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?

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:

> 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?
>
>
>

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:

>
> 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?

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*

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

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.