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class e rigs

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

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I am not trying to start trouble, but I have been talking to Steve (WA1QIX) and others about their class E rigs and how the negative limiting works.
    From what Steve tells me, he just hard limits the audio then filters it before it goes into the modulator?
    You have to remove (filter) the switching stuff from the pulse width modulator.
    But EVERY class E rig looks like this:

    [​IMG]

    The signal on the top is a normal AM signal from a plate modulated rig, the signal at the bottom is the class E rig.
    They ALL look 20 KHz wide like above, even when the high frequency is limited going in.
    Are other people seeing this?
    I see/hear it on every receiver/sdr.

    I think there is something in the design that causes the width...
     
  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page



    1. Waterfalls aren't spectrum analyzers.
    2. A properly designed PWM modulator does not create spurs.
    3. PWM modulated Class E rigs have excellent frequency response.
    4. If wide spectrum audio goes in, a wide RF bandwidth results.


    Phil
     
  3. W2VW

    W2VW Ham Member QRZ Page

    QIX' signal always occupies what seems like the right amount of bandwidth for the quality delivered which is excellent.

    There are a number of other class e users who have similar footprints but I don't hear much of them these days.

    A class e rig has at least one thing in common with other methods of AM. It can be a problem on the bands when not used properly.
     
  4. WA3VJB

    WA3VJB Ham Member QRZ Page

    The MOSFET basis for the "Class E" system dates back more than 40 years, and may have peaked in homebrew acceptance as people look toward other modern ways of getting on AM, such as store-bought Class D and SDR successors. Part of the reason is the lack of a production template that could help people who wish to build, which creates a risk that Phil alluded to of not being able to "properly design" a pulse-width modulator that does not create spurs.

    Another issue is that these older "Class E" transmitters seem to be more rigid, as Brett has noticed, with little change in passband among times these stations are on the air under varying conditions.

    The sharply defined passband of a modern SDR rig can be easily adjusted to suit conditions, and the result is quickly seen on a simple waterfall display. I have seen how Chuck, K1KW has demonstrated not only that flexibility, but also the cleanliness of signal transmitted by his "Anan" brand of radio. The rival "Flex" brand of SDR also has a distinctive look to its waveform monitored on even the most unsophisticated waterfall display, with sharp passband cutoffs that can be tailored by the operator to suit conditions and intent. Recently I saw how the "Super Senior" Class D AM transmitter could be adjusted toward better audio, going from a pinched passband to one that was more normal. The demonstration was part of a review for QST magazine, and the change was implemented by adjusting an external audio equalizer.

    Perhaps a representative "Class E" MOSFET transmitter could be explored by the ARRL Lab to shed some light on Brett's point.
     
  5. N2DTS

    N2DTS Ham Member QRZ Page

    Well, I don't know why a good sdr is not a spectrum analyzer.
    They are calibrated and some programs like spectraview have markers you can place like a spectrum analyzer.

    But anyway, Steve (and others) say they limit the audio to 7 KHz going in but always have a 20 KHz wide signal.
    A lot of the class E guys are very strong at my qth, Bob and Steve are always very strong, and the 20 KHz wide signals take everything out within that bandwidth and more, on any receiver.
    In the picture above (not Steve or Bob) you can see where the audio is, and the artifacts out beyond that.
    Another odd thing is the bandwidth is always very close to 20 KHz, not wider.
    When a single steady tone is made, there are no artifacts.
     
  6. K5UJ

    K5UJ Ham Member QRZ Page

    A ham not too far from me, Doug N9SHU has been homebrewing s.s. rigs lately and he seems to be a quick learner. While I have zero interest in that stuff, I applaud and admire his accomplishments. At first he was splattering all over the place but he kept working on it and got it dialed in to where I don't hear any grunge up or down the band when he transmits. His rigs sound clean. I am only 10, maybe 15 miles from him and his rigs are I think pretty high power. Class E and D seem to support heavy audio processing, the kind of compression density and negative limiting that would blow a vintage ham rig, and maybe even put some of the old broadcast rigs under some strain. The combination of power and audio and proximity give my old envelope detector receiver some challenges but he seems to have nothing over 5 kc up and down from zero beat. I can see how some hams running old receivers might think guys like Doug are wide, because with all that loudness they seem wide. But with a SDR and spectrum display, I don't know. I don't have one here but I guess a SDR can be overloaded like any other set?
     
  7. KA4KOE

    KA4KOE Ham Member QRZ Page

    20 KHz wide? Now what does that mean?? If the outliers are really down, then this statement doesn't mean too much. Just because you can see it doesn't necessarily mean you can hear it. Psychosomatic QRM perhaps??
     
  8. WA3VJB

    WA3VJB Ham Member QRZ Page

    They sure can, and they sound awful when it happens, mostly imaging, but also some really gnarly digitized audio that the D/A converter faithfully passes along.

    Overload can also happen with conventional, but modern transceivers too, especially if AGC settings are overwhelmed by a strong, well-modulated, and otherwise clean signal.

    But I have seen what Brett has highlighted on a variety of receiver nodes in various locales. In areas where signals are average, say peaking between S9 and 10+, the waterfall shows the same story.
     
  9. N2DTS

    N2DTS Ham Member QRZ Page

    Yes, the picture above was on a weaker signal. And as I said, those artifacts take out normal signals, its not just visual.
    The stronger the signal the worse it is of course.
    There are guys running sdr radios (Flex, Anan) into big amps and they always have a sharp cutoff at what they set the bandwidth at.
    Normal plate modulated rigs will have the high end taper off and may splatter a bit if over modulating, but I have never heard or seen anything as dense as the class E rigs.
     
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Case 1: There was some guy from the NE running a class E rig on 3.873 one night and we had a qso going on at 3.880 and we could hear his sideband platter, but he said he was running his audio up to 6.5kHz with a brickwall filter. My SA showed a 15kHz bandwidth so I sent him a friendly note with pics and I have not heard his sideband artifacts since.

    Case 2: A local ham and I made an AM contact one night and he said his waterfall was showing a 60 khz bandwidth from my Knight T-150 SGM (running 25 Watts). Now I had just upgraded and modded my T-150 so I was curious. After the qso I put my SA on the signal into both the antenna and a dummy load and it showed a 11.5kHz bandwidth with no spurs, about the bandwidth where I had designed it.

    I don't know what is is some receivers and their waterfalls but they don't always agree with a good Spectrum Analyzer.


    Phil
     
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