Gary Schafer wrote:
>
>
>Ian White, G3SEK wrote:
>
>> GW4FRX made some very useful tests using a borrowed analyser with a
>>digital peak hold facility. By continuing to sweep across the whole
>>signal for several minutes, holding the highest signal level found at
>>each frequency step, the analyser builds up a good statistical picture
>>of the overall bandwidth that the signal occupies.
>> The composite spectrum is composed of IMD products from all speech
>>frequencies, and it generally looks like a ragged triangle with the
>>higher-order IPs disappearing below the noise. The longer you sweep,
>>the better the statistics, and the smoother-edged the triangle becomes.
>>
>
>Hi Ian,
>
>The problem with peak hold analyses is that a particular peak only has
>to hit once and it becomes the same class of data as a frequency that
>is almost constantly present.
>It will tell you all the possible frequency "hits" that occurred but
>may give a false impression as to problems.
>
It does give equal weighting to splatter that only occurs on transient
peaks of the modulating waveform. But transient IMD could for example
include splatter that occurs on almost every word, which most other band
users would classify as annoying. Peak hold is a severe test, but not an
unfair one.
>Broadcast stations have different standards to adhere to than amateurs.
>A broadcast station must have a certain "frequency contour" that the
>signal conforms to. It must not transmit anything outside that contour.
>Sample and hold analyses is how that is measured.
>
Do you have access to any standard test procedures? We need to avoid
re-inventing the wheel here.
>
>Probably the simplest and most repeatable way to monitor inter mod is
>to just switch your receiver to the opposite side band. Im products are
>going to fall on the opposite side band as well as on the wanted side
>band. On the opposite side band you don't have the wanted signal
>components to interfere with hearing the IM products.
>
I thought we had agreed that levels of low-order IMD (which is what you
find in the opposite sideband) give no indication of the levels of
high-order IMD. That's why we need a test that goes outward to look for
high-order IMD.
--
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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