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Angle Demodulator using AM - PowerPoint PPT Presentation

Angle Demodulator using AM. FM demodulators first generate an AM signal and then use an AM demodulator to recover the message signal.

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Presentation Transcript

• 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.

• If the frequency response of such a system is given by

• And if the input to the system is

• 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.

• 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

• Such a circuit can be easily implemented, but usually the linear region of the frequency characteristic may not be wide enough.

• To obtain linear characteristics over a wide range of frequencies, usually two circuits tuned at two frequencies f1and f2are connected in a configuration, which is known as a balanced discriminator.

A balanced discriminator and the corresponding frequency response.

• A different approach to FM-signal demodulation is to use a phase-locked loop (PLL) => PLL-FM demodulator

• The input to the PLL is the angle-modulated signal

(where, for FM, )

• 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.

• 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

• where kvis a deviation constant

• Consequently, the VCO output may be expressed as

• where

• The phase comparator is a multiplier and a filter that rejects the signal component centered at 2fc.

• Hence, its output may be expressed as

• where the difference (t) -v(t)e(t) constitutes the phase error.

• The signal e(t) is the input to the loop filter.

• 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

• We may express the phase error as

• Or equivalently, either as

• Or as (Eq. 1)

Linearized PLL:

• The Fourier transform of (Eq. 1) is

• Hence

(Eq.2)

• Now, suppose that we design G(f) such that

• Then, from (Eq.2),

• Or equivalently,

• Since the control voltage of the VCO is proportional to the message signal, v(t) is the demodulated signal.

• 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 kHz and the peak frequency deviation is fixed at 75 kHz.

• Preemphasis is generally used, as described in Chapter 6, to improve the demodulator performance in the presence of noise in the received signal.

• The block diagram of such a receiver is shown in below

• 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 kHz, centered at fIF= 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 fIF= 10.7 MHz with a bandwidth of 200 kHz, is included in the limiter to remove higher-order frequency components introduced by the nonlinearity inherent in the hard limiter.

• A balanced frequency discriminator is used for frequency demodulation.

• The resulting message signal is then passed to the audio-frequency 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.

<Transmitter>

• Many FM-radio stations transmit music programs in stereo by using the outputs of two microphones

FM-stereo transmitter and signal spacing.

• 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 kHz.

• The difference signal ml(t)-mr(t) is used to AM modulate (DSB-SC) a 38 kHz carrier that is generated from a 19-kHz oscillator.

• A pilot tone at the frequency of 19 kHz is added to the signal for the purpose of demodulating the DSB-SC AM signal.

• We place the pilot tone at 19 kHz instead of 38 kHz because the pilot is more easily separated from the composite signal at the receiver.

• The combined signal is used to frequency modulate a carrier.

• 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-kHz bandwidth.

• The FM demodulator for FM stereo is basically the same as a conventional FM demodulator down to limiter/discriminator.

• 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.

• 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 kHz.

• Thus, it produces a monophonic output signal that consists of the sum of the signals at the two microphones.

• Commercial TV broadcasting began as black-and-white picture transmission in London in 1936 by the British Broadcasting Corporation (BBC).

• Color TV was demonstrated a few years later, but commercial TV stations were slow to develop the transmission of color-TV signals.

• 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 1960s, 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).