We did the following things:
- installed a 1/10 voltage divider such that the signal won't be saturated at the AA board (see here)
- put a Ithaco preamplifier 1201 as a whitening filter
- checked the entire beat detection system without using the real beat note
Here are some items to be done before the sun goes down tomorrow:
- calibration of ADC and the interfaces including the voltage divider and the whitening filter.
- fine matching of unwhitening filter at the digital side
- PLL response measurement ( freq to voltage response ) over the frequency range of interest
- plotting an well calibrated spectrum of the PLL output
The Ithaco 1201 was setup to have a zero at 0 Hz and two poles at 0.1 Hz and 10 Hz in order to emphasize the signal over the frequency range of interest.
Around 1Hz it is supposed to have a gain of 1000. These settings have done by tweaking the knobs on the front panel of the Ithaco 1201.
In addition to that, we made an unwhitening filter in digital filter banks. This filter was designed to cancel the analog whitening filter.
To check the entire beat detection system, we phase-locked the VCO to a Marconi running at 80 MHz, which is the center frequency of the VCO.
Then we imposed a frequency modulation on the Marconi to see if the signal is acquired to ADC successfully or not. It's quite healthy.
According to the spectra corrected by the unwhitening filter, we confirmed that the noise floor at 1Hz is order of 1Hz/sqrt Hz, which is already quite good.
Then we took several spectra while putting a modulation on the Marconi at a different frequency in each measurement.
The peak due to the artificial modulation essentially works as a calibration peak in the spectra.
So in this way we briefly checked the flatness of the response of the system in the frequency domain.
As a result we found that the response is not perfectly flat in the range of 0.05 - 30Hz, probably due to a mismatch of the combination of the whitening and unwhitening filters.
We will check it tomorrow.