[BC] BE 4M Modulation (from other list)

[Harold Hallikainen]

> This may be a breach of list protocol, but the broadcast.net list server
> keeps rejecting my posts as spam, so I'll try to reply to Harold over here
> at radiolists, since Harold is here too!
>
> The message rejected by broadcast.net follows...
>
> -----------------------------------------
>
>
>> Harold
>> Hallikainen writes:
>
>> I just read the article in the December RG about the new BE
>> AM transmitter
>> with "4M" modulation. From what I understand from the
>> article, it MAY be
>> similar to an idea I had 20 years ago or so.
>>
>> I had always thought that you could back down the pulse width
>> driving the
>> grid of a class C RF amplifier to back off the power without adversely
>> affecting the efficiency. The relationship between output
>> power (or output
>> amplitude) would not vary linearly with the pulse width, but you could
>> probably predistort audio to drive the pulse width generator
>> such that the
>> RF amplitude linearly followed the modulating audio amplitude.
>
> I did my MSEE thesis on RF pulse width modulation back in the mid 70s.
> That's what 4M is - RF duty cycle modulation. There is a nonlinear
> relationship between RF duty cycle and envelope amplitude, and it's a sine
> function. In other words, the amplitude of the output is proportional to
> the
> sine of the conduction angle. So, the predistortion must take the form of
> the arcsine function. This is easily done by comparing the modulating
> signal
> to a sinewave, much the same as conventional PWM is formed by comparing
> the
> modulating signal to a triangle function.
>
> Unfortunately, this technique does not work very well with tube type
> amplifiers. RF PWM (aka "4M") is only slightly more efficient than simple
> linear amplification or grid modulation. It's easy to understand why. When
> the tube's output amplitude is reduced, then the plate voltage is not
> swinging close to zero when plate current is flowing. So by Ohm's law you
> get a lot of dissipation in the tube when you cut back to lower
> amplitudes.
> There's a lot of voltage across the tube when the current is flowing. It's
> the same mechanism that limits efficiency in linear amplifiers and in
> simple
> (non-Doherty) grid modulation.
>
>>
>> I discussed this with someone at NAB. It MAY have been an
>> engineer from
>> AEL. I was looking at his new FM exciter at the time, I
>> think. Anyway, he
>> said that dealing with such narrow pulses would be difficult
>> and suggested
>> generating two pulse trains and vary the overlap between
>> them. This would
>> be the same as phase modulating the two pulse trains in opposite
>> directions. The two pulse trains would drive the grids of two tubes in
>> series so the series combination only conducted during the
>> overlap of the
>> pulse trains giving the desired variable RF pulse width "class C"
>> amplifier.  I commented that this sounded a lot like Ampliphase. He
>> agreed.
>
> Yes, it IS a lot like Ampliphase. According to Jerry Westberg's NAB
> presentation, it's done with a bridge amplifier. Each side of the bridge
> is
> driven with 50% duty cycle squarewaves. The phases of the two sides vary
> in
> opposite directions with the arcsine of the modulation function, and the
> output is taken differentially. The result is a series of pulses of
> alternating polarity, of varying width. That amplifier topology results in
> something that is a combination of RF pulse width modulation and
> Ampliphase.
>
> So just as you described, this is in fact two amplifiers in series with
> each
> other and with the load.
>
> The hard part of getting this to work is to make the transistors switch
> efficiently (and without blowing up) when load current is flowing. Each
> side
> of the bridge amplifier sees a load with a phase angle that varies as an
> instantaneous function of the modulating signal. The voltage and current
> in
> each side of the bridge amplifier are only in phase at the maximum
> positive
> modulation point. Below that amplitude, one side sees a capacitive load
> and
> the other side sees an inductive load. Jerry told me that BE has done
> something to make each side of the bridge see a better load. Whatever that
> is, that's the major engineering accomplishment that makes RF PWM work.
>
>>
>> Anyone have any details on 4M? It sounds interesting!
>>
>> Harold
>>
>
> Yes, it's very cool. The 4M PA topology is basically a balanced modulator.
> Modulation can theoretically pass beyond -100%, causing the carrier phase
> to
> flip 180 degrees as the envelope folds over. If BE has solved the
> "reactive
> load" problem then it opens up a new possible way of directly generating
> IQ
> signals such as COFDM, SSB, ISB, DRM, etc. Rather than using envelope
> elimination and restoration, it may be possible to directly generate IQ
> signals at high level by phase offsetting half the modules in a
> transmitter
> by 90 degrees, creating a highly efficient IQ modulator. The 0 degree
> modules would be modulated with the I channel signal and the 90 degree
> modules would be modulated with Q. (Or to equalize device dissipation, one
> set of modules could be modulated with I+Q while the other set is
> modulated
> with I-Q.) The whole transmitter would turn into a high efficiency
> arbitrary
> (but narrowband) signal generator. There would be no spectral
> growth/cancellation process the way there is in envelope elimination and
> restoration systems. This approach could carry a penalty of up to 3 dB in
> power or silicon (depending on the ratio of Q channel energy to I) but it
> would still be high efficiency. And it should be very clean too, again
> because of not having to split the signal into phase modulation and
> envelope
> components, with the bandwidth and time delay cancellation issues that
> ensue.
>
> Maybe we can all learn more from the Boston station that's using one.
>


In the tube case, it seems that if the grid is driven with a square pulse
train (not the normal sine wave of a class C amplifier biased beyond
cutoff), it seems that the tube would still be pulsed into saturation
maintaining efficiency.

The idea of a bridged output between two 50% duty cycle amplifiers is
CLEVER! Much simpler than my two tubes in series idea. I guess the bridged
output drives a transformer with a low pass filter after it (and
appropriate circuitry like a harmonic resonator to not pull the amplifier
output away from being a square wave, which would reduce efficiency).

On generating IQ, since the result of IQ modulation is a signal of varying
amplitude and phase, it seems much simpler to just vary the phase of the
drive and vary the overlap for the amplitude. No more parts!

I assume the drive is now being generated using DSP techniques instead of
a sine wave generator and a comparator, right?

On each side of the bridge driving a reactive load, it seems that as long
as the transistors are in saturation, we should not care what sort of a
load they are driving. Just pull that side of the bridge high or low and
let the transformer draw what it will.

Do you suppose the drivers are switching between ground and a single
supply (probably positive), or between positive and negative supplies?

Anyway, neat idea! I always wanted to play with this on ham radio, but
never got a chance. Since it'd be possible to IQ modulation, it'd be an
interesting way to generate SSB.

Harold


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