Marv,
After much consideration, I've abandoned the idea of using solid state
devices in an Envelope Modulated configuration.
Was this decision caused by the voltage-variable capacitances in them, or any
other reasons?
At present I wonder to what extent envelope modulation can be used with FETs,
without phase predistortion, before the unwanted phase modulation causes too
much IMD. Clearly it will be necessary to limit envelope modulation to the
higher parts of the envelope, and get linearity in the smaller amplitude range
through a different method.
I am still pursuing an Envelope Modulated architecture employing a tube
final. The test until will employ a 6146. The modulator is a standard
digital audio part operating at 200 KHz driving a bridge of power FET's. The
4-pole output filter(s) are at 25 KHz. Designing a single filter becomes
problematic at impedances greater than 1K or so due the self resonant
frequencies of available inductors. Therefore, four bridge rectifiers
driving four filters are seriesed on the output of the (switching supply)
output transformer.
So far it looks like a good configuration. I was considering something very
similar, using a 4CX1500B. I have two of them in my junk box. But no socket.
I can't imagine the resonance of inductors being a big problem. Using ferrite
cores, the Q should be low enough so they behave mostly as resistors, at the
frequencies where they would otherwise resonate. Splitting up the HV supply into
separate rectifier/filter groups and putting them in series is actually the most
common way to build a HV switching supply.
> This also conveniently allows sourcing the screen at 1/4
of the plate supply.
But is it convenient to modulate the screen voltage too?
An auxiliary supply and LF amplifier is connected in
series with the screen source which may be controlled to envelope correct
this final amp.
But that costs efficiency! What might be very reasonable is to keep the screen
voltage constant during mid/high amplitudes, modulating the plate voltage, and
in the low areas only freeze the plate voltage at a fixed level and modulate the
screen to get that part linear.
The grid can be left to self-bias through grid conduction and leak.
When/If this design becomes functional a legal limit version
utilizing a 4-400A will be attempted.
An interesting project!
I'm still contemplating a phase modulated bridge, similar to what's employed
in the Broadcast Electronics 4MX series, for a solid state final. The 4MX
employs ordinary power FET's since it only has to operate up to 1.8 Mhz.
(Much!) Better transistors will be required for HF.
Or just much smaller ones, using many in parallel!
> I plan a test unit using
ancient Siliconix VMP4's operating at 50 watts. Operating at 50 ohms eases
the output coupling and filter requirements. To translate this to the legal
limit level it may be best to pray for some appropriate GaN parts.
Don't be afraid of using many cheap small FETs in parallel. Using individual
source and gate resistors, they work fine as one big FET, but with far better
frequency range than if you actualy use a single big FET.
The input
of such a device may be opto, or dielectricly, isolated at the digital input
Optosiolation at RF isn't trivial. Dielectric or transformer isolation is much
easier.
and a simple 1:1 50 ohm transmission line transformer can isolate the RF
output from a direct line operated power supply.
Here you have me puzzled. How can you use a transmission line transformer to
provide DC isolation? That would need to be a conventional transformer!
If you mean winding a ferrite core with 50 ohm coax cable, and then using the
center conductor as primary and the shield as secondary, then that's a
conventional transformer, not a transmission line transformer!
> A series LC filter is
connected in the primary of this output transformer and a double Pi-Network
on the secondary will provide output matching as well as harmonic
suppression.
That should work. Or you can place a parallel resonant circuit over the
secondary, and match by having both this circuit's cap, and that of the series
circuit in the primary, implemented as variable caps.
At legal limit power, the caps will necessarily be small vacuum
units.
That would be best. But air variables should work, too.
Envelope correction for this scheme will require a small (50nS /
Freq. or so) adjustable delay inserted into one side of the bridge drive.
Here you caught me. I don't see why that's needed. Also I can't figure out how
much delay you mean. I suppose you mean "ns", not "nS", but even so it makes no
sense to me, since a time divided by a frequency is a time squared! And if you
mean 50ns divided by the frequency as a dimensionless number, we end up with
just some femtoseconds delay in the HF range, which doesn't seem reasonable!
So, can you explain this bit?
Both of the above designs will be driven by a DSP based exciter, capable of
AM, FM, CW & SSB, employing some standard Audio DSP parts, a few Microchip
PICs, and an Analog Devices Quadrature Up-Converter which can output directly
on frequency. The exciter portion of this design has been stable for about 5
years and is under slow construction. A receive function is also
contemplated with a "conventional" front end, including somewhat less than
standard IF gain, and a DSP baseband demodulator.
That sounds interesting, as you can put the required phase corrections, and also
the delay of the phase drive signal to compensate for the long power supply
delay, into the DSP. But it makes this design unsuitable as an add-on amplifier.
It can only be used with the DSP radio. Still a very worthy project.
Operating designs of this type to "zero" output in SSB mode is problematic.
I've considered adding a low level signal (say 35-40dB down) "out of band" in
the tens of Hz region. This would be just a curious artifact on the air and
be filtered out by anyone listening to the desired signal.
But it won't work!!! Even if you add such a "pilot tone", the envelope of your
SSB signal plus this tone _still_ crosses zero amplitude a few hundred times
every second! Such a pilot tone keeps the output from staying near zero for a
long time, but it absolutely doesn't avoid zero crossings! So it doesn't help in
any way.
Now if you can make the EER system work to -40dB, then you can simply cut off
any signal that's even lower! No ham operation requires more than 40dB S/N
ratio; in almost every operating condition there is enough noise and QRM so that
parts of the signal 40dB down can't be heard; and any IMD arising from curring
off what's below -40dB, is irrelevant, specially if that cutting off is done
smoothly.
The problem, of course, is getting the EER to work to -40dB. That will be hard,
I fear! I think it's a better approach to make the EER work only to -20dB, or
even just to -15dB, and control the output amplitude in the smaller portions by
screen modulation, or bias modulation, while keeping the plate supply at the
level corresponding to that -15 or -20dB level. The penalty in efficiency will
be small, and things will get much easier.
Just a thought on heatsinks. Check out the heatpipes from the Apple G5
(which are currently seeking their place in the landfill). They have an inch
square base and are probably good for a couple hundred watts each.
That would need to be checked. In general I'm a bit cautious about computer
cooling components. There is too much snake oil being sold in that field. For
example, many people belief their computers need to get rid of LOTS of heat,
when they see PC power supplies rated at 600 watts, and CPUs rated at over 100
watts. But this isn't true, in almost every case! My own computer does have a
550 watt rated supply, a CPU rated at 130 watts "maximum design power", several
hard disks, two optical drives, loads of memory, and is stuffed full of
expansion cards. But the power consumption, measured at the AC input, is 27
watts! The CPU has an average dissipation of less than 5 watts. I disconnected
the CPU fan, to avoid its ugly whining noise, and the CPU's heatsink gets barely
lukewarm. The CPU temperature, as reported by the BIOS, stays below 40°C. So, I
fear that computer coolers rated at hundreds of watts might actually not be what
they promise!
I have been thinking a lot about a high efficiency amp using one of my 4CX1500B
tubes, with a switching power supply. The good points: It would be robust, I
have the tube, no need to built a complex heat sink assembly. The downsides:
Some blower noise, even if less than when working in low efficiency modes; 3
minutes warm-up delay; Need for manual tuning, or a complex autotuning system; A
constant base power consumption of about 100 watts during all times, even RX,
for the filament, blower, and losses.
Specially this last thing, the significant power consumption during standby
times, is pretty much a show stopper for me. My idea of a high efficiency
amplifier just isn't compatible with 100 watts of power waste during RX! So I
guess the 4CX1500B is not for me... not even in a high efficiency scheme. Maybe
some instant-on tube. But I don't have any suitable one, and buying tubes means
spending more money than on FETs...
Manfred
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