Exp 3 FM HL24 (1) PDF - University of Sharjah

University of Sharjah College of Engineering Electrical Engineering Department TELECOMMUNICATION SYSTEMS I LAB. 0402347 Experiment # 3: FM Transmission and Reception 1 Table of Contents 1. Objectives..............

University of Sharjah College of Engineering Electrical Engineering Department TELECOMMUNICATION SYSTEMS I LAB. 0402347 Experiment # 3: FM Transmission and Reception 1 Table of Contents 1. Objectives............................................................................................................................3 2. Theoretical Background......................................................................................................3 3. Lab Work (Using TIMS)........................................................................................................8 Part I: Sensitivity and Linearity of VCO..........................................................................................8 Part II: FM Generation.................................................................................................................11 Part III: FM Demodulation........................................................................................................... 12 4. Lab Work (Using SIMULINK) Optional...............................................................................13 FM Generation.............................................................................................................................13 FM Demodulation........................................................................................................................17 5. Assignment........................................................................................................................18 2  FM TRANSMISSION And RECEPTION 1. Objectives In this experiment, the student will be able to use the Emona Telecoms-Trainer and Simulink to: ▪ Understand the concept of FM signal generation by using a voltage-controlled oscillator (VCO). ▪ Understand the concept of frequency demodulation by using a quadrature phase shift detector. 2. Theoretical Background If the frequency of carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal, then it is called as frequency modulation. In FM the amplitude of carrier remains constant. The variation in carrier frequency from the unmodulated carrier is called frequency deviation (∆f). Deriving the FM Equation General angle modulated signal is given by: x(t) = Ac cos (2 fct +  (t) ) (1) fc is the carrier frequency , The quantity 2 fc +  (t) = i(t) is called the instantaneous phase of x(t), while the quantity  (t) is called the phase deviation of x(t). The instantaneous angular frequency of x(t), defined as the rate of change of the instantaneous phase and having units of radians per second, is given by: di (t) d(2fct + (t)) d i (t) =  = = 2f c + (2) dt dt dt The quantity d is called the angular frequency deviation. The two basic types of angle modulation dt are Phase Modulation (PM) and Frequency Modulation (FM). 𝑑𝜃 Variation of 𝜃(𝑡) produces phase Modulation PM meanwhile variation of produces frequency 𝑑𝑡 modulation FM. 𝑑𝜃 Frequency modulation implies that is proportional to the magnitude of the modulation signal 𝑚(𝑡). 𝑑𝑡 This yield: 3  d = 2k m(t) (3) f dt Thus, in FM the instantaneous frequency varies linearly with the message signal and is given by: fi = fc + kf m(t). (4) The term kf , expressed in Hertz per unit of m(t), represents the frequency sensitivity of the FM signal. Since the sine operator acts on angles, not frequency. The phase angle  (t) of FM signal is given by:  (t) = 2k  m( )d t (5) f 0 ( Therefore, the time domain expression for FM is given by: t x(t) = A cos 2f t + 2k m( )d ) (6) C c f  0 Frequency deviation, modulation index and spectrum of FM Consider a sinusoidal modulating information signal given by: m(t) = Am cos(2 fm t ) (7) The instantaneous frequency of the resulting FM signal equals: fi(t) = fc + kf m(t) = fc + kf Am cos(2 fm t ) (8) The maximum change in instantaneous frequency fi from the carrier frequency fc, is known as frequency deviation f, where it is given by:  f = kf Am (9) Frequency deviation is a useful parameter for determining the bandwidth of FM signals. For example, an information signal of peak-to-peak voltage of 6 volts and a frequency of 10kHz with a frequency constant of 15 kHz/V would cause an FM carrier to change by a total of 90 kHz (45 kHz above and below the original carrier frequency). The carrier frequency would be swept over this range 10,000 times a second. Next, the FM-modulated signal is given by: x(t) = A cos (2f t +  sin(2f t)) (10) C c m 4 where kA f is the modulation index of the modulated signal. In general, for a non-sinusoidal = f m = f m f m m(t) signal, the modulation index is defined as: k f m(t) = max (11) W where, W is the bandwidth of the message signal, m(t). In case of a sinusoidal message signal, the modulated signal can be represented by:  x(t) =  A J ( ) cos(2 ( f n=− C n C + nfm )t) (12) where Jn() is known as Bessel functions in the order n and argument . Some of the selected values of Jn() is listed in Table 1. In the frequency domain, we have:  A J ( ) A J ( )   (13) X(f)=  n=− C n 2  ( f − ( f C + nf m )) + C n 2  ( f + ( f C + nf m))  From the equations (21) and (22), we observe that : 1. The spectrum consists of a carrier-frequency component and an infinite number of sideband components at frequencies fc  nfm (n = 1,2,3,4,5…..). 2. The relative amplitudes of the spectral lines depend on the value of Jn(), and the value of Jn() becomes very small for large values of n. 3. The number of significant spectral lines (that is, having appreciable relative amplitude) is a function of the modulation index . With  > 1, there will be many sideband lines. The amplitude spectrums of FM signals for several values of  are shown in the figure below. 5  1 0.5  = 0.2 fc - fm fc fc + fm 0.5 =1 fc =5 fc Amplitude spectrum of sinusoidal modulated FM signals (fm fixed). FM Generation A very simple and direct method of generating an FM signal is by the use of a voltage-controlled oscillator VCO. The frequency of such an oscillator can be varied by an amount proportional to the magnitude of an input (control) voltage. Such oscillators, in the form of an integrated circuit, have very linear characteristics over a frequency range that is a significant percentage of the center frequency. The block diagram of the VCO-FM generator is shown in Figure 1(a). Figure 1(b) shows a snapshot of an FM signal, together with the message from which it was derived. Note particularly that there are no amplitude variations - the envelope of an FM waveform is a constant. Figure 1: FM by VCO (a), and resulting output (b). 6 FM Demodulation FM demodulators are referred to as discriminators or frequency detectors. They convert the magnitude and rate of the changes in the transmitted signal frequency into the amplitude and frequency of the message signal. The quadrature detector includes the phase shifter, the phase detector(mixer or multiplier), and a low-pass filter. Figure 2: FM Quadrature demodulator The incoming signal is split into two components. The first enters the mixer( multiplier) directly, but the second is phase shifted. The phase shift is 90° for the carrier, but the deviation on the carrier will cause the phase shift to change slightly. Dependent upon the amount of deviation. The original signal and the phase shifted signal are then passed into a multiplier or mixer. The mixer output is dependent upon the phase difference between the two signals, i.e. it acts as a phase detector and produces a voltage output that is proportional to the phase difference and hence to the level of deviation on the signal. 7 3. Lab Work (Using TIMS) Equipment: In this experiment we will need the following equipment: 1. 1 X Variable DC module- Generates a variable DC voltage. 2. 1X VCO- Generates an FM signal. 3. 1 XAUDIO OSCILLAOR- generaste a sinusoidal signal. 4. 1X Multiplier – Multiplies two signals together 5. 1X Phase Shifter. Part I: Sensitivity and Linearity of VCO The Voltage Controlled Oscillator module functions in two modes: either as a VOLTAGE CONTROLLED OSCILLATOR with analog input voltage or as an FSK GENERATOR with digital input. Both modes have two frequency ranges of operation which are selected by a range switch. The VCO frequency and input sensitivity can be controlled from the front panel. The VCO output frequency is controlled by an analog input voltage. The input voltage, Vin, is scaled - amplified - by the front panel GAIN control. A DC voltage can be added to Vin internally, thus setting the CENTER FREQUENCY or FREE RUNNING FREQUENCY, fo. The CENTER FREQUENCY is defined as the VCO output frequency when no voltage is applied to the Vin connector. The Vin input is internally tied to the ground if no signal is applied. For this part, you will need to measure the sensitivity of the frequency of the VCO to an external control voltage, so that the frequency deviation can be set as desired. ❖ The CENTER FREQUENCY or FREE RUNNING FREQUENCY of the VCO is set with the front panel control labeled f . ❖ f can also 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 frequency sensitivity to the external control voltage is determined by the GAIN setting of the VCO. 8 A convenient way to set the sensitivity (and thus the GAIN control, which is not calibrated), to a definable value, is described below. I. VCO Configuration: Set the front panel switch of the Voltage-Controlled Oscillator (VCO) to 'LO' and turn the front panel GAIN control fully anti-clockwise. II. VCO Free Running Frequency (f0) Setting: Use the FREQUENCY COUNTER to monitor the VCO frequency. Adjust the front panel control to set the f0 frequency to 10 kHz. III. Sensitivity (Frequency Constant) Calibration: Figure 3 1. Use the VARIABLE DC module to set the DC at the VCO input voltage (Vin) to exactly 1.0 volts, with the VCO GAIN set fully anti-clockwise. 2. Gradually increase the VCO GAIN control from zero until the frequency changes by 2 kHz, reaching 8 kHz if using the TIMS device or 12 kHz if using the tutorTIMS software. The sensitivity (S) or frequency constant (𝐾𝑓) of the VCO is set, with values measured as 𝑲𝒇 = -2000 Hz/volt using EMONA TIMS device and 𝑲𝒇 = 2000 Hz/volt using tutorTims software. Ensure that the value of f0 and the Gain remain unchanged throughout the experiment 9 IV. VCO Linearity Measurement: 1. Take a series of readings of frequency versus DC voltage at Vin, covering a range from -2.5V to 2.5V in increments such as -2.5V, -2V, -1.5V,..., 2.5V. Measure the VCO frequency in each case. VCO-Input voltage VCO - output frequency (volt) (kHz) -2.5 14.62 kHz -2 13.73 kHz -1.5 12.80 kHz -1 11.89 kHz -0.5 10.99 kHz 0 10.05 kHz 0.5 9.12 kHz 1 8.01 kHz 1.5 7.19 kHz 2 6.18 kHz 2.5 5.28 kHz 2. Plot the output frequency versus the input voltage for each setting. 3. Determine the linear range by identifying the region on the curve where the relationship between DC volts and output frequency is linear. 10 Part II: FM Generation The objective of this procedure is to explore Frequency Modulation (FM) using a Voltage-Controlled Oscillator (VCO) and an Audio Oscillator. 1. Replace the DC voltage source with the output from an AUDIO OSCILLATOR tuned to 1000 Hz. Figure 4 2. Configure the oscilloscope, ensuring the time-based control displays three cycles of the Master Signals module's. Enabling channels A and B, and set both channels to DC coupling mode. 3. Modify the existing setup to match the configuration shown in Figure 4. 4. Observe the FM signal in the time domain and assess if the frequency changes correspond to the amplitude of the sinusoidal signal. Capture a screenshot of the results. 5. Use a spectrum analyzer to observe the frequency domain of the FM signal. Specify the frequencies where the signal is centered. Capture a screenshot of the results. 6. Measure the frequency difference between the carrier and its next replica, understanding its significance in the modulation process. 7. Vary the frequency of the AUDIO OSCILLATOR and observe changes in the modulated signal. 8. Explain the variations in the modulated signal resulting from the variation in the Audio Oscillator frequency. 11 Part III: FM Demodulation The objective of this procedure is to demodulate a Frequency Modulated (FM) signal and recover the original message signal. 1. Modify the existing setup as per Figure 5 to create a configuration suitable for FM demodulation. Figure 5 : FM Quadrature demodulator 2. Adjust the frequency of the message signal to 1 kHz, preparing it for the demodulation process. 3. Configure the Tunable Low-pass Filter module with its Gain control set to the midpoint of its range, preparing it for signal processing. 4. Set the Cut-off Frequency of the Tunable Low-pass Filter to 1 kHz, ensuring it aligns with the frequency of the message signal. Note: You can measure the 3-dB cutoff frequency of the LPF by connecting the TTL (CLK) output of the filter to the TTL input of FREQUENCY COUNTER, and divide the reading by 100. 5. Observe the output from the Low-pass Filter (LPF) and fine-tune the PHASE SHIFTER to achieve the best possible recovery of the original message signal. 6. Observe the demodulated signal's characteristics in both time and frequency domains. Capture a screenshot of the results. 12 13 4. Assignment 1. Draw a graph between frequency and voltage from the measurements taken in step 2 of Part I and write your comments. 2. Using the results, you get Part I; calculate the frequency deviation, modulation index, and bandwidth, and state whether it is a wideband or narrowband FM. 5. Lab work (Using Tims Software) Part I: Sensitivity and Linearity of VCO Figure 1:Adjusting the DC to 1V 14  Figure 2: Adjusting the gain until frequency=12kHz Observation: Each 1V will change the frequency by 2KHz Part II: FM Generation Figure 3: Message signal (blue), Modulated signal (red) Observation: When the magnitude of the message signal increases, the frequency of modulated signal increases and vice versa. 15  Figure 4: Measuring the amplitude of the message signal In order to calculate the deviation we need to get the amplitude of the message signal which is 2V and multiply it with kf which is 2KHz, so the deviation= 4KHz Figure 5: Spectrum of modulated signal Observation: The Distance between the center frequency Fo= 10KHz and the next signal F0+Fm= 11KHz is 1KHz which is Fm 16  Figure 6: Altering the message signal's frequency to 1.5KHz Observation: as u change the frequency of the original message, the distance between the center frequency and its next replica increases that is because the distance=Fm Part III: FM Demodulation Figure 7: phase shifting the carrier signal by 90 degrees First we need to phase shift the carrier signal to 90 degrees by adjusting the time to 25 µs, 90= 360 x tau x 10K = 25 µs 17  Figure 8: Recovered signal after LPF(red) and original signal (red) We have successfully recovered the signal since the frequency of the recovered signal is 1.037KHz which is the same frequency as the original message. Figure 9: Spectrum of recovered signal (blue) and original signal (red) 18