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Re: [Amps] IMD and spurious measurements

Subject: Re: [Amps] IMD and spurious measurements
From: "" <>
Date: Sat, 8 Aug 2015 20:00:08 +0200 (CEST)
List-post: <">>
In my opinion, the spurs are not generated in the amplifier, and not from
any external signals that are picked up from the antenna, and they are not 

Instead, the mechanism is the same that generated "modulation hum" in the 
heyday of the
regenerative receiver. Strong RF near fields are generated around the 
transmitting antenna that are picked up by
house wiring and test leads in the environment.

Non-linearites in all forms of power supplies create IM and modulation products 
that even may have their switching frequencies superimposed. 

This problem is commonly observed in multi-transmitter duplex installations on 
small platforms.
If small active antennas are used, which are very sensitive for near fields and 
common-mode currents,  it may severely limit the performance of the system.

It is however not the fault of the transmitters, the 400 W and 1 kW MOSFET 
amplifiers used on naval ships are mostly specified for "back-end 
intermodulation rejection" exceeding -60 dB down from the carrier when external 
signals of - 30 dB are fed into their outputs. The broadcast signals that you 
are receiving are far weaker than these levels.

As you correctly say, class C operation makes "back-end intermodulation" worse. 
I was once able to witness this when triplexing three 5 kW class C marine 
transmitters into one common log-periodic antenna. The isolation between the 
transmitters was about 35 dB, and the strongest in-band mixing product measured 
as -55 dB. Substituting 400W solid-state class AB amplifiers for a comparison 
netted a suppression of around - 70 dB which was on the limits of measurement.

If you repeat this measurement, using a well-shielded directional coupler as a 
signal pick-off between the amplifier and antenna, you will notice a marked 


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Datum : 2015-08-08 - 19:07 (V)
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Ämne : [Amps] IMD and spurious measurements

But now comes the most interesting discovery of the day, and the reason why I'm 
writing all this: Until here I was doing all tests into a dummy load, with my 
mixer's pickup antenna running along the coax to the load. I wanted to make 
that I'm getting no RF from the antenna into any place where it doesn't belong, 
so I switched to the antenna, found a clear spot in the upper portion of 40 
meters, nestled amidst lots of strong broadcast signals, and made a test 
transmission. Suprise! My signal had more spurs installed on it, than a 
Christmas tree has stuff attached! And some of that stuff was even moving...

It turns out that the whole mess of strong signals that is in the air gets fed 
from the antenna into the final stage, mixes with my transmitted signal, 
thousands of strong spurs, and all these get re-radiated!

That's life, folks. Transmitter final stages are very big and powerful mixer 
stages, connected to antennas through only broadband filters. Except while 
driving a final stage deep into saturation, the trash generated by them from 
mixing external signals onto new frequencies is FAR stronger than their 
internally sourced IMD! It makes one wonder how much sense it makes to strive 
for -40dB IMD products in a lab's perfect dummy-loaded environment, if this 
won't cure the -20dB spurs caused by external signals as soon as a real antenna 
is connected!

Comments, anyone? Do we have a case here for class A final stages, which should 
be a lot less prone to mixing external signals? Unfortunately I don't have any 
class-A radio at hand, to test this. It would be interesting if somebody who 
one, could do the test, and compare how much external signal mixing happens in 
class A as compared to class B.


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