Motivation:
We want to know the time delay of CDS in the IOP scheme.
Setup:
What I did:
1. Plugged out SCSI cable from ADC card 2 and DAC card 0 on C1SUS machine.
ADC card 2 is ADC 0
DAC card 0 is DAC 0
2. Measured tranfer function between ADC and DAC by SR785 and compared with the downsampling filter in IOP with 65534Hz(=4x16384Hz) sampling frequency.
As ADC_0_0 corresponds to PRM ULSEN input and DAC_0_0 corrsponds to ULCOIL output, we turned all the filters off and set gains to 0 or 1 so that TF between ULSEN to ULCOIL will be ideally 1. (see this wiki page for channel assigns)
The filter coefficients for the down sampling filter was found in;
/cvs/cds/rtcds/caltech/c1/core/advLigoRTS/src/fe/controller.c
It was named feCoeff4x.
static double feCoeff4x[9] =
{0.014805052402446,
-1.71662585474518, 0.78495484219691, -1.41346289716898, 0.99893884152400,
-1.68385964238855, 0.93734519457266, 0.00000127375260, 0.99819981588176};
3. Calculated the time delay dt using the following formula;
dt = [pm - pc]/f/360deg (pm: measured phase, pc: calculated phase from feCoeff4x, f: frequency)
4. Measured TF between the SCSI cables to estimate the effect of the cables and others.
Disconnected SCSI cables from ADC and DAC, and connected A aad B(see setup).
I measured both when input coupling of SR785 is DC and AC and see what happens.
Result:
[time delay of the CDS] (left, middle)
The time delay gets larger with frequency. The time delay seems to be -175 usec at DC.
However, the gain seems a little different from my expectation(feCoeff4x). So, there are maybe other filters I don't know.
I neglected TF of upsampling this time.
[cable and other effect] (right)
The effect to the time delay measurement was tiny by a factor of 10^4 to 10^3 (few nsec).
But the total cable length was about 5 m and assuming signal speed is 0.6c, delay will be about 30 nsec.
I don't know what's happening.
Plan:
- make a model that does not go through IOP and see the delay caused by IOP
By the way:
fb daqd is still running for hours!
Every FEs are running(c1sus,rms,mcs). |