08-01-2013, 02:58 PM
DESIGN OF FM TRANSCEIVER
AIM:
To transmit a modulating signal after frequency modulation using VCT-12 and receive the signal back after demodulating using VCT-13
APPARATUS REQUIRED:
5. VCT-12 trainer kit
6. VCT-13 trainer kit
7. CRO
8. Patch cards
HARDWARE DESCRIPTION OF FM TRANSMITTER TRAINER VCT-12:
The FM transmitter trainer kit VCT-12 has the following section:
1. On-board sine wave generator
2. MIC pre amplifier with a socket for external dynamic MIC
3. Audio amplifier for amplification of low level external input signal
4. Frequency modulation
5. Telescopic whip antenna
SINE WAVE GENERATOR:
A sine wave generator acts as an on board modulating signal source and generates an audio frequency sine wave .The amplitude of this sine wave generator varies from 0-5 V. However the output voltage from this source is controlled using a Trim pot to get an output signal in the range of 0-3V.The frequency of the signal varies form 300Hz to 15KHz.Since the amplitude of the source is large enough to modulate the carrier it need not be amplified ,instead it can be directly connected to the input of the amplitude modulator .
MIC PRE AMPLIFIER:
The MIC pre amplifier is capable of accurately amplifying even a very low level signal, picked up by the MIC to the required level to modulate the carrier. This section has a EP socket at its input stage where, in an external dynamic MIC can be plugged in the gain of the stage can be controlled by the user by adjusting the potentiometer Pot4.The maximum gain of this stage can be achieved in this is 200.The maximum level of the input signal to this amplifier, so as to produce an amplified output without saturation is 60mV.
AUDIO AMPLIFIER:
The audio amplifier stage has a BJT common emitter configuration. This audio amplifier can be used to amplify any lower level external modulating signal whose voltage level is below 100mV.The gain of this stage can also be controlled by the user by varying the pot meter POT-5.The maximum gain of this audio amplifier is 10.
FREQUENCY MODULATION:
The frequency modulator circuit is constructed around a BF495, high frequency small signal BJT. The collector circuit of the transistor consists of a tank circuit formed by a inductor and capacitor. This tank circuit together with the transistor acts as an oscillator and produces the carrier frequency .The transistor circuit appears to the oscillator as a variable capacitance. This capacitance adds to the capacitance of the oscillator-tuned circuit.
The size of this capacitance depends on the change in the collector current which occurs for a given change in base voltage and this is determined by the Trans conductance of the transistor .The transistor transconductance depends on the bias voltage applied to the transistor base. The larger the bias voltage, the larger the value of gm and the larger the value of gm and the larger capacitance which is added to the capacitance of the oscillator tuned circuit consequently the transistor circuit behaves as a voltage variable capacitance .The bias voltage applied to the transistor base determines the overall capacitance seen by the oscillator and hence the frequency of the carrier. This resulting in FM signal