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PLL design problem

C

cici

Jan 1, 1970
0
The output of the lowpass filter is a dc voltage. But the value is not
in the range of my VCO control voltage(0 to 1v). How ot pull down the
dc votage into 0 to 1v? I tried diode, not work.
Another thing is that the feedback affect the output of the Lowpass
filter. it makes the output of LPF a different value compared to using
2 independent input
at the PD. what is the reason? Thanks.
 
J

John Popelish

Jan 1, 1970
0
cici said:
The output of the lowpass filter is a dc voltage. But the value is not
in the range of my VCO control voltage(0 to 1v). How ot pull down the
dc votage into 0 to 1v? I tried diode, not work.
Another thing is that the feedback affect the output of the Lowpass
filter. it makes the output of LPF a different value compared to using
2 independent input
at the PD. what is the reason? Thanks.

If your low pass filter is just an RC network, you can replace ir with
an active (opamp based) low pass filter, yo get a low output impedance
that will make the output more immune to loading. Then you can use a
voltage divider to reduce the output swing to the lower range needed
by your voltage controlled oscillator. The phase detector output is
expected to be affected when you connect the low pass filter to the
VCO, because the comparison of the VCO to the input signal is what
determines the detector output average voltage which is what the low
pass filter is smoothing.
 
M

mdp

Jan 1, 1970
0
The phase detector produces a signal that is proportional to the phase
difference of its two inputs. If there is a frequency difference between
the inputs, the output of the phase detector will be periodic but has a DC
component that eventually "pulls in" the VCO to the correct frequency
(frequency acquisition), from there it pulls in the phase (phase
acquisition). The reason you have two different values out of the LPF could
be due to the VCO having both a different frequency and phase than that of
your second independent input.

The amount of steady state DC voltage out of the phase detector will be
dependent on your loop design. For example, a type 2 loop with an
integral-proportional loop filter has an average value out of zero out of
the phase detector when the loop is locked and the average value out of the
loop filter will be dependent on the free running frequency of your VCO - if
it matches the input it will be zero, if it is different than your input it
will be non-zero (the integral part of the filter - typically a capacitor -
holds the necessary DC value to keep the VCO locked in frequency).
 
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