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To study the specifications and working of a Transistor radio (AM & FM) kit and perform measurement on it.

Object: To study the specifications and working of a Transistor  radio (AM & FM) kit and perform measurement on it. 

Apparatus: AM/FM Radio receiver, millimeter, connecting probes.

Theory: The kit circuit consists of IC Version 2-Band AM/FM Radio Receiver and Tape recorder In AM receiver again there are two bands i.e. medium wave and shortwave. The shortwave frequency range 535 KHz - 1605 KHz. The FM receiver has frequency band width of 88MHz to 108 MHz. The tape recorder, tape transport mechanism and panel controls of tape recorder.

Fig 1.1: Block diagram of radio receiver 

The way in which the receiver works can be seen by following the signal as is passes through the receiver.

Front end amplifier and tuning block:   Signals enter the front end circuitry from the antenna. This circuit block performs two main functions:
Tuning:   Broadband tuning is applied to the RF stage. The purpose of this is to reject the signals on the image frequency and accept those on the wanted frequency. It must also be able to track the local oscillator so that as the receiver is tuned, so the RF tuning remains on the required frequency. Typically the selectivity provided at this stage is not high. Its main purpose is to reject signals on the image frequency which is at a frequency equal to twice that of the IF away from the wanted frequency. As the tuning within this block provides all the rejection for the image response, it must be at a sufficiently sharp to reduce the image to an acceptable level. However the RF tuning may also help in preventing strong off-channel signals from entering the receiver and overloading elements of the receiver, in particular the mixer or possibly even the RF amplifier.

Amplification:   In terms of amplification, the level is carefully chosen so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved. The amplifier must also be a low noise design. Any noise introduced in this block will be amplified later in the receiver.

Mixer / frequency translator block:   The tuned and amplified signal then enters one port of the mixer. The local oscillator signal enters the other port. The performance of the mixer is crucial to many elements of the overall receiver performance. It should be as linear as possible. If not, then spurious signals will be generated and these may appear as 'phantom' received signals.

Local oscillator:   The local oscillator may consist of a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor. Alternatively it may be a frequency synthesizer that will enable greater levels of stability and setting accuracy.

Intermediate frequency amplifier, IF block:   Once the signals leave the mixer they enter the IF stages. These stages contain most of the amplification in the receiver as well as the filtering that enables signals on one frequency to be separated from those on the next. Filters may consist simply of LC tuned transformers providing inter-stage coupling, or they may be much higher performance ceramic or even crystal filters, dependent upon what is required.

Detector / demodulator stage:   Once the signals have passed through the IF stages of the super heterodyne receiver, they need to be demodulated. Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered.

AM diode detector:   This is the most basic form of detector and this circuit block would simple consist of a diode and possibly a small capacitor to remove any remaining RF. The detector is cheap and its performance is adequate, requiring a sufficient voltage to overcome the diode forward drop. It is also not particularly linear, and finally it is subject to the effects of selective fading that can be apparent, especially on the HF bands.

Synchronous AM detector:   This form of AM detector block is used in where improved performance is needed. It mixes the incoming AM signal with another on the same frequency as the carrier. This second signal can be developed by passing the whole signal through a squaring amplifier. The advantages of the synchronous AM detector are that it provides a far more linear demodulation performance and it is far less subject to the problems of selective fading.

SSB product detector:   The SSB product detector block consists of a mixer and a local oscillator, often termed a beat frequency oscillator, BFO or carrier insertion oscillator, CIO. This form of detector is used for Morse code transmissions where the BFO is used to create an audible tone in line with the on-off keying of the transmitted carrier. Without this the carrier without modulation is difficult to detect. For SSB, the CIO re-inserts the carrier to make the modulation comprehensible.

Basic FM detector:   As an FM signal carries no amplitude variations a demodulator block that senses frequency variations is required. It should also be insensitive to amplitude variations as these could add extra noise. Simple FM detectors such as the Foster Seeley or ratio detectors can be made from discrete components although they do require the use of transformers.

PLL FM detector:   A phase locked loop can be used to make a very good FM demodulator. The incoming FM signal can be fed into the reference input, and the VCO drive voltage used to provide the detected audio output.

Quadrature FM detector:   This form of FM detector block is widely used within ICs. IT is simple to implement and provides a good linear output.

 

Audio amplifier:   The output from the demodulator is the recovered audio. This is passed into the audio stages where they are amplified and presented to the headphones or loudspeaker.

 

Fig 1.1: Circuit diagram of radio receiver

It is a supernetero dyne circuit with a tuned RF amplifier so that maximum signal sensitivity is 1 to 10mV. The RF stage tuned circuits and local oscillator are tuned by a three ganged variable capacitor controlled from a panel knob. The oscillator frequency can be varied from 98.7 MHz to 118.7 MHz. Yielding an intermediate frequency of 10.7 MHz. The IF amplifier section if several high gain stages, of which one or more amplitude limiters. The high gain non limiting input stage followed by one amplitude limiting stage. All stages are sunned to give the desired band pass characteristics this is centered on 10.7 MHz and has a 180 KHz band width to pass the desired signal. Amplitude limiting is usually arranged to have an on set threshold of about 1mV at the limiting - stage input, corresponding to the level of input signal. This may be set at 10mV or lower. In the Radio Receiver RF coil band switches are different remaining stages are same as in the case of super heterodyne receiver. The frequency band or the wave band covered is decided by the inductance of the coil and the capacitance of the variable capacitor that tuned circuits in the antenna section and the oscillator section of the converter stage. In modern super heterodyne receivers it becomes necessary to have shortwave bands in addition to the medium wave (MW), so that stations broadcasting on SW frequencies can also received.

One coil and a variable capacitor can be turned over one band of frequencies. For a second band of frequencies. For a second band of frequencies, a different number of tunes are required both for antenna section and the oscillator section of the converter stage. The variable capacitors, which are parts of a two gang capacitor, will remain the same. Thus for changing one wave band to another, one set of coils and trimmers should be connected to the circuit, simultaneously in the antenna section and the oscillator section of the converter stage of the receiver. This involves a number of switching operations that must be performed before a waveband can be changed. This is done with the help of a special multi contact switch known as the band change switch or a wave change switch. The common most switches used in multiband receivers is rotary switch.

Rotary Switch:

A rotary band switch consists of moveable contacts and fixed contacts are called positions. The number of positions per pole is equal to the number of wavebands in the multiband receiver. Fixed contacts MW, SW1, SW2 are punched don a circular plate of fiber or some other suitable insulating material. A smaller circular plate of the same insulating material carrier two semicircular metallic strips with projections on one end, which will make contact positions MS, SW1, SW2 one by one as the inner fiber plate is rotated with a central shaft. The fixed poles P1 and P2 punched on the outer fiber plate remain permanently in contact with their respective number of strips.

Procedure:

1. Switch on the demonstration board by connecting the power cord to the supply.

2. Check the stages as per the circuit and block diagrams measure the voltage data at each stage.

3. Measure output voltage Vo.

4. Connect the dc supply to the Receiver tune gain capacitor to reach station frequency programme comes from the loud speaker.

Result: We have studied working of Transistor radio (AM & FM).

Precautions:

  1. Make connections right and tight.
  2. Switch off power supply when not in use.
  3. Handle the equipments carefully.

Questions:

  1. Study the given IC version 2-band AM/FM radio receiver. Write testing procedure.

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