20-07-2013, 02:41 PM
FREQUENCY SHIFT KEYING (MODULATION AND DEMODULATION)
AIM:
To design and study the working of FSK modulation and demodulation with the help of a suitable circuit.
THEORY:
As its name suggests, a frequency shift keyed transmitter has its frequency shifted by the message.
Although there could be more than two frequencies involved in an FSK signal, in this experiment the
message will be a binary bit stream, and so only two frequencies will be involved. The word „keyed‟
suggests that the message is of the „on-off‟ (mark-space) variety, such as one (historically) generated by a
morse key, or more likely in the present context, a binary sequence.
Conceptually, and in fact, the transmitter could consist of two oscillators (on frequencies f1 and f2), with
only one being connected to the output at any one time.
Unless there are special relationships between the two oscillator frequencies and the bit clock there will
be abrupt phase discontinuities of the output waveform during transitions of the message.
Bandwidth:
Practice is for the tones f1 and f2 to bear special inter-relationships, and to be integer multiples of the bit
rate. This leads to the possibility of continuous phase, which offers advantages, especially with respect to
bandwidth control.
FSK signals can be generated at baseband, and transmitted over telephone lines (for example). In this
case, both f1 and f2 (of Figure 2) would be audio frequencies. Alternatively, this signal could be
translated to a higher frequency. Yet again, it may be generated directly at „carrier‟ frequencies.
PROCEDURE:
The connections are made as per the circuit diagram.
Message signal of amplitude 5v and frequency 150 Hz is applied to the base of the transistor
Carrier C1(t) of 200Hz and 7v is applied at the collector of the NPN transistor.
Another carrier C2(t) of 2 kHz and 7v is applied at the Collector of the PNP transistor.
After getting the FSK output waveform, calculate
t max and t min . Therefore f max = 1 / t max and
f min = 1 / t min.
Calculate the frequency deviation as f = f max – f min
Calculate the modulation index, = f / fm.
Then apply the FSK output to the input of the Demodulation circuits and gets the demodulated output.