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.

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Shamtam
    Oct 26, 2018 at 21:31
  • \$\begingroup\$ 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. \$\endgroup\$
    – Shamtam
    Oct 26, 2018 at 21:32
  • \$\begingroup\$ @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? \$\endgroup\$
    – lucenzo97
    Oct 26, 2018 at 23:24
  • \$\begingroup\$ Measure the channel power of your signal, or use a larger RBW \$\endgroup\$
    – sstobbe
    Oct 27, 2018 at 0:38
  • \$\begingroup\$ How does the generator's indicated Vout change as you switch in the 50 ohms? \$\endgroup\$ Oct 27, 2018 at 3:08

1 Answer 1


When your signal generator has a output impedance of 50 ohms, and your analyzer has an input impedance of 50 ohms, then you have a resulting voltage divider with ratio 1:2.

So the measured voltage amplitude on the crocodile connector will be 1/2 of the signal generator output voltage.


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