Agree,
I'd use the phrase "de-rated" like they do in aircraft engines.
Aircraft engines are rated for a certain horse power or thrust at sea
level. An engine thermally rated for 900 HP might only make three
quarters or half that amount at altitude.
The guys running those LDMOS devices full tilt, meaning past 1dB
compression, will have them fail, hopefully spectacularly, but then get
on the internet and bemoan SS amps in general as not being robust
enough. These are the people who have given SSPA's their reputation.
A friend of mine has an Ameritron 1306. 8 MRF-150's, amplifier rated for
1200W out SSB and CW. He runs it at 1KW. He doesn't vary. When doing
RTTY he knocks it back to 700W.
RF design of an SSPA is pretty straight forward, Most of the time and
money should be spent on the LPF and the thermal design. Everybody talks
about the price of the devices themselves. The most expensive part of
any SSPA is the LPF...by a long shot. Ceramic chip caps capable of
taking the high current involved are more expensive than silver if sold
by the ounce.
Why do we need four LDMOS devices for 1500W out? What's the 1dB
compression point on the devices? The LDMOS devices I know about have a
1KW compression point Run a pair at 800W out each. Should do it all day
long, have clean output, and be easier to cool.
On 4/23/2017 3:30 AM, Mark Hill wrote:
SPE should be in a good position to follow the 'multiple devices, under run'
approach. Their still current 2K-FA amplifier uses 6 pallets, each pallet
having a single 300 W, MRF151G, transistor. Then a combiner. There were some
pictures of the internals of this amp. on the FCC website when they were
going through type approval a few years back.
Unfortunately, SPE appears to be moving away from this approach - e.g. from
the six, oldie-but-goodie, MRF150 of their lower power amplifier (the 1K-FA)
to using only a single LDMOS in their 1K3 and 1K5 products. Sadly, many
users' see the rating, part of the amplifier model number, as something to
be chased, even on SSB. Some even report gleefully measuring power outputs
well in excess of the marketing department's rating. Independent test
reports suggest driving these amps. to circa 75%, or less, of the rating in
linear modes would make them much better RF neighbours.
I like the idea of four of the 1k8 LDMOS for a US legal-limit product. Being
generous, we could even allow the marketing department to call it a '2k'
product, before they feel tempted to conceive a model number with 7200, or
even 8000 in it! Four is a nice binary number for design. With care, and
enough copper and fans, it could even be CCS rated. As has been said, the
cost of the FET becomes less of a consideration when the product is in the
big $ class. User replacable pallets. Coverage to at least 70 MHz. Diplexed
output filters. Built in, -50 dB output sampler. etc.
Any manufacturers' willing to take up the challenge?
Regards,
Mark.
--
R. Kevin Stover AC0H
FISTS #11993
SKCC #215
NAQCC #3441
ARRL
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|