21-05-2013, 04:10 PM
PHASE LOCKED LOOP
Aim:
To measure the phase detection and measurement using phase locked loop and to find out the lock-in-range.
Apparatus:
1. Experimental kit of Phase Locked Loop
2. 20 MHz Dual Trace Oscilloscope
3. Patch Chords & CRO Probes
Procedure:
1. Set the Oscilloscope for the following settings: Channel 1-1V/division, Time base: 0.5ms/division
2. Patch the circuit as shown in the wiring diagram and apply power to the trainer. Adjust the output of the oscillator (sine wave) to approximately. 1 KHz, and the peak
to peak voltage to 6 volts (i.e., 6 vertical divisions). Now connect the oscilloscope to pins 3 and 4 of the 4046 device. The output frequency of the phase locked loop should be the same as the input.
3. Set the Oscilloscope time base to 2ms/division. Now, with a piece of wire, connect pin 9 of the 4046 integrated circuit to ground. Record the resultant output frequency of the phase locked loop.
a. FL-----------------------------------Hz
b. This output frequency is the lower range of the VCO, which is determined by the 0.1μF capacitor connected between pins 6 and 7, and the 100K ohms resistor connected between pin 12 and ground.
4. Set the Oscilloscope time base to 0.2ms/division. Now with the same wire, connect pin 9 to the +5 volts supply. Observe an output frequency that is higher than the one measured in Step 3. Record this frequency.
a. FH=-------------------------------------------Hz
b. This output frequency is upper range of VCO, which determined by the 0.1μF capacitor connected between pins 6 and 7, and the 560 ohms resistor connected between pin 11 and ground.
5. Now remove the connection between pin 9 and the +5 volt supply. Again measure an output frequency that is the same as the frequency of the function generator (approximately 1 KHz).
6. Now slowly increase the frequency of the function generator. Observe that the output frequency also increases. In fact, the output frequency follows the changes of the input frequency to confirm this.
7. While watching the output frequency of the phase locked loop, continue to slowly increase the input frequency and stop when the output frequency does not continue to increase. Measure the input frequency and record the result.
a. Fin(H)=-------------------------------------------Hz
It is find that this frequency is about the same as the frequency which is measured in step 4, the upper range of the VCO. The phase licked loop then follows input frequency changes for frequencies below this upper range.
8. Set the oscilloscope time base to 2ms/division. Now decrease the input frequency while observing the oscilloscope. At some point the output frequency will remain constant. Measure the input frequency and record the result.
Fin(L)=----------------------------------------------------------Hz
It should find that this frequency is about the same as the frequency which is measured in step 3, the lower range of the VCO. Consequently, the phase locked loop circuit follows changes in the input frequency for any frequency between the lower and upper range of the VCO. Therefore, the loop is locked. The range over which the phase locked loop follows changes in the input frequency is called the lock range.
9. To determine the lock-in-range, subtract the value determined in step 8 from the value in step 7 and record the result.
Lock-in-range=---------------------------------------------Hz
The lock-in-range can be changed by simply changing the value of the capacitor connected to pin 6 and C7. Decreasing the 0.1μF capacitor at pin 6 & 7 to 0.01μF increases the frequency.