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Measuring output power

This page was last updated : 26-Dec-22 10:49:14

This page is work in progress and will be updated frequently. 

When a OM wanted to buy a Crystal Measurement Adapter, I always ask which model of NanoVNA he has, or if he know the power output in dBm of his VNA. 

OM Derek has a NanoVNA from which I don't know the power output and he didn't have a dBm meter. But he told me that I did have a oscilloscope. And when you have that and a 50 ohm load, you good to go. 

Test setup

The idea behind it is, that you connect your RF source to a appropriate dummy load and measure the Peak-To-Peak or RMS voltage across it.  And with a osciloscope it's easy to measure the Peak-To-Peak voltage.

Make a test setup as shown below.

What are we looking for 

To convert voltage to power, we use the formula 

When you measure AC voltage (which RF) there is a catch. V should be Vrms (root-mean-square) and voltage peak-to-peak can be easily measured with a oscilloscope. I know some osciloscopes can measure voltage RMS but there is a catch to that but I'm coming back to that later. For a sinus wave to calculate Vrms from Vpp we use the formula

And with this information we are good to to.

Getting some measurement done

In the screen shot below, the RF source is sending out a clean sinus wave signal and in the above setup we measure 1.2V peak to peak (yeah I'm a QRP/QRPp guy).

When you have a modern digital scope like my Rigol, some can measure Vrms (as shown below), that is ok and you can use that value 

But be careful, for a good RMS measurement it's best to have a fixed number (1..2..3 etc) of periods on screen. 

If it's more of a square then you can use Vrms = Vpp / 2 and use that value.

When I used the digital options of my Rigol on the above signal,  Vpp is higher because of the overshoot.
So we use Vrms instead (the shape of the signal is a true square wave so the Vrms is low then expected).

Formulas by example

Wit the measured 1.2 volt peak-to-peak we work through a few formulas to get to the power output.

Step 1 : Convert Vpp to Vrms :

  

 

Step 2 : Calculate power :

When you want just to know the power, your done. But when you want to know dBm, there is one additional step to make.

 

Step 3 : Calculate power in dBm :

 That's all there is to it.

 

Cheat sheets and online calculators

Great cheat sheet including a table and all formulas needed (note : they use mW in there calculations) : 

https://www.markimicrowave.com/blog/wp-content/uploads/2016/11/power-to-voltage.pdf 

Online calculators : 

https://coretechgroup.com/dbm_calculator/ (note : vpp is for sinus wave signals)

The formulas are created in Latex and I used https://latex.codecogs.com/eqneditor/editor.php as a editor. Click to download latex_formulas.

 

 

Measuring my H4

25 dec 2022 Note : the text isn't done yet, but will be completed in the next week.

 When I wrote the above, I thought about measuring my NanoVNA H4 again.

The output signal of my NanoVNA H4 at 8.6 MHz. 

Using cursors to measure the Vpp and frequency. For Vpp I used the most flat part of the square (ignoring he overshoot).

Using the auto Measurement possibility of the Rigol : 

• Freq 8.59MHz (a bit off, but a osciloscoop isn't a high resolution counter).
• Vpp 256mV ( Vpp is much higher because of the overshoot of the square signal).
• Vrms 84.2mV (which is quite good looking at our manual 169mVpp measurement.

 

The Rigol can use the cursors to show how the signal is measured in the auto measurement mode.

Freq 8.59MHz (a bit off, but a osciloscoop isn't a high resolution counter)

Vpp note that the Vpp is much higher because of the overshoot of the square signal.

 Vrms looks ok because Vrms of a square is Vpp / 2 and 169mV / 2 is close to 84.1mV. 

So based on the Vrms, my H4 has a output power of aprx -8.5 dBm. I thought it used to be much more (+6dBm) when I measured it in the past with my dBm meter.
But maybe I did something wrong or the dBm meter had a magic smoke moment. I'm not sure. 

 Todo : add output power calculations, more background info and theorie. 

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