Re: Chebyshev low-pass / Amp questions / Etc.
- From: w8ji@xxxxxxxxx
- Date: 6 Apr 2006 12:23:40 -0700
KC2PIH@xxxxxxxxx wrote:
Hello - I've got my hands on an HF amp and I just don't know about the
quality of the low-pass filter in it. It appears to have a 5-element
Chebyshev low pass filter in it. I don't have anyway to measure the
values of the inductors or capacitors used in that circuit. When I
drive it above 25Mhz (into a dummy load of course), the SWR is low. It
has four MRF-454 transitors in it and when I drive it with 25-30 watts,
I see 400+ watts out PEP. The SSB audio sounds clear too, so without
understanding the circuitry fully, it seems to be biased "above" class
1.) First of all, the MRF454 gets very non-linear at 80 watts output if
you have a full 13.6 volts on the collector. If you are running 400
watts PEP, you are certainly splattering excessively. There is doubt
about that. That's just how that device is. That is why Motorola calls
it an 80 w device, and why four MRF-454 will barely make 300 w PEP on
PEAKS with marginally acceptable IM distortion.
2.) Second, the amp MUST have filters for each range. There isn't any
possible way to have that amp meet TA harmonic specs without several
switched filters. Harminc have nothing to do with having acceptable IM
performance.
3.) How it sounds on frequency, or how a few people tell you it sounds,
has little to do with legal linearity and splatter. There isn't a
person alive that can tell if an amp has unacceptable bandwidth unless
he knows how to tune your signal, has a good noise environment, and
your signal has acceptable strength. How you sound "on frequency" does
not indicate at all how clean you are with IM distortion or splatter,
and anyone checking you off frequency has to have a low noise floor,
good receiver, and know how to do it.
I hope this helps. It's very important to know what we are doing with
amps and radios!
Bands like 20 meters are just getting ruined by cheap 12 volt solid
state amps. Well designed solid state amps are generally bad enough,
but the poorly designed CB amps using transistors with poor bias
systems and driving the devices into non-linearity are ruining many
crowded phone bands.
If a solid state amp will run 150 watts peak power, it is generally
only acceptable to 100 watts or less on peaks for SSB. The general rule
of thumb for solid state amps is they have acceptable linearity only at
66 % of maximum peak power.
This is why most solid state 100 watt radios will run 150 watts when
people turn up internal controls without regard for bandwidth, and why
your MRF454's will run a bit over 400w saturated peak power but only be
linear to about 300 watts on peaks when power is measured on an
accurate peak meter.
73 Tom
.
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