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OBJECTS
1. to observe a frequency modulated signal and to measure the voltage to frequency conversion factor
2. to measure the frequency components in a carrier which is frequency modulated by a sinusoid.
EQUIPMENT
dual beam 10MHz oscilloscope, e.g. TecQuipment E32g or similar. Audio frequency signal generator with 0-5 Vpp output, e.g TecQuipment E16 or similar.
METHOD 1
to observe afrequency modulated signal and to measure the voltage to frequency conversion factor.
1. there are two frequency modulators included on the panel but for this preliminary investigation only the top modulator will be used. The carrier signal is internally generated at a frequency of fc. The output of the modulator is a signal at a frequency deviation from fc which is proportional to the amplitude ofthe input modulating signal.
2. to demonstrate this, connect a wire between the D.C. volts terminal (blue) and the black input terminal of the upper frequency modulator. Connect channel 1 of the oscilloscope to the modulator output terminals. Use a screened lead with the screen connected to earth (figure 1).
3. switch on the units and allow them to warm up for about 5 minutes. Set the D.C. voltsto about midposition. Set the channel 1 sensitivity to 1v/cm and the timebase to 2us/cm. Adjust the trigger setting and the trace position control so as to display the trace. This is the carrier, measure the peak to peak amplitude and estimate the frequency (approx. 470kHz)
4. set the D.C. volts to maximum + and observe that the frequency of the carrier increases. Set the D.C. volts to maximum -and observe that the carrier frequency decreases. This is the principle of frequency modulation.
5. disconnect the wire between the D.C. Volts terminal and the modulator. Connect the audio frequency signal generator to the signal input terminals. Use a screened lead with the screen connected to earth(figure 2)
6. set the channel 2 sensitivity to 2v/cm and connect this to the same terminals asthe signal generator. Adjust the oscilloscope controls to display both traces.
7. set the signal generator to about 10kHz and 10Vpp amplitude. Set the oscilloscope time base to 10us/cm and, with the trigger still on channel, obtain a steady (but blurred) trace of the carrier. Now carefully adjust the signal generator frequency until the two traces are both steady. You may have to vary the signalgenerator over quite a wide range. You should be able to see that the frequency of the carrier is changed by the signal generator waveform.
8. it is now necessary to find how much frequency deviation there is at the output of the modulator per volt at the input.
9. disconnect the signal generator and reconnect a wire between the D.C volts terminal and the black input terminal of the uppermodulator. Keep the oscilloscope channel 2 connected to the modulator input. Connect a wire between the modulator output red terminal and the analyser yellow input terminal. Connect the oscilloscope channel 1 to the yellow buffer output terminal using a screened lead, with the screen connected to a green earth terminal as in figure 3.
10. use the oscilloscope channel 2 to set the D.C. volts control toexactly 0, and the analyser control to exactly fc. Adjust the " Trim fc " control on the upper modulator to obtain a maximum response on the oscilloscope. The modulator centre frequency is now set to fc. Vary the analyser control to familiarise yourself with the " quality " of the resonant response. This is often known as the Q factor.
11. using the oscilloscope, set the D.C. volts control to +1volt and re-tune the analyser control for maximum response. Note the frequency given by the analyser. Do this at various settings of D.C volts both positive and negative. Plot a graph of frequency deviation versus D.C. voltage and by fitting a straight line obtain the voltage to frequency conversion factor in kHz per volt.
CONCLUSION 1
the frequency modulator converts voltage at the input to...