Phase-locked Loop and Frequency
1
A Voltage-Controlled Oscillator (VCO) is a circuit that provides a varying output signal (typically of square-wave or triangular-wave form) whose frequency can be adjusted over a range controlled by a dc voltage. An example of a VCO is the 566 IC unit, which contains circuitry to generate both squarewave and triangular-wave signals whose frequency is set by an external resistor and capacitor and then varied by an applied dc voltage. Figure (1) shows that the 566 contains current sources to charge and discharge an external capacitor C1 at a rate set by external resistor R1 and the control dc input voltage. A Schmitttrigger circuit is used to switch the current sources between charging and discharging the capacitor, and the triangular voltage developed across the capacitor and square wave from the Schmitt trigger are provided as outputs through buffer amplifiers.
Fig.(1) A block diagram of 566 IC.
2
A free-running or center-operating frequency, calculated from Eq.(1).
fo ,
can be
V VC 2 fo ) ……………..(1) ( R1 C1 V
With the following practical circuit value restrictions: 1. R1 should be within the range ( 2 K R1 20 K ). 2. VC should be within range ( 3 V VC V ).
4
3. f o should be below 1MHz. 4. V should range between 10 V and 24 V. Example 1:For the circuit shown in Fig.(2), R1 = 10 KΩ and Find: 1. The control voltage VC . 2. The free running-frequency f o .
C1 =820
pF,
Fig.(2) Connection of 566 VCO unit.
3
Solution:
1. VC
R3 10 K V (12V ) 10.4V 1.5 K 10 K R2 R3
2. Using equation (1) yields:
fo
2 12 10.4 ( ) 32.5KHz (10 103 )(820 10 12 ) 12
Exercise (1): The circuit of Fig.(3) shows 566 as a VCO unit. Potentiometer R3 allows varying VC from about 9 V to near 12 V. Find the control voltage VC and free-running frequency f o for the following condition:
1. With the potentiometer wiper set at the top of R3 . 2. With the potentiometer wiper set at the bottom of
A Voltage-Controlled Oscillator (VCO) is a circuit that provides a varying output signal (typically of square-wave or triangular-wave form) whose frequency can be adjusted over a range controlled by a dc voltage. An example of a VCO is the 566 IC unit, which contains circuitry to generate both squarewave and triangular-wave signals whose frequency is set by an external resistor and capacitor and then varied by an applied dc voltage. Figure (1) shows that the 566 contains current sources to charge and discharge an external capacitor C1 at a rate set by external resistor R1 and the control dc input voltage. A Schmitttrigger circuit is used to switch the current sources between charging and discharging the capacitor, and the triangular voltage developed across the capacitor and square wave from the Schmitt trigger are provided as outputs through buffer amplifiers.
Fig.(1) A block diagram of 566 IC.
2
A free-running or center-operating frequency, calculated from Eq.(1).
fo ,
can be
V VC 2 fo ) ……………..(1) ( R1 C1 V
With the following practical circuit value restrictions: 1. R1 should be within the range ( 2 K R1 20 K ). 2. VC should be within range ( 3 V VC V ).
4
3. f o should be below 1MHz. 4. V should range between 10 V and 24 V. Example 1:For the circuit shown in Fig.(2), R1 = 10 KΩ and Find: 1. The control voltage VC . 2. The free running-frequency f o .
C1 =820
pF,
Fig.(2) Connection of 566 VCO unit.
3
Solution:
1. VC
R3 10 K V (12V ) 10.4V 1.5 K 10 K R2 R3
2. Using equation (1) yields:
fo
2 12 10.4 ( ) 32.5KHz (10 103 )(820 10 12 ) 12
Exercise (1): The circuit of Fig.(3) shows 566 as a VCO unit. Potentiometer R3 allows varying VC from about 9 V to near 12 V. Find the control voltage VC and free-running frequency f o for the following condition:
1. With the potentiometer wiper set at the top of R3 . 2. With the potentiometer wiper set at the bottom of