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Author Topic: Mathematical correlation between Continuous and PEP  (Read 4234 times)
WY7CHY
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Posts: 1018




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« on: February 14, 2019, 08:22:48 PM »

I understand the difference between continuous and PEP. I have very good meters that read PEP pretty accurately. But I was wondering if anyone knew if there was a mathematical correlation between the two?

I know that with continuous; e.g. FM; you aren't speaking into the microphone as you would with SSB measuring PEP, so my question can't be really answered with 100% accuracy. But when amplifier specs show output wattage as: 600w cont/800w PEP; 800w cont/1000w PEP; 850w cont/1250w PEP; or 1000w cont/1300w PEP; are they actually measuring BOTH, or are they reading the EASY and ACCURATE "Continuous" power out, and mathematically extrapolating the PEP output power?

Again; I know the ACTUAL PEP power output will be different with different voice levels, different mic gain and compression levels, etc. But on "average", could a person who doesn't have a somewhat accurate PEP meter, basically figure out the PEP mathematically "on average"; or at least trust the manufacturer's specs?

I.e. The AL-80B is rated at 800w cont and 1000w PEP. If I measure at a given frequency 800w cont (FM); and I set the Mic gain and compression according to the transceiver's instructions; can I "Assume" with some accuracy, that my SSB PEP output is close to 1000w PEP?

I've never been able to accurately measure SSB/PEP with a average power out meter. Like I mentioned in the beginning, I have pretty accurate PEP meters. Not a problem. Just a curious question. I've had friends ask to borrow PEP meters and I was wondering if they could "With some accuracy" extrapolate what their "Speaking level " PEP output would be if they know with some accuracy their average continuous power is.

"It's winter; can't get up on the roof to play with antennas; too much time to think about things that probably don't matter". But thanks for any input.

Mike
 
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
KD9IQO
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Posts: 12




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« Reply #1 on: February 14, 2019, 08:45:02 PM »

"PEP is equal to the average power if an amplitude-modulated signal is not modulated"

The ARRL General Class License Manual page 4-7
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G3RZP
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Posts: 1323




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« Reply #2 on: February 15, 2019, 02:46:16 AM »

I would say that the statement

Quote
"PEP is equal to the average power if an amplitude-modulated signal is not modulated"

is not very meaningful. If an AM signal isn't modulated, it isn't AM - it's a straight forward carrier, and is 1/4 of the PEP of a 100% modulated AM signal. As  a carrier, the PEP is equal to the power measured by wattmeter or spectrum analyser or even oscilloscope.

The PEP to average power ratio of a voice modulated signal is dependent on many factors, including speaker's sex, the frequency response and shaping of the transmitted signal, and the disposition of vowels and consonants in the language being spoken. So I would argue that there is no accurate correlation possible.
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WY7CHY
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Posts: 1018




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« Reply #3 on: February 15, 2019, 06:15:06 AM »

Yea, I'm with you on this and totally agree. Was just wondering if a radio is spec'd at 800w cont and 1000w PEP and the average user could somewhat reliably measure the 800w cont, if they could with any accuracy, "Assume" that they were getting close to putting out the 1000w PEP without the use of a PEP wattmeter. "Assuming their mic gain and compression were adjusted according to the radio destruction manual".
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
W1VT
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Posts: 3404




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« Reply #4 on: February 15, 2019, 06:15:31 AM »

What you may be able to do, if  you have good Digital Signal Processing skills, is to run the audio waveform into a computer and calculate the average power, assuming a particular load impedance.  You may also be able to calculate what the meter should read based on that waveform.
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KD9IQO
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Posts: 12




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« Reply #5 on: February 15, 2019, 08:20:02 AM »

The problem is not the ARRL answer. 
The problem is an ill-defined question.
 You're not asking a question.........you're asking for an education.
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WY7CHY
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Posts: 1018




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« Reply #6 on: February 15, 2019, 11:09:58 AM »

The problem is not the ARRL answer. 
The problem is an ill-defined question.
 You're not asking a question.........you're asking for an education.
No, not really. But thanks for the feedback???

I have good meters, I have good scopes, I have access to pretty much any type of test or measuring equipment needed for any type of radio service, maintenance, repairs, etc. What I haven't compiled in the last 45 years, I have at work. I was simply asking if there was a simplified way for the average user, (Who asks a lot of questions about SSB PEP output because they don't have meters to read it), to measure.

G3RZP gave a good reply. I understand it. I was just trying to figure out how to explain to someone without the only answer being; "Buy a quality PEP Wattmeter". I've been doing some testing using CW keys and voice experiments; and only using a cont average power wattmeter (Not using a PEP meter or scope) to annotate readings, and then compare the same signal with the results of an accurate PEP wattmeter and/or scope. Then a correlation might be determined.

There are so many variables, that it will take a bit of experimenting. I was simply wondering if anyone had any experience with similar. But thanks KD9IQO; if I find myself needing to be "Educated", I'll make sure I call you.  Wink "All is good"
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Born Wild - Raised Proud: 73
Cheyenne, Wyoming
G3RZP
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Posts: 1323




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« Reply #7 on: February 15, 2019, 02:12:49 PM »

Even using a PEP wattmeter depends on how fast it responds - is the response time much greater than the voice peaks? A 'rule of thumb' is that if the speech top frequency is 3kHz, then the wattmeter should measure the peak in 30 microseconds or less It the needs to hold that value for long enough or the meter to read it.

An oscilloscope is probably the best way, but getting an average isn't that easy. I don't understand why people get all het up about extreme accuracy in power measurement - will it really make any difference if you are one half or three quarters of a dB below the maximum power allowed? Especially when you consider that the standard for a measurement lab to a 95% certainty is +/- 0.75 dB......and when people quote an 'accuracy' of anything, it's worth asking what the 'measurement uncertainty' is......
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K6STI
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Posts: 18




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« Reply #8 on: February 15, 2019, 03:22:18 PM »

... will it really make any difference if you are one half or three quarters of a dB below the maximum power allowed?

http://ham-radio.com/k6sti/pileup.htm
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G3RZP
Member

Posts: 1323




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« Reply #9 on: February 15, 2019, 04:09:00 PM »

however, it doesn't take into account the fact a that well  known DXer's call is more likely to be recognised than a 'runoff the mill# W or G or DL call...thus altering the chance of getting through.
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AA4PB
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Posts: 15067




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« Reply #10 on: February 15, 2019, 05:00:26 PM »

Yea, I'm with you on this and totally agree. Was just wondering if a radio is spec'd at 800w cont and 1000w PEP and the average user could somewhat reliably measure the 800w cont, if they could with any accuracy, "Assume" that they were getting close to putting out the 1000w PEP without the use of a PEP wattmeter. "Assuming their mic gain and compression were adjusted according to the radio destruction manual".

The fact that you accurately read 800W output with a steady signal (carrier) does NOT necessarily mean that will be putting out 1000W PEP when you modulate the radio. These are just the maximum power ratings that the amp can handle without overheating or damage. There is no magic formula that you can use to convert average power to PEP because the PEP power depends on the modulating signal characteristics (voice or digital waveform). The easiest way to measure PEP is with a PEP wattmeter. If you try to measure the output of a SSB transmitter with an average reading wattmeter, it will likely read quite low because it's response time is not quick enough to measure the voice peaks. How much low depends on your particular voice characteristics and the response time of your particular meter.

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Bob  AA4PB
Garrisonville, VA
N3DT
Member

Posts: 1793




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« Reply #11 on: February 15, 2019, 06:51:40 PM »

I could try reading my 80B with the LP100A in average measurement mode in FM TX and then measure the LP100A in PEP mode of my voice in SSB, but that would only tell my how my 80B/TS2000 responds to my voice, I reckon. I actually haven't tried that though. Might be interesting. According to LP the 100A has a very fast PEP measurement time. A scope measurement might be interesting too in comparison to meter tests.
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KM3F
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Posts: 910




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« Reply #12 on: February 15, 2019, 06:52:03 PM »

Consider this another way.
The peak reading meter has to accept the signal, convert it to DC by charging a capacitor at the input of a DC meter driver circuit in order to hold a meter reading for some prescribed time interval so it can be seen before the capacitor begins to discharge through a resistance.
Meter ballistics, hold time, driving wave form's peaks all come into play.
There is no easy way to relate peak to average unless the wave form is a repeating type such as a square wave, or a known repeating pulse at constant level and  duty time, or sign wave that can be calculated into a meaningful average.
A human 'talk signal' cannot be set to a reliable average.
All you need to do s look at a scope of voice and ask how an average could be calculated.
As W1VT offered, a computer to take many points and derive an average. Even this would be a bit slow on a full time basis and of questionable value for the effort unless you were analyzing a voice pattern for comparison.
An average reading meter does a pretty good job trying to follow complex  wave form considering it's slow speed response that was meant to register a  continuous  wave signal.
The best it usually does is display about 20 to 30% of peaks.
Good luck.
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WB6BYU
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Posts: 18534




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« Reply #13 on: February 15, 2019, 09:12:03 PM »

Quote from: WY7CHY

I understand the difference between continuous and PEP...



Perhaps there is less difference than you think...



Quote

But I was wondering if anyone knew if there was a mathematical correlation between the two?



Yes, there is a very simple correlation:
  100W continuous = 100W PEP
  800W continuous = 800W PEP
 1300W continuous = 1300W PEP


This is easy to verify:  connect an average-reading wattmeter and a PEP wattmeter in series between
your transmitter / amp and a dummy load.  Key the transmitter in CW mode.  Compare the meter
readings.  They should read the same thing.  That's because with an unmodulated carrier (or with FM)
the continuous power is the PEP power.
   



Quote

...But when amplifier specs show output wattage as: 600w cont/800w PEP; 800w cont/1000w PEP; 850w cont/1250w PEP; or 1000w cont/1300w PEP; are they actually measuring BOTH, or are they reading the EASY and ACCURATE "Continuous" power out, and mathematically extrapolating the PEP output power?



You are confusing power measurement with power rating.  They are totally different.

The difference isn't because the power is measured differently:  the difference is due to the
cooling characteristics of the amplifier.

Let's consider a typical linear amplifier that is capable of 800W PEP while meeting some
minimum standard for distortion.  That's limited by the tube characteristics and the bias
voltage, etc.  It means that the tube can't put out any more power without distorting.
And such an amplifier will not be perfectly efficient:  some amount of the power delivered
to the tube will be dissipated as heat:  for the purpose of this exercise, I'll assume it is
67% efficient, just to make the numbers easy to do in your head.  (In practice the
efficiency will vary with drive level, but I'll ignore that for this explanation.)

So at 800 watts PEP output, the input power to the tube is 1200 watts, and the amp
has to dissipate 400 watts of heat.

But unless we are running an advertisement, we don't leave a  brick on the key and
put out a continuous carrier.  For CW, we turn it on and off frequently, and the average
duty cycle is perhaps 40% to 50% during the time we are transmitting, and of course
0 when we are receiving.  So an hour-long QSO running 800W PEP of CW is only going
to require the amp to dissipate about 100 Watt-hours of accumulated heat.

On the other hand, if we are running SSB and our voice runs about 25% duty cycle,
then (assuming equal transmit and receive times) the amp only has to dissipate 50
Watt-hours of accumulated heat at 800 W PEP.

Then the question becomes, how much power can the amp safely dissipate before it
overheats?  If they expect that most operators are going to run it on SSB, then they
may scrimp on fans and copper heat spreaders and design it to dissipate 50 Watts
rather than 100 watts.  That permits 800 W of SSB, but the amp will overheat on
CW if it is run at more than 400W for any length of time.  On the other hand, if you
are running FM or digital, where it really is a continuous signal, you may be limited
to even less output power, because you don't have the 50% duty cycle of CW.


So when you see different ratings for output power for different modes, that typically
is because they are bumping into different limits.  For example, an amp rated for 800 W
PEP on SSB might have that limit because distortion products increase above that level.
But on CW, the thermal design may limit it to 600W PEP output at typical duty cycles:  the
amp is capable of 800W PEP on CW, but can't safely dissipate the heat for more than an
occasional short transmission.  Digital modes or FM would have a lower thermal limit, while
AM may push both limits:  maximum output without excessive distortion and thermal
dissipation, limiting carrier output power to  200 W (due to the 800W PEP limit at 100%
modulation), and possibly less due to the long-windedness of some AM operators causing
excessive heating.

The duty cycle calculation isn't as simple as it sounds:  you can't use an amp at full output
continuously for just one day a month and average your duty cycle across the whole month.
The screen grid of a tube, for example, heats up quickly because it is small, while heat
build-up in a ventilated cabinet will happen over a longer period of time.  So you may have
different limiting factors for different modes and power levels.




Quote

But on "average", could a person who doesn't have a somewhat accurate PEP meter, basically figure out the PEP mathematically "on average"; or at least trust the manufacturer's specs?
 


That, of course, is a totally different question.

Probably the best you could do would be to put your average wattmeter in line with an accurate PEP
meter and compare the readings on "average" speech.  Often it may be around 25 % of PEP or so,
depending on the voice, etc.  But that would give the operator an idea of what to expect with that
particular wattmeter on their own voice with that particular microphone
.

Or put the rig on a PEP meter and figure out where you have to set the transmitter ALC level to
generate the desired PEP output from the amplifier.  Then, under the same conditions, you could
reset the ALC level to that point and expect that you would have about the same PEP output
from the amp each time.
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K6BRN
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Posts: 1354




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« Reply #14 on: February 15, 2019, 09:57:07 PM »

Good Grief:

Quote
PEP is equal to steady carrier power, or radiotelegraph dot or dash average power, in a properly-formed CW transmission. PEP is also equal to average power in a steady FM, FSK, or RTTY transmission.

Although average power is the same as PEP for complex modulation forms, such as FSK, the peak envelope power bears no particular ratio or mathematical relationship to longer-term average power in distorted envelopes, such as a CW waveform with power overshoot, or with amplitude modulated waveforms, such as SSB or AM voice transmissions. Typical average power of a SSB voice transmission, for example, is 10-20% of PEP. The percentage of longer term average power to PEP increases with processing, and commonly reaches ~50% with extreme speech processing.

https://en.m.wikipedia.org/wiki/Peak_envelope_power

Brian - K6BRN
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