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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

    OK I'll try and clarify my view :)

    By the way I 'think' where we divert on this issue is that you interpret the IMD contribution as an unwanted thing and are seeking to quantify its contribution to PEP as an error. I'm just interested in accurately measuring PEP according to the classic definition and IMD contributions are valid even though they are unwanted.

    Is that what we disagree on?

    Anyway, here's the way I see things:
    First of all I'm assuming there is typically a big difference in terms of the RF frequency vs the highest modulation frequency.
    eg many MHz vs just a few kHz.

    Let's first assume the transmitter has a really good LPF and the harmonic terms have been removed to -60dBc or better.
    We know the period of the modulation waveform is going to be typically hundreds of microseconds or more and the period of the RF is going to be hundreds of nanoseconds or less. Now we can both agree that any unwanted IMD terms can cause summing and cancelling and so can the individual tones in our speech.

    We can also agree at some point they can all phase together and we get a crest in 'modulation' (and sometimes we get a null)
    Now this next bit is where I think we have a different view. Both are technically valid views but I am being a bit of a pedant and sticking to the classic definition of PEP.

    For the determination of PEP I think this RF cycle at the modulation crest is perfectly valid despite it having unwanted IMD contribution. This is because PEP is only interested in what the average power is at that top RF cycle (not how clean the modulation is)

    Deep inside that crest is the single RF cycle that is subtly bigger than it neighbours. It is the daddy RF cycle where PEP is ideally measured.Because the period of any IMD term and the wanted modulation is so large (thousands of times bigger?) it can't realistically distort this RF cycle away from being a sine wave. The IMD will have had a part to play in its amplitude but it won't realistically affect the purity of the sinewave.

    So if it were possible to extract this single sinewave and measure its Vpeak you could still calculate its Vrms with good accuracy because it is still a sine wave. So the readacross from Vpk to PEP in the meter would give a good result.


    If we now look at the case where the harmonics aren't as well filtered then there won't be 'just' a pure fundamental sine wave in that little timeslot of a few hundred nanoseconds. There could be two (or more) sine waves squeezed in there and they might not be fitting into the slot with the same starting phase as the fundamental sinewave.

    So you get a slight summing and cancelling effect depending on the phases of the fundamental and the harmonic. So the Vpk of the composite waveform in that little slot becomes uncertain wrt the true Vrms of the composite waveform. This is because we don't know the relative phase of these two sinewaves.

    When this happens then you can't accurately convert from Vpk to Vrms anymore.
    if you explore all possibilities for the phase angle then you get a surprisingly large error in the simple Vpk to Vrms conversion and this makes the meter suffer very large measurement uncertainty due to the (uncertain) phase angle of the harmonic(s).
    In reality the angle will be set 'somewhere' and should stay like that but there is no knowing if it is at an angle that makes the meter read slightly low or makes it optimistic or anything inbetween.

    If you arrange a test setup that lets you rotate the phase of the harmonic you can explore the size of the uncertainty window. You can do it all with a SPICE simulator as well. You can also predict it using some simple sums. All three should agree pretty closely.
     
  2. G0HZU

    G0HZU QRZ Member

    I can't edit my posts so I'd like to clarify that when you change the phase of the harmonic the shape of the composite waveform of F + 3F (where 3F = third harmonic) you will see the composite waveform change from a squashed sinewave (where the meter will ready artificially low) across to a spiky peaked waveform where the meter will read artificially high .

    The true rms of the composite waveform hasn't changed but the Vpk will be markedly different and this is what causes the measurement uncertainty with phase angle.
     
  3. G0HZU

    G0HZU QRZ Member

    I hope nobody minds me trying to inform about this source of error with a Vpk based meter. I'm surprised hat this source of error isn't better known by hams so forgive me for being so persistent :)

    Here's the basic maths to predict the impact of a -30dBc harmonic on the measurement uncertainty of a Vpk detector used in a cheap power meter.

    If you have a 100W transmitter and a 50R system then the Vrms is 70.71V and if the waveform is a pure sinewave then the Vpk will be 100V.

    Because it is a pure sinewave the power can be calculated from Vpk as (100Vpk*100Vpk)/100 = 100W

    If you introduce a third harmonic at -30dBc the Vp of the harmonic will be 100/31.6 = 3.16Vpk
    So depending on the phase of the harmonic the Vpk of the composite waveform could be 100+3.16V or 100-3.16V or anything in between.

    If it is 103.16V then the Vpk based meter will report (103.16*103.16)/100 =106.4W ( = +0.27dB)
    If it is 96.84V then the Vpk based meter will report (96.84*96.84)/100 = 93.8W ( = -0.28dB)

    You can see that this is an uncertainty window of about +/-6% for a -30dBc harmonic.

    If you go down to a -40dBc harmonic the uncertainty is +/- 2%
    If you go down to a -50dBc harmonic the uncertainty is +/- 0.6% and this is where Bird recommend you keep the harmonic level below if you are using a Vpk detector based power meter that is operating in the high level (linear) region.

    Note that the above is well known in the RF engineering world and isn't something I've simply made up myself.
     
  4. W1BR

    W1BR Ham Member QRZ Page

    I got my butt handed to me for daring to use average and PEP in the same sentence, but I agree.

    Pete
     
  5. NM8D

    NM8D Ham Member QRZ Page

    We're going in circles and im getting Dizzy!!!:p

    Michael
    nm8d
     
  6. K4FMX

    K4FMX Ham Member QRZ Page

    Yes I am considering IMD as part of the error. I understand what you are saying as to the difference with harmonics distorting the measured peak cycle. But with the common peak detecting wattmeter we don’t have much choice as to what time frame it actually looks at. The envelope/RF cycle gets distorted by all signals present. The harmonics and IM products are not readily apparent when viewing the envelope as they are very small compared to the main modulation envelope. We no longer have only that nice long low frequency envelope.

    But note that when we have more than just a two tone signal involved we have many other envelopes with the main envelope. So the more signals involved the shorter the time there is for all peaks to sum at the same time.

    It is not readily apparent that IM products have any influence on PEP as we are usually operating / tuning the amplifier for maximum output and all harmonics, IM products etc. all add together to show a particular PEP as read on the common peak detecting meter.
    But if we want to know the true PEP of the wanted products we need to be aware that all signals that approach the wattmeter will influence the reading.

    It could very well be that one daddy RF cycle that you are wanting to measure is the one that is influenced by the sum of the root cycle, its harmonics and the IM products. After all they will all be in phase at some point in time and our wattmeter has no way to discern when that time frame is. All it knows is that is the highest peak of the signal.

    I concur with what you are saying as to seeing the difference in the wattmeter when the phase or amplitude of the harmonic is changed. But I also will assert that you will see a similar change in the wattmeter when IMD products are changed.

    Here is a link to a pretty good paper about two tone testing and the caveats. It also gives insight to harmonics and IM products causing power reading errors like we have been discussing.

    http://www.macomtech.com/static/PDFs/TechnicaArticles/RFcharacterization.pdf

    73
    Gary K4FMX


     
  7. G0HZU

    G0HZU QRZ Member

    I've only skimmed through the pdf but I don't think the error analysis in that article applies here. They are using a different measurement method.
    They are not trying to measure Vpk with a Vpk detector in the way a typical ham power meter operates. They are doing two tone testing and predicting PEP based on measuring average power of two tones over a very long time frame. The idea is that PEP is approx = average power x 2.

    For sure IMD and other spurious will mess up the accuracy of that approach but of course, they are not trying to measure PEP at a modulation crest in the same way a Vpk based ham meter operates. They are using a different approach which will have a different set of error/uncertainty sources. Basically you can't read across the pitfalls of that 'average power of two tones' approach and then apply them to a Vpk based system.

    They are trying to go from average power (eg seen on a thermocouple type meter) to PEP. But you cannot predict PEP accurately by simply doubling the average power seen on the meter. (because there aren't always just two tones present)

    However, if you switch across to a Vpk based meter then as long as the harmonics are well suppressed then you can measure PEP quite accurately by detecting Vpk even if there are (or are not) any IMD terms. You just have to be able to detect and hold the Vpk faithfully.

    The measurement uncertainty analysis for their approach is completely different compared to a Vpk based meter. Basically, their method DOES suffer errors because of both IMD and harmonics. The Vpk system is affected by uncertainty caused by harmonics (especially by the phase angle of the harmonic) and not by IMD.

    I hope you don't think I'm being deliberately awkward here. I'm trying to be technically correct :)
     
  8. K4FMX

    K4FMX Ham Member QRZ Page

    If you read on down the page, I think on page 3 or so they discuss a little and compare errors using peak reading meters compared to average reading type power meters and mention errors caused by harmonics and IMD products.

    73
    Gary K4FMX

     
  9. G0HZU

    G0HZU QRZ Member

    I've read it again and there's still nothing that states that a Vpk reading meter will suffer errors in measuring PEP due to IMD. IMD contribution is a valid component of PEP so you should not see its contribution to the amplitude of the peak RF cycle as an error. However, harmonics of the RF cycle definitely WILL upset the conversion from Vpk to Vrms process as per my earlier analysis because the peak RF cycle will no longer be a pure sinewave.

    If you can find a paragraph in the pdf article that also states that IMD terms contribute to PEP measuring uncertainty for a Vpk based meter then you win a prize. I don't think the paragraph exists but please can you cut and paste from the article to make it easier for me :)
     
  10. W7MMQ

    W7MMQ Ham Member QRZ Page

    jeez, 120 pages on PEP measurement and the FCC said it would simplify things lol. Just go back to DC Input, How hard is looking at your HV meter and IP meter. How much simpler could it get. 800ma x 4400 volts is not rocket science.
     
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