their receiver overload!
>
>Rx connected to a PC with a decent sound card and some known
>attenuators is a reasonable stab for things like amplifier IMD (where
>you are not trying for monster IMD3DR, it gets much harder when looking
>at things like good first mixers as everything becomes very critical.
>
RX overload and purity of 2-tone sources are only important if the TX
IMD is good (ie low).
But there are no such problems when measuring BAD signals. These only
require a dynamic range of 30-40dB, and in some cases even less. Most
modern SDRs can achieve this very easily, or will signal an error
message if they can't.
The most useful measurement on an on-air SSB signal is a 'peak hold'
spectrum showing total occupancy, because this tests everything at the
same time - the exciter, the PA, all combinations of IMD products from
all speech frequencies, and also the effects of poor PSU regulation
(which a static two-tone test fails to capture).
It is very easy to grab a 'peak hold' spectrum off-air from a single
transmission. It's also important to have a comparison spectrum from a
'clean' signal (hopefully, your own) at a similar maximum level.
Then the message is: "Your signal has much worse IMD than others
received here at the same level. Fix it!"
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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