class e rigs

Discussion in 'Amplitude Modulation' started by N2DTS, Oct 23, 2017.

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  1. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    If'n you don't have an oscilliscope but have a decent computer, this tool will show your envelope and Modulation indices:

    http://www.radioassociates.com/


    Phil
     
    Last edited: Nov 2, 2017
    K6CLS likes this.
  2. N2DTS

    N2DTS Ham Member QRZ Page

    You don't need a good computer, it will run on almost anything...


     
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    OK, I found the information that supports the above, see pdf file if interested.

    An analysis of the FFT equations yields the following information:

    A PDM wave includes:

    1) the expected DC component,
    2) the message or information itself,
    3) terms related to phase-modulated carrier waves at all harmonics of F0.
    4) the phase modulation is proportional to x(t),
    5) the phase deviation increases with harmonic order n while the carrier amplitude decreases,
    6) the sidebands progressively spread out in frequency and diminish in magnitude at the higher frequencies.


    Phil
     

    Attached Files:

  4. N2DTS

    N2DTS Ham Member QRZ Page

    What does that mean in English?
    The carrier needs to have a low phase noise?
    One person uses a flex 5000 which has a good amount of phase noise on TX.
    Most older solid state radios have a good amount of phase noise.

    Some of the newer stuff is getting quite clean...I wonder how a good vfo (coil, cap and tube) does...
    How do you measure phase noise of a carrier?




     
  5. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I uploaded a more explanatory pdf but here is the crux:

    Spectrum Analyzers sample the AM waveform envelope, which is a Voltage verses Time envelope, and convert it to an Amplitude verses Frequency display using mathematical techniques such as Fourier Transforms.

    Note: 70 kHz was chosen as the sampling frequency fS in the earlier PWM broadcast transmitters for a number of reasons: 1) the sampling frequency satisfied the “Nyquist criteria” where the sampling frequency has to be at least two (2) times the frequency of the modulating audio, 2) we were dealing with the problem of switching vacuum tube modulator grids and their respective switching characteristics, 3) in a hot and high level RF environment, PWM generation circuits at the time weren’t stable at higher sampling frequencies.

    An analysis of the FT Equation 2 yields the following information:

    A PDM wave includes:
    1) the expected DC component,
    2) the message or modulating information itself,
    3) terms related to phase-modulated carrier waves at all harmonics of
    fS.
    4) the resulting phase modulation is proportional to
    x(t),
    5) the phase deviation increases with harmonic order
    n while the carrier amplitude decreases,
    6) the sidebands progressively spread out in frequency and diminish in magnitude at the higher frequencies.

    Now all of this is for a system where the PWM
    directly modulates the Final.

    However, in modern PWM transmitters, the PWM signal is highly filtered and converted into an analog signal before being applied to the final stage, the same kind of signal as one would see at the “high” side of the modulation transformer of a plate modulated transmitter.

    In broadcast and in most amateur Class E rigs, the PWM filter has a sharp roll-off characteristic around 35kHz for a 70 kHz sampling (switching) frequency.

    If there is any phase information left after filtering, it should be down at least 24 dB from the carrier.

    In addition, the final stage’s tuned circuit would further reduce any harmonics generated in the process.



    Phil
     

    Attached Files:

  6. N2DTS

    N2DTS Ham Member QRZ Page

    So it should be really clean.
     
  7. WA3VJB

    WA3VJB Ham Member QRZ Page

    Well, well, well. Haven't we gone the long way around the bush?
    If there is anyone on here who is affiliated with the builders of these homebrew transmitters, let's recommend that they slip the word to selected operators to check their settings and refine any adjustments to stay friends with the neighbors. Not only is that expected as responsible operators, it is very important for the rest of us to maintain the impression of clean, responsible operating.
    Steve, WA1QIX is the only participant in this thread. I believe he may enjoy a certain position of influence. Steve, are you willing to step in, even if it means a confrontation to get across this important issue?
     
    N6YW likes this.
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

  9. N2DTS

    N2DTS Ham Member QRZ Page

    Steve's signal has been clean, so I don't know what the other guys are doing different, maybe pushing the audio into the limiter harder, or some other reason.
     
  10. K4KYV

    K4KYV Premium Subscriber QRZ Page

    Some of them, a certain station in particular, openly boasts of boosting his audio response up to 20 kHz, making the signal 40 kHz wide even without distortion.
     
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