Measuring the voltage noise and frequency response of the Analog Delay-line Frequency Discriminator (DFD)
The schematic and an actual photo of the setup is shown below. The setup was checked to be physically sturdy with no loose connections or moving parts.
The voltage noise at the output of the DFD was measured using an SR785 signal analyzer while simultaneously monitoring the signal on an oscilloscope.
The noise at the output of the DFD was measured for no RF input and at several RF input frequencies including the zero crossing frequency and the optimum operating frequency of the DFD (20MHz).
The plot below show the voltage noise for different RF inputs to the DFD. It can be seen that the noise level is slightly lower at the zero crossing frequency where the amplitude noise is eliminated by the DFD.
I also did measurements to obtain the frequency response of the setup as the cable length difference has changed from the prior setup. The cable length difference is 21cm and the obtained linear signal at the output of the DFD extends over ~ 380MHz which is good enough for our purposes in FOL. A cosine fit to the data was done as before. //edit- Manasa: The gain of SR560 was set to 20 to obtain the data shown below//
Fit Coefficients (with 95% confidence bounds):
a = -0.8763 (-1.076, -0.6763)
b = 3.771 (3.441, 4.102)
Data and matlab scripts are zipped and attached.