Another Monday, by John renbourn from the LP Another Monday.
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FM Modulator & Demodulator - a knol by Abdalla Sokar
Page 1 of 3
FM Modulator & Demodulator Two different Architectures Circuit 1:FM Modulator:
In Figure-1 -We are only using the VCO section of the IC (LM565) as the modulator in which the frequency of the VCO is governed by the following equation
Where Rt: the resistance connected to pin 8 Ct: the capacitor connected to pin 9 Vc: voltage on pin 7 -C3 is to remove the DC from the i/p signal -R6 &C4 are used to mainly block the effect of the circuit on the i/p signal The i/p signal is added to the voltage on pin 7 to form an AC signal with a DC voltage controlled by R3 such that the voltage on pin 7 will control the operation of the VCO as when Vc increases the frequency decreases and vice versa which indicate that we frequency modulated the i/p signal ----------------------------------------------------------------------------------------------------------------------------------------------------------------FM Demodulator:
http://knol.google.com/k/fm-modulator-demodulator
12/27/2010
FM Modulator & Demodulator - a knol by Abdalla Sokar
Page 2 of 3
-By C104, R107 & R106 we control the free running frequency of the PLL in which R106 is used to adjust the free running frequency to be exactly like that of the FM modulator. -C104 & R105 are used to isolate the 2 inputs of the phase detector as we the signal must enters one input at a time -R102 & R103 are used to make a voltage divider as we only have one voltage source. -C102 is used to bypass the RF i/p to the biasing point. -The control voltage of the VCO which was used in the PLL to track the i/p is the o/p of the Demodulator as it consists of AC and DC, where the AC signal is the detected signal & the DC corresponds to the average frequency on the FM input, it also consists of some traces of the carrier used which is removed by the low pass filter (R109, R110 & C108). -C106, C107 & R108 are used as a loop filter that controls the Capture range of the PLL. ---------------------------------------------------------------------------------------------------------------------------------Audio Amplifier:
This simple amplifier shows the LM386 in a high-gain configuration (A = 200). For a maximum gain of only 20, we used a 10 uF capacitor to connect pin 1 to pin 8. Maximum gains between 20 and 200 may be realized by adding a selected resistor in series with the same 10 uF capacitor. The 10k potentiometer will give the amplifier a variable gain from zero up to the maximum.
The input signal go through the Vin node, by passing the signal through the 10uF capacitor ,we can block the DC voltage so that we can add the desired DC voltage which is 5 volts using the voltage divider by two resistor each is 10Kohm with the 10 volts supply. We use the VCO in this CD4046 IC as a frequency modulator, for each frequency the VCO give as a certain corresponding voltage, these voltages can be sent to the Second PLL (frequency demodulator).
http://knol.google.com/k/fm-modulator-demodulator
12/27/2010
FM Modulator & Demodulator - a knol by Abdalla Sokar
Page 3 of 3
For the demodulator, the CD4046 IC works with the low-pass filter (the 10k resistance and the 1000pF capacitor after the second IC) to perform a closed loop PLL so that it can works as a FM demodulator. The op-amp is a unity-gain voltage follower and the final RC low-pass filter (the 5k resistance and the 100pF capacitor) filters the carrier frequency. A filtering capacitor (0.1 uF ceramic disc) should be connected between +10V and ground of each IC.
