UNIT-III AM & FM RECEIVERS

1. UNIT-III AM & FM RECEIVERS Subject:-Analog Communication By Shweta Suryawanshi.

2. Objectives • Draw a simplified block diagram of an AM tuned radio frequency receiver. • Explain the theory of superheterodyne. • Draw & identify the parts of a simple superherterodyne block diagram. • Know the reasons for including an RF amp in a receiver. • Define the term selectivity, sensitivity, Image frequency • Understand the theory and operation of a communication receiver. • Draw a simplified block diagram of an FM receiver.

3. AM Transmitter • AM transmitters are used in medium wave (MW) and short wave (SW) frequency bands for AM broadcast. The two types of AM transmitters that are used based on their transmitting powers are: • High Level • Low Level • Choice between the two modulation schemes depends on the transmitting power of the AM transmitter. In broadcast transmitters, where the transmitting power may be of the order of kilowatts, high level modulation is employed. In low power transmitters, where only a few watts of transmitting power are required , low level modulation is used.

4. High Level Transmitter Figure: High Level Transmitter

5. High Level Transmitter contd.. Carrier oscillator • Generates the carrier signal in the RF range. • The carrier oscillator generates a sub multiple of the required carrier frequency which is then multiplied by the frequency multiplier stage to get the required carrier frequency. • This is done to achieve frequency stability

6. High Level Transmitter contd.. Buffer Amplifier • The purpose of the buffer amplifier is two fold. It first matches the output impedance of the carrier oscillator with the input impedance of the frequency multiplier, the next stage of the carrier oscillator. It then isolates the carrier oscillator and frequency multiplier. • This is required so that the multiplier does not draw a large current from the carrier oscillator. If this occurs, the frequency of the carrier oscillator will not remain stable.

7. High Level Transmitter contd.. Frequency Multiplier • This stage is also known as harmonic generator. • It generates higher harmonics of carrier oscillator frequency. • The frequency multiplier is a tuned circuit that can be tuned to get the requisite carrier frequency that is to be transmitted.

8. High Level Transmitter contd.. Power Amplifier • The power of the carrier signal is then amplified in the power amplifier stage. A class C power amplifier gives high power current pulses of the carrier signal at its output. Audio Chain • The audio signal to be transmitted is obtained from the microphone, as shown in figure. The audio driver amplifier amplifies the voltage of this signal. A class A or a class B power amplifier amplifies the power of the audio signal.

9. High Level Transmitter contd.. Modulated Class C Amplifier • This is the output stage of the transmitter. The modulating audio signal and the carrier signal, after power amplification, are applied to this modulating stage. • Modulation takes place at this stage. The class C amplifier also amplifies the power of the AM signal to the required transmitting power.

10. Low Level Transmitter Figure: Low Level Transmitter

11. Low Level Transmitter contd.. • It is similar to a high-level transmitter, except that the power of the carrier and audio signals are not amplified. These two signals are directly applied to the modulated class C power amplifier. • Modulation takes place at this stage, and the power of the modulated signal is amplified to the required transmitting power level. • The transmitting antenna then transmits the signal.

12. RECEIVERS Receiver is an electronic equipment which pick ups the desired signal, reject the unwanted signal and demodulate the carrier signal to get back the original modulating signal.

13. Function of Receivers • Intercept the incoming modulated signal • Select desired signal and reject unwanted signals • Amplify selected R.F signal • Detect modulated signal to get back original modulating signal • Amplify modulating frequency signal

14. Classification of Radio Receivers Depending upon application • AM Receivers - receive broadcast of speech or music from AM transmitters which operate on long wave, medium wave or short wave bands. • FM Receivers – receive broadcast programs from FM transmitters which operate in VHF or UHF bands.

15. Communication Receivers - used for reception of telegraph and short wave telephone signals. Television Receivers - used to receive television broadcast in VHF or UHF bands. Radar Receivers – used to receive radio detection and ranging signals.

16. Depending upon fundamental aspects Tuned Radio Frequency (TRF)Receivers Super-heterodyne Receivers

17. Tuned Radio Frequency (TRF) Receiver: Composed of RF amplifiers and detectors. No frequency conversion It is not often used. Difficult to design tunable RF stages. Difficult to obtain high gain RF amplifiers Super-hetrodyne Receiver Downconvert RF signal to lower IF frequency Main amplifixcation takes place at IF Communication Receiver Downconvert RF signal to two IF frequency RECEIVER TYPES

18. TRF (Tuned Radio frequency) RECEIVER Speaker Power amplifier 2nd RF Amplifier Detector 1st RF Amplifier Audio amplifier Ganged Tuning

20. TRF receiver includes an RF stage a detector stage and an audio stage . Two or three RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage.

21. RF section (Receiver front end) used to detect the signal bandlimit the received RF signal and amplifying the received RF signal. AM detector Demodulates the AM wave and converts it to the original information signal. Audio section Used to amplify the recovered signal

22. TRF receivers are simple to design and allow the broadcast frequency 535 KHz to 1640 KHz. High senstivity. Advantages of TRF

23. At the higher frequency, it produces difficulty in design. It has poor audio quality. Drawbacks Instability Variation in BW Poor Selectivity Disadvantages of TRF

24. INSTABILITY Due to high gain, multi stage amplifiers are susceptible to breaking into oscillation. As gain of RF amplifier is very high ,a small feedback from output to input with correct phase can lead to oscillations. Correct phase means a positive feedback and it takes place due through stray capacitances As reactance of stray capacitances decreases at higher frequencies resulting in increased feedback. Forcing the device to work as an oscillator instead of an amplifier.

25. VARIATION IN BANDWIDTH The bandwidth is inconsistent and varies with the center frequency when tuned over a wide range of input frequencies. As frequency increases, the bandwidth ( f/Q) increases. Thus, the selectivity of the input filter changes over any appreciable range of input frequencies.

26. Example Suppose required BW=10KHz We have f1=545KHz,f2=1640KHz Q1= f1/BW= 54.5 , Q2=f2/BW=164 But practically Q is limited upto 120 Considering Q limit 120 , BW changes to13.6 KHz ( as BW=f2/Q2=1640/120) So Adjacent channel is picked up resulting in variation in bandwidth.

27. POOR SELECTIVITY The gains are not uniform over a very wide frequency range. Due to higher frequencies ability to select desired signal is affected. Due to these drawbacks TRF are rarely used.

28. IF=fo- fs RF amplifier fs mixer IF amplifier detector AF & P amplifier fo Local Oscillator (ωc+ωIF) Ganged tuning SUPER HETRODYNE RECEIVER • The shortcomings of the TRF receiver are overcome by the super heterodyne receiver. [A+ m(t)] cosωIFt [A+ m(t)] cosωct Km(t)

30. Heterodyne – to mix two frequencies together in a nonlinear device or to transmit one frequency to another using nonlinear mixing. Also known as frequency conversion , high frequency down converted to low frequency.(IF) A super heterodyne receiver converts all incoming radio frequency (RF) signals to a lower frequency known as an intermediate frequency (IF).

31. RF section Consists of a pre-selector and an amplifier Pre-selector is a broad-tuned bandpass filter with an adjustable center frequency used to reject unwanted radio frequency and to reduce the noise bandwidth. RF amplifier determines the sensitivity of the receiver and a predominant factor in determining the noise figure for the receiver.

32. Figure: Circuit Diagram of Antenna coupled with Tuner

33. RF section Advantages Greater gain i.e. better selectivity Improved Image rejection Improved SNR Improved adjacent channel rejection Better coupling of receiver with antenna Preventing spurious frequencies from entering mixer and avoiding heterodyning there Prevention of reradiation of Local Oscillator through the antenna of the receiver

34. Mixer/converter section Consists of a radio-frequency oscillator and a mixer. Choice of oscillator depends on the stability and accuracy desired. Mixer is a nonlinear device to convert radio frequency to intermediate frequencies (i.e. heterodyning process). The shape of the envelope, the bandwidth and the original information contained in the envelope remains unchanged although the carrier and sideband frequencies are translated from RF to IF.

35. Figure: Circuit Diagram of Mixer

36. Frequency Conversion • Frequency conversion is the process of translating a modulated signal to a higher or lower frequency while retaining all the originally transmitted information. • In radio receivers, high-frequency signals are converted to a lower, intermediate frequency. This is called down conversion. • In satellite communications, the original signal is generated at a lower frequency and then converted to a higher frequency. This is called up conversion.

37. Frequency Conversion Mixing Principles • Frequency conversion is a form of amplitude modulation carried out by a mixer circuit or converter. • The function performed by the mixer is called heterodyning. • Mixers accept two inputs: The signal to be translated to another frequency is applied to one input, and the sine wave from a local oscillator is applied to the other input. • Like an amplitude modulator, a mixer essentially performs a mathematical multiplication of its two input signals. • The oscillator is the carrier, and the signal to be translated is the modulating signal. • The output contains not only the carrier signal but also sidebands formed when the local oscillator and input signal are mixed.

38. Frequency Conversion Concept of a mixer.

39. Local Oscillator The local oscillator is designed such that its frequency of oscillation is always above or below the desired RF carrier by an amount equal to the IF center frequency. Therefore the difference of RF and oscillator frequency is always equal to the IF frequency

40. Figure: Circuit Diagram of Local Oscillator

41. The adjustment for the center frequency of the pre-selector and the local oscillator frequency are gang-tune (the two adjustments are tied together so that single adjustment will change the center frequency of the pre-selector and at the same time change the local oscillator) when local oscillator frequency is tuned above the RF – high side injection when local oscillator frequency is tuned below the RF – low side injection Mathematically expressed : High side injection Low side injection

43. Why Local Oscillator Frequency is Higher Than Signal Frequency • Normal tunable capacitor has a capacitance ratio of apprx. 10:1 giving frequency ratio of 3.2:1. If Local Oscillator(fo) is designaed to be below signal frequency( fs) the ratio will be 14:1 whereas if fo is greater than fs the frequency ratio is 2.2:1. • The entire frequency range can be covered in one sweep. • The ratio of fo/fs is: 995/540=1.84---Lower Broadcast range 2105/1650=1.28-- Higher Broadcast range Tracking problem disappear due to lesser variation in frequency ratio.

44. Tuning Range • The tuning range of a tunable radio receiver is decided by the range over which capacitance of the resonant circuits can be varied. Typically this has a maximum of about 10:1 • The resonant frequency of a high Q tuned circuit is given by fo = 1/(2π√(LC)) Also Rc = Cmax/ Cmin and Rf = fmax/fmin= √Rc Where Rf = circuit frequency tuning range And Rc = corresponding tuning range ratio

45. Tracking Padder Tracking Figure: Padder Tracking Figure: Tracking Error Co = (Cs * Cp)/(Cs + Cp)

46. Tracking • Trimmer Tracking Figure: Tracking Error Figure: Trimmer Tracking Co = Cs + Ct

47. Tracking • Three Point Tracking Figure: Three Point Tracking Figure: Tracking Error Co = Cp(Cs * Ct)/(Cp + Cs + Ct)

48. IF section Consists of a series of IF amplifiers and bandpass filters to achieve most of the receiver gain and selectivity. The IF is always lower than the RF because it is easier and less expensive to construct high-gain, stable amplifiers for low frequency signals. IF amplifiers are also less likely to oscillate than their RF counterparts.

49. Intermediate Frequency and Images Signal, local oscillator, and image frequencies in a superheterodyne.

50. Figure: Circuit diagram of IF section