Tom W8JI wrote:
>> thread. I summarized some of their comments and included a method
>> that Rich suggested for measuring AVERAGE IMD with another receiver
>> and a calibrated step attenuator at <http://gs35b.com/imd.html>. Of
>> course this method can't be used to quote specific numbers for the
>> 3rd and 5th order products but produces an overall look at your
>> system performance.
>
> I'm pretty sure that does not give you the average IM level Tony.
>
> I'll have to think about that a little bit more, but off the top it
> seems wrong because the meter on our receivers detects peaks, not
> average. I'm pretty sure it actually would measure the highest value
> of peak power contained in the bandwidth of the receiver filter,
> assuming the receiver is perfect.
>
> Average makes no sense to me, since a receiver S meter is not an
> averaging measurement device.
That makes sense and it really isn't an issue of peak versus average to
me but rather a means to compare the levels
>
> I would add a correction that the level is not the average, but
> rather the peak power contained in the filter bandwidth. I'd add a
> second caution that the S meter should be set as low as possible to
> give a reading, not half scale. Many receivers fall totally apart at
> 30-40 over nine. If you had 40 dB of attenuation to make S-9 at the
> main signal, that would place the main signal at 9 +40 when you are
> on the adjacent channel. That would not be very smart. On the other
> hand if you set the reading on the unwanted trash to a minimal but
> reliable S meter movement of S-2, you would have much less
> fundamental signal to deal with.
>
> Let's look at my ICOM 751A data ( I happen to have it on my desk).
> S1= -130dBm S2= -110dBm S5= -100dBm S7= -90dBm S9= -80dBm S9+10=
> -70dBm S9+20= -60dBm S9+30= -50dBm S9+40= -40dBm S9+50= -25dBm S9+60=
> -17dBm
>
>> From S2 to S3 is about 1dB, so resolution would be good. If
> we were measuring peak power 40 dB down, S2 would place the main
> signal 40 dB above -110dBm at S9 +10 and we would have good
> confidence the 751A would not be non-linear.
>
> On the other hand if we placed the signal as suggested at S-9, the
> main signal would be at 40 dB over nine and the 751 is wiping out.
> Using mid-scale is not a good idea.
Ok... in my case I use the old TS-940 and I would have to put a
generator on it to see how it responded. The point was a convenient
point to compare the signals and your suggestion of another point on the
meter is well taken. The important point being that the thing isn't in
compression and you can compare levels there.
>
> The whole thing really needs looked at carefully. Obviously there
> already are a few problems.
>
> 73 Tom
Obviously this isn't supposed to be a substitute for good test equipment
but a method that could be used that could reveal something usable
without having thousands of dollars in expensive gear.
Thanks for the suggestions.
73, Tony W4ZT
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