CHAPTER 2

Amplitude Modulation2-3 AM RECEIVERS

Introduction AM demodulation – reverse process of AM

modulation. Demodulator: converts a received

modulated-wave back to the original source information.

Basic understanding of the terminology commonly used to describe radio receivers & their characteristics is needed to understand demodulation process

Simplified block diagram of an AM receiver

Receiver Parameters Selectivity Bandwidth improvement Sensitivity Dynamic range Fidelity Insertion Loss Noise temperature & Equivalent noise

temperature

Selectivity Used to measure the ability of the

receiver to accept a given band of frequencies and reject all others.

Way to describe selectivity is to simply give the bandwidth of the receiver at the -3dB points.

Not necessarily a good means of determining how well the receiver will reject unwanted frequencies.

Cont’d… Give the receiver bandwidth at two

levels of attenuation. Eg: -3dB, -60dB The ratio of two BW ~ Shape factor

SF = B(-60 dB) / B(- 3dB)

Where SF – Shape factor B(-60dB) – BW 60dB below max signal level

B(-3dB) – BW 3dB below max signal level

Cont’d… If both BW equal, the shape factor

would be 1. Impossible to achieve in practical circuit Example application for SF nearly 1

Satellite Microwave Two way radio Rx

Bandwidth Improvement

Thermal noise directly proportional to bandwidth.

Reduce BW ~ reduce noise, improving system performance.

Reducing BW = improving the noise figure of the RX

Cont’d…

Bandwidth Improvement, BIBI = BRF /BIF

Where BRF = RF Bandwidth (Hz)

BIF = IF Bandwidth (Hz)

Noise figure improvement,NF = 10 log BI

Sensitivity The minimum RF signal level that can

be detected at the input to the Rx and still produce a usable demodulated information signal.

Usually stated in micro volts of received signal.

Rx sensitivity also called Rx threshold.

Cont’d… Depends on:

The noise power present at the input to the Rx.

Rx noise figure. AM detector sensitivity. BI factor of the Rx

To improve ~ reduce the noise level Reducing the temperature or Rx BW or RX noise

figure

Dynamic range

The difference (in dB) between the minimum input level necessary to discern a signal and the input level that will overdrive the Rx and produce distortion.

Input power range over which the Rx is useful.

Cont’d…

A dynamic range of 100dB is considered about the highest possible.

A low dynamic range can cause a desensitizing of the RF amplifiers and result in severe intermodulation distortion of the weaker input signal.

Fidelity

A measure of the ability of a communication system to produce (at the output of the Rx) an exact replica of the original source information.

Cont’d…

Forms of distortion that can deteriorate the fidelity of a communication system:- Amplitude Frequency Phase

Noise Temperature & Equivalent noise Temperature

Thermal noise directly proportional to temperature ~ can be expressed in degrees, watts or volts.

Environmental temperature, T (kelvin) T = N/KB

Where N = noise power (watts) K = Boltzman’s Constant

(1.38 X 10-23 J/K) B = Bandwidth (Hz)

Cont’d…

Equivalent noise temperature, (Te)

Te = T(F-1)

Where T = environmental temperature (kelvin)

F = Noise factor Te often used in low noise, sophisticated

radio receivers rather than noise figure.

Insertion loss IL is a parameter associated with the

frequencies that fall within the passband of a filter.

The ratio of the power transferred to a load with a filter in the circuit to the power transferred to a load without the filter.

IL (dB) = 10 log (Pout /Pin)

AM RECEIVERS Two basic types of radio receivers.1. Coherent

Synchronous receivers The frequencies generated in the Rx & used for

demodulation are synchronized to oscillator frequencies generated in Tx.

2. Non-coherent Asynchronous receivers Either no frequencies are generated in the Rx or the

frequencies used for demodulation completely independent from the Tx’s carrier frequency.

Non-coherent detection = envelope detection.

COHERENT EXAMPLE OF COHERENT DEMODULATION: SSB

The received signal is heterodyned /mixed with a local carrier signal which is synchronous (coherent) with the carrier used at the transmitting end.

LPF XSSB

cos wct

Coherent demodulation

Carrier=9MHz Modulating signal=2kHz=0.002MHz After modulation, and SSB generated,

only upper sideband=9.002MHz transmitted.

In receiver, add with local oscillator at 9MHz, output is sum and difference:18.002MHz, and 0.002MHz=modulating signal.

Use filter to filter everything else

Example of how product detector works..

Non-Coherent Rx

Tuned Radio Frequency Rx Superheterodyne Rx

Non-coherent tuned radio frequency receiver (TRF Rx) block diagram

Cont’d… Earliest types of AM Rx. Figure shows the block diagram of a

three stage TRF Rx. Consists of RF stage, detector stage and

audio stage. Simple and high sensitivity. BW inconsistent & varies with the

center frequency.

Cont’d…

Skin effect phenomenon. B = f/QWhere Q is quality factor. TRF Rx is useful to single-channel,

low frequency application.

AM superheterodyne receiver block diagram

Cont’d…

Non uniform selectivity of TRF led to the development of the Superheterodyne Rx.

Its gain, selectivity and sensitivity characteristics are superior to those of other Rx configurations.

Cont’d…

Frequency conversion.High side injection,

flo = fRF + fIF

Low side injection flo = fRF -fIf

INTERMEDIATE FREQUENCY (IF)

Mixers generate signals that are the sum and difference of the incoming signal frequency (fS) and the frequency of the local oscillator (fLO).

The difference frequency is more commonly chosen as the IF.

Some receivers use the sum frequency for the IF.

IMAGES

An image (fIM) is an undesired signal that is separated from the desired signal frequency (frf) by two times the IF (fIF).

fI = frf + 2fIF or frf - 2fIF

Images interfere with the desired signal. Images can be eliminated or minimized by:

Proper selection of the IF in design. Use of highly selective filters before the

mixer. Use of a dual conversion receiver.

con’t’d…

Image frequencyfim = fRF + 2fIF

Image Frequency rejection ratio IFRR = √ (1 + Q²ρ²)Where ρ = (fim/fRF) –(fRF/fim)

AM APPLICATION

AM Radio broadcasting Commercial AM radio broadcasting

utilizes he frequency band 535 – 1605 kHz for transmission voice and music.

Carrier frequency allocation range, 540-1600 kHz with 10 kHz spacing.

Cont’d… Radio stations employ conventional AM

for signal transmission – to reduce the cost of implementing the Rx.

Used superheterodyne Rx. Every AM radio signal is converted to a

common IF frequency of fIF = 455 kHz.

END OF CHAPTER 2 :AMPLITUDE MODULATION