Data Communication Circuits, Data Communication Codes

When 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 narrowband FM.

When 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 narrowband FM.

Reactance 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.

In 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.

If 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.

 i)  The amplitude of FM is constant. It is independent of depth of modulation. Hence transmitter power remains constant in FM whereas it varies in AM.

 ii) Since 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.

 iii)  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.

 iv)  Since guard bands are provided in FM, there is less possibility of adjacent channel interference.

 v) 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 interference.

 vi)  Since FM uses UHF and VHF ranges, the noise interference is minimum compared to AM which uses MF and HF ranges.

Carson‟s rule:

 Carson‟s rule of FM bandwidth is given as,

BW = 2(D f+ fm (max))

Here _f is the maximum frequency deviation and fm (max)) is the maximum signal frequency.

In 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.

In 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

Coherent (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.

Non 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 exploited.

ASCII code

ASCII in ANSI X3.4

Corresponding CCITT recommendation is IA5 (International Alphabet No.5)

·     –ISO specification is ISO 646

Total 128 codes

·     –96 codes are graphic symbols (in Col. 2~7).

·     94 codes are printable

·     And 2 codes viz. SPACE & DEL characters arenon printable

·     32 codes control symbols (Col. 0 & 1) All are non printable

EBCDIC code

·     It is an 8-bit code with 256symbols

·     No parity bit for error checking

·     The graphic symbols are almost same as ASCII

·     Several differences in Control characters as compared to ASCII

BAUDOT code

·     It is a 5-bit code also known asITA2 (International TelegraphAlphabet No. 2).

·     32 codes are possible. With the help of Letter shift & Figure shift key same

code is used torepresent two symbols.

·     Maximum symbols in this code are58

·  Used in Telegraphy/ Telex

Antipodal Signals

 In BPSK, the two symbols are transmitted with the help of following signals,

Symbol‟s1 (t)„1=2P cos (2Df0 t)

Symbol‟s2 (t)„0=2P cos (2Df0 t + _)

Here observe that above two signals differ only in a relative phase shift of 1800. Such signals are called antipodal signals.