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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. WG7X

    WG7X Premium Subscriber QRZ Page

    Yes, but this is normal for the QRZ crowd. They will discuss and argue about anything you care to bring up.

    As you no doubt knew before you started this.

    Cee Ya!

    Gary
     
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    I disagree. :p ;)
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I disagree with your disagreement.:p
     
  4. G0HZU

    G0HZU QRZ Member

    It's a very simple explanation :)

    Let's look at the -20dBc case first as it causes a huge fat error window on a Vpeak meter (approx +/-20%) even though the harmonic is only 1/100th of the power.

    Let's also look at 'just' a cw signal to keep it simple.

    If you have a fundamental of 100Vpk and a -20dBc 2nd harmonic so the harmonic voltage is only down by a factor of 10 in voltage (=10Vpk) then you could adjust the phase of the harmonic such that in enhances the peak voltage of the combined waveform to about 110Vpk. Remember, this type of meter measure PEAK VOLTAGE. It isn't a proper power meter. That's the problem.

    You could also adjust the phase such that the harmonic reduces the peak voltage by about 10V so you get a combined waveform of about 90Vpk.
    If you compute the power (in a 50R system) for each case you get

    (90^2)/100 = 81W
    (100^2)/100 = 100W
    or
    (110^2)/100 = 121W

    So with a -20dBc harmonic you get an error window of about +/- 20% with a Vpk reading based power meter. The correct answer if measured on a proper power meter is about 101W as you have 100W plus 1W. The harmonic causes only a tiny increase in the reading on a proper power meter.

    If you repeat the sums for -30dB harmonics with a Vpk based meter you get an error window of about +/- 6% in power estimation as the phase is
    changed.

    For -40dBc you get about +/-2% on a Vpk type power meter.

    It's easy to demonstrate the above if you get a cheap and basic 'power' meter like a Bird 43. Give it a signal with a -20dBc harmonic and adjust the phase of the harmonic and the needle on the Bird will fluctuate 'about' +/-20% in power indication. This is because it uses a Vpk detector in the linear region on the upper half of its scale. it isn't a proper power meter. It just measures peak voltage and fudges the scaling on the meter to read across to 'power'

    i.e. the Bird 43 meter (and a typical ham power meter) is reduced to junk if there are high harmonics and you demand high measurement certainty.

    You have to use a different meter in such a case!

    A regular lab grade power meter using a thermistor or thermocouple head would hardly blink as the phase was slowly altered. In theory it should
    be unaffected.
     
  5. G0HZU

    G0HZU QRZ Member

    If it helps I can demo all the above with practical tests on a cheapo Bird 43 or Welz SP220 meter to show how much the measurement uncertainty creeps up if there are significant harmonics present on these Vpk reading meters.

    It's really easy to make the needle swing approx +/-20% in power indication if a -20dBc harmonic has its phase rotated. Also, +/-6% can be demoed for a -30dBc harmonic.

    Now both meters use a directional coupler for sampling some of the signal and that isn't the bit that is the issue. It's the fact they put a very crude Vpk detector on the end of the coupled port. in the case of the Bird the detector operates in the linear region over a fair bit of the dial. So this makes it very prone to large errors caused by harmonics.

    Probably the best demo is to simply look at a scope with a fundamental and a -20dBc third harmonic with the phase slowly rotating. Even the human eye can spot the significant change in Vpk that the harmonic causes.

    So it's no surprise the Vpk meter can tell lies if it gets a signal with harmonics present as the relative phase of the harmonic could be at any angle. It could make Vpk higher, lower or the same.

    Finally, if you still don't believe me then ask Bird to explain it to you :) They stipulate -50dBc harmonics are required on their meters that use Vpk detectors. I'm just trying to explain to you why this is the case :)

    I have a Bird 43 here at the minute that I have borrowed and I've tested it this evening and the meter moves about +/- 19% for a -20dBc harmonic if the phase is rotated. But then I have done this test on a Bird before... and the Welz meter :)

    I also repeated the test on an Anritsu lab grade power meter (thermocouple) and also an HP thermistor type meter and these reported the power correctly with no problem caused by the angle of the harmonic.
     
  6. K4FMX

    K4FMX Ham Member QRZ Page

    Yes for measuring steady power but you can not measure PEP with a thermistor or thermocouple type power meter.

    73
    Gary K4FMX

     
  7. G0HZU

    G0HZU QRZ Member

    I understand your argument but the strict definition of PEP doesn't care about contribution from IMD terms. It just cherry picks the largest single
    RF cycle at the modulation crest (whether there is IMD affecting the crest or not) and PEP is the average power in the timeslot of that one 'biggest' RF
    cycle. It doesn't matter if the modulation is dirty or clean. PEP is PEP.

    When the timeslot is that short with respect to the period of the modulation then the modulation or associated IMD can't contribute distortion in
    that single (biggest) RF cycle. So you can use a Vpeak reading meter and do a conversion of Vpeak to PEP in the meter.

    What can contribute to the downfall of the accuracy of the conversion are harmonics and other high frequency spurious terms (eg spurious from
    the synthesiser) that could also be transmitted at -30dBc on a dirty radio. These could affect the accuracy of the conversion from Vpeak to PEP in
    the meter because the waveform inside the period of the single RF cycle is no longer a sinewave and you can't read across from Vpk to Vrms with
    a simple equation any more.


    For typical ham SSB transmissions I agree :)

    I guess that's what makes this subject interesting because there is no perfect power meter that can cope with all types of waveform. But it is definitely worth investigating the limitations of each type in order to assess the measurement uncertainty when applied to a specific task. My background is RF engineering and I have to always ask myself "am I using the right tool for the job?" "am I making a valid measurement?" "what are the main contributors to measurement uncertainty here?"

    To most hams it isn't really that important to explore those questions too deeply as accurate power measurement is rarely required. However it obviously can be important to measure power accurately if an amplifer is being tested for efficiency because even small amounts of measurement uncertainty can be significant and can cause confusing results if the amp is being developed for good efficiency.
     
  8. K4FMX

    K4FMX Ham Member QRZ Page

    According to the PEP definition it is the average power contained in a single RF cycle at the crest (peak) of the modulation envelope.

    Since any other signals that are present (side bands, harmonics, IM
    products) will create an envelope, at some point in time all those signals will fall in phase (because there is constant rotation as they are all on different frequencies) and cause a maximum peak in the envelope which is where PEP is to be measured.

    So our measurement has to be at the peak of the envelope time. This will encompass all signals present. As a practical matter when making measurements via the peak voltage method our meter just finds the largest peak over a rather long period of time and that gets us the envelope peak.
    So I don't see how IM products are not included in PEP measurements.

    It takes two or more RF signals to create an envelope. This can be the result of two single tones or multiple tones or voice to generate them from the audio level in the transmitter.
    It all gets converted to RF and when we look at those signals at RF we see an envelope that looks like an audio signal.

    We can see the same looking envelope if we look at the carriers of two separate CW transmitters separated by say 1kHZ. The envelope that we would see would be exactly the same as if we had a single SSB transmitter that was modulated with a two tone signal.
    In both cases the peak envelope power of the composite signals would be read at the peak of those envelopes. Because the two signals are on different frequencies they are in phase at one point in time (peak) and out of phase some time latter (valley or null). This is how the envelope of a modulated transmitter is seen.

    You can look at it as if one of those signals is the carrier and the other is the modulating signal. The difference in frequency is an audio signal that produces the envelope that we see.
    This is exactly what an AM transmitted signal looks like except there will be two side bands rather than one. There will be 3 separate signals present.
    The amplitude of both side bands add together and add to the carrier amplitude to give us as a peak in the RF voltage as we see it on a scope when they are all in phase. This is the crest of the modulation envelope.
    Later in time, depending on modulation frequency, we see a dip or null in the modulation envelope. This is where the independent signals are out of phase.

    As we look at these three separate signals on a scope or a broad band meter it looks as though there is one signal with an envelope that varies as an audio frequency. This is the composite signal.
    These signals do not have to be close together in frequency. As long as your meter or scope can see both signals it will look as though one is modulating the other and there will be an envelope with a frequency of the envelope equal to the spacing of the two RF signals.
     
    Distortion of an RF cycle appears outside of the amplifier as we view the signal. As signals are amplified they are very distorted within the amplifier. (class B amplifier only conducts on 1/2 of each cycle) Harmonics and IM signals are generated within the amplifier but exit the amplifier as individual sin waves on their respective frequencies.
    Normally the tank circuit cleans up any distortion that happens within the amplifier. That is how we get a sin wave out of a class B or class C amplifier.
    If you were to look at the main signal with a band pass filter where you do not see any harmonics you will see a pure sin wave. But look at it and the harmonics (broad band) and it looks distorted. Same with IM products present.

    What I am saying is that when other signals are present and using broadband peak reading meter, you are always going to see the sum of all signals at some point in time. And the highest peak (sum of all signals) will be what we read.

    73
    Gary K4FMX





     
    Last edited: Jan 12, 2013
  9. G0HZU

    G0HZU QRZ Member

    I think we are going in circles here. Sure the IMD is unwanted and it will affect the amplitude of the signal but the PEP measurement is only concerned with the largest single RF cycle in the time domain. How it got to be the biggest RF cycle doesn't matter. It doesn't matter if it was contributed to by audio from a squeaky microphone PTT switch or a dog barking in the background or by unwanted IMD terms.


    It doesn't matter if IMD has influenced the amplitude of this cycle. It is the daddy RF cycle and it is used to define PEP.
    It really is that simple. You can argue the case that IMD are causing errors because IMD is unwanted power but that isn't relevant to the classic PEP definition.

    Harmonics are a problem if you try and use a Vpk meter to read across to PEP. You seem to be arguing that IMD terms will give similar errors and I disagree because PEP doesn't care about what makes up the biggest RF cycle (See above).

    It DOES care if you can measure the average power in the timeslot of that cycle correctly and harmonics will mess up the conversion from Vpeak to Vrms if you use a Vpk reading meter. This is because you can't convert from Vpk to Vrms with a simple equation anymore.
     
  10. K4FMX

    K4FMX Ham Member QRZ Page

    Maybe I am missing something here? How would harmonics distorting the waveform be any different than IM products distorting the waveform? They all add the same way.

    73
    Gary K4FMX


     
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