1. In a radio receiver, which of the following stages does not need alignment _____

1. TRF stage
2. IF stage
3. Antenna input stage
4. Audio stage

Explanation

Alignment is a process where several stages of the receiver are properly tuned to the desired frequency.

The input to the audio stage is a demodulated message signal and does not require any alignment to receive the desired frequency.

In electronics, a super-heterodyne receiver uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original carrier frequency and the audio stage does not need alignment in a radio receiver.

2. Which of the following is the earliest type of AM receiver?

4. None of the above

Explanation

3. The stages of TRF receiver include

1. RF stage
2. Detector Stage
3. Audio stage
4. All of the above

Explanation

TRF receiver includes 3 stages i.e an RF stage, a detector stage, and an audio stage. Generally, two or three RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage.

The detector converts RF signals directly to information, and the audio stage amplifies the information signals to a usable level.

1. Inconsistent Bandwidth
2. High Instability
3. Non-uniform Gain
4. All of the above

Explanation

Although TRF receivers are simple and have a relatively high sensitivity, they have three distinct disadvantages that limit their usefulness to single-channel, low-frequency applications.

Inconsistent Bandwidth:- The primary disadvantage is their 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. At radio frequencies, current flow is limited to the outermost area of a conductor; thus, the higher the frequency, the smaller the effective area and the greater the resistance.

Instability:- The second disadvantage of TRF receivers is instability due to the large number of RF amplifiers all tuned to the same center frequency. High-frequency, multistage amplifiers are susceptible to breaking into oscillations. This problem can be reduced somewhat by tuning each amplifier to a slightly different frequency, slightly above or below the desired center frequency.

Non-uniform Gain:- The third disadvantage of TRF receivers is their gains are not uniform over a very wide frequency range because of the nonuniform L/C ratios of the transformer-coupled tank circuits in the RF amplifiers.

1. Simplicity
2. High sensitivity
3. Both 1 and 2
4. None of the above

Explanation

1. Simplest type of receiver since it does not involve mixing and IF operation.

2. Very much suitable to receive a single frequency.

3. TRF receives have good sensitivity.

6. A RF carrier of 700 kHz at 10 V peak value is amplitude modulated by an audio signal of 2 kHz at 5 V. The sidebands produced are:

1. 702 kHz
2. 698 kHz
3. 702, 698 kHz
4. 72 kHz

Explanation

The sideband frequencies are fc ± fm = (700±2) KHz

∴ The sidebands produced are: 702, 698 kHz

7. To improve selectivity, the bandwidth of a receiver can be reduced by which of the following methods?

1. Using fewer tuned circuits
2. Using a loudspeaker
4. Using more tuned circuits

Explanation

Selectivity is the measure for the receiver to check the desired signal is captured at the output or not.

• Receiver bandwidth should match with the modulating signal for better sensitivity.
• in digital systems, the minimum theoretical bandwidth is Nyquist bandwidth.
• The selectivity of a receiver is evaluated by the relative intensity of a signal received from an external source.

Tuned circuit

This is defined as any electrical conducting path between the Inductor and Capacitor. The circuit consists of both the elements as inductor and capacitor.

Simultaneous variation of multiple frequencies is not possible with the tuned circuits. To determine the suppression factor IRR (Image Rejection Ratio)  is used.

$IRR\: = \:\alpha = \frac{{{G_{fs}}}}{{{G_{fsi}}}} = \sqrt {1 + {Q^2}{\rho ^2}}$

$\rho = \frac{{{f_{si}}}}{{{f_s}}} – \frac{{{f_s}}}{{{f_{si}}}}$

fs: Tuned frequency

fsi: image frequency

Practically this is high for the better receivers.

The IRR can be improved by the tuned circuits connected in cascade mode.

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