Low frequency noise measurement

Hi.

I want to measure output noise at 7 kHz for my circuit using USB-SA44B. The circuit’s output noise density (theoritically calculated) is 125 nV/Hz^-1/2. Measurment RBW will be 1 Hz. For DC block, I am using 3.3 uF capacitor within PCB. But I cant get output! Even though noise floor for this range is -130 dbm (=71 nV).
Can you please provide me the solution?

Thanks in advance.

Jubayer,

Setup for this type of measurement would be something like:
RBW 100 Hz, VBW 10 Hz, detector settings power, average. Center frequency 7 KHz, span 10 kHz. Reference level -50 dBm.
A 3.3 uF DC block should be reasonably flat down to 3 kHz or so.
Let me know if this works for you.

The other thing to note is your circuit should have a 50 ohm output impedance. If it does not, the SA44B will load it down and the noise reading will be much lower.

Dear Mr. Crooks,

I have attached the DUT. I have used 4.7 uF nonpolarized Al electrolytic capacitor for DC block at the output. Then using coaxial cable directly from PCB output to the analyzer input.
My quextion is still do I need to connect 50 resistor after 4.7 uF?
Thanks in advance
Jubayer

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I would add two resistors:
1) A 49.9 ohm series resistor for a 50 ohm output impedance
2) A 10k shunt resistor to ground after the DC blocking cap to bleed off residual DC.

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