# Why the amplitude of the 90 shifted phase signal when use the analytic signal block is not equal to the original signal?

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### Answers (1)

Jyotsna Talluri
on 16 Jun 2020

You can try variable phase shift block in simulink.

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Walter Roberson
on 20 Jun 2020

Suppose you leave the program running for two weeks. Then a frequency as low as one cycle per week could be detected, mathematically. That frequency has to have its phase shifted by 90 degrees, just like all the other frequencies do.Is it mathematically possible for something like a hilbert transform to properly predict when to make the appropriate output adjustments before the system has seen one cycle of the wave? Nyquist would say NO.

I would argue that it is therefore not possible to do what you want in a real-time system, unless

- You put a lower-frequency boundary for detection (upper frequency is according to sampling frequency); and
- You have a transport delay corresponding to one cycle of that low frequency.

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