Regenerative Repeater
In telegraphy, a device that receives current pulses and corrects their shape and duration and further transmits them. Regenerative repeaters are used during many pulse retransmissions in order to improve the quality of data transmission.
Repeaters are actually amplifiers with suitable frequency response characteristics, placed at regular intervals along the transmission channel to overcome signal. The repeaters used in a digital link are necessarily regenerative, i.e., they amplify and as well as regenerate the error free transmitted signal from distorted and attenuated received signal through a decision deice at the receiver end. Three basic functions are performed by regenerative repeater as follows. (i) Reshaping of the incoming pulse train using an equalizing filter. (ii) Extracting necessary timing information for sampling. (iii) Decision making based on the state of transmission from sampled values.
Equalizer
The function of the equalizer is to shape the received pulse so as to compensate the phase and amplitude distortion by the transmission channel. Definition of Equalizer: An equalizer is a filter that compensates for the dispersion effect of the transmission channel.
Equalizer restores higher frequency components and eliminates pulse dispersion. Distortion is the form of dispersion which is caused by an attenuation of high frequency components of the pulse train. Equalizer: remove ISI Drawbacks: --it also increases received channel noise by boosting its high frequency components --Pulse dispersion results in ISI and consequence increases in detection error probability.
To minimize the interferences by the transversal filter equalizer also known as zero forcing equalizer.
Zero forcing equalizer: zero forcing equalizer forces the equalizer output pulse to have zero
value at the sampling instant (or decision making). I.e. equalizer output pulses should have satisfies the Nyquist criterion or controlled ISI criterion. Another approach to equalization
1. LMS error equalizer: this do not force pulse samples to zero at 2N point i.e. mean of
squared errors over set of output samples minimized
2. Automatic equalizer: automatic means the setting of tap gain of equalizer can be done
automatically by using a special sequence of pulses prior to data transmission.
3. Adaptive equalizer: In adaptive equalization the tap gain are adjusted continuously during
transmission. The timing circuitry provides a periodic pulse train derived from the received pulse at instants where the SNR is maximized. The decision making device (regenerator) which is enabled at the sampling instant determined by the timing circuit makes its decision based on whether or not the amplitude of the quantized signal plus noise exceeds a pre-determined (threshold) voltage level i.e. it makes a decision whether a positive or negative pulse is present. Decision device is actually made of averaging circuit with a comparator with one end connected with the threshold level. In this way the noise and distortion introduced in a repeater space is completely removed provided the distortion is not too long to produce an error in decision making.
Eye Pattern
The ISI and other signal degradation on the oscilloscope (CRO) called eye diagram. The oscilloscope (CRO) shows the super-impose of several traces.
An operation tools for evaluating the effect of ISI in an insightful manner, so it is called eye pattern.
1. The width of the eye opening: It defines the time interval over which the received wave can be sampled, without an error due to ISI. 2. The middle of the eye lid with maximum eye opening: It is the best sampling instant. 3. The rate of closure of the eye: It defines the sensitivity of the system to the timing error. 4. The eye is completely close: It signifies severe effect of ISI and it is impossible to overcome the combined effect of ISI and noise. 5. Squinted (asymmetric) eye: It signifies non-linear transmission distortion.
Interpretation of Eye Diagram
Eye diagram allows one study about the effect of jitter.
Timing Jitter: Small random deviation of the incoming pulses from their ideal location called timing
jitter. Random forms (or cause) of jitter: noise, interferences, and mistuning of the clock circuits. Pattern dependent jitter results from clock mistuning and, amplitude-to-phase conversion in the clock circuit, And ISI, which alters the position of the peaks of the input signal according to the pattern. jitter reduction Anti-jitter circuits Jitter buffers De-jitterizer The rms value of jitter over a long chain of N repeater: J J I J J
Condition: every 200 miles in a long digital link jitter reduction is necessary.
Timing Extraction:
