G3RZP wrote:
>
>It is interesting that in landline SSB, IMD was measured by loading the
>channel with a noise signal: this had a 'slot' in the spectrum with no
>noise in it, and the IMD was measured as the amount of noise appearing in
>that slot. This is (provided the noise has the correct characteristics - a
>fair amount of LF energy modulating it) a good simulation of the voice
>signal. For similar reasons, some tx specs requiring 2 tone testing get
>nasty with requirements for tone spacings of around 50Hz or less: this
>really shows up power supply dynamic characteristic faults. There was some
>stuff published on noise testing in the early 1970's and I know the
>Russian adminstration is keen on it as a method of testing
>commercial/professional SSB transmitters today
>
Yes, noise testing is a possible test method... but if our objective is
to measure spectrum occupancy with voice signals, there is absolutely no
point in simulating a voice signal when we can use a *real* one!
Sorry for not replying to Gary's messages (yesterday was very busy) but
Tom has already picked up all the main points.
> I wonder how repeatable voice testing is unless you have a
>standard voice
(That is understood - you must have missed that in a message several
days ago.)
>- certainly, the words spoken can give interesting
>variations in the energy per word and thus how hard the tx is driven.
The beauty of combining a standard digital voice file with peak-hold
testing is that the file can include several *different* voices saying
several different things. All you're looking for are that highest TIM
peaks that any one of them produces.
I admit that this is a severe test - well of course it's more severe
than a two-tone test... if we didn't *need* a more severe test, we
wouldn't be having this discussion!
But I think it is also a fair test. It does not disadvantage any kind of
transmitter that wouldn't also cause splatter on the air.
Obviously this is not a test from which you can extract more fundamental
information (I fully agree with Eric about that) but it *can* be
standardized.
Above all, it directly targets what other users of the amateur bands
want to know - the level of splatter.
--
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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