We suspected if the wideband RF detector New Focus 1811-FC could be the source of noise floor through its dark noise. So I took dark noise spectrum measurements today of this photodetector.

Method:

• I blocked the photodetector with a beam dump.
• First I took the spectrum directly from the RF port using HP 4395A in spectrum analyzer mode, from 0 to 130 MHz with 3kHz ResBW and 50 averages.
• I converted dBm to V_rms using 10**((P_dbm-13)/20) and then divided by np.sqrt(3e3) to get spectrum in Vrms/rtHz.
• Next, I measured the dark noise spectrum by mixing it down with a local oscillator using the same ZX05-1MHW mixer we use in the PLL.
• First I used the Wenzel crystal oscillator at 24.483 MHz as LO.
• Then I used Marconi at the same frequency for comparison.
• I also took mixed down measurements for 27.34 MHz(Resonant RFPD peak point), 50MHz, 75 MHz, 100MHz and 125MHz.
• All these later measurements were taken by SR785 from 0 to 102.4 kHz (in 3 steps) with rms averaging of 100 factor and BMH windowing.

Conclusions:

• Contrary to our expectation, mixed down dark noise is more than the direct dark noise from RF port. The mixer should have same conversion loss for dark noise and should also be suppressing one quadrature, so the output from the mixer should be less noisy.
• I think I have missed something in these measurements. It looks too bad to be true. In past, we have been running beat note measurements with S*A (Actuator Slop x Amplifier gain) of 200-500 kHz. With the measured noise after mixing down, it would correspond to 0.6 - 1.5 Hz/rtHz.
• We do have a noise floor around these values but we also have measurements going below 1 Hz/rtHz, so something seems fishy.
• At the time of writing this post, we quickly did a dark noise measurement with the resonant RFPD at 27.34 MHz with Marconi and its dark noise after mixing down is around 12 nV/Hz. Much much better!

Edit Tue Jan 22 20:06:00 2019

Correction:

• When we measured resonant RFPD, measurements made more sense. The direct noise from RF port was higher and the noise after mixing down was lower by conversion loss and factor of sqrt(2) due to the suppression of one quadrature.
• So we decided to measure the wideband RFPD again. To our surprise, this time the measurements made sense. PFA the new plots with new measurements.
• The mystery is still on for what caused the measurements to be bad earlier. We can not find anything that we changed except for switching RFPDs back and forth.
• So from new measurements, it seems like mixed down dark noise is at the level of 100 nV/rtHz. It is higher at other frequencies mostly because of Marconi's inherent noise. We should trust the result with crystal oscillator (black curve) more.
• This would correspond to 50 mHz/rtHz of flat noise contribution from the dark noise. This is way below the measured noise floor in our beat note spectrums. So phot detector dark noise is off the hook (for now).
• We need to continue our search for the culprit.