Question: What are the limitations of TRF receiver. Explain how these limitations are avoided using superheterodyne receiver.
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Mumbai University > Information Technology > Sem 3 > Principles of analog and digital communication

Marks:- 5M

Year:- May 2016

 modified 2.8 years ago  • written 2.8 years ago by
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• Poor selectivity and low sensitivity in proportion to the number of tuned amplifiers used.

• Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections.

• An additional problem for the TRF receiver is tuning different frequencies. All the tuned circuits need to tune together to the same frequency or track very closely. Another problem to keep the narrow bandwidth tuning. Keeping several tuned circuits aligned is difficult.

• The bandwidth of a tuned circuit doesn’t remain constant and increases with the frequency increase.

• The need to have all RF stages track one another

• Instability due to large number of RF stages.

• Received bandwidth increases with frequency (varies with center frequency)

• Gain is non-uniform over a wide range of frequencies.

• IF stage permits use at very high frequencies.

• Because many components operate at the fixed IF, they can be optimized.

• Less expensive.

• Better selectivity

• Improved circuit stability.

• Uniform gain over a wide range of frequencies

Use of the fixed lower IF channel gives the following advantages:

1. for a given Q factor in the tuned circuits, the bandwidth is lower making it easier to achieve the required selectivity.

2. At lower frequencies, circuit losses are often lower allowing higher Q factors to be achieved and hence, even greater selectivity and higher gain in the tuned circuits.

3. It is easier to control, or shape, the bandwidth characteristic at one fixed frequency. Filters can be easily designed with a desired Bandpass characteristic and slope characteristic, an impossible task for circuits which tune over a range of frequencies.

4. Since the receiver selectivity and most of the receiver pre-detection gain, are both controlled by the fixed IF stages, the selectivity and gain of the superheterodyne receiver are more consistent over its tuning range than in the TRF receiver.