Date: Wed, 10 May 2017 11:23:45 +0930
From: "Leigh Turner" <invertech@frontierisp.net.au>
To: "'AMPS'" <amps@contesting.com>
Subject: Re: [Amps] HV MOSFETs for RF
<Yeah Peter's Class-A amp is likely run with considerable back-off so the
<expected efficiency figures seem worse. Nevertheless I'm sure it runs
<squeaky-clean with very low IMD at 25 Watts PEP.
The "average operator" to whom you refer is often clueless in adjusting
their rig and amp combos for attaining good end-to-end Tx IMD.
New SS amps like the OM-600 etc are primarily designed with modern SDR
exciters in mind, those owners being their target audience rather than
conventional 100/200W rigs. The latter requiring a power attenuator pad to
properly interface and achieve the systemic operational sweet-spot and no
nasty power overshoot possibilities. In some respects this is not unlike the
situation with a high-gain grid-driven AB1 tetrode linear amplifier.
In the ham HF context I would regard IMD levels attained by the classic
Yaesu FT-767GX or the Icom IC-751A rigs as perfectly acceptable with their
2-tone IMD performance and cleanness of their on-air signal. Sadly few
successive modern ham rigs managed to attain / retain this good benchmark in
respect of IM3 and curtailing the more troublesome higher order stuff :-(
Marine HF radios of virtually any era routinely achieve this good level of
IMD performance because all Tx user controls are removed from the front
panel so the rig always stays in its sweet spot mode no matter what.
Similarly the commercial and military HF radio manufacturers like Codan, etc
routinely attain excellent Tx IMD performance from the very same families of
RF power devices used by the ham radio manufacturers; so it can be done if
the will and commitment to do it is there.
Leigh
VK5KLT
## Class A can easily be done with 25% eff. When you talk about..back off,
shane nailed it.
If you want to ..back off, then at least design the output network around the
reduced power level, so
eff is high. IE: MRF-422s with mating output circuit, designed to operate
at no more than 100 w pep out.
Not designing the same pair of MRF-422s to operate at 200w pep out, then back
off to 100 w pep out, thats
fubar, the eff will drop to 35% every time. If you are gonna run 35% eff,
you may as well run 25%....and run it class A.
## A MK-V run at 200w has a diss of 200watts..at 50% eff. Same MK-V run
at 100 watts will dissipate 186 watts with 35% eff.
No wonder the MK-V runs just as hot at 100w as it does with 200w.
If it was designed correctly, it could be run at 100w out...and 100 w of
diss..with 50% eff. BIG difference between 100w diss..and 186 watts
of diss....for the same 100w out.
## marine radios achieve good IMD cuz they are designed correctly, nothing to
do with lack of front panel mic gain or processor knobs.
Marine gear always has way better 2 tone results vs ham junk...using the same
2 tone test sequence on each. Check out the current
ITU marine specs for SSB IMD products, its an eye opener. Then look at the
original FCC marine specs, which were even more stringent.
Kicker is... marine radios run on 12 vdc, not 30 or 50 vdc ! Marine radios
will still meet ITU specs with just 10.5 vdc...and a dying battery.
## problem with ham gear is.... it not only has worse IMD than its marine
counterparts.... it gets even worse still, with the processor and
mic gain controls. So you then have ham gear with bad 2 tone imd specs to
begin with, that in turn gets further degraded with the mic gain
and processor controls. Now that all these xcvrs are DSP, you no longer even
have a real RF clipper. You cant do clipping in the DSP domain,
so plan B is to screw with the ALC time constants instead. A fast attack,
fast release RF compressor, done right, will have the same basic effect as
a RF clipper. The ALC on any xcvr is never compatible with the ALC on most
amplifiers, they all use different time constants.
## Modern Xcvrs with their sky high RX specs are basicly rendered semi useless
these days. I have yet to even hear an IMD product on CW.
I require 2 x cw signals, each 50 db over S-9, and 2 khz apart, to even get an
IMD product thats equal to the MDS of the RX. Problem is,
the typ –140 db MDS of todays xcvrs is rendered useless, since the noise floor
in urban areas is way higher than that. Same deal with astronomy
+ light pollution. I gave up on that a while back. If you cant even see
the milky way from your own back yard, throw in the towel, you are wasting
your time. Ditto with 160M and S7 to S9+ noise levels. The dedicated 160M
RX arrays dont work too good from my location. If I point it at
say VK land, Im also pointed right at a source of noise. Ditto with every
other direction. 160M these days, is for the folks who live out in the bush.
## HUGE difference between a collins S line or KWM-2 with its –58db pep
IMD3 and a K3, with its –29db pep. Thats a 30 db difference.
Those 2 radios are not even in the same ballpark. Thats a full 3 magnitudes
of difference...apples + oranges. Perhaps the fix is to put amateurs
on dedicated freqs, like say every 7 to 10 khz apart on SSB. Ever listen to
a SSB contest ? The band is chock full of IMD. A 30 over S9 ssb signal
will have IMD products that are S9+. A collins would be S4 on a collins
RX....and S1 on any icom / yaesu / kenwood.
Nobody can hear anything, cuz the band is full of imd.
## the OM-600 was designed to run on anything from 100-265 volts..and was
tailor made for folks who want to use just 120 vac. Being the lightest SS
amp out there, its good for dxpeditions, and taking on an airplane etc, or
using anywhere where the commercial AC power varies in both freq + voltage.
You dont even have to change the wiring taps, there isnt any. Only thing that
gets changed is the fuses, big ones for c120 vac..and small ones for c240 use.
Jim VE7RF
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