Thanks Manfred!
I honestly do appreciate the clarifications and although I did say
capable of 3 KW, I should have created the distinction between rated for
1500W capable of 3KW. Of course as a SWAG I'd say about 50 to 75 would
run an amp at what ever it's capable of doing
On 1/25/2019 10:47 AM, Manfred Mornhinweg wrote:
Hi Roger, and all,
SS amps (including those in many high priced 100 and 200 W rigs) have
historically had poor IMD.
That's correct, unfortunately. And the reasons are various. Among them
the fact that bipolar transistors aren't complitely linear, MOSFETs even
amplifiers copied from early pioneering work done at
Motorola in the 1970s, where the goal was to get any RF power at all,
rather than getting clean RF power.
That's about the time I was teaching Transistor Circuit design to the
trades. The Instrument shop guys at least were interested. I think this
was about where the electricians and trades other than the Instrument
shop opted out. They should have stayed as they missed all the
excitement inherent to Boolean Algebra and logic circuits:-D That's
about as far as we went before the workload (major expansion) made it
darn near impossible to schedule classes
Add to this that it's mighty hard to find ways to correctly handle the
very low impedance of power transistors delivering 100W from a 13.8V
supply. As a result almost every of those final stages is only a very
poor approximation to what a true linear amplifier should be.
At least going to 50 V helped that
All this can be improved quite dramatically, but hams these days buy
radios instead of building them,
True, but today's rigs are getting mighty complex. OTOH "It's my
opinion" that modern rigs and particularly the top end rigs are giving
the user far too much control, allowing them to make a pretty decent rig
sound like a pile of "fertilizer"? Then there's the ham who moves the
top and bottom of his signal 100 Hz lower to get more natural sounding
lows and then wonder why stations calling are 100 Hz off his frequency.,
or the ham who purchases a state-of-the-art rig with super steep filters
and blame the rig when he's hearing a station 5 KHz away with an IMD in
the 20s
and don't ask the manufacturers loudly
enough (that is, with their wallet and feet) to build better linearity
into the transmitters.
We are a nitch market
To build a SS amp with a CLEAN output requires an amp capable of
running almost twice the desired output. So an amp for a clean 1500W
legal limit should be capable of close to 3 KW out. Adding the
REQUIRED low pass filters capable of 3KW may cost as much or more
than the rest of the >amp.
I have to state some corrections to this. First, what exactly is an
amplifier's power rating?
Agreed
Typically linear amplifiers are rated for the power at which the IMD
degrades to just the acceptable level. That could be anywhere from
-28dBc to -40dBc, depending on the quality desired, and the type of
signal fed through the amplifier.
When they sell tube amps rated at two to three time the capability of
the tube and the users run them that way with crappy IMD and then
complain about short tube life, it follows that they will run SS amps
the same way.
For testing purposes, often an amplifier is rated for the power at which
the gain is 1dB lower than at mid power. That's the "1dB compression
point".
Instead non-linear amplifiers are typically rated either for the maximum
power they can produce under practical conditions, or a certain power
level where the trade-off between efficiency, gain and power is good.
IMD is irrelevant, of course.
I think you are being overly optimistic and giving some too much credit
(more power for less money).:-D Many will run an amp until it can't put
out any more and 811s get a bad rep.
So, Roger, if you want a 3kW amplifier to use at 1.5kW producing a clean
signal, the 3kW is probably the saturated power output, and at 1.5kW the
amplifier might be capable of an IMD level of -35dBc or better. But in
this case, this being a LINEAR amplifier, the power rating would be
1.5kW, not 3kW!
Agreed
Also neither the low pass filters nor the power supply need to be
dimensioned for 3kW operation, because the amplifier will actually never
run at more than 1.5kW - provided its control system is working
correctly, keeping it in its linear range.
If it's capable many will push it to the limits
These transistors are NOT expensive, compared to tubes. A transistor
(actually two transistors in one package) rated at 1.5kW output costs
$172 in single quantity, and less if you buy several:
https://www.digikey.com/product-detail/en/nxp-usa-inc/MRF1K50NR5/568-13079-1-ND/6562303
And if you get into more modern technology, using switchmode amplifiers
with envelope modulation, you can use gallium nitride transistors that
are even much cheaper than this.
What is true is that you won't usually find them at swap meets. But it's
quite possible that eventually somebody will buy 100 of them, or more,
at quantity discount, and sell them at swap meets cheaper than the
quantity for a single one at a distributor.
This means a LOT of work, skill, and knowledge in heat transfer.
Yes.
The mating surfaces on the heat spreader and the transistors need to
be flat! Really flat! Lapping surfaces close to being close to
optically flat takes skill!? Sometimes hours of effort and a great
deal of mechanical aptitude.
Forget that. Solder down the transistors.
Another plus for the new silver solder paste
And the copper base needs to be soldered to the circuit board, to
complete the source circuit! Many, many homebrewers forget this.
Many of these capabilities/requirements are outside the realm of the
sharpest people in electronics and old timers.? I know how, but no
longer have the physical abilities.
I feel like you... The older one gets, the more one knows, but the less
does the body help to turn knowledge into working equipment.
SS amp control? It can be built in or done by computer. Neglecting
the software, we still need to get the required signals, in the
proper form to the computer which may require substantial hardware in
the SS amp adding still more cost.
The only control that's really required is TX/RX switching with proper
timing, low pass filter selection, and SWR/thermal protection. This
doesn't need any computer. On the other hand, considering that a
self-contained microcontroller containing several analog inputs digital
inputs and outputs, various internal specialized hardware, along with
copious memory and processor capacity, costs under $2, it's not such a
bad idea to use one to replace discrete circuitry!
I was thinking of my Yaesu SS amp using 8 transistors
Of course one can follow the modern trend and make the amplifier look
like a computer with an RF stage added in the back corner. But I don't
see the point in doing so.
Now where I disagree on a couple of points. With today's licensing
structure (needed to build our numbers to justify our use of very
expensive frequencies) bringing in many younger and less educated
people with little? money or interest in electronics into ham radio,
lowering the average age of hams. Yes there are those few who are
expanding the envelope, bringing with them things like dynamic
predistortion. STILL the knowledge level (and abilities) of the
average new ham just like the average new college graduates, is
abysmal. (I taught at the university while working toward a masters
in CS and was dismayed at the abysmal intelligence in those classes)?
I can't ignore the inability of many old timers to adapt to the new
technology either.
Well, we don't really disagree a lot there... The lack of knowledge and
the superficiality of many young people is pretty obvious - but when I
look around old hams, it becomes obvious that those are in average not
much more knowledgable!
Agreed and they (in general) lack the adaptability / ability to embrace
the new technology
In my country the vast majority of new hams are so-called
"emergencists". They joined ham radio for the sole purpose of providing
emergency communications in the event of a catastrophe that renders
every other communication system inoperable.
"I think" that is somewhat true here as well. I've just never heard that
reference used
But were the old hams all technically savvy? No! Those were mostly
ragchewers, while a few were DXers and contesters. In the 1980s there
were also a lot of "radioneeders"
The CB band absorbed those for a while, but the grain elevator operators
got smart and installed business band repeaters and leased rugged
transceivers to the farmers
At my age, I'm now a appliance operator.?
I fight becoming one! My main transceiver is an appliance, a TS-450SAT.
But it's modified, with phase-lock to a GPS standard, improved filtering
(Collins mechanical filters), and so on. The power supply is a homebrew
That was my goal too, but two strokes and one heart attack
40A switcher. The station and home is powered with "homebrew
electricity". And I'm working (or at least pretending so!) on a high
power, high efficiency SDR transceiver project.
Those large displays are often necessary to display many important
functions in advanced SS amps.
Like which? And are they really necessary?
Depends on the amp and transistors. With the Quadra I'd appreciate more.
Particularly Emitter current and temp as well as drive. Some swear by
them and I swear at them.
Some rigs are expensive and over priced, but some bring new technology
with extremely clean signals.?
Like which? I'm not aware of commercial ham equipment having extremely
good IMD performance. I would like to know of any!
Anan has a group of transceivers and circuit boards from around 10W to
200W using dynamic predistortion with better than -70 db IMD with "Clear
Signal" software https://apache-labs.com/1001/Ham-Radio-Products.html
and they use "open source" software for most functions. Pricey but
still less than the competition and much cheaper in general than the big
three.
We technically interested hams should take the lead in developing a
type of modern legal-limit amplifier, that overcomes the problems
making them so expensive now. For example, instead of having one
lowpass filter per band, which is the reason for much of the bulk and
cost of amplifiers, we should develop a transmitter that can work with
a single lowpass filter. How so? Simple: Make use of the UHF
capabilities of modern transistors to modulate the entire HF signal
onto an UHF carrier, as a PWM signal or some other digital code, then
pass the UHF signal through a high efficiency, high power switching
stage, and then pass the output through a simple, single lowpass
filter that strips the UHF signal and passes everything HF and lower.
The harmonics of this amplifier would fall on UHF frequencies and be
filtered out easily.
We should definitely NOT keep copying the class AB push-pull linear
amp circuits developed in the 1970s, with all their problems and
shortcomings, as if there could be no other way in the world to
generate clean HF signals.
Agreed, although your depth of knowledge far exceeds mine. Ability too
as my left side has limited mobility. That now takes me hours to do what
used to take minutes. I could touch type over 70 WPM, now it's one
finger typing.
73, and again thanks
Roger (K8RI)
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
--
Roger (K8RI)
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|