EXPERIMENT 2
GENERATION AND DETECTION OF FSK SIGNAL
Aim: To design and test the circuit for the generation and detection of Frequency Shift Keying
signal.
Components Required:
Sl.No Components Quantity
Theory
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is
transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK
(BFSK). BFSK literally implies using a couple of discrete frequencies, one for symbol “1” and
another for symbol “0.” With this scheme, the "1" is called the mark frequency and the "0" is called
the space frequency. The waveform of an FSK modulated carrier is illustrated in Figure 2.1.
Audio frequency-shift keying (AFSK) is a modulation technique by which digital data is
represented by changes in the frequency (pitch) of an audio tone, yielding an encoded signal
suitable for transmission via radio or telephone. Normally, the transmission medium dictates the
use of AFSK tones (unlike radio frequency-shift keying, in performing the modulation at baseband
frequency). In radio applications, the AFSK-modulated signal normally is used to modulate an HF
carrier (using a conventional technique, such as AM or FM) for transmission.
AFSK is not always being used for high-speed data communications, since it is far less efficient in
both power and bandwidth than most other modulation schemes. In addition, since AFSK is carried
in the audio band, it has the disadvantage that encoded signals will pass through AC-coupled
telephone links, including most equipment originally designed for carrying voice.
The simplest and most common form of FSK operates on two switches, using the presence of one
carrier wave to indicate a binary one and another one to indicate a binary zero.
Applications
Most early telephone-line modems used audio frequency-shift keying to send and receive data at up
to rates of about 300 bits per second. The common Bell 103 modem used this technique. Bell 202
standard, for 1200 baud communication, used AFSK using 1200 Hz to represent mark and 2200 Hz
for space. Some early microcomputers used a specific form of AFSK modulation, the Kansas City
standard, to store data on audio cassettes. AFSK is still widely used in amateur radio, as it allows
data transmission through unmodified voice band equipment. Radio control gear uses FSK for all
its RF link purposes.
Implementation
To transmit two carrier signals that have different frequencies, we have used two transistors that are
complementary to each other. Either SL100 or SK100 will be ON depending on whether m(t) is
high or low respectively. A circuit diagram for FSK generation and detection is shown.
When the square wave message is high or logic „1‟, carrier wave c1(t) gets transmitted to the emitter
of the transistor and carrier c2(t) is transmitted when the message signal is low or logic „0‟.
Therefore, the output will be a continuous wave but with different frequencies.
In the demodulation process, the input RC combination filters out the low frequency part and input
to the diode becomes an ASK wave form. Now we have the task of only detecting the ASK that is
already explained in the earlier experiment.
The expected waveforms at different points of the generation and detection circuits are shown.
Circuit Diagram:
Design:
Let IC = IE = 1mA , Collector voltage VC = 2.5 V , VCEsat = 0.3 V , VBEsat = 0.7 V .
Hence VRE = VC - VCEsat = 2.2 V.
RE = VRE / IE = 2.2 / 1 x = 2.2 KΩ = R3 = R4
IB = IE / 5 = 0.2 m A . Transistor is used in saturation and current gain is 5.
Using KVL , VB = VRB + VBESAT +VRE
4 = IB RB + 2.9 , RB = 5.5 KΩ = R1 , Choose 6.2 KΩ .
Detection :
LPF : FC = 1.5 KHz ,
FC = , Let C = 0.1µF , R = 1KΩ = R7
LPF : Fm = 300 Hz,
Fm = , Let C = 0.1µF , R = 5.3 KΩ = R9 , Choose 5.6 KΩ.
Wave forms at different stages of ASK generation and detection.
Procedure :
FSK generation :
1. Setup the circuit as shown in figure for the FSK generation part.
2. Apply m(t) , 8V(p-p) , fm = 300 Hz Square wave
3. Apply c1(t) = 4V(p-p) , fc1 = 5KHz Sine wave and c2(t) = 4(p-p) , fc2 = 1.5 KHz Sine wave.
4. Observe the FSK signal at the output of summer circuit .
FSK detection:
1. Setup circuit for FSK detection.
2. Observe the signal at LPF and envelope detector.
3. Vary the reference DC voltage from 0V to desired value (approximately 0.2V) such that the
square wave is observed at output.
4. Observe the amplitude and frequency of the output signal.
5. Note all the waveforms in graph sheet and tabulate the results.
Result
Measurement m(t) C1(t) C2(t) FSK Envelope Reference m’(t)
Vp-p Vp-p Vp-p Output detector DC Vp-p
Vp-p Output Voltage
Vp-p V
Amplitude
Frequency(Hz) NA NA NA
