1. AM RECEPTION Introduction
AM demodulation – reverse process of AM modulation.
Basic understanding of the terminology commonly used to describe radio receivers & their characteristics.

2. SIMPLIFIED BLOCK DIAGRAM OF AN AM RECEIVER(RX)

3. Receiver (RX) Parameters
Selectivity
Bandwidth improvement
Sensitivity
Dynamic range
Fidelity
Insertion Loss
Noise temperature & Equivalent noise temperature

4. 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

5. If both BW equal, the shape factor would be 1.
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

6. 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,

7. 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

8. 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.

9. 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

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

11. 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.

12. 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 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

13. AM RECEIVERS Synchronous receivers
Two basic types of radio receivers.
Coherent
Synchronous receivers
The frequencies generated in the Rx & used for demodulation are synchronized to oscillator frequencies generated in Tx.
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

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

15. 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.

16. AM superheterodyne receiver block diagram

17. Image frequency; fim = fRF + 2fIF Image Frequency Rejection Ratio;
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;

18. 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, 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.

19. END OF AMPLITUDE MODULATION