# phase lock loop (PLL)

B

#### biras

Jan 1, 1970
0
hi
i choosed the VCO resistor & capacitor According to the formula (
f min= 1/(R2(C1+32pf)), f max =(1/(R1(C1+32pf)))+f min)
my Fmin is 10KHZ and Fmax is 20KHZ.

And i have not connected any loop filter.
i have checked the offset frequency.

But the output is very worst, I'm not getting the exact squrwave(VCO
OUT).
I have checked the connection, every thing is correct.

If I connect the capacitor between the VCOout-pin and ground, then the
output is somewhat ok.

Warm Regards

J

#### John Popelish

Jan 1, 1970
0
biras said:
hi
i choosed the VCO resistor & capacitor According to the formula (
f min= 1/(R2(C1+32pf)), f max =(1/(R1(C1+32pf)))+f min)
my Fmin is 10KHZ and Fmax is 20KHZ.

And i have not connected any loop filter.
i have checked the offset frequency.

But the output is very worst, I'm not getting the exact squrwave(VCO
OUT).
I have checked the connection, every thing is correct.

If I connect the capacitor between the VCOout-pin and ground, then the
output is somewhat ok.

What chip are you using?

B

#### biras

Jan 1, 1970
0
i am using 4046 pll chip

J

#### John Popelish

Jan 1, 1970
0
biras said:
hi
i choosed the VCO resistor & capacitor According to the formula (
f min= 1/(R2(C1+32pf)), f max =(1/(R1(C1+32pf)))+f min)
my Fmin is 10KHZ and Fmax is 20KHZ.

And i have not connected any loop filter.
i have checked the offset frequency.

But the output is very worst, I'm not getting the exact squrwave(VCO
OUT).
I have checked the connection, every thing is correct.

If I connect the capacitor between the VCOout-pin and ground, then the
output is somewhat ok.

Warm Regards
First point. This PLL is an analog circuit with some digital parts,
so it needs a very clean power supply. If other chips operate across
the same supply, you may have noise from them getting into the PLL.

Check that your VCO is programmed (by R1, R2 and C1) to operate at the
correct range of frequency. Connect the VCO control line to the
positive supply line and measure the frequency of the VCO output
frequency. This should be the expected maximum frequency. Then
connect the VCO control line to the negative supply line. The VCO
should produce the expected minimum frequency. The output is not
guaranteed to be precisely a 50% duty cycle square wave, but it should
be very close.

If these two measurements are correct, then the VCO part of the PLL is
set up correctly, and all that remains is to choose the PLL phase
detector and design the loop filter.

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