Roger, Tom, and all,
I can't help giving my 5 cents (or whatever) on this...
With first generation SS amps, auto tune was almost a necessity, BUT
with the new amps?
It totally depends. If you only operate in the band segments where your
antennas have a low SWR, say, under 1.5:1, then of course you don't need
an autotuner. If instead you want to operate with antennas having higher
SWR, _and_ you want your amp to actually work as it should, then you
need some sort of tuner. It can be automatic or manual, internal or
external, located in the shack or built into the antenna - but you do
need one.
If the amp has good, effective protection, then a tuner is _not_
required for safety. The protection handles that. At high SWR you would
get lower output power, or even the ampo might shut down, but there
should be no smoke. But the news for some people are that a tuner _does
not_ improve safety! Even with a tuner, the amp still needs the
protective circuitry!
So a tuner is not a way to avoid damage to the amplifier, but a way to
allow operation at full power and normal IMD and efficiency into
antennas having high SWR.
With the current flock of transistors that can handle the wrong
antenna or even an open circuit, auto tune is just a convenient, but
expensive accessory.
Not so. Please, everybody, _DO NOT BELIEVE THE FALSE CLAIMS ABOUT
INDESTRUCTIBLE LDMOSFETS!_ There has been much advertising lately about
LDMOSFETs that can survive 65:1 SWR or even more, and that cannot be
destroyed. This is false advertising! There are videos circulating that
fool people into believing things the video does not actually show. The
reality about these "extra rugged" LDMOSFETs is actually quite simple:
They can handle such a degree of avalanching that they are SWR-proof _as
long as not overheated_. Only if the input power to an amplifier is
strictly limited (in the power supply) to a level that the
device/heatsink combination can dissipate continuously, then the device
is indeed pretty much indestructible by load mismatch. But in a ham
amplifier this is very rarely the case! The amplifiers are about 50 to
65% efficient, which means that the dissipation is about as much, or
less than the ouput power; and then the designers usually employ the
smallest, lightest, least expensive way to provide enough dissipation
capability for this power level.
For example, a 60% efficient, 1500W output amp would have a maximum
normal dissipation of 1000W, and the designer would use two LDMOSFETs
mounted on a copper heat spreader and a heatsink with fans, that keeps
the junctions at a safe but high temperature under these conditions. The
power supply needs to deliver 2500W. If the amplifier is running like
this, with the junctions nearly borderline hot, then there is almost no
additional dissipation capability left. And this means that if now a
mismatch happens, for example because a bird sat on the antenna tip, the
dissipation increases, possibly all the way to 2500W, the junctions
overheat, and the "indestructible" LDMOSFETs blow up. They take a time
to blow up - this time is given by the amount of additional dissipation,
and the difference between the normal operating temperature and the
absolute maximum one. The time might vary from minutes to milliseconds.
So, even "indestructible" LDMOSFETs do need protective circuitry. The
good news is that a proper combination of their ruggedness with the
protections makes the whole amp pretty much indestructible. But it
still cannot work at full power into a high SWR.
50% Vs 60%. Who cares?
I do care! Better efficiency means lower power consumption, the ability
to run from a smaller genset in portable use, a smaller power supply,
smaller heatsink, so a much smaller and lighter amp, and less heat and
noise in the shack. It also makes thermal handling much easier. It's
also better for the planet and for the future of humanity, although I
admit that the effect of us hams on total worldwide energy use isn't
such a big factor... :-)
Anyway, higher efficiency has such big advantages, that I made it the
first and foremost goal in my own amplifier development. If we can drive
the efficiency close to 100%, then we can basically do away with
heatsinks and noisy fans, and we can use power supplies of half the size
and cost - all while improving reliability!
They are easily water cooled without the headaches of water cooling a
tube amp..
Yes. And indeed we should use more water cooling in high power amps,
while we don't yet have high efficiency ones.
I agree with a lot of Tom's comments, but not all:
1. The LDMOS parts are a huge step forward. The prior parts,
typically needing matched pairs or quads were far more fragile and,
failing one is failing two (or four). This was expensive,
frustrating and frequent. As improved as the LDMOS part are, they
are far from indestructible and no match for tube reliability. The
65:1 SWR spec does not apply to CW where far more modest mismatches
will destroy the parts nearly instantly. I have had one of the
1.25KW parts fail on me.
Exactly. In CW you typically have the junctions running at a temperature
that doesn't allow much additional power dissipation.
2. A key to reliability is headroom. The closer you are to their
rated limits the more delicate they are.
Exactly, again.
I am running two 1.25KW parts at 1KW (total) out. I believe it has
helped reliability, but efficiency goes way down when you run that
far below max output. That means more heat
No, efficiency shouldn't be lower. You probably didn't run them at the
optimal load impedance, in that circuit. If you run a stage designed for
1kW peak output at 500W, of course the efficiency suffers a lot. But if
you re-design the impedance matching for the optimum impedance resulting
at 500W load, then the efficiency will be back up - and even a tiny bit
higher than at 1kW, due to a lower drain saturation voltage!
3. The good news is that you do not need a matching network. The bad
news is you do not have a matching network. Output varies quite a
bit as the load impedance changes and you can't do anything about it.
My solution is to flatten the SWR of all my antennas so I can switch
between them without seeing big excursions in the output power. The
worst my amps will see is 1.5:1 and most of the operating occurs
below 1.3:1. Manufacturers will disagree, but I think if your SWR
exceeds 1.5:1, you should use a tuner.
I fully agree.
4. The filter has been a bigger challenge than the RF module. I have
had multiple cap failures early on. Some have been due to other
design issues, RF in the selection logic etc.. I have not had a cap
failure this year, but I really do not know how much headroom I have
since the specs for RF current are sparse.
Yes, this is a problem. Many hams fail to understand it. Just a few days
ago I saw a web site of a ham who built an amp with low pass filters,
and he wrote that he "used 3kV rated capacitors because there is such a
high current there". He got it wrong! The point is that a capacitor
rated for a higher voltage _does not_ necessarily have a higher current
rating too! Very often higher voltage ratings go along with _lower_
current ratings!
For low pass filters, and many other RF power applications, designers
need to equally watch both the voltage and current ratings of the
capacitors used. A 1kV rating should give plenty of headroom in a legal
limit low pass filter used at reasonable SWR, but the capacitors will
need to have current ratings of 5 to 10 amperes, perhaps even a little
more in some cases. And there are very few capacitors available that
meet this requirement. And they tend to be expensive. Which makes
homebrew metal clad mica or metal clad teflon capacitors an attractive
possibility. They are physically larger, but are easy to make for those
ratings.
Instead of a bank of low pass filters, possibly followed by an antenna
tuner, I see no reason why one shouldn't use a higher Q, band-switched
tank circuit, much like the one in a tube amp, proviidng both the
filtering and matching functions. Just like in a tube amp, this could be
a manually or automatically tuned one.
Due to impedance reasons, the tank needs to come after the broadband
transformers, so it cannot be directly connected to the LDMOSFETs. At
least, not while they run from only 50V. When we get practical, usable
300V devices, we will be able to do away with the broadband
transformers, as the drain impedance will then be in a range where band
switching is practical.
Also, I concur with the comment that a diplexer is a must. The third
harmonic is only about 10-11 dB down. It is good to know where that
power is going.
Tom, I would like to dig deeper on this point. I do not believe that a
diplexer with a dummy load for harmonics is a good idea. So it might be
productive to discuss this.
My point is that a transistor does not "produce" a specific ratio of
fundamental and harmonic power. What it does, simply, is varying its own
conductivity according to the drive signal, following certain nonlinear
rules. How much power is developed in the process depends a huge lot on
the load impedance seen by the transistor. If it's loaded by a diplexer,
the load impedance will look mainly resistive at both the fundamental
and the harmonic frequencies, and a lot of power will be developed on
the harmonics, and dissipated in the diplexer's dummy load. That power
is lost, and represents a reduction in efficiency. If instead a plain
simple low pass filter is used, the load seen by the transistor will be
mainly resistive on the fundamental frequency only, but will be mostly
reactive on the harmonics, resulting in almost no power generated and
dissipated on the harmonic frequencies, and thus a better efficiency.
A low pass filter without a diplexer needs to be of a type that doesn't
cause the amplifier to develop excessive peak voltages or currents. An
amp with little or no negative feedback behaves largely as a current
source, and thus requires a capacitor-input low pass filter. Instead an
amp with heavy feedback works more like a voltage source, and thus
requires a filter with inductive input. And if the amp has a medium
level of feedback, performing like a "soft" current or voltage source,
it can be hard to obtain good operation with a plain lowpass filter of
any kind! That's why some designers have resorted to diplexers - but
it's not a good reason!
5. The splitter/combiners have been simple and reliable.
Yes. And in some cases they can even be skipped. It is possible to
design the output transformers for 25 ohm, and place the secondaries in
series. The main disadvantage is that emergency operation
with one half of the amp burned out is not possible. And there is a
larger chance for phasing issues, unless all wires are very short.
6. IMD is not good. There may be clever feedback schemes to help
with this, but it is over my head.
LDMOSFETs, just like other MOSFETs, bipolar transistors, and triodes,
are very non-linear. Tetrodes are better, but need a huge idling power
for it, resulting in ultra-poor average efficiency. So, indeed it's best
to linearize amplifiers by using negative feedback. And the very high
gain of LDMOSFETs makes them highly suitable for simple, effective
linearization by feedback.
That said, I find it highly desirable to operate amplifiers well into
saturation, for the sake of efficiency. This requires additional
negative feedback, to achieve decent linearity. Basically the feedback
circuit has to stabilize the gain at all drive levels to the exact
amount the amplifier has when saturated to the level decided by the
designer.
Such a feedback circuit can no longer operate directly at RF, from drain
to gate. Instead it needs an external feedback loop: An envelope
detector at the output, another at the drive signal, an error amplifier
comparing the outputs of both detectors and amplifying the result, and
applying it to some point that controls the gain. This could be the bias
of the LDMOSFET, or it could be a PIN diode attenuator in the drive
circuit, among other possibilities.
The result should be very good IMD, along with a full-power efficiency
around 80%, and a corresponding improvement in average efficiency. The
circuit looks moderately complex on paper, but uses only cheap, small parts.
My RF modules have been purchased as pallets. I do not think these
parts are optimized for linearity and they are nowhere near as good
as tubes.
Most pallets are indeed for broadcast or industrial uses, that don't
require linearity, and not for SSB transmission. So many of them have
modest to poor linearity. But surely not all. And of course, they cannot
be driven into saturation and still provide any acceptable linearity!
I really am hoping that some clever "predistortion" algorithms will
find their way into the SDR radios to offset some of the deficiencies
of these parts
This is slowly but certainly finding its way into ham radio. It's an
alternative to the external feedback proposed above, and has the
advantage that it can also predistort and thus correct the phase
distortion introduced by amplifiers. At least in principle, it can...
But it's a technique mostly suited to amplifier stages integrated into
the radios, and not to add-on amplifiers.
7. Heat removal is manageable for CW and SSB. I have not tried RTTY,
but there is a lot of heat in a very small area so it needs serious
heat conduction and airflow. If I were to build one for home use, I
would try water cooling. Some of the pallets now come mounted on
heat spreaders milled for water flow.
Yes. And the thermal bottleneck is usually the parts closest to the
silicon. That's the metal flange of the device, its interface to the
heat spreader or watercooled block, and its metal immediately under and
around the device. A large block of copper with water flowing through
channels on its other side is often far from optimal. It's better to
find one milled such that the water flows through narrow channels very
close to the device. Like directly underneath the device mounting area,
with the block milled such that it has a star-like structure of metal
fins protruding from the mounting surface into the water, combining the
most metal-water interface area with the shortest and widest possible
thermal path through metal. And the water should flow fast through that
part, causing high turbulence, which reduces the thermal resistance of
the metal-water interface. The fast flow is achieved by having narrow
water passages, and a sufficiently powerful circulation pump.
8. While energy is still energy, it is a lot easier to work with the
cover off when there is no 3500V exposed. That said, it is still
dangerous if you are not careful
Working under the hood of a running engine is far more dangerous, and
many rather modestly educated mechanics have survived that for many
years. No big deal. One just has to use reasonable caution.
This is a must. IF the amp does not have automatic band selection,
you will blow it up some day, protection circuitry notwithstanding.
If the protection circuit is good enough, there should be no way to blow
it up. But that means some sensing _before_ the low pass filter! Many
amps don't have it.
10. No doubt solid state is the future and, in a very controlled
situation, can be made to work well under demanding circumstances.
However, will they be as reliable and as linear as tubes for a wide
range of load mismatches, high duty cycles and operator errors? Not
today
I think they can well be MORE reliable than tubes. I have seen more than
enough tubes destroyed because of operator error. Of course that's
mostly because many tube amps don't have much protection circuitry. The
basic point is: With correctly implemented protection, an amp should be
impossible to blow up. But no amplifying device - neither solid state
nor hollow state - is indestructible on its own. Tubes tend to be far
more forgiving than transistors, in terms of mismatch and the resulting
overheating, but transistors are enormously more forgiving in terms of
rough handling, like impact, shock, vibration, etc. Shipping a tube amp
can be a problem. Shipping a solid state amp is not.
Like I said, I am speaking as an integrator, not as a designer. Many
of the shortcomings can be improved with better designs, but the
underlying technology still has its limits.
I see it from the designer's point of view. It is possible to embed an
LDMOSFET in an amp in such a way that it is protected from all sorts
of mishandling the designer thought about. And it isn't even expensive
to do that. But designers are fallible humans, and often overlook some
combination of conditions that can result in blowing up an amp despite
the protection circuitry. So it comes down to quality in design and
construction, and lots of testing under all imaginable conditions.
Engineering is a combination of brains and material. The more you use of
one, the less you need of the other. In low-volume products like ham
amplifiers, companies tend to invest rather modest amounts of brains, so
there are many components for a modest result. If ham consumers want
bullet proof, highly linear amplifiers, they have to pay either for huge
amounts of expensive parts in an amplifier that follows the brute force
principle, or they have to pay for lots of engineering hours that get
diluted among only relatively few amplifiers sold. That explains the
high price of today's solid state amps.
If there were a hundred million demanding hams in the world wanting to
buy a good amplifier, soon there would be many companies offering
excellent products at ever lower, competing prices. But given our real
numbers, and the lack of interest in high IMD performance and efficiency
that most hams show, there are only a few companies serving this
market, and they can't invest enormous engineering resources in it. It
simply doesn't pay.
A technically minded ham who is willing and able to spend lots of hours
designing and optimizing a good amp, can end up with an efficient,
highly linear amp that uses relatively few parts, and avoids high cost
ones. But there are very few such hams remaining. And that's the core
problem.
Phew, this post got longer than intended. Sorry for boring you all!
Manfred
========================
Visit my hobby homepage!
http://ludens.cl
========================
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|