AM to PM conversion in plate modulated transmitters

[QUOTE


The fact that this happens on all three transmitters makes me think it's an inherent property of AM plate modulation of vacuum tube amplifiers. I need to brush up on vacuum tube physics, but I think the varying B+ changes the distribution and density of electrons within the tube, varying the plate-to-grid capacitance and thus the phase shift of the amplifier.[/QUOTE]

Bingo! Indeed the phase shift of a tank circuit changes with the resistive loading, (and reactance) which is constantly changing in a plate modulated stage. The FCC never specified incident phase modulation on an AM transmitter, since nobody had any means to measure it until quite recently. And it has no audible effect on a conventional demodulator. (The FCC, on the other hand DOES have severe limits on the incidental AM on an FM transmitter!)

Where this REALLY has a nasty effect is on NTSC video transmitters. The chroma phase can depend a great deal on the underlying luminence...and the effect is called differential phase. Incredibly complicated and convoluted circuitry is generally used to compensate for differential phase. Or at least WAS, when analog TV was still around.

 

You know, I remember this stuff too. I worked as an engineering intern at the Maryland Center for Public Broadcasting in the mid 1970s when I was a Cornell student. I hadn't yet learned all the theory needed to understand everything but I definitely remember the term "differential phase". I knew what it meant and how to detect and measure it, but I didn't know the mechanisms that generated it. I think it could show up in almost anything that processed an NTSC signal, not just the VSB broadcast transmitter. Microwave links, video tape machines, camera electronics, video processing amplifiers, etc.

I remember watching the other engineers adjusting video pre-distortion circuits in the transmitter to correct for the distortions of the Eimac power klystrons. I was especially impressed by the hugely stretched sync pulse needed to drive the nonlinear klystrons to the correct peak power.

In those days, WMPB operated on UHF channel 67 with an RCA TTU-60B (?) I vividly remember the operating parameters since I had to log them every hour: emitter voltage -18 kV (the anode was grounded to facilitate water cooling); 4.5A beam current for each of two visual klystrons; 2.25A for the single aural klystron, later reduce to 1.75 A when they modified their license because the aural signal was out-talking the visual.

 

Well, sort of. Yes, simultaneous amplitude and phase modulation can be used to generate SSB. It can also be used to generate any kind of signal: AM, PM, FM, SSB, ISB, DSB, QAM, whatever; that's what I/Q processing in SDRs is all about. I was thinking of the special case where the phase shift increases with the amplitude in a nonlinear but predictable way. (There's only one degree of freedom, vs two in the general case.)

I'll have to think about that. I think generating SSB or even lopsided AM requires more than a simple, direct relationship between amplitude and phase. Viewed in complex coordinates (i.e., I and Q) at baseband (i.e., referenced to the suppressed carrier), an upper sideband SSB signal vector will only rotate counter clockwise and a LSB signal vector will rotate only clockwise. That's because the USB signal only contains positive frequency components and the LSB signal contains only negative frequency components. Of course the amplitude and rotation rate of each signal vector will vary a lot, but it will always rotate in the same direction. In the AM-to-PM conversion case, the instantaneous rotation of the signal vector is in one direction as the amplitude is increasing, and back in the other direction as the amplitude decreases.

 

I still see some signals with a lot more audio on one side of the carrier.
Its always an old tube rig, but it does not seem to sound bad.

 

Now you really have me wondering about this.

 

One of the nice things about the demise of NTSC is that a bunch of really nice vectorscopes can be had for bargain basement prices. These can be twiddled to create a great RTTY monitor....or ANY phase/amplitude modulated digital signal.

 

https://hackaday.com/tag/vectorscope/

 

George Woodard stated that there was a concern with IPM with the advent of the then Analog Stereo (Motorola) system and phase noise:

"...Stereo operation is incidental phase modulation
(IPM) which has been previously discussed in section
7.1.5. Excessive IPM can affect stereo
separation, single channel distortion, and the occupied
bandwidth of a stereo transmission; with
the most significant of the three being single channel
distortion.

Recognizing the various causes of IPM is the
first step in correcting the trouble. There are
many potential sources of IPM but the most common
source is insufficient amplifier neutralization,
either of a final modulated RF amplifier or
of a lower power driver stage. The solution is,
of course, to perform better neutralization of the
offending amplifier stages. This is easier said than
done in most cases. Since the inception of AMStereo
broadcasting, manufacturers of the transmitters
have paid more attention to the problems
of IPM and in most cases have reduced production
levels of IPM in current model transmitters
to acceptable levels for AM- Stereo operation.

In some cases, for especially difficult problems, field
engineers from the manufacturers factory must
make special on site adjustment of individual
transmitters which are particularly susceptible to
excessive IPM. Several engineering consulting
firms have developed special knowledge in the
AM- Stereo field, having collected data on many
transmitters in current use, some of which have
been out of production for several years. It is suggested
that such a firm be contacted to solve particularly
difficult neutralization procedures on old
or new equipment which may not have been satisfactorily
performed by the original manufacturer.

Phase Noise
Residual phase modulated noise cannot normally
be detected by a standard AM broadcast
receiver employing envelope detection, while
stereophonic AM receivers are sensitive to PM
noise. PM to AM conversion can occur on the
medium wave band over multi- ionospheric
"hops" (night -time skywave propagation) which
can then be detected by receivers employing standard
envelope detection. Very early transmitters
sometimes produced more residual phase noise
sidebands than those produced by the desired program
amplitude modulation. Night -time reception
of distant stations sometimes was accompanied
by a 120 -360 Hz "roar" which was caused by
phase modulation from the master oscillator filament
supply. A somewhat subdued "roar" can
be heard even today from many shortwave stations
using older transmitting equipment, especially
on the higher shortwave bands where multi -
hop propagation is more prevalent.
Significant phase modulated noise in modern
transmitters is virtually nonexistent due to the use
of high quality quartz crystal oscillator.

Phase noise modulation of 0.6
degrees (0.01 radians) avg. is fully acceptable for
monophonic AM broadcasting. Phase noise modulation
of approximately 0.2 degrees (0.0032 radians)
avg. is usually considered acceptable for
AM stereophonic broadcasting. As with measurements
of Incidental Phase Modulation (IPM), an
HP -8901, or equivalent, Modulation Analyzer is
recommended for phase noise modulation measurements..."

Another source (can't seem to find it right now) said that in Amplitude Modulation if the Third Harmonic is allowed to be present with the fundamental AM wave, phase modulation will be present.

Jerry Whitaker in, Power Vacuum Tubes Handbook states that Incidental Phase Modulation can be caused by poor neutralization and or by improper loading of the IPA stage.

Pheel

 

Yeah, but....cathode ray tubes???

Seriously, can they work well away from 3.58 MHz?

 

Ah, that's useful!