Here's more about IMD than you may want to know...
I used to work with very low IMD amplifiers for the cellular industry
and spent a lot oftime studying IMD sources and means of control.
I learned the most about the causes of IMD from a PhD thesis, and
later book, by Joel Vuolevi...
http://herkules.oulu.fi/isbn9514265149/
Most literature describes Intermodulation distortion as coming from
harmonics of the fundamental mixing with the main signal.
3rd order IMD means
2 x fundamental +/- 1 x fundamental. (2+1 = 3rd order)
But, the total contribution to the IMD also comes from 3 F +/- 2F,
4F +/- 3F, etc up to the 9th harmonic and beyond. All those products
stack up on the same frequency at the output.
Each of these products has a phase and the resulting IMD depends on
how all the phases line up. This can make the IMD level very sensitive
to the test signal, power level, temperature, bias, etc. It also can
make the high and low side IMD have different levels.
The FCC recognizes this effect and mandates that RF transmitter tests
uses a random modulation to measure IMD, so the testers don't tweak the
test to find a sweet spot in IMD performance where some of the components
cancel out.
The level of harmonics and their phases in the output matching
network affects the total resulting IMD. But it's nearly impossible to
design for or control all those effects. Every matching and bias network
presents a different impedance at all those harmonics.
Most UHF power transistors designed for very low IMD (-50 dB) have an
output match specification that is _NOT_ the same at the optimum power
transfer match.
So, the optimum IMD setting of the tune capacitor is _probably_ not
peak power output but there's not enough information available to set
it...and it would drift around anyway.
The biggest contributor to IMD is the amplifier going into compression
on peaks. This produces a nonlinear input vs output response and those
non-linearities produce the harmonic mixing effects that create IMD.
The shape of the compression curve affects the level of IMD. A curve
with a sharp break has more IMD than a more gradual curve. As a guess,
a more heavily load amplifier will have a softer compression curve and
produce less IMD when driven into compression.
A heavily loaded amplifier limits on current and a lightly loaded amp
limits on voltage swing. Intuitively, voltage clipping seems "sharper"
than current limiting.
The cellular industry implements predistortion of the input signal
to compensate for amplifier compression. The amplifier compression
curve bends down, so the predistortion circuit bends the the gain
curve up to compensate.
The future of amplifiers for _should_ include an IMD monitoring
circuit and predistortion. This would produce significantly more power
with a smaller and more efficient amplifier.
IMD levels should be set to about -40 to -50 dB. This would have to
apply to both transceivers and amplifiers, because much of the IMD these
days comes from the crummy amplifiers in the transceiver.
Crowded bands would seem completely different if everybody's IMD
dropped 10-20 dB.
jeff, wa1hco
-------- Original Message --------
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
TexasRF@aol.com [TexasRF@aol.com]
Sent: Wednesday, December 21, 2011 8:12 PM
To: deswynar@xplornet.ca; donroden@hiwaay.net
Cc: amps@contesting.com
Subject: Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!
Hi All, it makes no sense to not peak the tune C as the last step in a
matching procedure.
The tuning network is fully adjustable, allowing a match from your nominal
50 ohm load to what ever plate load impedance you want to use. If the tube
needs 2000 ohms for best efficiency and power output, it is a simple matter
to provide that. But, the transformed impedance is resistive only when the
complete network is resonant. The only way that can happen is when the
plate tune C is adjusted last, for maximum power transfer. The complete
network includes all sources of reactance, including tube output C and stray C
as
well as the effect of the rf choke and tank inductor and tune C.
When you tweak the tune C for maximum output, you are making all of the
reactance contributors parallel resonant, leaving only a resistive load for
the tube.
Sure the plate load impedance changes when you tweak the tune C, that is
what is supposed to happen. Over coupling slightly is just another way to say
that the plate load impedance is reduced slightly. If you don't end up
with the desired over coupled value, then the plate load C is not set to the
required capacity and needs further adjustment.
Operating a tube into a reactive load just doesn't make any sense in my
mind. It may not be reactive enough to hurt anything but how can it help in
any way?
Standing by for other view points and arguments.
73,
Gerald K5GW
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