Inaccuracy problem in MT Phase Locked Loop VI

Question

Hello,
I'm using MT Phase Locked Loop VI(PLL VI hereafter) in LabVIEW with NI USRP2920(Kind of IQ converter) to measure carrier frequency and phase of input signal.
The PLL VI works fine when relatively low frequency signals like 500Hz are input, but when it comes to high frequency like 30kHz, the accuracy of frequency and phase estimation in PLL VI is significantly degraded. In my implementation, MT Baseband PLL is selected as an instance.
Can anyone describe why this things happen?
If it is difficult to measure high frequency with this PLL VI, does anyone know appropriate way to measure input IQ signal's carrier frequency and phase?
Thank you for reading through.
P.S.
In my implementation, FDMA is adopted. So, above "carrier" means kind of subcarrier which is downconverted to the baseband as IQ.

Answer 1

Aha! I found a solution! For future people looking for the answer - I highly recommend THIS link as a set of files that will get you 90% of the way there
https://decibel.ni.com/content/docs/DOC-1121
For people still reading - the difference is see in my attached JPG. The top example (mine) uses a multiply followed by a low pass filter. The problem is that that filter is non-monotonic with changing phase (i.e. that filter's DC output is maximum when phase is matched but decreases with both leading and lagging phase). The NI solution (bottom) is monotonic with phase (that is, phase difference between the LO and the input).
I can tell the NI solution is doing some complex (as in non-real numbers) math to extract the phase information. It looks a lot like in-phase & quadtrature extraction and maybe thats important. The key question I have going forward is - what does a blessed Hilbert Transform help me with ? What is its function? Reading the help - I do get a little buried in the Math of it.
-Thanks
Attachments:
ExampleCode.jpg ‏60 KB
PLL_Test2.vi ‏88 KB