Analog Circuits and Systems

Prof. K Radhakrishna Rao

Lecture 2 Role of Analog Signal Processing in

Electronic Products – Part 1

1

Structure of an electronic product

2

Electronic Products

o  Process analog signals and digital data o  This involve transmission and reception of signals and data o  It is generally necessary to code signal and data to transmit over

channels o  Transmission can be over wires or wireless o  Processing and storage are efficient in digital form o  Several human interface technologies are available

3

Products considered

o  Radio Receiver o  Modem o  Cell Phone o  ECG

4

Radio Receiver

o  AM Receiver o  FM Receiver

5

Radio waves are classified as

o  Low frequency (LF): 30 kHz – 300 kHz, o  Medium frequency (MF): 300 kHz – 3 MHz, o  High frequency (HF): 3 MHz – 30 MHz, o  Very high frequency (VHF): 30 MHz – 300 MHz, o  Ultra high frequency (UHF): 300 MHz – 3 GHz, o  Super high frequency (SHF): 3 GHz – 30 GHz, o  Extremely high frequency (EHF): 30 GHz to 300 GHz.

6

Radio broadcasting

o  one-way wireless transmission over radio waves to reach a wide audience

o  takes place in MF (300 kHz – 3 MHz), HF (3 MHz – 30 MHz) and VHF (30 MHz – 300 MHz) regions

7

Major modes of radio broadcasting

o  Sine wave (single tone) represented by

( )( )( )( )

( )( )

( )( )

sin

PM Analogwhere phase in radians PSK Digital

QPSK Digital

FM Analog frequency in rad/sec

FSK Digital

AM, DSB Analog peak magnitude in volts

ASK Digital

p

p

V t

V

ω φ

φ

ω

+

=

=

=

8

AM broadcasting

o  Amplitude of the carrier signal is varied in response to the amplitude of the signal to be transmitted

o  Amplitude modulation is done by a unit called mixer (nothing but a multiplier) which produces an output

( sin )sin

where is the modulating frequency is the carrier frequency is the amplitude of the carrier and

is the amplitude of the modulating signal

= +pc pm m c

m

c

pc

pm

output V V t t

VV

ω ωω

ω is known as the

modulation index.

pm

pc

Vm

V=

9

AM broadcasting

Carrier Signal

Modulating Sine Wave Signal

Amplitude Modulated Signal

10

AM broadcasting

o  Medium wave range: 540 kHz -1.7 MHz ◦  can be received over thousands of miles (especially at night) ◦  very sensitive to atmospheric conditions and solar activity

o  Short wave range: 1.6 MHz – 30 MHz

11

AM Receiver

12

AM Receiver

o  Antenna with RF amplifier: Tunable band pass filter cum amplifier o  Mixer: Shifts the variable carrier frequency to a lower fixed

intermediate frequency. It is a multiplier of carrier and the local oscillator generated sine wave (�c + �IF) where ��IF = 455 kHz

o  IF amplifier: Tuned amplifier with centre frequency at IF. Bandwidth = 2 x signal bandwidth

o  AM detector: AM demodulator (multiplier/diode peak detector) o  AGC (Automatic Gain Control): Uses the IF signal strength

information to control the gains of RF amplifier/mixer/ IF amplifier o  Power amplifier feeds the speakers

13

FM Broadcasting

o  Frequency of the carrier signal is varied in response to the amplitude of the signal to be transmitted

o  Frequency modulation is achieved by designing an oscillator whose frequency is controlled by the modulating signal using voltage controlled oscillator (VCO)

o  FM broadcasting is over 88 MHz – 108 MHz o  FM channels are separated by 0.2 MHz

14

FM Broadcasting

( )sin sinwhere is the carrier frequency,

is the frequency deviation and is the modulating frequency.

pc c d m

c

d

m

VCO V t tω ω ωω

ωω

= +Δ

Δ

15

FM Receiver

16

FM Receiver

o  Antenna with RF amplifier: Tunable band pass filter cum amplifier o  Mixer: Shifts the carrier frequency that is variable to a lower

intermediate frequency. It is a multiplier of carrier and the local oscillator generated sine wave (�c + � IF) where �IF = 10.7 MHz

o  IF amplifier: Tuned amplifier with centre frequency at IF. Bandwidth = 2 x signal bandwidth

o  FM detector: FM demodulator (Phase Locked Loop/frequency discriminator)

o  AGC (Automatic Gain Control): Uses the IF signal strength information to control the gains of RF amplifier/mixer/IF amplifier

o  Stereo Power Amplifier

17

Modem

o  Short for MOdulator - DEModulator o  The Modem is a hardware device that enables a computer to send

and receive information over telephone  lines, or any other communication channel

o  It converts the digital data used by the computer into an analog signal used on phone lines, and converts the received analog signal into digital data to be used by the computer

o  Modems are asynchronous devices: the device transmits data in an intermittent stream of small packets. Once received, the receiving system then takes the data in the packets and reassembles it into a form the computer can use.

18

Modem (contd..)

o  1 or more byte/s (8 bits) is transferred within 1 packet, which is equivalent to one character.

o  For the computer to receive information each packet must contain a Start and a Stop bit; therefore, the complete packet would be 10 bits.

19

Generic Functional Block Diagram of a Modem

20