NON-LINEAR CIRCUIT
Goal:
Construct a VCO and create a frequency-modulated waveform of your voice.
Set Up:
VOLTAGE CONTROLLED OSCILLATOR
+10 V
1k
0.5 R
0.5 R
output
2R
2R
C
3.3k
i/p
R
R
PROCEDURE:
1) Connect the elements as in figure.
2) Increase the input from 1 volt to 7 volts, by varying the 3.3k pot.
3) Observe the waveform in CRO.
4) Notice what happens if any of the diodes is removed?
5) Draw a graph of Vin vs. frequency.
Vin
Frequency
Justify why is it nonlinear?
[Notice that the charging and the discharging current equations, of a capacitor
through a resistor, control the period of the waveform.]
Now connect a sine wave [5Vpp – 5Hz – zero DC offset] to the input and notice the
VCO output waveform.
MIKE PRE-AMPLIFIER
Set Up:
+10 V
10k
1Meg
Condenser mike
0.01F
10k
output
2k
Procedure:
1) Fix the DC bias point of this pre-amplifier.
2) Now connect the output of this amplifier to the input of the VCO, as in the
next figure.
3) The output DC bias voltage of this amplifier decides the free running
frequency of the VCO i.e. the frequency of the VCO, when there is no voice
input to the mike.
4) Note this frequency. You can try to adjust R or C and fix this frequency.
Say, lets fix this frequency around 100 KHz.
5) Now talk in front of the mike and notice the frequency modulated waveform
at the output of the CRO.
6) Notice the frequency shift about the free running frequency.
FREQUENCY MODULATED WAVEFORM OF YOUR VOICE
+10 V
0.5 R
0.5 R
output
2R
2R
+10 V
10k
0.01F
10k
C
1Meg
i/p
Mike
2k
R
Report the free running frequency of your VCO, as a printout from the digital CRO.
R