AM Reception

FIGURE 5-1 Simplified block diagram of an AM receiver

Tomasi

Electronic Communications Systems, 5eCopyright 2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

Demodulation reverse process of modulation

DSBFC simply converts modulated signal back to original source information.

To do this requires receiving, amplifying, demodulating, bandlimiting and filtering.

Refer Figure 5.1

From Figure 5.1

Functions: RF section/Receiver Front end:

Mixer/converter: Intermediate Freq: freq between RF and

information freq.

IF section: amplify & selectivity

AM detector: demodulates

Audio section:

Receiver Parameters To evaluate the ability of the receiver performance

to demodulate signal

1. Selectivity: ability to accept a given band of freq & reject all others.

Shape Factor

2. Bandwidth Improvement: ability to reduce bandwidth to minimize noise.

BI

3. Sensitivity: Minimum RF signal level that can be detected at the i/p & still be able to be demodulated.

SNR Threshold

4. Dynamic Range: difference in dB bet.

Min. i/p level necessary to distinguish a

signal and the i/p level to overdrive the rx

& create distortion.

5. Fidelity: ability to produce at the o/p of rx

an exac replica of the original source

info.

6. Insertion Loss

7. Noise

AM Receiver Types

TRF

Superhetrodyne

Tuned-Radio-Frequency (TRF) Receiver

The TRF receiver is the simplest receiver that meets all the basic requirements.

Drawbacks of TRF Receivers

Difficulty in tuning all the stages to exactlythe same frequency simultaneously.

Very high Q for the tuning coils are

required for good selectivity BW=fo/Q.

Selectivity is not constant for a wide range of frequencies due to skin effect which

causes the BW to vary with fo.

FIGURE 5-4 AM superheterodyne receiver block diagram

Tomasi

Electronic Communications Systems, 5eCopyright 2004 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

Antenna and Front End

The antenna consists of an inductor in the form of a large number of turns of wire

around a ferrite rod. The inductance forms

part of the input tuning circuit.

Low-cost receivers sometimes omit the RF amplifier.

Main advantages of having RF amplifier: improves sensitivity and image frequency

rejection.

Mixer and Local Oscillator

The mixer and LO frequency convert the input frequency, fc, to a fixed fIF:

High-side injection: fLO = fc + fIF

Refer

to

page

171

IF Amplifier, Detector, & AGC

IF Amplifier and AGC

Most receivers have two or more IF stages to provide the bulk of their gain (i.e.

sensitivity) and their selectivity.

Automatic gain control (AGC) is obtained from the detector stage to adjusts the gain

of the IF (and sometimes the RF) stages

inversely to the input signal level. This

enables the receiver to cope with large

variations in input signal.

Diode Detector Waveforms

Sensitivity and Selectivity

Sensitivity is expressed as the minimum input signal required to produce a

specified output level for a given (S+N)/N

ratio.

Selectivity is the ability of the receiver to reject unwanted or interfering signals. It

may be defined by the shape factor of the

IF filter or by the amount of adjacent

channel rejection.

Shape Factor

dB

dB

B

BSF

6

60

Image Frequency

One of the problems with the superhet receiver is that an image frequency signal

could interfere with the reception of the

desired signal. The image frequency is

given by: fimage = fsig + 2fIF

where fsig = desired signal.

An image signal must be rejected by tuning circuits prior to mixing.

Image Frequency Rejection

For a tuned circuit with a quality factor of Q, then the image frequency rejection is:

image

sig

sig

image

f

f

f

fx

wherexQIR

,1 22

In dB, IR (dB) = 20 log IR

Principle of Mixer & Local

Oscillator

In Superhetrodyne Receiver

Function

Take the signal from the RF stage (antenna & front end) & convert it to the IF (intermediate frequency)

Mixing two signal frequencies produces SUM (+) and DIFFERENCE (-)

LO produces very stable & pure sine waves. LO frequency changes according to the desired

frequency (carrier frequency) + IF (which is fixed).

Hence, if the receiver is tuned to a new desired frequency, LO frequency automatically changed so that the difference between the new desired frequency & LO frequency is maintained.

How things work in LO & Mixer

Example:

Desired radio station frequency = 1MHz

Bandwidth = 20kHz

Hence,

3 components of the AM signal are:

Lower sideband = 990kHz

Carrier freq. = 1MHz or 1000kHz

Upper sideband = 1010kHz

When go into the mixer, they will mix with the LO signal, say of 0.8MHz.

The result will be the SUM & the DIFFERENCE:

990 800 = 190 & 1790 kHz (mixing of lsb)

1000 800 = 200 & 1800 kHz (mixing of carrier)

1010 800 = 210 & 1810 kHz (mixing of usb)

DIFFERENCE = (190, 200, 210)

SUM = (1790, 1800, 1810)

No information is loss it just being converted to a new set of frequencies.

Now say we want to tune to another channel which is 0.2MHz higher than the first radio

channel.

New desired signal = 1.2 MHz

To track the new set of input frequency, LO frequency is increased by 0.2 MHz, to 1MHz.

New output of mixer then equals to

1190 1000 = 190 & 2190 kHz

1200 1000 = 200 & 2200 kHz

1210 1000 = 210 & 2210 kHz

DIFFERENCE = (190, 200, 210)

SUM = (2190, 2200, 2210)

Now, let us look back the set of frequencies for the first channel (1MHz) & for the second

channel (1.2MHz)

1MHz radio station:

DIFFERENCE = (190, 200, 210)

SUM = (1790, 1800, 1810)

1.2MHz radio station:

DIFFERENCE = (190, 200, 210)

SUM = (2190, 2200, 2210)

SAME

This is the uniqueness of superhetrodyne receiver: The DIFFERENCE between received frequency component

and LO frequency is always the same regardless of what the tuned frequency is.

This DIFFERENCE is known as INTERMEDIATE FREQUENCY (IF).

So.

Since all tuned signals (or radio channels) now have the same IF value, regardless of where they are in the received band (say 20 channels in a bandwidth range from 50MHz to 150MHz), the receiver is designed to handle signals of this frequency (IF) ONLY.

Compared to the TRF receiver, if the first channel is at 50MHz & we want to change to a new

channel at 100MHz, TRF must shift +50MHz to get

the new channel.

In superhetrodyne, IF is maintained and only LO frequency is changed, but its OK because LO is very stable & easy to control.

What about the SUM?

It is easily eliminate by having a Bandpass Filter that remove the SUM (which is a set

of high frequency components) & only

allow the DIFFERENCE to pass through.

Image Frequency Problem

Say the IF used in AM system = 455kHz

Channel that we interested to = 25MHz

But there are also another unwanted signal from other transmitter at 24.09MHz

Normally, RF amplifier will allow both signals

At mixer & LO, LO frequency = 25MHz - 455kHz (low tracking)

= 24 545kHz

The DIFFERENCE for both signals will be Channel A = 25000 24545 = 455 kHz

Unwanted signal = 24545 24090 = 455 kHz!!

In other words, an undesired signal on the other side of the LO output will have the same

difference frequency and pass into the IF

amplifier.

This undesired signal image frequency.

From the previous example (25MHz signal vs 24.09MHz image signal), they are so close &

will be very difficult to filter out by RF amplifier

Solution: Use higher IF frequency (like 10.7MHz) for higher desired signal

Using higher IF = 10.7MHz

Desired signal = 25MHz

LO frequency = 25MHz-10.7MHz

= 14.3 MHz

The image freq. for this desired signal should also gives IF = 10.7MHz when mixed with LO

freq = 14.3MHz

Hence, image freq. = 14.3 -10.7MHz

= 3.6MHz

Desired signal (25MHz) and its image frequency (3.6MHz) is further apart & hence it

is easier to be filtered at RF amplifier.

Image signal is not a problem when a 455kHz IF is used for the low AM

broadcast band. Why?