Experiment One: Generating Frequency Modulation (FM) Using Voltage Controlled Oscillator (VCO) Modified from original TIMS Manual experiment by Mr. Faisel Tubbal. Objectives 1) Learn about VCO and how it can be calibrated 2) Learn how to generate FM signal using VCO 3) Display and measures of FM spectrum Equipment Required 1) For VCO calibration: Variable DC, VCO and Frequency Counter 2) For FM generating: Audio Oscillator, Buffer Amplifier and VCO Essential Reading 1) Watch the video, read the introduction, read the tutorial questions, and read any necessary data sheets. Introduction The Voltage Control Oscillator (VCO) is available as a low-cost integrated circuit (IC), and its performance is remarkable, The VCO IC is generally based on a bi-stable 'flip-flop', or 'multivibrator' type of circuit. Thus its output waveform is a rectangular wave. However, ICs are available with this converted to a sinusoid. The mean frequency of these oscillators is determined by an RC circuit. The controllable part of the VCO is its frequency, which may be varied about a mean by an external control voltage. This experiment is an introduction to the FM signal. You will measure the properties of a VCO then use the VCO to generate a wideband FM signal. This could be demodulated with a simple zerocrossing counter-type demodulator but won t be for this laboratory. Experiment Section 1: VCO Calibration (measuring the VCO sensitivity) For this experiment you will need to measure the sensitivity of the frequency of the VCO to an external control voltage, so that the frequency deviation can later be set as desired. The mean frequency of the VCO is set with the front panel control labelled f o. The mean frequency can as well be varied by a DC control voltage connected to the Vin socket. Internally this control voltage can be amplified by an amount determined by the setting of the front panel GAIN control. Thus the overall frequency sensitivity to the external control voltage is determined both by the amplitude of the voltage at Vin and the GAIN setting. A convenient way to set the sensitivity (and thus the GAIN control, which is not calibrated), to a definable value, is described below. 1
Task 1 Voltage Controlled Oscillator (VCO) a) As shown in Fig. 1.1 (a) and (b) set the mode of operation to VCO with on-board switch and set the front panel switch to HI (when it is on high position, the carrier frequency is 100 khz and when it is on LO, the carrier frequency is 10 khz). (a) Figure 1.1. (a) On-board switch of VCO and (b) front panel switch b) With no input to the Vin socket. It is effectively connected to ground. In this state the GAIN control setting does not affect the oscillator frequency. c) Use the FREQUENCY COUNTER to monitor the VCO frequency. Use the front panel control f 0 to set the frequency to about 100 khz as shown in Fig. 1.2 below. (b) Figure 1.2. Measuring the frequency of VCO Task 2 VCO sensitivity calculation a) Set the VARIABLE DC module output to minus 2 volts (this is the TIMS ANALOG REFERENCE LEVEL) and connect this DC voltage to the Vin socket of the VCO as shown in Fig. 1.3 below. 2
Figure 1.3. Measuring the VCO sensitivity (Calibration) b) Adjust the VCO gain control until the frequency changes by 10 khz (110 khz or 90 khz). Note that the direction of change will depend upon the polarity of the DC voltage. The negative DC used in the previous Task should give a frequency increase for a clockwise rotation of the GAIN control. c) The GAIN control of the VCO is now set to give a 10 khz peak frequency deviation for a modulating signal peak input of 2 volts. You have now calibrated the sensitivity, S, of the VCO for the purposes of the work to follow. What is the reading on frequency counter after adjusting VCO gain? f = Calculate the sensitivity using the following equation? S= Frequency deviation / Signal peak input (1) Calculation: Section 2: FM generation using VCO Task 1 FM spectrum a) After finishing calibrating VCO, in this section VCO will be used to generate FM signal. AUDIO OSCILLATOR is used as input signal source and The BUFFER AMPLIFIER serves as the depth-of-modulation control. 3
b) As shown in Fig. 1.4, connect the output of the AUDIO OSCILLATOR to the FREQUENCY METER and adjust the frequency of the AUDIO OSCILLATOR so that the FREQUENCY METER reads about 5 khz. Figure 1.4 Measuring and adjusting the frequency of audio oscillator c) Replace the DC voltage to the VCO with the output from an AUDIO OSCILLATOR, connected via a BUFFER AMPLIFIER, to the Vin socket of the VCO. The model is illustrated in Fig. 1.5 below. Figure 1.5. Generating FM signal using VCO d) Now use the Oscilloscope in Math FFT mode and using an appropriate time base selection (see provided video for the FM laboratory or read manual on oscilloscope on how to do this basically select MATH button and select FFT mode) look at the carrier and the first side-lobe to the left or right of the carrier (it should have the carrier at about 100kHz in the FFT measurement). Adjust the gain on the BUFFER AMPLIFIER so that at least one of the side lobes is about 2dB below the magnitude measured for the carrier. For Oscilloscope, it is recommended to put the sample frequency at 500 khz/sec. e) Sketch all significant components of the resulting FM spectrum (this could include components as far down as -40dB, though -30dB is probably satisfactory). We will need this sketch later when applying Carson s rule. 4
Task 2 FM bandwidth a) Examine and sketch the input sinusoidal waveform signal (pure tone) at the output of the BUFFER AMPLIFIER. Include both the amplitude excursion from ground (0V DC) and the time taken for one period. We are particularly interested in the peak amplitude in volts. Amplitude ( ) Time ( ) -V Graph 1-1 Voltage waveform 5
b) Now we have a rule of thumb called Carson s rule (you may want to do some internet browsing and lookup Carson s rule). Basically, it provides a rule that can be used to give a first draft estimate of the spectral bandwidth occupied by the FM signal. In words it states that the bandwidth occupied by an FM signal is approximately equal to twice the sum of the highest message frequency and the frequency deviation caused by the amplitude of the modulating signal. FM bandwidth = 2 [(Input signal peak * S)+ Input signal frequency] (2) c) Calculate the FM bandwidth for your measured results using equation (2)? Calculation: d) Compare the estimate bandwidth with the significant spectral components you observed in task 1-e. Comparison: e) Label and sketch the FM bandwidth on the FM spectrum observed in task 1-e. f) Try and adjust the oscilloscope time base and trigger (probably trigger on the VCO s output) so that you can see the FM modulation in the time domain. This may require some trial and error adjustments on the oscilloscope (you may also need to adjust the trigger level. g) Sketch FM signal in time domain. Amplitude ( ) Time ( ) -V Graph 1-2 Voltage Waveform 6