Power detector

Thread Starter

lucasN6676

Joined Feb 20, 2024
12
Hello folks, I have a question regarding rf power detector.
Why are peak detectors and logarithmic detectors not suitable for large papr?
 

KeithWalker

Joined Jul 10, 2017
3,095
Why are you assuming that they are not?
What do you mean, "PAPR" and "PD"?. My understanding is that "PAPR" is "powered air purifying respirator".
Can you give us a bit of background context so we don't have to make wild guesses?
 
Last edited:

Thread Starter

lucasN6676

Joined Feb 20, 2024
12
Why are you assuming that they are not?
Do you mean "powered air purifying respirator" and what do you mean "PD"?.
Can you give us a bit of background context so we don't have to make wild guesses?
I mean power detector, I have coupler after a power amplifier to couple the signal to a power detector. The thing is that in ofdm systems the signals have large papr, and I have read in several papers that rms power detector is the most suitable one for large papr but I couldn't really know why peak detector and logarithmic detector are not suitable here
 

Ya’akov

Joined Jan 27, 2019
9,165
While I won’t claim to be an expert, my understanding is that a peak detector is precisely the correct option for PAPR characterization.

Why do you think it is not?
 

Ya’akov

Joined Jan 27, 2019
9,165
Why are you assuming that they are not?
What do you mean, "PAPR" and "PD"?. My understanding is that "PAPR" is "powered air purifying respirator".
Can you give us a bit of background context so we don't have to make wild guesses?
PAPR is Peak to Average Power Ratio. A high PAPR means the transmitter is going to be using a lot of bandwidth so there are several techniques to mitigate this. It is particularly important for OFDM.

Measuring PAPR is the first step in making the changes to reduce it (if needed).
 

Ya’akov

Joined Jan 27, 2019
9,165
I mean power detector, I have coupler after a power amplifier to couple the signal to a power detector. The thing is that in ofdm systems the signals have large papr, and I have read in several papers that rms power detector is the most suitable one for large papr but I couldn't really know why peak detector and logarithmic detector are not suitable here
Can you provide a cite or two for those papers?
 

KeithWalker

Joined Jul 10, 2017
3,095
PAPR is Peak to Average Power Ratio. A high PAPR means the transmitter is going to be using a lot of bandwidth so there are several techniques to mitigate this. It is particularly important for OFDM.

Measuring PAPR is the first step in making the changes to reduce it (if needed).
Thanks for the explanation. RF power is not one of my specialities.
 

Ya’akov

Joined Jan 27, 2019
9,165
I've read several papers saying that rms PD is the most suitable one, and not offering a deep reason for it. here's a paper
So, that paper is on VSWR measurement. The requirements for it are not necessarily the same as PAPR calculation. In fact, since to calculate PAPR you need peak information (as in the case of PA protection), the requirements are diametrically opposed.

If you are trying to measure VSWR, the paper itself gives you a clear explanation.

From the paper:

A. Power Detector​
There are two categories of detectors: peak and Root Mean​
Square (RMS). The first ones give information on the peak​
power whereas the second ones give information on the​
average power. A peak detector would have been used to​
realize a protection loop to avoid the PA breakdown.​
However, in order to regulate the average power at the PA​
output for Automatic Level Control and VSWR compensation​
as detailed in the previous section, a RMS detector will be​
preferred. Furthermore, workgroup IEEE 802.15.3c is working​
seriously towards the modulation standard OFDM [2], which​
implies a high Peak-to-Average Power Ratio (PAPR) – as​
high as 6dB – that means a waveform shape well-different​
from a sine wave. So the power detector should be a Root
Mean Square (RMS) one, rather than a peak one which would
not output the true average power of the transmitter.

For PAPR calculations, many samples have to be taken so the peak can be determined and the average calculated. An RMS detector would not be suitable because it would not provide the peak values. For VSWR the average power is the critical thing, and while a peak detector can provide that if integrated over time, that process would be a silly exercise when an RMS detector provides it directly.

So, if you want to calculate PAPR, use a peak detector but if you want to know the instantaneous average power, use an RMS detector.

[EDITED to fix autocorrect mangling of PAPR]
 

Thread Starter

lucasN6676

Joined Feb 20, 2024
12
So, that paper is on VSWR measurement. The requirements for it are not necessarily the same as PAPR calculation. In fact, since to calculate PAPR you need peak information (as in the case of PA protection), the requirements are diametrically opposed.

If you are trying to measure VSWR, the paper itself gives you a clear explanation.

From the paper:

A. Power Detector​
There are two categories of detectors: peak and Root Mean​
Square (RMS). The first ones give information on the peak​
power whereas the second ones give information on the​
average power. A peak detector would have been used to​
realize a protection loop to avoid the PA breakdown.​
However, in order to regulate the average power at the PA​
output for Automatic Level Control and VSWR compensation​
as detailed in the previous section, a RMS detector will be​
preferred. Furthermore, workgroup IEEE 802.15.3c is working​
seriously towards the modulation standard OFDM [2], which​
implies a high Peak-to-Average Power Ratio (PAPR) – as​
high as 6dB – that means a waveform shape well-different​
from a sine wave. So the power detector should be a Root
Mean Square (RMS) one, rather than a peak one which would
not output the true average power of the transmitter.

For PAPR calculations, many samples have to be taken so the peak can be determined and the average calculated. An RMS detector would not be suitable because it would not provide the peak values. For VSWR the average power is the critical thing, and while a peak detector can provide that if integrated over time, that process would be a silly exercise when an RMS detector provides it directly.

So, if you want to calculate PAPR, use a peak detector but if you want to know the instantaneous average power, use an RMS detector.

[EDITED to fix autocorrect mangling of PAPR]
here's another paper, it also said the same thing in the introduction
 

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Ya’akov

Joined Jan 27, 2019
9,165
here's another paper, it also said the same thing in the introduction
The explanation above applies here, too. The paper says:
RMS detection is insensitive to Peak-to-Average
envelope power Ratio (PAR) and offers better accuracy​
compared to peak detection. Therefore, RMS detection is​
more suited for modulated signals with high PAR​

So clearly, for applications where a high PAPR is present and the average power is the value of interest, an RMS detector is the proper choice. On the other hand and RMS detector doesn’t measure peak power and for PAPR calculation, you need that.

If you are not measuring PAPR, or doing something else where peak values are important (such as PA protection), the RMS detector will not return values distorted by the high PAPR of the the OFDM modulated signal.
 

Thread Starter

lucasN6676

Joined Feb 20, 2024
12
The explanation above applies here, too. The paper says:
RMS detection is insensitive to Peak-to-Average
envelope power Ratio (PAR) and offers better accuracy​
compared to peak detection. Therefore, RMS detection is​
more suited for modulated signals with high PAR​

So clearly, for applications where a high PAPR is present and the average power is the value of interest, an RMS detector is the proper choice. On the other hand and RMS detector doesn’t measure peak power and for PAPR calculation, you need that.

If you are not measuring PAPR, or doing something else where peak values are important (such as PA protection), the RMS detector will not return values distorted by the high PAPR of the the OFDM modulated signal.
Aha
Are you following my explanation? It is actually in the papers themselves.
Yes, I understand now. I don't want to measure PAPR, I just want to measure PA output instantly to control the transmitted power, so here rms PD is the best one
 

Thread Starter

lucasN6676

Joined Feb 20, 2024
12
Aha

Yes, I understand now. I don't want to measure PAPR, I just want to measure PA output instantly to control the transmitted power, so here rms PD is the best one
and we don't use peak detector in that application because it will return distorted values that will make it hard to control the transmitted power?
 
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