On Thu, 2012-08-02 at 07:53 +0100, Ian White GM3SEK wrote:
>
> RX overload and purity of 2-tone sources are only important if the TX
> IMD is good (ie low).
Rx overload can be an issue that gets mistaken by those who do not
understand the use of the attenuator for transmitter generated IMD if
the receiver is seeing a massive signal from the subject transmitter.
> 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.
Yea, but you still have to get the input level somewhere sane.
> 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).
Linrad has a good mode for this test in its transmitter testing mode,
but two of three tone testing also has its place during development
(much easier to make repeatable).
> Then the message is: "Your signal has much worse IMD than others
> received here at the same level. Fix it!"
I always liked just sending QSD (Your keying is defective), says it all
really.
Regards, Dan.
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