> So from the above the IM products from 300 and 3000 Hz
> tones are:
> 3rd = 5.7 KHz and -2.4 KHz.
> 5th = 8.4 KHz and -5.1 KHz.
> 7th = 11.1 KHz and -7.8 KHz.
>
> All of the negative products represent the products that
> fall on the
> opposite side band.
The extremes of IM3 product are the *highest frequency
transmitted* plus 2.7 kHz and the *lowest frequency
transmitted* minus 2.7kHz.
Consider a 7 MHz USB signal . The upper IM3 would be 7.003
plus .0027 = 7.0057
The lower IM3 would be 7.0003-.0027= 6.9976
If we had a receiver at 7.0000 with EXACTLY the same carrier
offset and filter BW the LSB passband on a sideband switch
would be 6.9997 down to 6.997.
The highest tone spread IM3 product from the TX would fall
at 6.9976 and be covered. The lowest tone IM3 found on LSB
could be well above 6.9997.
The passband of the receiver does not exactly align with the
IM3 range because of the 300Hz lowest tone offset.
Most likely all the closer spaced IM5 and higher will fall
within the passband, but none of the wider ones will.
We really can't say this is any kind of IM measurement. It
simply checks opposite sideband with whatever peak
accumulated power the S meter is detecting. Any good AGC
detects the peaks and holds the sample, but also has some
tolerance to ignore very short momentary peaks. So the S
meter is an almost peak, but not totally peak, indicator.
It definitely isn't anywhere near average unless the
receiver has almost useless AGC.
This thing really needs some thought as to what to call it.
As for receivers, many are terrible for IM within the
roofing filter BW. The FT757 is around 55dB IM3 DR with two
pure tones. The Collins 75S3 around 60dB (no roofing filter
so that is 3kHz spaced IM3). An R4C would be useless for
this method. While fortunately they are mostly gone, it
still illustrates we have to know the receiver.
The problem is IM3 DR is measured by audio and not by S
meter reading. The fact the meter does not move at the
bottom end pushes the lower end of threshold up. It isn't
good enough to just hear the signal, now it has to move the
S meter to a useful level to be a useful signal.
I'm not sure what that would do to the useful dynamic range,
but at first glance it doesn't seem good. My 751A for
example has a noise floor about 20-30dB below the lowest
possible S meter reading. Any IM3 DR test I would do would
look for IM3 products near that noise floor. An IM3
transmitter measurement would require a starting level at
least 30dB higher than noise floor. That will take DR away
from the receiver.
Do you follow what I am saying Gary?
I sure hope people don't use this method over the air unless
they fully understand what they are doing. It looks like the
initial thoughts didn't even understand that S meters read
the peak power, not the average.
73 Tom
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