COMMUNICATION CIRCUITS, DATA COMMUNICATION CODES.
the modulation index is less than 1, the angle modulated systems are called low
index. The bandwidth requirement of low index systems is approximately twice of
the modulating signal frequency. Therefore low index systems are called
modulator is direct FM, where as Armstrong method is indirect FM. Armstrong
method generates FM from PM. Hence crystal oscillators can be used in Armstrong
method. Therefore frequency stability is better than reactance modulator.
narrow band FM, the frequency deviation is very small. Hence the frequency
spectrum consists of two major sidebands like AM. Other sidebands are
negligible and hence they can be neglected. Therefore the bandwidth of
narrowband FM is limited only to twice of the highest modulating frequency.
the deviation in carrier frequency is large enough so that other sidebands
cannot be neglected, then it is called wideband FM. The bandwidth of wideband
FM is calculated as per Carson‟s rule.
FM has following
advantages over AM.
amplitude of FM is constant. It is independent of depth of modulation. Hence
transmitter power remains constant in FM whereas it varies in AM.
amplitude of FM constant, the noise interference is minimum in FM. Any noise
superimposing amplitude can be removed with the help of amplitude limits.
Whereas it is difficult to remove amplitude variations due to noise in AM.
The depth of modulation has limitation
in AM. But in FM the depth of modulation can be increased to any value by
increasing the deviation. This does not cause any distortion in FM signal.
guard bands are provided in FM, there is less possibility of adjacent channel
Since space waves are used for FM, the
radius of propagation is limited to line of sight. Hence it is possible to
operate several independent transmitters on same frequency with minimum
Since FM uses UHF and VHF ranges, the
noise interference is minimum compared to AM which uses MF and HF ranges.
Carson‟s rule of FM
bandwidth is given as,
BW = 2(D f+ fm
Here _f is the maximum
frequency deviation and fm (max)) is the maximum signal frequency.
direct FM type of angle modulation, the frequency of the carrier is varied directly
by the modulating signal. This means; an instantaneous frequency deviation is
directly proportional to amplitude of the modulating signal.
indirect FM type of angle modulation, FM is obtained by phase modulation of the
carrier. This means, an instantaneous phase of the carrier directly
proportional to amplitude of the modulating signal
(synchronous) detection: In coherent detection, the local
carrier generated at the receiver is phase locked with the carrier at
the transmitter. The detection is done by correlating received noisy signal and
locally generated carrier. The coherent detection is a synchronous detection.
coherent (envelope) detection: This type of
detection does not need receiver carrier to be phase locked with
transmitter carrier. The advantage of such a system is that the system becomes
simple, but the drawback is that error probability increases. The different
digital modulation techniques are used for specific application areas. The
choice is made such that the transmitted power and channel bandwidth are best
ASCII in ANSI X3.4
recommendation is IA5 (International Alphabet No.5)
specification is ISO 646
Total 128 codes
codes are graphic symbols (in Col. 2~7).
codes are printable
2 codes viz. SPACE & DEL characters arenon printable
codes control symbols (Col. 0 & 1) All are non printable
is an 8-bit code with 256symbols
parity bit for error checking
graphic symbols are almost same as ASCII
differences in Control characters as compared to ASCII
is a 5-bit code also known asITA2 (International TelegraphAlphabet No. 2).
codes are possible. With the help of Letter shift & Figure shift key same
code is used
torepresent two symbols.
symbols in this code are58
in Telegraphy/ Telex
In BPSK, the two
symbols are transmitted with the help of following signals,
Symbol‟s1 (t)„1=2P cos
(t)„0=2P cos (2Df0 t + _)
observe that above two signals differ only in a relative phase shift of 1800.
Such signals are called antipodal signals.