[Amps] IMD

[Tom Rauch]

>> I disagree.  There is a local ham who, during calls to dx 
>> stations, runs
>> his
>> amp into a region above where he should.  It is those 
>> peaks that cause
>> terrible buckshot.  If the amplifier is tested at 600 
>> watts, and the
>> distortion level is rated at that level, then that is all 
>> it should be
>> rated
>> at - period.    Any power output above guaranteed 
>> linearity is worthless.
>> Colin  K7FM


> Thats a bunch of BS as usual from you.

Why is that line necessary in a technical exchange? IMO he 
was making a valid point.


> Any decent commercial amp will do more than its advertised 
> at. Its called
> overhead and allows for tube aging. Using you distorted 
> logic (pun intended)
> an AL-1500 cant be run over 1500W otherwise you will have 
> a poor signal.


Please check this link:

http://www.ameritron.com/man/pdf/AL-1500.pdf


If we actually read the specs, it was tested in excess of 
2500W PEP in a two tone test. The IM3 at that level was 
better than -34dB from one tone of the two. That's -40dB PEP 
(crummy ARRL method).

It is a little cleaner at 1500W, but not much.

By the way the tested IM3 was worse case drive level using 
two isolated exciters through a combiner rather than a sweet 
spot caused by using a typical radio nto the amp. It is 
possible if you raise and lower power (especially using a 
regular radio as a test generator) to find "sweet spots" 
where certain IM products are better over a narrow range. 
Many systems don't follow a classic 3:1 IM vs. carrier level 
change. In other words if you increase carrier power by 1 dB 
IM should increase 3dB, but that doesn't often work in real 
systems.

If someone is measuring IM using a radio that has IM 
anywhere remotely close to the PA IM levels, then they are 
fooling themselves. They aren't measuring anything. It takes 
an exceptionally clean source to obtain accurate data, and 
you won't find that in a modern radio (although a class A 
FT1000MP MKV kept well out of ALC gets close).

I also have the opinion that IM should be determined at the 
maximum specified or expected power, not some lower number.

73 Tom