6. A radio receiver used in the AM system is shown in Figure 2. The mixer transl

Question

6. A radio receiver used in the AM system is shown in Figure 2. The mixer translates the carrier frequency fc to a fixed IF of 455 kHz by using a local oscillator of frequency fLo The broadcast-band frequencies range from 540 to 1600 kHz. Antenna IF sectionEnv Envelope detector RF section Common uning Audio amplifier Local Speaker -oscillator Figure 2 A superheterodyne AM receiver (a) Determine the range of tuning that must be provided in the local oscillator When fO is higher than the fe (superheterodyne receiver) and 453 k h- (i) When fio is lower than f (b) Explain why the usual AM radio receiver uses a superheterodyne system. (c) What is the main role of the IF section in Figure 2? 17] 17] Activate Wingon n

Explanation / Answer

We know local oscillator is used with mixer to fluctuate the frquency values, which is called heterodyning.

Given that mixer translates the carrier frequency fc to a fixed Intermediate frequency (IF) of 455khz using a local oscillator of frequency fLo .

(a)

When fLo is higher than fc it is called high side injection. and the equation is given by fLO= fRF+fIF

where fLO is frequency of local oscillator, fRF is frequency of radio signal , fIF is the frequency of intermediate signal.

fLO= fRF +fIF given 540<fRF<1600

hence by using the above equation we get fLO values as 540+455<fLO<1600+455

995<fLO<2055

therefore when fLO is higher than fc the ranhe of tuning is 995khz<fLO<2055khz

(ii)

when fLO is lower than fc the equation is given as fLO= fRF - fIF

therefore the range of tuning is given by 540-455< fLO< 1600-455

85<fLO<1145

(b) We know in amplitude modulation (AM) the variation of strength of radio signal is done by audio signal. When the local oscillator frequency is greater than the carrier frequency, the audio signal is generated and the variation of the radio signal quite easy and hence it uses the superheterodyne system. other major reason to use the superheterodyne system is it has many advantages such as

1. high side injection and low side injection process.

2.local oscillator radiation is reduced.

3. local oscillator side band noise is reduced.

(c)

IF section used here is for the variation of the local oscillator frequency. As we know fLO= fRF - fIF , the change in frequency of intermediate frequency section will effect the frequency of the local oscillator which helps the process of AM receiver.

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