FREQUENCY SHIFT KEYING

FSK:SIGNALS AND DEMODULATION

CONCEPTS

Existing concept: Design a FSK modulator using 555/XR 2206 and convert it to sine wave using filter and transmit the same using IR LED and demodulate the same PLL NE 565/XR 2212.

Proposed concept: To design a FSK modulator using XR 2206 monolithic function generator to produce a sine wave and transmit the signal using IR LED over wireless channel and to demodulate it using PLL NE 565.

WHAT IS FSK?

Frequency-shift keying or 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) . The "1" is called the mark frequency and

the "0" is called the space frequency.

MODULATION

The minimum duration of a mark or space condition is called the element Length.

Typical values for element length are between 5 and 22 milliseconds.

An alternate way of specifying element length is in terms of the keying speed(Bauds).

Baud = ______1______ Element length

COMPONENTS IN FSK:

XR-2206: It is a monolithic function generator IC capable of producing high quality square, triangle, ramp and pulse waveform of high stability and accuracy.

PLL or Phase Locked Loop in FSK e.g. PLL 565 is used as a demodulator.

IR LED for transmitting over wireless channelsInstead of PLL 565 we can also use XR-2212 also

as a precision Phase locked loop.

BLOCK DIAGRAM

Laser APD

Digital Information Digital output Source

EncoderLaser Drivercircit

Amplifier and

equalizerDecoder

XR-2206:FUNCTION GENERATOR:

FEATURES Low-Sine Wave Distortion, 0.5%, Typical Excellent Temperature Stability, 20ppm/°C, Typ. Wide Sweep Range, 2000:1, Typical Low-Supply Sensitivity, 0.01%V, Typ. Linear Amplitude Modulation TTL Compatible FSK Controls Wide Supply Range, 10V to 26V Adjustable Duty Cycle, 1% TO 99%

PIN DETAILS OF XR2206

SYSTEM DESCRIPTION:

The XR-2206 is comprised of four functional blocks:

a voltage-controlled oscillator (VCO), an analog multiplier and sine-shaper;a unity gain buffer amplifier;a set of current switches.

XR-2206 BLOCK DIAGRAM

MODULATION PROCESS:

The XR-2206 can be operated with two separate timing resistors, R1 and R2, connected to the timing Pin 7 and 8, respectively.

Depending on the polarity of the logic signal at Pin 9, either one or the other of these timing resistors is activated. Thus, the output frequency can be keyed between two levels.

f1 = 1/R1C and f2 = 1/R2C

NE/SE565:PLL

The NE/SE565 Phase locked loop (PLL) is a self-contained, adaptable filter and demodulator for the frequency range from 0.001Hz to 500kHz.

The circuit comprises a voltage-controlled oscillator of exceptional

stability and linearity, a phase comparator, an amplifier a low pass filter.

BLOCK DIAGRAM:

DEMODULATION USING NE/SE565 PLL

DEMODULATION PROCESS:

The demodulator receives a signal at one of the two distinct carrier frequencies, 1,270 Hz or 1,070 Hz.

Capacitance coupling is used at the input to remove a dc level.

As the signal appears at the input of 565 PLL, the PLL tracks between the two possible frequencies with a corresponding dc shift at the output.

R1 and C1 determine the free-running frequency of the VCO.

A three-stage RC ladder filter is employed for removing the sum frequency component from the output.

Comparator IC 710 is used to compare the decoded output with the reference.

An input at frequency 1,070 Hz and 1,270 Hz drives the demodulator output voltage to a more positive voltage level and less positive output, respectively.

WIRELESS TRANSMISSION:

Any wireless communication system has three important parts.

Transmitter: IR LED

Receiver : Photo diode

Transmission Medium: Air

IR(INFRARED) TRANSMISSION:

Infrared (IR) light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometers, and extending conventionally to 300 micrometres. These wavelengths correspond to a frequency range of approximately 1 to 400THz, and include most of the thermal radiation emitted by objects near room temperature.

BLOCK DIAGRAM:

CONCLUSION

FSK signals take many different forms depending on their intended application where binary FSK is the simplest of them.

With the increasing sophistication of electronics and signal detection theory, it is clear that FSK modulators and demodulators, can be utilized to optimally demodulate the many different digital signal formats.