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 of
time 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
________________________________________
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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