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. WA8ZYT

    WA8ZYT Ham Member QRZ Page

    How about leaving well enough alone? You can buy a PEP board for most watt meters for around $30. Or hook up an oscilloscope to monitor your output. What is so hard about that? Good god, man, you are making a mountain out of a mole hill.
     
  2. WG7X

    WG7X Premium Subscriber QRZ Page

    Let's Keep it simple.

    Even the FCC decided years ago that your method was too difficult to implement and to enforce.

    Why make something that has been made easier more difficult again?

    Lets see... Read two meters while in QSO then take the data from the meters, do some math for a final result, versus buying a good peak reading meter and simply reading the output?

    Molehills into mountains is right...

    My opinion only, which you did ask for.

    73 Gary
     
  3. K4FMX

    K4FMX Ham Member QRZ Page

    If you have a meter capable of accurately reading PEP I don't think the detection method much matters. However as to your point of additional errors with harmonics present, there is also the matter of IM products present that will give power reading errors.
    If you have intermod products that are down 30 dB that get added or subtracted to the main signal that will also give an additional 6% error in the PEP reading.
    The voltages of the main signal and those of the IM products will add together and what you get is the composite PEP.

    73
    Gary K4FMX

     
  4. G0HZU

    G0HZU QRZ Member

    I'm slightly nervous about giving my view on this because it could take the thread off on a tangent but here goes... :)

    I think it gets complicated when you consider IMD terms because we know they will affect the signal amplitude in the time domain compared
    to what would be seen on a perfectly linear amplifier. i.e. you usually see flat topping in the modulation envelope.

    But... the PEP definition is average power in the timeslot of 1 RF cycle so any IMD contribution (that causes flat topping) won't be fast enough to
    cause measurable distortion of the single RF cycle sinewave at the flat topped modulation crest. So IMD effects are just 'dirty modulation' and 'yes' they appear as flat
    topping but the PEP you measure at the flat top is still a valid PEP measurement and you can still measure it fairly accurately with a Vpeak detector if the harmonics are well filtered.

    i.e. PEP is still PEP even with high levels of IMD?

    Hope I've typed all that correct as it is late here and I've drunk a beer as well LOL
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think drinking beer clarifies the mind and its thinking processes.

    For example, after drinking a lot of beer, I proposed marriage to my girlfriend, who was my first wife. I say "first," because now I'm on my third, and drinking much less beer.:p

    I find it difficult to believe IMD products, or harmonics, or anything else that's down 30-40 dB will really influence power readings. The power levels are just too small.
     
  6. K4FMX

    K4FMX Ham Member QRZ Page

    My point is that the PEP that you might read will consist of the sum of the wanted signal and the IM products. So the 100 watt PEP reading that you might see with a perfectly linear amplifier could still show 100 watts PEP with IM products that are 30 db down (and only .1 watt in level). But the wanted signal would only be aproximatly 94 watts.

    I don't think that you have to flat top to have IM products. They can be present at any power level and not just at maximum capability of the amplifier.

    The one cycle rule for PEP is only there to keep people from finding ways to get around the true intent of PEP measurements such as in our old rules when input power was measured the plate current meter had to have a time constant of no greater than .05 seconds (if I remember right).

    When measureing PEP we are most always looking at (measuring) many cycles of the RF frequency for even short time envelope peaks.
    The IM products are present at frequencies removed from the wanted signal just like any harmonic would be although not quite so far removed. As I see it they will add to the wanted signal amplitude just the same as harmonics would or the side bands of an AM transmitter add to the carrier to give a PEP that consist of the composite of all.

    73
    Gary K4FMX



     
  7. K7JEM

    K7JEM Ham Member QRZ Page

    Right. I think some people don't know how much 30dB down actually is. If you are running 1000 watt transmitter, and your IMD or harmonics, or whatever, is down 30dB, that means they are at a level of 1 watt. I find it hard to believe that a single watt will change your 1000 watt reading by 6% or even 1%, since 1% of 1000 watts is 10 watts. Even 1% would be hard to read on a wattmeter.

    Joe
     
  8. K4FMX

    K4FMX Ham Member QRZ Page

    But.. you do not add powers directly. You must first add the voltages present in each signal and then compute power from that.

    If you look at an AM signal with a 375 watt carrier it will have a total of 93.75 watts in each side band. If you add 375 and 93.75 plus 93.75 you would think that the PEP would be 562.5 watts but in reality the PEP of that transmitter is 1500 watts!

    To find PEP you must first add the voltage of each together. 375 watts = 136.9 volts across 50 ohms. 93.75 watts = 68.47 volts across 50 ohms.
    Add the voltages together (136.9 + 68.47 + 68.47) and you have 273.84 volts.
    E^2 /50 = 273.84x273.84 = 74988.35 / 50 = 1500 watts PEP.

    So even a small signal that is 30 dB down, 1 watt from 1000 watts will be a considerable contributer to PEP.

    73
    Gary K4FMX

     
  9. K7JEM

    K7JEM Ham Member QRZ Page


    Yes, you posted your clarification while I was making my post. I was more referring to your previous comment about error in reading, which is really not an error, it is just a composite snap shot. The meter will still read PEP, it will just include those IMD products too.

    Joe
     
  10. KQ4X

    KQ4X Ham Member QRZ Page

    To those who think I'm making a mountain out of a mole hill...so far we have 8 pages and 74 posts explaining how PEP "really" works. LOL!
     
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