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. K4FMX

    K4FMX Ham Member QRZ Page

    Let's try this and see if it posts ok. This is from the top of the 3rd collum on the 4th page.

    industry practiceuses an averagereading powermeter and calculatesthe PEP astwice the average power reading. This isnot technically correct because the PEP isalways less than twice the average powerin the presence of harmonic and intermodulationdistortion. However, at thetypical IMD specification levels, around–30 dBc, the error is negligible. If a peakreading power meter is used to measurethe PEP directly, that fact should benoted with the results so that measurementscan be compared with another system​
    that uses average reading meters.

    They are telling you that harmonic and IMD both will skew the PEP readings.

    73
    Gary K4FMX

     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    However, it states that if harmonics and IMD are at normal levels the error is negligible. So in a pure sense, yes, pep readings are skewed by them but as hams running amps that are typically within IMD of -30db and sufficient harmonic suppression a typical peak reading meter of good quality should be fairly accurate in reading pep output.
     
  3. G0HZU

    G0HZU QRZ Member


    I think you are misinterpreting what is meant by that pasted quote :)

    I'll explain the thought process that leads up to your pasted quote from the article.

    The pdf article is aimed at measuring IMD terms ACCURATELY. Now one thing it mentions near the start is that some people like to measure IMD terms in -dBc wrt the tone level and some state IMD should be measured in -dBc wrt the true PEP.

    So it explains an INDIRECT means to measure PEP to cater for both views. This indirect method is that you can approximate PEP to twice the average power of the two test tones by using a 'slow' power meter that displays average power.

    But this indirect route to PEP is flawed wrt measurement certainty and the article states this. It can be affected by IMD and harmonics. These are sources of error that affect this method of PEP measurement. It then shows the paragraph you pasted. The purpose of this paragraph is to explain that if you were to also measure PEP by DIRECT means i.e. with a power meter than can measure this directly then you 'could' get a subtly different answer to the flawed INDIRECT method and this might cause confusion when comparing test results.

    The implication is that the DIRECT method of measuring PEP won't suffer the same IMD error so if you were to use both types of meter to measure PEP you could get a confusing result because IMD affects the indirect method of determining PEP.

    The whole point of that paragraph is to highlight the difference between the two measurement methods for PEP. IMD causes an error on the INDIRECT (2* average power) method but NOT the direct method. So they advise with Rule #3 that you to stick to ONE method of power measurement if you want to compare results and avoid confusion.

    Do you understand now? (pleeeeeeease say yes :) )
     
  4. KM1H

    KM1H Ham Member QRZ Page

    Thats pure BS, that amp and others of the era were designed and marketed when 2000W PEP Input on SSB was legal. Nobody thought in terms of output back then.

    As far as measuring power dont even worry about it, the FCC could care less if you are a hair over 1500W. They dont even bother those running twice that HL-2200 power capability (it might do 1300W on a good day) as do many of the currently marketed amps.

    Carl
     
  5. K4FMX

    K4FMX Ham Member QRZ Page

    I guess you are interpreting differently. In past posts that you made you agree that harmonics can influence the PEP readings and also IM products can influence it. But I think that your argument consist of IM products being a natural part of PEP and harmonics are not.

    This quote says that errors are negligible when IM products are -30 dB or so. They are talking about measuring average power here.
    The reason the comparison is negligible is when measuring average power, -30 dB levels are barley seen on the power meter. A 1000 watt transmitter with -30 dB IM or harmonic would be only 1 watt added to the 1000 watt reading which is .1%. But if the same 1000 watt and the 1 watt IM product or harmonic voltages are added together and PEP is measured that is a 6% difference as you have shown in the past.

    When the article says that "PEP is always less than 2 times the average power" I take that to mean that they are talking about the true PEP which would be minus any errors from IM products or harmonics.

    In other words they are saying that IM and harmonics if not very small, being read along with the main tones in the average power mode will look like the average power is greater than just the main tone signals are minus any IM or harmonics.

    You have already agreed and profess that harmonics will influence PEP measurements with a peak reading meter. From what the article says, if there are significant enough harmonics and IM products to influence average power readings it should stand to reason that they should also have influence on PEP readings, at least harmonics according to you.

    My point is, if they are saying that PEP will be a little less than 2 times average power readings in the presents of harmonics and IM products (which Increases average power), then they are talking about pure wanted signal powers only in PEP and not considering IM or harmonics as part of true PEP.

    We know that an average power meter such as a thermister will read and add ALL signals together and show the sum of those averages directly.

    "The whole point of that paragraph is to highlight the difference between the two measurement methods for PEP. IMD causes an error on the INDIRECT (2* average power) method but NOT the direct method. So they advise with Rule #3 that you to stick to ONE method of power measurement if you want to compare results and avoid confusion."

    To your quote: They are including HARMONICS along with IM products. So if the above quote were true then harmonics would not cause an error in direct measurements either!

    73
    Gary K4FMX

     
  6. G0HZU

    G0HZU QRZ Member

    Here's a very good recommendation for you Gary because otherwise this unnecessary diversion is just going to go on and on....

    Why not read up on the various forms of power sensor/detector available? Each type of power sensor and PEP measurement technique has its own set of measurement uncertainties. Some sensors or techniques are prone to large uncertainties that other sensors or techniques are immune to.
    Until you read up on this then I am wasting my time trying to help you because you clearly don't trust my input. At the moment you seem convinced that you can apply the same measurement uncertainties across from differing PEP measuring methods and also you seem convinced that the sensors all suffer the same uncertainties wrt IMD and harmonics.

    That's why you think I'm being inconsistent in my arguments.

    Here's the first clue:
    Not all sensors in meters that can measure PEP 'directly' are cheapo meters that use a diode operating in the linear region. Diodes operating in the linear region are Vpk detectors and are extremely sensitive to uncertainty caused by harmonics when trying to measure power. Ham meters usually use this type of detector/sensor.

    So, when doing a 'direct' measurement of PEP the author of the article could well be using something different. eg a PEP indicating power meter that has a square law response. This will capture and measure PEP correctly as it will be able to read the true rms voltage of the waveform. The square law detector may still be using a diode but in a different part of its operating curve and with a very sophisticated circuit around the diode(s). Usually this means operating the diode with tiny signals that are below the Vt for a semiconductor junction where Vt = 0.026V.

    Yes, but this is for typical cheapo ham power meters that operate the detector diode up in the linear region and this introduces extreme sensitivity to harmonics. By extreme sensitivity I mean indicated power errors of about +/- 20% when the harmonic is only -20dBc! Or indicated power errors of +/-6% when the harmonic is at -30dBc.

    You started this diversion off by claiming that IMD terms will also introduce the same +/- 6% of uncertainty for this type of meter. Well you are wrong and you now seem to be trying to support your claim by posting up uncertainties that affect a different type of power sensor and different method of calculating PEP. That isn't logical.

    What matters here is technical correctness and not who is right and who is wrong. So please don't think I am being inconsistent or stubborn here :)
     
  7. KA5S

    KA5S Premium Subscriber QRZ Page

    I use directional couplers and a scope to monitor real peak power, keeping the output to the level of 100 watts CW. That's quicker and more IMO accurate than wattmeters to see when the envelope looks funny and adjust the sound card levels Microswat decided to change. It also lets me keep an eye on mike settings.

    (Lately I am using something called Soundlock, which is supposed to not let SC levels be increased. It does have to be running on startup.)

    Cortland
    KA5S
     
  8. KA5S

    KA5S Premium Subscriber QRZ Page

    Gary,

    If harmonics noticeably affect wattmeter readings, they're too high to put on the air. We're supposed to keep harmonics to -40 dB or lower, which would be just 150 mW with 1500 watts output. That's hard to see.

    Cortland
    KA5S
     
  9. W2VW

    W2VW Ham Member QRZ Page

    And some hams want bandwidth regulations which would require measurement.....
     
  10. W2VW

    W2VW Ham Member QRZ Page

    Guess you missed the ebay listing of Bill's 4CX15000 about a year before he became an sk....
     
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