Hi Daniel, Thank you so much for all the info, very useful. I tried the Keep M in N block, straightforward and works great! Now I also prefer using this block.
Thanks again Karen On Wed, Dec 28, 2022 at 2:39 PM Daniel Estévez <dan...@destevez.net> wrote: > Hi Karen, > > Good you got this working. > > I now see that something I said is wrong. I said: to get FFT bin 128, we > should use Skip Head 127. This is not correct. We need to skip FFT bins > 0 through 127 (both included), which is a total of 128 samples, so we > should use Skip Head 128. > > There is another thing I was misunderstanding. The Keep 1 in N block > keeps the last element out of every N, rather than the first element of > every N, which is what I expected. This explains why you need to use > Skip Head 129 rather than Skip Head 129 as the above paragraph would > seem to indicate. > > Since I find this a bit confusing, I prefer to use the Keep M in N > block, which keeps the first M elements out of every N, starting with an > initial offset. I find it more straightforward to think in terms of > keeping the first element than in terms of keeping the last element. > Here, using M = 1, N = 1024, and an initial offset of 128 will get you > the FFT bin you want. > > Best, > Daniel. > > On 28/12/2022 16:31, Karen young wrote: > > Hi Daniel, > > > > Thanks a lot. This is so helpful. I tried both methods you suggested. > > The phase is correct now. > > > > For "Shift: No", compared to the Skip Head value 127, value 129 has the > > max magnitude. I guess it is a zero-based system. Please correct me if > > wrong. > > > > Thanks and best regards > > Karen > > > > > > > > On Tue, Dec 27, 2022 at 12:42 PM Daniel Estévez <dan...@destevez.net > > <mailto:dan...@destevez.net>> wrote: > > > > 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. > > > > > >
