1. CHAPTER 2 Amplitude Modulation 2-3 AM RECEIVERS

2. 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

3. Simplified block diagram of an AM receiver

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

5. 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.

6. 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

7. 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

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

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

10. 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.

11. 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

12. 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.

13. 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.

14. 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.

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

16. 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)

17. Cont’d… Equivalent noise temperature, (T e ) T e = T(F-1) Where T = environmental temperature (kelvin) F = Noise factor T e often used in low noise, sophisticated radio receivers rather than noise figure.

18. 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 (P out /P in )

19. 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.

20. 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 X SSB cos w c t Coherent demodulation

21. 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..

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

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

24. 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.

25. Cont’d… Skin effect phenomenon. B = f/Q Where Q is quality factor. TRF Rx is useful to single-channel, low frequency application.

26. AM superheterodyne receiver block diagram

27. 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.

28. Cont’d… Frequency conversion. High side injection, f lo = f RF + f IF Low side injection f lo = f RF -f If

29. INTERMEDIATE FREQUENCY (IF ) Mixers generate signals that are the sum and difference of the incoming signal frequency (f S ) and the frequency of the local oscillator (f LO ). The difference frequency is more commonly chosen as the IF. Some receivers use the sum frequency for the IF.

30. IMAGES An image (f IM ) is an undesired signal that is separated from the desired signal frequency (f rf ) by two times the IF (f IF ). f I = f rf + 2f IF or f rf - 2f IF 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.

31. con’t’d… Image frequency f im = f RF + 2f IF Image Frequency rejection ratio IFRR = √ (1 + Q ² ρ ² ) Where ρ = (f im /f RF ) – (f RF /f im )

32. 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.

33. 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 f IF = 455 kHz.

34. END OF CHAPTER 2 : AMPLITUDE MODULATION