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

    Amplifier designers do have the proper equipment as to get an FCC certification (required in the U.S. for HF amplifiers) they perform their own testing and provide detailed lab reports to the FCC.

    You would not normally use an "attenuator" for this, as that would be a really BIG attenuator. Usually you use a directional coupler with -20 dB (or more) decoupled port outputs and then use smaller attenuators after that to get down to the +10 dBm (10 mW) level or so for the spectrum analyzer. Way cheaper, and less hardware (and a lot less heat in the attenuator).

    I can make these measurements here on my bench and have all the hardware; but it's impossible to keep all that in the hamshack and active and also a bit fruitless. Takes two very low distortion signal sources, directional couplers, attenuators, very linear mixer, and a spectrum analyzer you can trust to be accurately calibrated and operating at low enough signal levels to not introduce its own distortion. Takes me a while to even set up to do this.

    In my experience, most SS amplifiers aren't great. GG power triode tube amps can be really good if properly loaded and operated below compression. My own 4-1000A grid-driven tetrode amp is "fair," but not as good as, say, an 8877 triode amp at the same power level. The "12Vdc" operated SS amps are about the worst, and unfortunately that's what's in most of our rigs!:p
     
    KB4QZH likes this.
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Hi Joe,
    The "lab" way to test 2 tone is to get 2 power oscillators and a mixer to drive the power amp into a dummy load and directional coupler to feed the spectrum analyzer. Any transceiver is going to put a bit of IMD into the mix and then you can't really tell what the amp is doing. This will give you a number that is repeatable and such, but still won't really tell you what your whole system as built is doing while on the air (Steve posted while I'm typing and he's much better at all of this than I will ever be!)

    The LP500 looks really cool and gets sterling reviews. There is quite a waiting list and it's a bit expensive (but cool! Bigger toys for bigger boys, etc.)

    I use a Wavenode WN-2 in my shack and paired with a cheapo in today's world Tek scope, the combo is really nice to tell me what's going out to the antenna. I built a sampler right into my TS-820 that feeds the vertical input of the scope and the Wavenode's sampler feeds the horizontal. I put a blanking signal from the TR sequencer to kill the scope trace when not transmitting to not burn a hole in the screen.

    This setup works "amateurishly" well for a few hundred bucks. I got the Wavenode because I flipped the topology of my amp (SB-200) around to really ground the grids (and a bunch of other mods, including a shunt reg in the TS-820 for screens and driver... the amp is fantastic, and the radio's IMD dropped several dB with the mods) and wanted to know that it wasn't a band polluter.

    I'll shoot a 1 minute video as soon as one of my ham buddies shows up of how it all works and I think you will like it. The cool thing about the WN-2 is that in about 20 minutes of playing with it, I recognized what the scope looks like when the distortion is low (below -35dB) and now can nail the tuning and such in about 5 seconds while putting out a call.
     
    Last edited: Jan 22, 2017
  3. NL7IB

    NL7IB Ham Member QRZ Page

    Thankfully!
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's great, but won't show IMD.
     
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    WB2WIK,

    If you have a moment could you comment on the test procedure given in this link, in the section labeled "Measurement of Intermodulation Distortion at Audio Frequencies" third paragraph in the section and the accompanying figure.

    http://users.tpg.com.au/users/ldbutler/Intermodulation.htm

    Not very quantifiable in the absolute, but useful?

     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That describes a nice solution but has nothing to do with modulated radio frequency amplifiers.:)

    The Transmitter Tests section does.
     
  7. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Oh, wow... duh. An example of wishful thinking/confirmation bias! Interesting procedure nonetheless.
     

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