Re: phase measurement

[Daniel Estévez]

On 27/12/2022 17:49, Karen young wrote:
> Hi,
> Attached is the model I built for phase measurement.
> Since the samp_rate (5M) is 8 times of signal source freq (625k),  the 
> phase should be expected as shown in the table, while the measured phase 
> is only correct when there is half cycle or full cycle delay.
> What can I do to make it correct?
> Delay step      Expected phase delay    Measured phase delay
> 0       0       0
> 1       -0.79   -1.5
> 2       -1.57   -4.7
> 3       -2.36   -1.5
> 4       -3.14   -3.14
> 5       -3.93   -1.57
> 6       -4.71   -1.57
> 7       -5.5    -1.57
> 8       0       0

Hi Karen,

I think that the problem might be with your Skip Head block. I see that, 
in combination with the Keep 1 in N block, you're using it to select the 
correct FFT bin in the vectors output by the FFT blocks. However, it 
seems to me that you're not selecting the correct bin.

Since you're using "Shift: Yes" in your FFTs, the DC bin would be at 
index 512. The bin corresponding to the positive frequency of the cosine 
(you're using a real cosine, so it has a positive and a negative 
frequency component, if that makes sense), would be at index 512 + 
1024/8 = 640. Therefore, I think it's necessary to use 639 as the number 
of items to skip.

If you used "Shift: No" in the FFTs, the the positive frequency of the 
cosine would be at index 1024/8 = 128, so the number of items to skip 
should be 127 rather than 129.

To make sure that you're selecting the correct FFT bin, I suggest you 
look at its magnitude and compare it with a plot of the magnitudes of 
all the FFT bins. This will show if you're in fact selecting the correct 
bin where the maximum of the FFT is, or something else.

Best,
Daniel.

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