DESIGN OF FM TRANSCEIVER - Computer Programming


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Tuesday, June 28, 2011



EXPT : 3(ii)


To transmit a modulating signal after frequency modulation using VCT-12 and receive the signal back after demodulating using VCT-13


1.      VCT-12 trainer kit
2.      VCT-13 trainer kit
3.      CRO
4.      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

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 .

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.


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.


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

A telescopic whip antenna is used to radiate the AM signal generated by the amplitude modulator.




The Fm receiver trainer VCT-13 has the following sections
                  1.FM super heterodyne receiver
                  2.Buffer and filter
                  3.Audio power amplifier        

FM super heterodyne receiver:
The FM receiver is built with the dedicated FM receiver IC-CXA1619IC consists of the following sections namely RF amplifier, Mixer and oscillator, IF amplifier and quadrature detector .The circuit details and the description of IC-CXA1619IC are given in appendix

Buffer and filter:

A buffer is used to prevent any loading to the previous stage .The filter section consists of a BPF with a Pass band to 20KHZ –15MHZ.A notch filter is also included to eliminate the 50Hz power supply noise

Audio power amplifier: 

            The Audio power amplifier     is constructed using ICTBA810 to increase the power level of the demodulated message signal to the required level. The gain of this amplifier can be adjusted by the user by varying the pot meter POT-1.the maximum gain of this audio amplifier is 25.the amplified signal can be given to a loud signal which can be extremely plugged into the VCT-13 trainer


Amplitude (V)
Time Period (msec)

Modulating Signal

Demodulated signal


1.      The circuit wiring is done as shown in diagram
2.      A modulating signal input  given to the Frequency modulator can also be given from a external function generator or an AFO
3.      If an external signal source with every low voltage level is used then this signal can be amplified using the audio amplifier before connecting to the input of the FM modulator
4.      Now increase the amplitude of the modulated signal to the required level.
5.      The amplitude and the time duration of the modulating signal are observed using CRO.
6.      the amplitude and time duration of the modulated signal are observed using a CRO and tabulated.
7.      The final demodulated signal is viewed using a CRO Also the amplitude and time duration of the demodulated wave are noted down


 The modulating signal is transmitted after frequency modulation using VCT-12 and the signal is received back after demodulation using VCT-13

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