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
reradiated.
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
improvement.
73/
Karl-Arne
SM0AOM
----Ursprungligt meddelande----
Från : manfred@ludens.cl
Datum : 2015-08-08 - 19:07 (V)
Till : amps@contesting.com
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
sure
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,
causing
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
has
one, could do the test, and compare how much external signal mixing happens in
class A as compared to class B.
Manfred
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