BINARY FREQUENCY SHIFT KEYING

BINARY FREQUENCY SHIFT KEYING

What is BFSK?

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 uses a pair of discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the "1" is called the mark frequency and the "0" is called the space frequency.

The baseband signal or modulating signal is in binary, whereas the carrier signal is analog.

E002

E006

E012

E014

E022

E023

M-ary FSK

The other type of digital modulation scheme is the M-ary scheme.

In this, we can send any one of the M possible signals during each signalling interval

In an M-ary FSK system the symbols are not one bit long like bfsk but each symbol is N bit long

no. Of possible signals M=2N

Receiver

There are different methods of demodulating FSK. A natural classification is into synchronous (coherent) or asynchronous (non-coherent).

Transmitter

Coherent FSK

Binary FSK Signal and Modulator

Coherent FSK: two signals have the same initial phase Φ at t=0

s1(t) = Acos(2πf1t +Φ), kT ≤t ≤(k +1)T for1

s2(t) = Acos(2πf2t +Φ), kT ≤t ≤(k +1)T for0

In its most general form, the binary FSK scheme uses two signals with different frequencies to represent binary 0 and 1

s1(t) = Acos(2πf1t +Φ1), kT ≤t ≤(k +1)T for1

s2(t) = Acos(2πf2t +Φ2), kT ≤t ≤(k +1)T for 0

Where Φ1 and Φ2 are initial phases at t=0, and T is the bit period of the binary data.

These two signals are not coherent since Φ1 and Φ2 are not the same in general.

The signal is not continuous at bit transitions.

This form of FSK is called noncoherent or discontinuous FSK.

Non-Coherent BFSK Receiver

Any questions?

The receiver consists of two bandpass filters, one with centre frequency fH and other with centre frequency fL.

Since fH - fL = 2fb,

the outputs of filters do not overlap.

The bandpass filters pass their corresponding main lobes without much distortion.

The outputs of filters are applied to envelope detectors. The outputs of detectors are compared by the comparator. If unipolar comparator is used, then the output of comparator is the bit sequence b(t).

Applications

The frequency synthesizer generates two frequencies, f1 and f2 which are synchronized.

The binary input data controls the multiplexer.

The bit timing must be synchronized with the carrier frequencies.

If a 1 is present, s1(t) will pass and if a 0 is present, s2(t) will pass.

1. They are used in point to point military communication.

2. FSK is used in caller id and remote metering.

3. FSK is also used in the united states' emergency alert system to transmit warning information.

4. They are used in car alarm systems and garage door openers.

5. They are used in security systems.

6. They are used in smoke alarm systems.

The noncoherent or discontinuous FSK can be generated by switching the modulator output line between two different oscillators

Block Diagram of N-bit M-ary FSK

Thank You!

BFSK Spectrum

FL is the frequency used for binary 0, also called space frequency

Fh is the frequency used for binary 1, also called mark frequency

Fb is the bit period frequency.

Disadvantages

1. The major disadvantage is its high bandwidth requirement as discussed earlier. Therefore, it is extensively used in low speed modems having bit rates below 1200 bits/sec.

2. Coherent FSK is not used in practice due to the difficulty in generating two reference frequencies close together at the receiver.

3. A large number of correctly tuned filters are required for the reception of m-ary FSK.

4. The FSK is not preferred for the high speed modems because with increase in speed, the bit rate increases as the telephone lines have a very low bandwidth, it is not possible to satisfy the bandwidth requirement of FSK at higher speed. Therefore FSK is preferred only for the low speed modems

Operation:

Coherent BFSK Receiver

 At the transmitter an N bit symbol is presented after a period of Ts¬¬ to an N bit D/A converter

 The output of the D/A converter is applied to a frequency modulator. The frequency modulator generates a carrier waveform whose frequency is determined by the analog modulating signal at the output of D/A converter.

 The transmitted signal, for the duration of the symbol interval has frequencies f0, f1..... or fM-1 where M = 2N. For example if N = 2, then M = 22 = 4. Then there will be four different frequencies corresponding to the four different possible messages i.e. 00, 01, 10, 11.

 The M-ary FSK signal is demodulated using envelope detectors as shown in the figure. At the receiver the incoming signal is applied to M paralleled bandpass filters each followed by an envelope detector.

 The bandpass filters have centre frequencies f0, f1..... fM-1 . Therefore these filters will pass only the signals of their centre frequencies. Remember that at any given instant of time only one frequency is being received.

 The output of bandpass fitters are then applied to the envelope detectors. They produce dc voltage proportional to their ac input voltage.

 Outputs of all the envelope detectors are applied to a device which determines which of the detector is the largest of all at the given time.

 The output of this device is applied to an N bit A to B converter which reproduces the originally transmitted N bit message.

There are two correlators for two frequencies of

FSK signal. These correlators are supplied with locally generated carriers Φ1(t) and Φ2(t).

If the transmitted frequency is fH, then output s1(t) will be higher than s2(t).

Hence, y(t) >0

The device then decides in favour of binary '1'

If s2(t)>s1(t),

then y(y)<0

and the device decides in favour of binary '0'

Advantages

1. It is relatively easy to implement.

2.One of the major advantage is that its better noise immunity due to which probability of error reduces

3. It has constant power requirement.

4. It rejects the unwanted signals that are weaker than the desired signals.

5. It is used in long distance communications.

6. FSK is insensitive to channel fluctuations.

7. Frequency shift keying is used to send teletype and can also be used to send radio control signals