Simulink Spectrum Analyzer channel power wrong (impedance?)

Hi,

I have the following part if a SIMULINK model:

I assume the signals are voltages and I measure the signal power assuming 50-ohm matched system. Then the signal power is given by

P[dBm] = 10*log10(std(signal)^2 / 50 * 1000)

This is what I do in the small white blocks in front of the value displays. It can be seen that "RX in [dBm]" is -30dBm as I designed and expect.

Now I connect the SIMULINK Spectrum Analyzer in parallel and measure channel power. This is what I get:

-15.76dBm! This is far off. Now subtracting 10*log10(50) from this would give roughly -33dBm, close to -30 dBm.

Can it be that the Spectrum Analyzer block assumes 1 Ohm impedance? Can I set the input impedance to 50 Ohm?

Even then, the result would still be off by >3dB. Any idea for the discrepancies?

Answer 1

[Pertti Tolonen]

Yes: In order to get correct 50ohm dBm numbers, you must change the "Reference load" -setting in "Trace options". (n the screen grab above, the trace options menu is minimized.)

Struggled with the very same problem myself...