[Amps] Nonlinear Thoughts

Radio WC6W via Amps amps at contesting.com
Thu Oct 23 15:38:18 EDT 2014


*** Manfred,

> 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?

*** Several reasons.   

    I wanted to run the system at 50 ohms.  This required high voltage parts.  The one's from APT, IXYS & DEI (now part of IXYS) all required massive gate drive.   

    I considered some silly schemes such as cascoding a HV FET with a MRF140 in the source.  This gets rather partsy for a bridge design and still requires considerable gate drive.   I also considered ancient VHF bipolars, which don't require the voltage excursion of the FET gates, for the cascode.   It's unfortunate that we have to design with only parts that are available. :-)

    It never really looked practical on paper.

    And it's unfortunate that a current mode bridge doesn't appear to be a realizable geometry.

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 have considered a variable mode scheme for both tubes and transistors ... several times.

> 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.

*** It eases the voltage requirements on the secondary windings, rectifiers, capacitors, etc.

 > This also conveniently allows sourcing the screen at 1/4
> of the plate supply.  

But is it convenient to modulate the screen voltage too?

***   By virtue of the 1/4 supply tap, the Screen is coarsely modulated in sympathy with the plate supply.  

> 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.

***   Not much.   The aux supply is 50 volts at 20 mA.   As a starting point, it is designed to track at 60% of the plate modulation value.  Envelope correction may then be applied to correct the foibles of an otherwise open loop system.

The grid can be left to self-bias through grid conduction and leak.

***   A small fixed supply with a shunt regulator takes care of that here.

> When/If this design becomes functional a legal limit version
> utilizing a 4-400A will be attempted.

An interesting project!

***  The 4-400A construction was actually started many years ago intended to be a standalone EER amplifier before I knew what I was doing!  

> 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.

*** To do it right, and output a 30 Mhz square wave, the parts need to be capable of a couple hundred MHz.  

> 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 

***  The input isolation is performed on digital signals.  The RF frequency, with separate generators for each side of the bridge, are in the amplifier rather than the exciter.

> 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!

***  It's a piece of RG402 with ferrite cores on the outside.  The shield is the primary and the center conductor is the secondary.  An extension of this line, with more external cores, forms a balun.

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.

*** A nice bonus of placing the series trap in the primary is preventing DC flow across the bridge in the event of ... something bad happening!   

> 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?

***  At 30 Mhz the bridge is operating at a 30ns. rate.   At this frequency, a ns or two should be sufficient to correct any errors.  At 2 Mhz, several tens of ns may be required.   

      A fixed delay of appropriate length may be inserted by the exciter into the reference side of the bridge.   Then a variable delay in the modulated side of the bridge may allow both positive and negative excursions, from the calculated timing, controlled by the envelope correction circuitry.   Some form of protection will be necessary to avoid overrunning a cycle which would be verrrry ugly.

> 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.

***  These designs may only be used with the matching exciter.   Neither was intended to be a standalone amplifier.

      The delays in the RF path and Power path in the Envelope scheme are both in the millisecond range.  Curiously, the RF delay is a mite longer and therefore it is the Power path which requires time padding.      

> 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 

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.

*** We'll see.   I expect the 6146 version to perform well.

> 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!

***  The G5 had a kilowatt supply.   IIRC, each processor ran at 100 watts max. though Apple controlled the clock speed to greatly reduce the dissipation unless the thing was actually processing something more than a couple of keystrokes.   

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.

*** I have a 4CX1500B amplifier with a conventional (100 lb) power supply.   I have considered making an envelope tracking switching supply for it just for ... fun.   The delay in the SPS output filters requires some look ahead from the input and if that's done correctly it becomes somewhat partsy.

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...

*** Can't do much about that heater!



73 & Good afternoon,
    Marv WC6W



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