NanoVNA V2 / Forum

Note: this page is a mirror of https://groups.io/g/NanoVNAV2.
Click here to join and see most recent posts.

Measuring power (how to interpret what I see)


ploegmma 2021/01/16 12:23

Hi. I would like to do power measurements with the NanoVNA but I'm not sure how to interpret the readings I see.

This is what I did:

Calibrate (full calibration with open, short, load and thru) for 15kHz - 15MHz
only trace CH1 LOGMAG active
Signal generator 7 MHz sine, connected to port2 (vary the amplitude setting on the signal generator from  10mV - 1V)

This is what I see:

Peak at 7 MHz, dBm varies but not with expected values.

This is what I expected:

I measured the signal of the generator with an oscilloscope and calculated the dBm values for the signal using the peak-to-peak values measured by the scope. I assume 50 ohms and used log(10)((Vpp squared * 1000) / 400) * 10. I double checked my results with on-line calculators as well.

The values I read from the NanoVNA are about 33 dBm off (lower).

Does that mean that the value it calibrated against (from port1) is 33 dBm? Did I make a mistake in my setup or calculation? Or is it not possible what I wanted to do here?

Dragan Milivojevic 2021/01/17 04:42

The values reported on port2 are relative and normalized to port1.
As the output on port1 varies with frequency ...

On Sun, 17 Jan 2021 at 04:38, <ploegmma@gmail.com> wrote:

John Gord 2021/01/16 21:09

The NanoVNA receives with a narrow bandwidth at a limited number of frequencies.  If one of those frequencies is not identical to your input signal, you will miss it or see it at a low amplitude.  Try measuring over a much smaller range, say 6.95MHz to 7.05MHz for your 7 MHz signal.
Something like the tinySA is much more suited for looking for signals over a wide frequency span.
--John Gord

On Sat, Jan 16, 2021 at 07:38 PM, <ploegmma@gmail.com> wrote:

David Eckhardt 2021/01/17 17:06

Unless you set your o'scope to 50-ohms or use a 50-ohm through terminator,
the input is high impedance, usually 10 Meg.

Dave - WØLEV




On Sun, Jan 17, 2021 at 3:38 AM <ploegmma@gmail.com> wrote:

> Hi. I would like to do power measurements with the NanoVNA but I'm not
> sure how to interpret the readings I see.
>
> This is what I did:
>
> Calibrate (full calibration with open, short, load and thru) for 15kHz -
> 15MHz
> only trace CH1 LOGMAG active
> Signal generator 7 MHz sine, connected to port2 (vary the amplitude
> setting on the signal generator from 10mV - 1V)
>
> This is what I see:
>
> Peak at 7 MHz, dBm varies but not with expected values.
>
> This is what I expected:
>
> I measured the signal of the generator with an oscilloscope and calculated
> the dBm values for the signal using the peak-to-peak values measured by the
> scope. I assume 50 ohms and used log(10)((Vpp squared * 1000) / 400) * 10.
> I double checked my results with on-line calculators as well.
>
> The values I read from the NanoVNA are about 33 dBm off (lower).
>
> Does that mean that the value it calibrated against (from port1) is 33
> dBm? Did I make a mistake in my setup or calculation? Or is it not possible
> what I wanted to do here?
>
>
>

--
*Dave - WØLEV*
*Just Let Darwin Work*

ploegmma 2021/01/17 09:57

Ah, yes, that makes sense. I will try that and see what results I get. Thnx

ploegmma 2021/01/17 10:16

That's something I also didn't think of. I feel pretty stupid by now. I will use a T-BNC with 50 ohm termination and 1X setting and see what difference that makes. Thnx.

To reply to this topic, join https://groups.io/g/NanoVNAV2

View this thread on groups.io