FSK Modulators using IC 555

FSK means Frequency Shift Keying. It’s one of the Modulation Scheme used to Transmit Digital Data using High Frequency carrier signal. In this Modulation Scheme bit 1 is transmitted using one carrier signal and bit 0 is transmitted using another carrier signal like

· Bit 1 (logic high) is Transmitted using Carrier Frequency F_c1

· Bit 0 (logic low) is Transmitted using Carrier Frequency F_c2

It means Series of bits (bit stream) are transmitted with two different carrier signals used for bit 1 and bit 0.

The figure given below shows waveforms for FSK Modulation

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The First Waveform is bit pattern – data bits. 2^nd Waveform shows FSK Modulation – for data bit 1 it transmits one carrier (frequency Fc1) and for data bit 0 it transmits another carrier (frequency Fc2). Usually carrier frequencies are higher than (in terms of 100s of KHz) data rate. So in single bit period several cycles of carrier are transmitted.

The Circuit given here illustrates how FSK Modulated Wave can be generated. It is build using IC555. A Square Pulses are given as input to represent bit 1 and bit 0, and as an output IC555 generates FSK Modulated Wave. To generate Square Pulses one more IC555 is used.

FSK Modulator

As shown in figure IC 555 is configured as Astable Multi-vibrator. It generates two different frequencies. It generates one frequency F1 when transistor Q1 is OFF and another frequency F2 when transistor Q1 is ON.

· When Q1 is OFF R3 is not connected in the circuit. The frequency is given by

F1 = 1.44 / (R1 + 2×R2) × C1

· When Q1 is ON, R3 get connected in parallel with R1. So frequency is given by

F2 = 1.44 / (R’ + 2×R2) × C1

Where R’ = R1×R3 / (R1+R3)

The Square Pulses (bit stream) are applied at the base input of Q1. So for bit 1 input to Q1 is high, so it is ON, so IC555 generates frequency F2 in the output. Similarly for bit 0 – input to Q2 is low, so it is OFF, so IC555 generates frequency F1. So we get one frequency F1 for bit 1 and other frequency F2 for bit 0 as per requirement.

Please refer Circuit Diagram in the Circuit Diagram Tab above

Pulse Generator Circuit

Here also IC555 is connected in Astable mode. It will generate continuous pulses. It generates pulses at 1 KHz frequency. The frequency is determined by RC components R4, R5 and C3 as

F = 1.44 / (R4 + 2×R5) × C3

Circuit Designing

Let us see how values of R1, R2, R3 and C1 can be found out or calculated. I have selected F2 as 120 KHz for bit 1 and F1 as 90 KHz for bit 0.

Assume C1 = 1 nF = 1 × 10^-9F

Substituting this value in to above equation

F1 = 1.44 / (R1 + 2×R2) × 10^-9

Because F1 is 90 KHz

90 × 10³= 1.44 / (R1 + 2×R2) × 10^-9

R1 + 2×R2 = 1.44 /( 9 ×10⁴× 10^-9)

R1 + 2×R2 = 16000

Now to get around 50% duty cycle R2 > R1 so let us take R1 = 4.5 K. So

2×R2 = 16000-4500 = 11500

So R2 = 5750 = 5.75 K

The nearest practical value can be 5.6 K.

Thus finally R1 = 4.5 K, R2 = 5.6 K and C1 = 1 nF. Instead of selecting fixed value of R1, a Potentiometer is used to tune the frequency and get exact value of 90 KHz.

Now let us find out value of R3 for frequency F2 = 120 KHz

F2 = 1.44 / (R’ + 2×5600) × 10^-9

Because F1 is 90 KHz

120 × 10³= 1.44 / (R’ + 11200) × 10^-9

R’ + 11200 = 1.44 /( 12 ×10⁴× 10^-9)

R’ + 11200 = 12000

R’ = 800

But R’ = R1 × R3 / (R1+R3). So

(R1 × R3) / (R1+R3) = 800

4500 × R3 / (4500+R3) = 800

3700 × R3 = 4500 × 800

Finally R3 = 973

We can select nearest practical value as 1K. But instead of using fixed value 1 K Resistor, a Potentiometer of 1 K is used so that the frequency can be adjusted to exactly 120 KHz.

Similarly we can calculate values of R4, R5 and C3 in pulse generator circuit to generate pulses at 1 KHz. The values are calculated as R4 = 3.3 K, R5 = 5.6 K and C3 = 0.1 µF.

Circuit Setup

The circuit can be built on bread board or on general purpose PCB.

· On bread board, place all the components and make necessary connections as shown in circuit diagram

· On general purpose PCB, mount and solder all the components and connect components as per circuit diagram

Next, apply 5 V to the circuit through power supply and connect output of pulse generator and FSK Modulator to both the channels of DSO (or CRO) using CRO probes.

Circuit Operation

· As 5 V supply is given to circuit, Pulse Generator will generate square pulses of 1 KHz

· These pulses are given to base input of Transistor. Because Transistor is connected in switch mode, it will switch ON and OFF

· When pulse input to Transistor is high – Transistor is ON – R3 connects in parallel with R1 – IC555 generates 120 KHz frequency in the output

· When pulse input Transistor is low – it is OFF – only R1 is connected in IC555 circuit – it generates 90 KHz frequency in the output

· In place of giving Square Pulses as input we can give bit stream of 1’s and 0’s also

Here is the Image of Circuit Output on DSO