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How to measure power for a low power RF Power Amplifier?

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sharethewell

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I am wondering how to measure output power for a low power RF Power Amplifier. (Signal is sinusoidal. Load resistor is 50 ohms)

The RF power amplifier's operation frequency is 433MHz.

So should I use which of the two following methods?

1. Measure RMS voltage across the load resister, then measure RMS current through the load resister. So the power is derived by RMS Voltage X RMS Current.

2. Because V*I is only suitable for low frequency or DC signal, so the RF output power should be derived by measuring RMS voltage across the load resister. Then Power = (RMS voltage squared) / load resister.

Which one is correct? Thanks!
 

The two methods are the same.
In RF, I = V / 50R
 

At frequencies above ~100 MHz, it is not a good idea to use volt- and ampere meters due to stray reactances. This fact is why power meters are used preferably, and using the Smith diagram, with measurement of standing waves offers better results.

For 433 MHz, there are good low-cost power meters available based on calibrated detectors. If you can find good calibrated attenuators, then you can build a simple diode detector and calibrate it against a good power meter.

Using diode detectors with attenuators is good if you make sure that the detector is not overloaded; usually a good limit rule is 1...5 mW max.
Otherwise, good power meters must be also handled with a caution; some have a 100 mW limit, others, only 10 mW.
Using good attenuators is always recommended to measure RF, VHF, UHF and microwave power.
 

The most convenient way would be to go from the circuit, via coaxial cable, into a dedicated RF power meter. Make sure the terminating of the coax. onto the PCB is done with the shortest connections from the inner to the "hot" part of the circuit and likewise for the braid. In fact, use RG178 and remove the oouter FEP jacket - then fill the undisturbed braid with solder, then cut it to be maybe 1mm longer than the FEP outer. Then cut the dielectric to 1mm and the inner to 1mm and solder it down.

You could use a spectrum analyser - make sure you measure the totalpower of the modulated carrier including sidebands.

You could measure (using a reasonable scope probe and scope) at 433MHz - as you said, look at the voltage across the resistor which you could make from a non inductive 50R chip resistor.

This all assumes you are measuring a few mW or W and it is on a PCB, rather than higher powers from test equipment of some sort in which case coaxial measurements would be very easy.
 

Roger Freeman said:
The most convenient way would be to go from the circuit, via coaxial cable, into a dedicated RF power meter. Make sure the terminating of the coax. onto the PCB is done with the shortest connections from the inner to the "hot" part of the circuit and likewise for the braid. In fact, use RG178 and remove the oouter FEP jacket - then fill the undisturbed braid with solder, then cut it to be maybe 1mm longer than the FEP outer. Then cut the dielectric to 1mm and the inner to 1mm and solder it down.

You could use a spectrum analyser - make sure you measure the totalpower of the modulated carrier including sidebands.

You could measure (using a reasonable scope probe and scope) at 433MHz - as you said, look at the voltage across the resistor which you could make from a non inductive 50R chip resistor.

This all assumes you are measuring a few mW or W and it is on a PCB, rather than higher powers from test equipment of some sort in which case coaxial measurements would be very easy.
Click to expand...

Thanks.

How accurate is it to measure the RF power by using a power meter compared to using a spectrum analyzer?

Another question, can I use a RF power meter to measure power consumption from the power supply in my RF class E power amplifier? How should I connect the power meter to the circuit to measure the power supply power?

Thanks a lot!
 

biff44 said:
Power meters speak'um truth.
Click to expand...

"Speak'um truth" means "speak some truth"?

Do you have any idea how to test the power supply power by using the power meter? How to set it up? I have no experience at all. Thanks a lot.
 

BigBoss said:
Power meters are sufficiently accurate.
Click to expand...

Cool. Do you know how to hook up the power meter with a RF power amplifier IC?

The PA IC is packaged and is connected to a costumed test PC board. The PC board has BNC connectors of 50ohms termination on the PC board. Then for measuring the power supply power, how do I measure it with a power meter?

As much as I can understand, the BNC connector for power supply is connect to a power source/power supply. Then how can a RF power meter fit in this setup? It seems most RF power meters use BNC connectors.
 

sharethewell said:
Cool. Do you know how to hook up the power meter with a RF power amplifier IC?

The PA IC is packaged and is connected to a costumed test PC board. The PC board has BNC connectors of 50ohms termination on the PC board. Then for measuring the power supply power, how do I measure it with a power meter?

As much as I can understand, the BNC connector for power supply is connect to a power source/power supply. Then how can a RF power meter fit in this setup? It seems most RF power meters use BNC connectors.
Click to expand...

I don't understand what you intend to measure ??? Power supply DC Power or RF Power of your PA ??
If you talk about RF Power Meters, they are not for DC power measurements.
For power consumption, simply connect a ammeter and voltmeter and measure them.. and multiply..
 
Last edited:

BigBoss said:
I don't understand what you intend to measure ??? Power supply DC Power or RF Power of your PA ??
If you draw a skecth, we can say something concrete....
Click to expand...

Sorry for the confusion. For power supply power, it is DC power. For RF PA output power, it is RF(AC) power.

Let me put the questions this way.
For RF PA output power measurement, I can just hook the BNC connector of the output of the PA with the BNC connector of the RF power meter.

Since the power supply power is DC power, that means I can't just use a RF power meter to measure it, right? Does it mean I can only use voltmeter and ammeter to measure the DC power supply power?

Is that clear? Thanks for your help!
 

sharethewell said:
Sorry for the confusion. For power supply power, it is DC power. For RF PA output power, it is RF(AC) power.

Let me put the questions this way.
For RF PA output power measurement, I can just hook the BNC connector of the output of the PA with the BNC connector of the RF power meter.

Since the power supply power is DC power, that means I can't just use a RF power meter to measure it, right? Does it mean I can only use voltmeter and ammeter to measure the DC power supply power?

Is that clear? Thanks for your help!
Click to expand...

Let's clarify your question..
You can use an ammeter and voltmeter to measure DC Power Consumption.That's it..

For PA RF Power, you should of course use Power Meter but pay some attention for those points..

-Most of Power Meters have N-Type Male connectors..Soem of them have SMA and other high quality connectors..
-BNC Connectors have relatively poor Return Losses and they are NOT preferred for Power Amplifier Outputs.Because high power PA need quite good RLs.However for lower power levels, they are also used..
-Maximum Input Power for Sensor of Power Meter is defined and therefore if your RF Power exceeds that maximum, you MUST use a Attenuator to reduce this power for Power Meter..
-Generally applicable maximum TOTAL power for the sensor of Power Meter is around +20dBm, don't exceed this value otherwise the sensor will be damaged.
 
BigBoss said:
Let's clarify your question..
You can use an ammeter and voltmeter to measure DC Power Consumption.That's it..

For PA RF Power, you should of course use Power Meter but pay some attention for those points..

-Most of Power Meters have N-Type Male connectors..Soem of them have SMA and other high quality connectors..
-BNC Connectors have relatively poor Return Losses and they are NOT preferred for Power Amplifier Outputs.Because high power PA need quite good RLs.However for lower power levels, they are also used..
-Maximum Input Power for Sensor of Power Meter is defined and therefore if your RF Power exceeds that maximum, you MUST use a Attenuator to reduce this power for Power Meter..
-Generally applicable maximum TOTAL power for the sensor of Power Meter is around +20dBm, don't exceed this value otherwise the sensor will be damaged.
Click to expand...

Thank you very much. This is very helpful.

Yes. The output RF power is low power which is around 7 or 8 mW.

Can I use a voltmeter and ammeter to measure this RF output power? Accurate?

If I can, then is voltmeter+ammeter or a spectrum analyzer more accurate to measure the RF output power?

Thanks again.

Thanks.
 
Last edited:

sharethewell said:
Thank you very much. This is very helpful.

Yes. The output RF power is low power which is around 7 or 8 mW.

Can I use a voltmeter and ammeter to measure this RF output power? Accurate?

If I can, then is voltmeter+ammeter or a spectrum analyzer more accurate to measure the RF output power?

Thanks again.

Thanks.
Click to expand...

You can absolutely NOT use voltmeter and ammeter to measure RF Power..
RF Power Meters are MORE accurate than Spectrum Analyzers but if your signal has harmonics-probably it has- the Power Meter in this case will measure total power including harmonic powers even the harmonics have relatively much lower power levels.In that case, use Spectrum Analyzer to measure the peak power of your carrier.
They will be around in \[\pm 0.2....1dB\] uncertainty range depending on your SA quality.
 

BigBoss said:
You can absolutely NOT use voltmeter and ammeter to measure RF Power..
RF Power Meters are MORE accurate than Spectrum Analyzers but if your signal has harmonics-probably it has- the Power Meter in this case will measure total power including harmonic powers even the harmonics have relatively much lower power levels.In that case, use Spectrum Analyzer to measure the peak power of your carrier.
They will be around in \[\pm 0.2....1dB\] uncertainty range depending on your SA quality.
Click to expand...

Thanks a lot.
 

We, erroneously, assumed from the thread title that you understood some basics of microwave measurement.

1) RF Power is mesured with a different piece of test equipment than DC power.

2) you measure RF power with either a "spectrum analyzer" or an "RF Power Meter". These instruments almost always have a coaxial connector as their input port. You must connect the microwave/RF signal to that input terminal using a 50 ohm coaxial cable with the appropriate connector on each end.

3) If you apply a DC voltage to your RF Measurement Instrument, you will probably blow it up at great expense. So make absolutely sure there is no DC voltage present at your RF output port before hooking anything up to it!

4) To measure the DC volts and DC current coming from your power supply to your RF amplifier, you would use a digital voltmeter that had DC volts and DC amps capability. You can just run a wire from power supply, to/thru voltmeter, and on to the RF amp for that measurement.

5) If you try to measure your RF power using your DC voltmeter, it will probably show nothing at all. At best, the reading will be very far off in accuracy.
 

biff44 said:
We, erroneously, assumed from the thread title that you understood some basics of microwave measurement.

4) To measure the DC volts and DC current coming from your power supply to your RF amplifier, you would use a digital voltmeter that had DC volts and DC amps capability. You can just run a wire from power supply, to/thru voltmeter, and on to the RF amp for that measurement.
Click to expand...

Since most power supplies will display current and voltage used when connected to the power amplifier, do you think I can just rely on the voltage and current reading on the power supply?

If I use the digital voltmeter to measure the power and connect it to the power amplifier just like the way what you said, I am wondering how accurate the measurement would be, since the digital voltmeter is connected in series between the power supply and the power amplifier.

Thank you.
 

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