0
\$\begingroup\$

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.

enter image description hereenter image description hereenter image description here

\$\endgroup\$
  • \$\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 '18 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 '18 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\$ – Keno Oct 26 '18 at 23:24
  • \$\begingroup\$ Measure the channel power of your signal, or use a larger RBW \$\endgroup\$ – sstobbe Oct 27 '18 at 0:38
  • \$\begingroup\$ How does the generator's indicated Vout change as you switch in the 50 ohms? \$\endgroup\$ – analogsystemsrf Oct 27 '18 at 3:08
1
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.