Modulation Toolkit PLL filter parameter

[KlausS]

Hello

I'm trying to use the PLL vi from the  Modulation Toolkit to lock to an external frequency acquired by a a 24bit DAQ card. The external freq. is in the range 30-800Hz (optical chopper) and sampling rate is 20kS at the moment. The ext signal is a square wave. Finally I need to generate a sine wave (only in software) locked to the ext freq.

With the default loop filter parameters I can't get the thing to work properly, means no stable freqency and the BW of the PLL seems to be too high (sin(phase) tries to follow the square input signal if the vco gain is above 0.001). I have also looked at the example vi 'MT Passband PLL Measurements' . Problem is that I dont have the faintest idea on how to modify the IIR loop filterparameters. I checked the web for some details on pll's but until now I couldnt figure things out. Looks like I need an in depth course on control systems....

 Can someone give me a hint on how to proceed?  Do I really need a full DSP course or can I get away with some experimenting?

 thanks

klaus

[BerndF]

Hi KlausS,

please have a look at this thread, there is some good suggestions aswell as a VI that will help you to caculate the coefficients for a smoothing filter.

Regards,
Bernd 

[KlausS · Accepted solution]

Thank you Bernd,

I tried some filters and now finally ended up with the default coefficients! It turned out they work quite well for my purpose. However, the default filter params for some reason require a signal that crosses 0 ( a 0-5V TTL wont work!). I now use the 'Scale 1D' vi to before I send the signal to the pll processor. The nonlinear phase output at higher gains can be dealt with by linearizing the phase in a specfic fitting routine. One needs sufficiant oversampling to do that.  Now I have at least 80dB supression of the harmonics in my pll output (independent on vco gain). 

To really improve things, a PLL filter design vi would be needed that implements all the theory.

klaus

p.s. I found the tutorials on this site very helpful:

http://www.complextoreal.com/tutorial.htm

[BerndF]

Hi KausS,

thanks for posting your feedback and the link to the tutorial, seems to be a pretty good website.

Regards,
Bernd