What is pep?

Discussion in 'General Technical Questions and Answers' started by G4ALA, Jul 24, 2008.

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  1. G4ALA

    G4ALA Ham Member QRZ Page

    Hello everybody.

    I have a fundamental question concerning the definition of Peak Envelope Power (PEP) by our beloved supervisory authorities. In you case it is the FCC. In our case it is OFCOM.

    Audio amplifiers frequently quote (deceptively I feel) a peak power which is, in fact, twice their peak RMS power but represents their peak Instantaneous power.

    What then is PEP? How do you set up a power meter to show PEP?

    Do you set it to show

    a) The peak RMS power (of the envelope)

    or

    b) The peak instantaneous power (of the envelope), which reflects an RMS power which is only half the meter reading.

    This is quite critical when a meter has to be calibrated. It also reflects upon the alleged power output of linear amplfiers.

    Thank you

    John

    G4ALA
     
  2. AI3V

    AI3V Ham Member QRZ Page

    A meter is much too slow to show PEP , You want to measure with a scope.

    PEP Reading meters all use some sort of a sample-and-hold circut,Which may be of questionable accuracy.

    Rege
     
  3. AG3Y

    AG3Y Guest

    Hi John, first you must remember that the "normal" meter cannot respond fast enough to indicate peaks of a waveform, but will in almost all cases respond to the average power output of a voice waveform. There are some special circuits and meters that will detect the peak and allow it to be displayed, but these devices need to be powered with a built-in power source.

    Unlike the "instantaneous peak power" readings that some hi-fi manufacturers claim for their audio products, PEP is an actual entity which can be accurately measured and observed with the proper instruments, typically an oscilloscope, or some sort of "peak holding" meter.

    My understanding is that the "puffed" instantaneous peak power that audio manufacturers often quote is based on the absolutely greatest amount of instantaneous power that the amp can put out before the voltage in the power supply starts to "sag" etc. This instantaneous power may only last for milliseconds, or even less, and really contributes nothing to the total audio output over a longer period of time.

    PEP, on the other hand, can reflect the true power being put out by an RF amplifier, and is something that should be carefully monitored, by the aformentioned oscilloscope or peak holding meters, if you are running an amp that is capable of delivering maximum legal power to an antenna.

    Now, the experts will come in and start another 100+ post string about PEP etc. I betcha.

    73, Jim
     
  4. N5RFX

    N5RFX Ham Member QRZ Page

    A is correct.

    A is correct. Peak envelope power is the average power supplied to the antenna transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope, under normal operating conditions. RMS power and average power are the same value.

    The best way to determine the accuracy of a PEP meter is to first use a single sine wave. Since the PEP and average power of a single sine wave are equal you can establish the relationship of PEP to average power using more complex waves Next use two equal amplitude sine waves to see the average power drop 3dB. Three equal amplitude tones will make the average power drop 4.8dB. 10LOG(1/n) tells you the dB difference between PEP and average power where n = the number of equal amplitude sine waves.

    73,
    Mark N5RFX
     
    Last edited: Jul 24, 2008
  5. AG3Y

    AG3Y Guest

  6. G4ALA

    G4ALA Ham Member QRZ Page

    Muddier Yet

    Thank you all for you observations so far.

    I found the following definition of PEP. It is one of the few definitions that even approaches clarity.

    "peak envelope power (of a radio transmitter) [PEP, pX, PX]: The average power supplied to the antenna transmission line by a transmitter during one radio frequency cycle at the crest of the modulation envelope taken under normal operating conditions. [NTIA] [RR] (188)"

    But that is not the question, nor is how to measure PEP on different instruments. The question is.........

    How do the FCC/OFCOM define PEP?. Is it as uncertain as I suspect it is?

    Yes, I use an Oscilloscope to set up my meters and yes, they are all peak reading. However, I desire to remain within the "officially set" limits for power. What setting do I use? One is twice the other.

    73


    John
    G4ALA
     
  7. AC0FP

    AC0FP Ham Member QRZ Page

    G4ALA,

    Well the first four posts are all technically correct and Jim's assessment of the number of answers is in the ballpark. I have noticed that on QRZ that this subject seems to be avoided for some reason, probably because there are several different ways to say the same thing.

    For amateur radio purposes PEP is a method to measure transmitter linearity. An example is a transmitter which happens to put out 50 watts with a single audio CW tone. If an audio two tone generator is used (two equal amplitude non-harmonically related audio signals) the wattmeter will still read 50 watts but with an oscilloscope and spectrum analyzer you will observe a waveform that has twice the peak amplitude of the original CW signal. As the two tone audio signal is increased there will be a point that the two signals on the spectrum analyzer will become four, at the point where the two new signals, known as third order intermodulation products are 30 dB below the original two signals we have the PEP limit (linearity limit) of this amplifier. The ratio of PEP to CW power with a two tone signal is 2:1 and as been previously stated if three signals were used the ratio would be 3:1, etc...

    Hope this helps a little.

    73,

    Frank:)
     
    Last edited: Jul 24, 2008
  8. N5RFX

    N5RFX Ham Member QRZ Page

    Terminolgy can get you in trouble

    RMS deals with voltage. So when RMS voltage is used to determine power, the result is average power. The article you quoted is does not use the standard definition of RMS power. RMS power is derived from RMS voltage. RMS voltage is the square root of the mean of the squares of voltage. The square of voltage is power. Thus you have the square root of the average power. Square that number to convert from RMS voltage to power, and you now have the average power. I don't like the term RMS power, but it is used and does have a standard meaning in the RF engineering world. The quoted article does not use the standard definition used in RF engineering.

    73,
    Mark N5RFX
     
    Last edited: Jul 24, 2008
  9. K0CMH

    K0CMH Ham Member QRZ Page

  10. WD0GOF

    WD0GOF Ham Member QRZ Page

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