1. Amplitude Modulation 2.2 AM RECEIVERS
CHAPTER 2
Amplitude Modulation
2.2 AM RECEIVERS

2. OBJECTIVES To define AM demodulation
To define and describe the receiver parameters
To describe the operation of a tuned radio frequency (TRF) receiver
To describe the operation of a superheterodyne receiver

3. LECTURE OVERVIEW Demodulation Receiver parameters
Tuned radio frequency (TRF) receiver
Superheterodyne receiver

4. 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

5. Simplified block diagram of an AM receiver

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

7. 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.

8. 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

9. 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

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

11. Cont’d…
Bandwidth Improvement, BI BI = BRF /BIF Where BRF = RF Bandwidth (Hz) BIF = IF Bandwidth (Hz) Noise figure improvement, NF = 10 log BI

12. 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.

13. Cont’d… Depends on: To improve ~ reduce the noise level
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

14. 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.

15. 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.

16. 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.

17. Cont’d…
Forms of distortion that can deteriorate the fidelity of a communication system:-
Amplitude
Frequency
Phase

18. 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 J/K)
B = Bandwidth (Hz)

19. 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.

20. 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)

21. AM RECEIVERS Two basic types of radio receivers. Coherent Non-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.

22. Coherent demodulation
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 X
SSB
cos wct
Coherent demodulation

23. Non-Coherent Rx
Tuned Radio Frequency Rx
Superheterodyne Rx

24. Non-coherent tuned radio frequency receiver (TRF Rx)

25. Cont’D
RF amplifier - to filter and amplify the received signal to a level sufficient to drive the detector
Audio detector - converts RF signals directly to information
Audio stage – amplifies the information signals to a usable level
Advantages – simple and have relatively high sensitivity

26. DisAdvantages of trf
Bandwidth is inconsistent and varies with center frequency when tuned over a wide range of input frequencies
This is caused by a phenomenon called the skin effect
Skin effect phenomenon:
B = f/Q
Where Q is quality factor.

27. Cont’d…
Instability due to large number of RF amplifiers all tuned to the same center frequency.
Can be reduced by tuning each amplifier to a slightly different frequency, slightly above or below the desired center frequency.
Their gains are not uniform over a very wide frequency range because of the non-uniform L/C ratios of the transformer-coupled tank circuits in the RF amplifiers

28. AM superheterodyne receiver

29. Cont’d…
RF section:
Preselector is use to provide enough initial bandlimiting to prevent a specific unwanted radio frequency called the image frequency from entering the receiver
Preselector also reduces the noise bandwidth of the receiver
RF amplifier determines the sensitivity of the receiver

30. CONt’d… Mixer/ converter section: IF section:
Is a nonlinear device and its purpose is to convert radio frequencies to intermediate frequencies (RF-to-IF translation)
IF section:
Most of the receiver gain and selectivity is achieved in the IF section
IF is always lower in frequency than the RF because it is easier and less expensive to construct high-gain, stable amplifiers for low- frequency signals.

31. Cont’d… Detector section: Audio amplifier section:
To convert the IF signals back to the original source information
Audio amplifier section:
Comprises several cascaded audio amplifiers and one or more speakers

32. Frequency conversion High side injection, flo = fRF + fIF
Low side injection
flo = fRF - fIf

33. 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.

34. 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.

35. con’t’d… Image frequency fim = fRF + 2fIF
Image Frequency rejection ratio
IFRR = √ (1 + Q²ρ²)
Where ρ = (fim/fRF) –(fRF/fim)

36. Example
For a citizens band receiver using high-side injection with an RF carrier of 27 MHz and an IF center frequency of 455 kHz, determine a. Local oscillator frequency b. Image frequency c. IFRR for a preselector Q of 100

37. AM APPLICATION AM Radio broadcasting
Commercial AM radio broadcasting utilizes the frequency band 535 – 1605 kHz for transmission voice and music.
Carrier frequency allocation range, kHz with 10 kHz spacing.

38. 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.

39. AT THE END OF THIS CHAPTER, YOU SHOULD BE ABLE
To define AM demodulation
To define and describe the receiver parameters
To describe the operation of a tuned radio frequency (TRF) receiver
To describe the operation of a superheterodyne receiver

40. END OF CHAPTER 2.2 : AM RECEIVER