Angle Demodulator using AM n FM demodulators first

Angle Demodulator using AM n FM demodulators first generate an AM signal and then use an AM demodulator to recover the message signal. ¨ ¨ To transform the FM signal into an AM signal, Pass the FM signal through an LTI system, whose frequency response is approximately a straight line in the frequency band of the FM signal. n If the frequency response of such a system is given by n And if the input to the system is n Then the output will be the signal The next step is to demodulate this AM signal to obtain Ac(Vo+kkfm(t)), from which the message m(t) can be recovered. 1

Angle Demodulators using AM n Many circuits can be used to implement the first stage of an FM demodulator, i. e. , FM to AM conversion. One candidate is a simple differentiator with ¨ Another candidate is the rising half of the frequency characteristics of a tuned circuit, as shown in below n Such a circuit can be easily implemented, but usually the linear region of the frequency characteristic may not be wide enough. n To obtain linear characteristics over a wide range of frequencies, usually two circuits tuned at two frequencies f 1 and f 2 are connected in a configuration, which is known as a balanced discriminator. ¨ 2

Angle Demodulator using AM A balanced discriminator and the corresponding frequency response. 3

Angle Demodulator using PLL n A different approach to FM-signal demodulation is to use a phase-locked loop (PLL) => PLL-FM demodulator n The input to the PLL is the angle-modulated signal (where, for FM, n ) The VCO generates a sinusoid of a fixed frequency; in this case, it generates the carrier frequency fc, in the absence of an input control voltage. 4

Angle Demodulator using PLL n n Now, suppose that the control voltage to the VCO is the loop filter's output, denoted as v(t). Then, the instantaneous frequency of the VCO is ¨ n Consequently, the VCO output may be expressed as ¨ n n where The phase comparator is a multiplier and a filter that rejects the signal component centered at 2 fc. Hence, its output may be expressed as ¨ n where kv is a deviation constant where the difference (t) - v(t) e(t) constitutes the phase error. The signal e(t) is the input to the loop filter. 5

Angle Demodulators using PLL n Let us assume that the PLL is in lock position, so the phase error is small. Then, ¨ under this condition, so we may deal with the linearized model of the PLL, shown in below ¨ n We may express the phase error as ¨ Or equivalently, either as ¨ Or as (Eq. 1) Linearized PLL: 6

Angle Demodulators using PLL n The Fourier transform of (Eq. 1) is n Hence (Eq. 2) n Now, suppose that we design G(f) such that n Then, from (Eq. 2), ¨ n Or equivalently, Since the control voltage of the VCO is proportional to the message signal, v(t) is the demodulated signal. 7

FM-Radio Broadcasting n n n Commercial FM-radio broadcasting utilizes the frequency band 88 -108 MHz for the transmission of voice and music signals. The carrier frequencies are separated by 200 k. Hz and the peak frequency deviation is fixed at 75 k. Hz. Preemphasis is generally used, as described in Chapter 6, to improve the demodulator performance in the presence of noise in the received signal. The receiver most commonly used in FM-radio broadcast is a superheterodyne type. The block diagram of such a receiver is shown in below 8

FM-Radio Broadcasting Block diagram of a superheterodyne FM-radio receiver. 9

FM-Radio Broadcasting n n As in AM-radio reception, common tuning between the RF amplifier and the local oscillator allows the mixer to bring all FM-radio signals to a common IF bandwidth of 200 k. Hz, centered at f. IF = 10. 7 MHz. Since the message signal m(t) is embedded in the frequency of the carrier, any amplitude variations in the received signal are a result of additive noise and interference. The amplitude limiter removes any amplitude variations in the received signal at the output of the IF amplifier by bandlimiting the signal. A bandpass filter, which is centered at f. IF = 10. 7 MHz with a bandwidth of 200 k. Hz, is included in the limiter to remove higher-order frequency components introduced by the nonlinearity inherent in the hard limiter. 10

FM-Radio Broadcasting n n n A balanced frequency discriminator is used for frequency demodulation. The resulting message signal is then passed to the audiofrequency amplifier, which performs the functions of deemphasis and amplification. The output of the audio amplifier is further filtered by a lowpass filter to remove out-of-band noise, and this output is used to drive a loudspeaker. 11

FM-Stereo Broadcasting <Transmitter> n Many FM-radio stations transmit music programs in stereo by using the outputs of two microphones FM-stereo transmitter and signal spacing. 12

FM-Stereo Broadcasting n n n The signals from the left and right microphones, ml(t) and mr(t), are added and subtracted. The sum signal ml(t)+mr(t) is left unchanged and occupies the frequency band 0 -15 k. Hz. The difference signal ml(t)-mr(t) is used to AM modulate (DSB -SC) a 38 k. Hz carrier that is generated from a 19 -k. Hz oscillator. A pilot tone at the frequency of 19 k. Hz is added to the signal for the purpose of demodulating the DSB-SC AM signal. We place the pilot tone at 19 k. Hz instead of 38 k. Hz because the pilot is more easily separated from the composite signal at the receiver. The combined signal is used to frequency modulate a carrier. 13

FM-Stereo Broadcasting n By configuring the baseband signal as an FDM signal, a monophonic FM receiver can recover the sum signal ml(t)+mr(t) by using a conventional FM demodulator. Hence, FM-stereo broadcasting is compatible with conventional FM. ¨ In addition, the resulting FM signal does not exceed the allocated 200 k. Hz bandwidth. ¨ <Receiver> n The FM demodulator for FM stereo is basically the same as a conventional FM demodulator down to limiter/discriminator. ¨ n Thus, the received signal is converted to baseband. Following the discriminator, the baseband message signal is separated into the two signals, ml(t)+mr(t) and ml(t)-mr(t), and passed through deemphasis filters, as shown in Figure 4. 18. 14

FM-Stereo Broadcasting n n n The difference signal is obtained from the DSB-SC signal via a synchronous demodulator using the pilot tone. By taking the sum and difference of the two composite signals, we recover the two signals, ml(t) and mr(t). These audio signals are amplified by audio-band amplifiers, and the two outputs drive dual loudspeakers. As indicated, an FM receiver that is not configured to receive the FM stereo sees only the baseband signal ml(t)+mr(t) in the frequency range 0 -15 k. Hz. Thus, it produces a monophonic output signal that consists of the sum of the signals at the two microphones. 15

FM-Stereo Broadcasting Figure 4. 18 FM-stereo receiver. 16

Television Broadcasting n Commercial TV broadcasting began as black-and-white picture transmission in London in 1936 by the British Broadcasting Corporation (BBC). n Color TV was demonstrated a few years later, but commercial TV stations were slow to develop the transmission of color-TV signals. ¨ ¨ ¨ n This was due to the high cost of color-TV receivers. With the development of the transistor, the cost of color TV decreased significantly. By the middle 1960 s, color TV broadcasting was widely used by the industry. The frequencies allocated for TV broadcasting fall in the VHF and UHF bands. Table 4. 2 lists the TV channels allocated in the United States. The channel bandwidth allocated for the transmission of TV signals is 6 MHz. In contrast to radio broadcasting, standards for television-signal transmission vary from country to country. ¨ The US standard was set by the National Television Systems Committee (NTSC). ¨ ¨ ¨ 17

Television Broadcasting 18