Gary Schafer wrote:
>
>We are not talking about precise measurements here but getting in the
>ball park as to how bad a signal is by an easy method.
>
Forgive me, but you're still missing the point: measuring low-order IMD
simply doesn't tell us everything we need to know - not even
approximately.
On the air, there is a vital difference between these two cases:
Transmitter #1, whose higher-order IMDs progressively reduce in
amplitude with frequency offset, and go down rapidly so you can
comfortably listen to weak signals only a few kHz away;
Transmitter #2, which has almost exactly the same levels of 3rds and
5ths, but whose higher-order IMDs reduce only gradually - or even worse,
only come down to a "plateau" level, and then audible splatter ( =
high-order IMD on speech peaks) continues around that same level for
100kHz or more, on both sides of the main signal.
You can live on the bands with TX #1, but not with TX #2... but a simple
measurement of low-order IMD can *not* tell which is which. Since we're
interested in high-order IMD, we need to measure it directly.
Some of us have done these measurements, with both spectrum analysers
and receivers, and seen the difference. More to the point, most band
users have heard the difference. Therefore we need a lab test that will
*show* us that difference.
>Any method that uses voice as a signal source is not going to be very
>precise anyway.
>
With a standard voice signal that is digitally recorded and reproduced,
combined the peak-hold method of measurement, the results can be highly
repeatable. (Those two features of the test should not be separated.)
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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