## Measuring output power

*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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