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  Converter Operation

Introduction to Converter Operation

Within this stage, a single transistor (BF 194) is employed. This stage contains local oscillator and mixer. The antenna coil is connected in the input section (Base) and IF transformer is connected in the output section (Collector). The oscillator coil is connected in emitter section.

A Hartely oscillator is Local oscillator. This oscillator comprises got two sections, an amplifying section and frequency determining. The frequency determining section contains coil (L), tuning trimmer and condenser. The amplifier section contains transistor. The feedback network is employed to maintain the oscillations.

Antenna coil, trimmer and Tuning condenser are connected in the input (Antenna) tuning section.

It chose wanted signal and similar is feed to the base. The oscillator signal is feed to the emitter. The two signals are merged in series. While mixing, five frequencies are generated.

Intermediate frequency signal is chosen in the output. In collector section the IF transformer is a tuned transformer. It is tuned to IF signal (455 KHz).

IF signal = Fo - Fs

Fo = Oscillator Frequency

Fs = Signal Frequency

To the IF amplifier the output IF signal is applied.

 

It is a tuned RF amplifier. It is planned like a transformer coupled amplifier. Two stages of amplifier are employed in receiver.

FUNCTIONS

1st   function it amplifies the strength of IF signal

2nd function it enhances the sensitivity ot the receiver.

3rd function it keeps the bandwidth.

WORKING

2 x BF 195 transistors are employed. In the input and output sections the IF transformers are connected. In this stage they are tuned transformers and tuned to IF signal (455 KHz). Input signal is feed to the transistor's base. The output signal is occupied from the collector. Both of the two transistors amplify the strength of IF signals.

The primary section In IF transformer works like parallel tuned circuit. Tight coupling is applied among primary and secondary coils to preserve the band width (10 Khz). The output signal of the amplified is applied to the detector.

NECESSITIES OF COMMON INTERMEDIATE FREQUENCY

In the dial If any radio station is tuned, it is intended to obtain common IF. if not, we must use 100s of IF stages for 100s of radio stations. It is impossible. Therefore common IF is employed.

FACTORS DECIDING THE IF

The factors that are deciding the intermediate frequency are as follows:

1. In the broadcast band the intermediate frequency should not fall.

2. To prevent adjacent channel interference, low I.F should be selected.

3. To prevent image frequency interference, high IF should be selected.

4. Lower intermediate frequencies are more stable.

By referring the above factors, 455 to 460 KHz is employed as IF of AM radio receivers. Several AM receivers use 455 KHz as IF.

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