Zyg Skrobanski wrote:
>>>Does a transmitter with poor 3rd order IMD therefore have any noticeable
>>>distortion in the voice quality due to the speech intermod products within
>>>the audio bandwidth?
>>
>>Why would it have audible distortion? The garbage being measured is already
>>20-30dB down, is outside the passband of the receiver fileters, and the
>>receive detector and audio system already has IM products. Not only that,
>>many locations have high noise levels masking IMD that can be heard in quiet
>>locations.
>
>
> Looking at the ARRL graphs of transceiver IMD testing, and comparing
> different rigs by their 3rd order products, if one rig has poor 3rd IMD
> results it does not necessarily follow (according to the graphs) that the
> higher order products are also going to be worse. For example, for the 160
> meter result at -4 KHz, the TS2000 has a spur at -55 dB, and the FT1000MP
> is at -50 dB. Both rigs are below 60 dB at 10 KHz. So, if the 3rd order
> distortion is not audible (not that I thought it would be;) and its value
> does not indicate the amount of the higher order IMD, then what is the
> purpose of having the 3rd order IMD specification?
>
>
>>
3rd order distortion is usually quite a bit higher than 5th or higher
levels. 3rd order products are usually the ones that cause the most
problems, so are the ones looked at most.
Remember that 3rd order products are generated from the mixing of the
frequencies within the audio pass band of the rig. So the distortion
(3rd order products) will be close in frequency to the wanted signal.
2A + B + C. So 1kc x 2 + 1.5kc = 3.5kc etc. The 3.5 kc product falls
just outside the wanted pass band of the signal. That equals adjacent
channel splatter.
If the wanted signal is 30 over S9 and the transmitter has a -30 db 3rd
order performance, that puts the 3.5 kc unwanted product at S9 level.
Do the multiplication with many different frequencies within the audio
band width and you can see how far out some of them fall. 1.8kc x 2 +
2kc = 5.6kc. Also some 3rd order products of the lower audio frequencies
fall within the wanted audio band. As Rich said, that is why distortion
is sometimes heard on frequency. The 3rd order product being 25 or 30 db
below the wanted signal gets covered up by the wanted signal if it is
within the pass band so it is hard to hear. But outside the pass band
the 3rd order products are all by themselves without the wanted signal
to cover them up.
A decent receiver makes a pretty good spectrum analyzer. Tuning off to
the side of a signal will reveal unwanted products. Just remember to
account for where the filter edges are in relation to what your dial says.
73
Gary K4FMX
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