Self-clocking signal

The clock recovery (also symbol clock synchronization) in the digital transmission technique has the objective to determine from a received digital signal to the transmit clock of the transmitter and therefore to enable the timely sampling the received signal, or the returned in the opposite direction digital signal properly time align ( sync ). Clock recovery is necessary on the receiver side, to determine the periodic sampling of the received data stream. Without this required precise alignment of the digital receive signal can not be correctly evaluated in accordance massive bit error would result. Provides a method of transfer clock recovery, it can be dispensed with the elaborate parallel clock transmission via a private communication channel for synchronization.

In order to win from the received signal to the transmit clock, the received signal must be sufficiently many signal edges have. This means that long sequences of '1 'or '0' should be avoided without edges in the transmission. This can be achieved through an appropriate channel coding and the use of appropriate line codes such as the biphase mark code or the 4B5B code. Further statistical process on the transmission side as scrambler can be used which transform the transmitted pseudo-random signal, and thus provide a sufficient number of signal edges in the middle of the clock recovery available.

Method

In principle, for clock recovery, two different methods are used:

• Non-linear distortion of the input signal,
• Decision feedback method with adaptive filters.

They are briefly described in the following sections.

Clock recovery by non-linear distortion

In this method, the signal edges are obtained by a non-linear distortion of the input signal, which is then band-pass or a PLL, followed by threshold decision result in the symbol clock. In principle, various non-linear functions can be used here, such as squaring or differentiation with subsequent formation amount ( rectification) of the input signal.

In practice, however, almost all the differentiation with magnitude formation in combination with a PLL has significant meaning. That is, the temporal variations of the received signal are rectified and used as a reference frequency for the PLL capture range roughly matched. In the steady state of the PLL clock thus obtained is then the same frequency and phase locked to the transmit clock.

The disadvantage of this method is that the exact phase can not therefore be recovered. In practical systems, the exact sampling of reception data is then usually carried out by a fixed threshold in the receiver phase offset ( skew ). This time offset is the exact sampling within the time duration of a symbol time offset dar. This must therefore be of multi -frequency system of the respective frequency, for example the table, are derived.

Clock recovery by decision feedback

In this method of clock recovery will be made of the so-called Nyquist condition first use. This condition indicates that the time -symbol interference, also known as intersymbol interference disappears. The prerequisite for virtually all transmission channels and their distortions, the use of adaptive Kanalentzerrern for compensation.

If the transmission channel equalized accordingly and meets the inter-symbol of freedom, can be recovered exactly after the individual transmit pulses of transmit clock before and by synchronizing on the zero crossing of the received signal. This information is averaged over time and serves as a control signal for a controllable oscillator.

The advantage of this process is that it recovers exactly not only the sampling frequency, but also the sampling phase. The disadvantage is the cost, which is connected to the adaptive filters. In particular, this necessary training sequence at the beginning of a transfer must be provided as part of the transmission protocol.

Another form of clock recovery by means of a feedback control loop is to apply a modified Costas loop. Usually, this loop is for coherent demodulation of digital angle modulation such as phase shift keying used. Rather than to synchronize to the carrier frequency, an appropriately modified Costas loop can also be used to recover the symbol clock. Here, too, the phase angle is recovered correctly.