[Amps] Checking for IMD

[R L Measures]

On Aug 12, 2006, at 6:09 PM, 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,

Good idea, Tom

> 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.

Since the same S-meter is used for both measurements, and step- 
attenuators are quite impartial, the method works.
>
> Average makes no sense to me, since a receiver S meter is
> not an averaging measurement device.

D-arsonval meters don't semi-average?
>
> 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.

good point

> 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.

S-meters typically have fewer db per S-unit below S-5
>
> Let's look at my ...

R L MEASURES, AG6K. 805-386-3734
r at somis.org