AM RECEPTION

Introduction

AM demodulation – reverse process of AM modulation.

Basic understanding of the terminology commonly used to describe radio receivers & their characteristics.

SIMPLIFIED BLOCK DIAGRAM OF AN AM RECEIVER(RX)

Receiver (RX) Parameters

1. Selectivity2. Bandwidth improvement3. Sensitivity4. Dynamic range5. Fidelity6. Insertion Loss7. Noise temperature & Equivalent

noise temperature

1. 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. 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 ~ SF = 2 Example application for SF nearly 1

Satellite Microwave Two way radio Rx

2. Bandwidth Improvement (BI)

Thermal noise directly proportional to bandwidth. Reduce BW ~ reduce noise, improving system performance. Reducing BW = improving the noise figure of the RX

Where BRF = RF Bandwidth (Hz)

BIF = IF Bandwidth (Hz)

Noise figure improvement,

IF

RF

B

BBI

BINF log10

3. 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. 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

4. 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. 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.

5. 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.

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

Linear gain, 1-dB compression point, and third-order intercept distortion for a typical amplifier

6. Insertion loss (IL)

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.

in

outdB P

PIL log10)(

7. Noise Temperature & Equivalent noise Temperature

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

Environmental temperature, T (kelvin)

Where N = noise power (watts) K = Boltzman’s Constant (1.38 X 10-23 J/K) B = Bandwidth (Hz) Equivalent noise temperature, (Te) often used in low noise,

sophisticated radio receivers rather than noise figure.

Where T = environmental temperature (kelvin)

F = Noise factor

KB

NT

)1( FTTe

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. 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. Skin effect phenomenon.

Where Q is quality factor. TRF Rx is useful to single-channel, low frequency application.

Q

fB

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.

Frequency conversion. High side injection, flo = fRF + fIF

Low side injection, flo = fRF – fIf

Image frequency; fim = fRF + 2fIF

Image Frequency Rejection Ratio;

im

RF

RF

im

f

f

f

fwhereQIFFR )1( 22

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.

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 AMPLITUDE MODULATION