Tara and I measured the transfer function for the TCS ISS Box we got from Livingston.
**Schematic:**
We had to make a cable for the three pronged power input for the box before we could test anything. After making the cable, we found differences between the pdf schematic and the values of the components in the box. Specifically, the capacitors in op amp stages defining the transfer function, C38, C44, C59, and C63 are all a factor of ten higher than reported in the schematic. I updated the schematic with the correct values and attached it to this elog. The following picture shows the circuitry which defines the shape of the transfer function:
There are currently only two op amp stages being used, and then a flip sign stage at the end.
**Measurements and Calculations:**
We measured the transfer function for the two filter stages individually, as well as the overall transfer function for both stages. I also modeled the three circuits in LISO and compared the generated transfer function with what we measured. The following three plots show the results:
The results for the transfer function generated with LISO agree with the measured TF in all three cases. The overall transfer function for both stages does not include the flip sign stage, which when turned on just flips the phase.
**Modifications:**
Since the transfer function for both stages is not quite the shape we want for our ISS, we will need to make a few modifications:
1. We want a gain of ~60dB for a factor of 1000 of intensity noise suppression
2. We want the transfer function to block DC components, and thus fall off at low frequencies
In order to get a gain of ~60dB at the peak, we can use variable gain stage preceding the filters. The data sheet for the vga is: AD8336 VGA Data Sheet. We tested this and confirmed that a gain of 60dB is easily achievable.
A high pass filter will have to be added in one of the unused op amp stages, in order to get a transfer function that blocks the DC component and falls off at low frequencies. I should be able to design a high pass filter and model the whole TF in LISO to optimize the shape, and then filling one or two of the unused stages with the design should be pretty easy. |