How to read voltage from spectrum analyzer

[damian_s]

how to read a spectrum analyzer

Dear all,

I'm new to spectrum analyzer and having difficulty to calculate the measured signal voltage received by spectrum analyzer. Here's the case :

- I tap a signal to an oscilloscope and it read 2 Vpp
- I tap the signal to a spectrum analyzer and it read -23.37 dBm in a 50 ohm system.

My calculation for -23.37 dBm is about 0.42 Vpp.

Is there something wrong I did in my calculation or there is something else I didn't consider in my measurement using spectrum analyzer?

FYI, I forget to mention that there is 20 dB Attenuation measurement result in spectrum analyzer'.

 

[FvM]

how to read voltage

My calculation for -23.37 dBm is about 0.42 Vpp.

This would be the case (considering the attenuator) for a CW sine signal.

 

[backdoor_]

how to read spectral scope

Make sure the scope input Z is set to 50 not 1M.

 

[dipnirvana]

how to read spectrum

let's consider your test circuit as voltage source . Now tell me what is source impedance of your circuit. Is it comparable to 50 ohm spectrum analyzer probe?? if yes, it will load the circuit u r trying to measure.

But as u use normal probe with oscilloscope it will not load the circuit as normally it's impedance is in the order of Mohm.

So, consider these impedances and you won't find the inconsistence in your measurement.

 

[E-design]

how to read spectrum analyzer

2Vpp on the scope with 1M input impedance will read -16dBm on a 50 Ohm SA with a 20dB pad in series. You may be double terminating the signal somehow giving approx -20dBm. Most SA can have up to a few dB gain error if not properly or recently calibrated. That may give you near -23dBm you read.