I compared results from three different readout techniques, it seems that my Schmidtt algorithm does not work at frequency above 10Hz for our required sensitivity, but IQ readout is very promising.

First, to check if I can produce the same results from the same data analyzed by two different technique  (Schmidtt, and IQ). I used 2 data set (chI and chQ) for IQ read out technique, then for Schmidtt technique, I used only chI data. Then I compared the results with what I got from PLL (I used the old data because I have not measured the new one yet, but it should give a rough idea).

fig1: comparison between IQ readout and Schmidtt technique. Black line is the old measurement for Marconi noise, 10khz input range.

==comments==

The signal was demodulated down to ~1kHz and obtained by 32kHz DAQ. The data was taken with 150s time stretch. It turned out that the Schmidtt method is not sensitive enough.  IQ readout seems to be senstive down to 2 x10^-2 Hz/rtHz. The mismatch between the IQ and PLL was probably the setup between the two are different. (I have not measured the noise level of the current setup with PLL yet).

==next==

So I moved on to use IQ readout with other data. I chose marconi input frequency modulation range to be 1kHz and 100Hz. IQ method can measure down to 3x10^-3 Hz/rtHz. (I'll still have to verify this with PLL, but from a quick look it seems that the results are very reliable (I don't use any delay line in IQ read out at all and I can get sensitivity better than 10^-2 Hz/rtHz). It seems to be very promising for our beat readout.

I will try to compare this will results from PLL, if they agree. I'll move on to use this method to measure the noise level of marconi with frequency modulation function off (we can not do that by PLL technique). The result will give us an upper limit of the Marconi noise plus the readout technique noise which can be used in the noise budget.

fig2: Marconi noise at different settings, measured by IQ methods. The demodulated frequency is 1kHz. This plot is intended to show the sensitivity of IQ method which can be used to measure coating noise upto 1kHz (if the noise from an oscillator used for demodulation is not too noisy).  I have not plot the results from PLL on the same plot yet, but I have attached Marconi noise measured by PLL in fig3 below (data from 2010).

fig3: Marconi noise at different setting (Carrier @160MHz). The noise level for 1kHz and 100 Hz input range are almost the same and agree with what I got from IQ readout.

==note==

•  I will check how sensitive IQ read out be (by compared it with PLL). However, I also notice one possible problem with this technique. The peaks around 1kHz, from demodulated signal, and its harmonics can be seen clearly in the plot . (there is also this mysterious peak at90 Hz as well, I'm not sure yet where it comes from). It means that if our beat frequency drifts around, the psd of the beat will have weird bumps all over those frequencies.
• Think about digital delay line, I have not succeeded yet. I'm still confused with the calibration of this technique, but if IQ readout is working ok, I can use this method for beat measurement.