AM to PM conversion in plate modulated transmitters

Discussion in 'Amplitude Modulation' started by KA9Q, Oct 10, 2019.

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  1. KL7AJ

    KL7AJ Ham Member QRZ Page

    [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. :)
     
    KA9Q and WD4IGX like this.
  2. KA9Q

    KA9Q Ham Member QRZ Page

    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.
     
    Last edited: Oct 11, 2019
  3. KA9Q

    KA9Q Ham Member QRZ Page

    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.
     
    Last edited: Oct 11, 2019
  4. N2DTS

    N2DTS Ham Member QRZ Page

    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.
     
  5. KA9Q

    KA9Q Ham Member QRZ Page

    Now you really have me wondering about this.
     
  6. KL7AJ

    KL7AJ Ham Member QRZ Page

    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.
     
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  7. KL7AJ

    KL7AJ Ham Member QRZ Page

    https://hackaday.com/tag/vectorscope/
     
  8. AC0OB

    AC0OB Subscriber QRZ Page

    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
     
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  9. KA9Q

    KA9Q Ham Member QRZ Page

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

    Seriously, can they work well away from 3.58 MHz?
     
  10. KA9Q

    KA9Q Ham Member QRZ Page

    Ah, that's useful!
     

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