PEP? Are Those Power Meters Showing Us PEP Peak Envelope Power, or just Peak Power?

Discussion in 'Amateur Radio Amplifiers' started by KQ4X, Dec 31, 2012.

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  1. G0HZU

    G0HZU QRZ Member

    Sorry to be a bit evasive on this because you are again asking for a simple answer to something that can be quite complex.
    I would expect IMD terms to consist of a mix of AM-AM IMD and also AM-PM IMD so it all gets a bit muddy when trying to define the behaviour of IMD terms at -30dBc. For example AM-PM IMD terms tend to be in phase quadrature with the main RF cycle so they could be at -30dBc and in this case these IMD terms would not add or detract from PEP.

    But I certainly agree that with a simple model of an amplifier it is possible for IMD to add to PEP if the phase is right. In this case I guess you can simply add the voltage as per your sums but only if the amplifier's non linear characteristic places the IMD term at the right phase. I'd usually expect the IMD to be caused by some compression of the peak part of the waveform as the large signal gain of the amplifier is usually 'less' at the peak voltage point of the time dependent waveform compared to parts of the waveform that are at lower voltages. In this case the IMD terms won't be at the right phase to add to PEP. They would reduce it.

    However, I guess some amplifiers can achieve the phase that adds the IMD to PEP at certain drive levels. I think I could easily design an amplifier that would break the PEP rule in that pdf article.

    Note that you don't need much in the way of AM-PM to get -30dBc IMD and the amount of AM-AM and AM-PM effect varies according to what part of the RF cycle the amplifier is at. The AM-PM IMD tend to be in quadrature and the AM-AM IMD can be at a wide range of angles I guess and there are also issues with memory effect when trying to characterise IMD behaviour. so it all gets far too complicated for me to want to analyse fully...

    If you want a clearer and better answer than I can give then you could try emailing an RF amplifier design house? Where I work we design RF power amplifiers (as modules within our product line) but we aren't really that concerned with detailed IMD analysis so I only know the basics.

    Yes, it's not the correct term, that's why we ended up arguing for the last few pages :)
    However none of this stuff about IMD is relevant to this thread so it's probably time to stop talking about IMD...

    Yes, I appreciate that you knew this and that's the bit that is really relevant to ham power meters. I did look to see what meters are available these days and the nicest one appears to be the LP100A. This looks to be very well engineeered and is very versatile. But it does look to use a Vpeak detector so I would expect the measurement uncertainty to degrade the accuracy spec if there are harmonics appearing at -40dBc.
     
  2. K4FMX

    K4FMX Ham Member QRZ Page

    I scanned thru a few papers on AM-AM, AM-PM and from what I understand the 3rd and 5th IM products are out of phase by 35-40 degrees until you start getting into compression and then there seems to be a rather sharp curve going up to 180 degrees difference.

    It seems as though the problem may be in the GHz range and maybe not so much at HF. There is mention of a TWT tube causing delay problems where this shows up. They also mention that it is mostly caused by capacitance change in the amplifier transistors as the current varies.
    I agree that analyses can get muddy rather quickly.
    But I am sure that you know more about it than I do.

    You seem to agree that if the phase is right the IM terms can add to the PEP just as harmonics can add to the PEP reading if the phase is right.
    Although the harmonics do their work in a little different manor.

    I do agree that the LP100A (by N8LP) looks like a very nice meter. One of these days I may buy one. He seems to be doing all the right things with it. I recently read on his web site that he had looked at the direct RMS reading chips but that they were not as accurate as the one that he uses. But yes it is a Vpeak reading instrument as far as I can tell and he does some very fast sampling in the circuit.
    RF power measurement accuracy is difficult to achieve with any instrument.

    Nice discussion!

    73
    Gary K4FMX



     
  3. G0HZU

    G0HZU QRZ Member

    Yes it is always interesting to explore and understand the limits of test gear.

    One thing I've manged to do is borrow a Bird 43 wattmeter again.The chap who owns it has two of them and I have borrowed them both before for doing a few tests on. This one has a 400-1000MHz 10W slug in it and I already know the accuracy is poor.

    When I had both of the meters here I tried swapping the slugs (both were 400-1000MHz 10W ) because one of the meters wouldn't display any power. Once I swapped the slugs a few times both meters started working but they were indicating quite different power levels for the same pure 432MHz signal.

    I'm hoping he will be able to give me a few more slugs next week to check out on other bands as well. I already know that this slug displays the predicted +/- 6% uncertainty with a -30dBc harmonic as I've tested it before and watched the needle on the Bird 43 dial move +/- 6% in sympathy with the phase of the harmonic.
     
  4. K4FMX

    K4FMX Ham Member QRZ Page

    The Bird meters will read slightly different as they get a little wear on them. As you have seen the contact sometimes gets dirty and does not make good connection to the slug terminal. Also the ground is sometimes poor around the whole slug.
    As they are used a lot the slug gets a little sloppy in its fit and also the depth can change slightly from wear which moves the slug a little closer to the line section. Fooling with the slug to change slightly how it seats will change readings.

    Dropping a slug can change calibration too.
    I have several Bird 43 meters here and they all read slightly different even with the same slug. The meter movements get tired and don't settle in the same spot always either.

    I also have several HP and Boonton thermal type power meters and an old HP selective level meter that is pretty good at reading power levels but it is quite a trick to get everything to be in the same ball park for readings.
    Some day I am going to get the old HP calorimeter working and see how that does.

    73
    Gary K4FMX


     
  5. G0HZU

    G0HZU QRZ Member

    Well one thing is for sure these Bird 43 meters are massively overrated by a lot of hams.

    I guess they are quite versatile in that they can cover wide frequency ranges and power levels if someone buys lots of slugs. However, having played with two of them and seen how unreliable and inconsistent they are I wouldn't give these dinosaurs the bench space.

    What kills the Bird 43 as a serious meter is:
    It can be intermittent depending on the slug connection. (this alone is enough for me to dismiss it)
    The detector diode operates below the linear region for a fair bit of the dial and has no temperature compensation.
    The slug sharing system relies on each meter having an identical meter movement.
    The slugs can easily be dropped or abused.
    It uses the same passive (Vpk) detector diode system as a CB power meter so can't read true Vrms across most of the dial range so the accuracy falls away badly when harmonics are present.


    I tested this meter over temperature a while back and taking it from a warm room to below 0 degC made it change the reading by 20% for low to medium dial readings where the detector diode is operating below the linear region. I did the same test on a cheapo Welz ham meter and it fared much better over temperature as it keeps the detector diode close to or well inside the linear region.

    Both the Bird and the Welz meters suffered poor measurement uncertainty in the presence of harmonics.

    However, I'm sure that if someone calibrated their Bird 43 meter with their favourite slug and always used it within a restricted temperature range and they always used it on radios with low harmonics and they kept the slug contacts clean and solid then they could get quite consistent results :)
     
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