Post by njattack on Jan 25, 2006 10:17:40 GMT -5
The PLL tutorial by C. Langton cleared up many points on the PLL operation. The question I have is:
The basic equation for the phase detector where the phase difference equals the sine of the input signal minus the PLL signal results in a numerical output. What are the UNITS for this numerical output? I assume Volts, but from the fundamental definition of frequency which is the change of phase with respect to time, it would seem to me that the units would be Volts-Hertz.
Further, this numerical output, i.e., Phase Error, is being integrated so it would have to have the units of Volts-Hertz.
However, the VCO gain multiplier has units of Hertz/Volts, so that when multiplied the final units would be Hertz^2!
Can anyone clear that up?
2. The second question is about the loop filter itself. It was stated that the entire PLL loop is a filter. What filtered out the double frequency term that one gets after the multiplication takes place in the phase detector? That is, if the PLL is modeled as a second order IIR filter I didn't see any equation implementing the standard difference equation that one usually uses for a 2nd order IIR. Perhaps it is embedded somehow. If so, it would be helpful to show that so that one can identify the coefficients b0,b1,b2 and a0,a1, a2 with the PLL. I am unable to tell what is what.
Thank you.
The basic equation for the phase detector where the phase difference equals the sine of the input signal minus the PLL signal results in a numerical output. What are the UNITS for this numerical output? I assume Volts, but from the fundamental definition of frequency which is the change of phase with respect to time, it would seem to me that the units would be Volts-Hertz.
Further, this numerical output, i.e., Phase Error, is being integrated so it would have to have the units of Volts-Hertz.
However, the VCO gain multiplier has units of Hertz/Volts, so that when multiplied the final units would be Hertz^2!
Can anyone clear that up?
2. The second question is about the loop filter itself. It was stated that the entire PLL loop is a filter. What filtered out the double frequency term that one gets after the multiplication takes place in the phase detector? That is, if the PLL is modeled as a second order IIR filter I didn't see any equation implementing the standard difference equation that one usually uses for a 2nd order IIR. Perhaps it is embedded somehow. If so, it would be helpful to show that so that one can identify the coefficients b0,b1,b2 and a0,a1, a2 with the PLL. I am unable to tell what is what.
Thank you.