[Amps] Checking for IMD

[Tony King - W4ZT]

Gary Schafer wrote:
<snip>
> It is no poorer than tuning the receiver to another frequency to 
> listen for IM products. The same kind of noise and interference will 
> exist on other frequencies too.
> 
> My contention of using the other side band is that you are always 
> moving the receiver pass band the same amount every time you switch 
> side bands as opposed to tuning around. You can quickly switch back 
> and forth to compare levels of wanted and unwanted signals. It give 
> you pretty much the same information as you get by tuning but it is 
> very repeatable when checking various signals on the band. You can 
> get a feel for what the signal levels should be on the opposite side 
> band with some practice.
> 
> 73 Gary  K4FMX

Gary,

Perhaps that is where the confusion is/was all along.

My point was that, as you pointed out, there is a very large portion of
the IM that will yield mix frequencies outside the normal listening
passband 2A+B, 2B+A and higher order combinations. Those same
frequencies will be outside BOTH sideband windows so the ONLY way to
listen for them is to tune away from the carrier frequency.  The
distinct advantage to tuning away would be to insure that there was no
question of sideband suppression issues and it farther separates you
from the fundamental energies. Tuning the receiver will have to be done
so those IM products fall inside your bandpass.  In the case of USB and
using your 3 KHz example, it would be necessary to either tune DOWN 2.7
KHz minimum or more (preferably more to get the fundamentals well out of
your filter) or, if you switch to LSB, to tune UP 5.7 KHz minimum or 
more as above. In either case, listening and hearing doesn't tell you 
anything. Nor does a "S" meter reading unless you have controlled 
conditions where you can monitor the signal in isolation, adjust levels 
to give you a non saturating fundamental signal, and then comparing that 
signal with the level of the IM products. The basic premise of Rich's 
original method. Attempting to make any kind of measurement or judgment 
with signals we hear on the bands is, in my opinion, a total waste of time.

Good IM hunting!

73, Tony W4ZT
I sure wish I had a good spectrum analyzer ;)