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Acom or Ameritron...??

Discussion in 'Amateur Radio Amplifiers' started by W3AMM, Jan 3, 2017.

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Some strange ideas there.:)

    I won't go into all of it, but monitoring your output waveform, whether a sinusoidal observation or a trapezoid, doesn't tell you much about your signal unless you're seriously non-linear...enough to observe on a 4-5" scope screen (it has to be pretty bad to be observed).

    Much, if not most, of the signals on the bands occupying more than necessary bandwidth on modulated modes suffer excessive intermodulation distortion, which cannot be observed by an analog detector and oscilloscope. You can look "perfect" on a scope (nice, squared up trapezoid pattern with sides so straight they'd make a ruler blush) and still have a horrible signal. IMD cannot be observed this way.
     
  2. K0OKS

    K0OKS Premium Subscriber QRZ Page

    So it sounds like an accurate power meter would tell me a fair amount. Eventually I would like an LP-100A or their fancier models, but I think that may be overkill at this point.

    I have numerous scopes and am building an RF sampler box (two actually). I know I could just wrap a wire around the coax, but I want something consistent.

    IMD should be able to be measured well enough with a two-tone test once I get the scope hooked up.

    My main question was, in order to preserve the 3-500ZG tube life, but still actually get amplification, where would a sweet spot be to use as an everyday ragchew SSB maximum power setting for output from the 7300? I have been using 60% except when the band is terrible in which case I go to 70%.

    At least one meter seems to indicate the 7300 actually puts out more like 113W at 100% with a CW carrier.

    Thanks. This is all interesting info.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    IMD cannot be measured with an oscilloscope.

    It's a frequency-domain measurement, not a time-domain one. You need a spectrum analyzer, and a very good one, as well as very pure (very low distortion) tone sources.

    Keeping an amplifier "clean" isn't so much a matter of how much drive is applied as a matter of proper loading. Underloading (which often happens if you just tune for maximum output power, or a plate current dip) is likely the leading cause of distortion (IMD).
     
  4. K0OKS

    K0OKS Premium Subscriber QRZ Page

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Nope.

    He was attempting to correlate a time domain scope display to what might happen in frequency domain, but that's a wild guess. Roger made an interesting attempt at correlation, but I find more examples where it doesn't work at all.

    He's an ex-Tek guy, but I'm an ex-HP guy and we never agreed on anything.:p
     
    KA0HCP likes this.
  6. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I see. Well some day I will get a nice spectrum analyzer, most likely an old HP... but I have a bunch of old Tek scopes!

    How well can you judge IMD from looking at a waterfall? (Either on another receiver, or a Web SDR receiver) I often see faint lines extending past the ends of the main signal bandwidth on the waterfall (for their people's signals that sound like something is clipping). Is that flat topping/clipping, or IMD? Given how easy it is to see your own over the air signal on a Web SDR receiver I wonder how useful that is for troubleshoot/tuning? Thoughts?

    Thanks for all the info. Just st trying to learn all I can.
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think it might be somewhat useful but using voice or tone modulation that's constantly changing makes it impossible to get any kind of accurate reading. Should be useful in comparing one signal to another, though.

    Another issue is when using a receiver connected to an antenna with a spectrum full of signals, your receiver itself will contribute IMD so what you see is a combination of the other guy's signal plus your receiver; as such, I'd never blame anyone for anything due to my receiver indications. A good lab-grade spectrum analyzer in a closed system with controlled signal levels to assure maximum purity of the apparatus is really the only way to go. I think 99%+ of all hams are not equipped to measure TX IMD, so it's not a big deal. There also isn't any specific regulation in Part 97 regarding IMD, so even if it's terrible it would likely be legal unless you're using it to intentionally interfere with somebody.
     
  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Relative comparisons should definitely be good.

    What is your opinion of the LP-500/700 for this purpose? I certainly don't need such detailed info at this point, but perhaps in the future.
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Those aren't spectrum analyzers, they add a spectrum display to a receiver; you're still totally at the mercy of the receiver itself and whatever characteristics it has.
     
  10. K0OKS

    K0OKS Premium Subscriber QRZ Page

    So, just out of curiosity... the *proper* lab-grade method would be to attach the output of the amplifier to an attenuator and then directly to the spectrum analyzer, yes?

    This is likely way beyond the realm of Amateur Radio, and more into the professional realm, but I would think an amplifier designer would want this type of setup for sure.

    Again, thanks for all the info.
     

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