# R&S Spectrum Analyzer Voltage Measurement Error

I just got FPC1000 spectrum analyzer that goes from 5 kHz to 1 GHz. I didn't had any spectrum analyzer experiences before this one so everything is very new to me.

Among few things that I don't understand about it is also a significant error seen at SA when measuring a signal directly from function generator.

I applied 1,05Vpp/10kHz sine wave directly to input of SA (using BNC to crocodile connector cable since I don't have the real probe yet). 1,05Vpp approximately equals 4 dBm (power unit). But SA shows me exactly -2 dBm, which equals approximately 500 mVpp (which is approximately a factor of 2 smaller than the real value)!

Even if I set output function generator impedance to 50 Ohms, SA shows me 1.8 dBm, which is 760 mVpp.

Are the probes making an error? Or am I missing something here? I cannot find the solution for this case.

• It looks like you should be using a 7mm coax cable to connect to the SA... using alligator clips to hook onto a precision connector would make a microwave test tech cringe. It's possible the uncontrolled impedance and cables you're losing are introducing loss at 10kHz, but without that being characterized it's hard to say. – Shamtam Oct 26 '18 at 21:31
• That said, when you do get the correct cables, you will need to set your function generator to a $50\Omega$ output impedance to get the correct reading, as otherwise it's likely looking to output to a high-Z input, and the SA will load the signal generator more than it expects. – Shamtam Oct 26 '18 at 21:32
• @Shamtam How about if I would measure a few kOhm or few 10kOhm load in circuit with SA? Would there be a mismatch too? Or does always measured load has to be 50 Ohms? – Keno Oct 26 '18 at 23:24
• Measure the channel power of your signal, or use a larger RBW – sstobbe Oct 27 '18 at 0:38
• How does the generator's indicated Vout change as you switch in the 50 ohms? – analogsystemsrf Oct 27 '18 at 3:08