1. Field of the Invention
This invention relates in general to the recovery of the timing of a digital input signal at a receiving end of a digital communication network, and more particularly to a phase/frequency comparator included in a timing recovering circuit.
A basic function of any receiver included in a digital transmission system is to recover a local clock signal to regenerate the bits of a received digital input signal The receiving end decides the probable value of a binary element of said input signal by sampling the input signal as a function of the clock signal. The input signal is e.g. a video signal transmitted in a digital form via an optical fiber medium.
2. State of the Prior Art
The timing (or clock) recovering circuit is usually devised in the form of a phase locked loop.
Such a timing recovering circuit essentially comprises a phase comparator, an intermediary means principally for low pass filtering, and a voltage-controlled local oscillator producing a clock signal. The characteristics of the recovering circuit depend notably on the comparator used. As one of the essential problems with digital transmission is the generation of the clock signal for sampling the input signal received in the receiving end, the frequency and phase of the clock signal must be synchronized with those of the input signal.
The local clock signal is initially asynchronous with the input signal, and the timing recovering circuit is intended to lock the clock signal onto the frequency of said input signal.
The frequency locking takes places at the end of an acquisition process if the initial difference between the respective frequencies of the clock and input signals at the transmission and receiving ends is lower than a predetermined value, the latter being a function of parameters of the timing recovering circuit.
The maximum frequency difference for which the locking of the local clock signal with the received input signal is acquired is called the acquisition or locking range. For the timing recovering circuit in the receiving end to function properly, the acquisition range must be greater than an uncertainty interval related to the precision of the oscillator used in the transmission end.
For instance, for a 140-Mbit/s transmission system in which a transmission end uses a clock with a frequency precision of .+-.10.sup.-4, the acquisition range of the local oscillator must not be less than 2.times.140.times.(10.sup.3 .times.10.sup.-4).ident.28 kHz.
In this way, proper functioning of the receiving circuit is guaranteed for given variations of the transmission frequency by "matching" the local oscillator in the receiving end to the frequency of the signal received during the acquisition process in response to e.g. a frequency jump.
After this frequency locking is achieved, the recovered local clock signal must have low phase jitter. Indeed, phase shifts between the locked regenerated clock signal and the received signal to be regenerated must be sufficiently low to avoid deteriorating the performance of the transmission system in the steady state after frequency acquisition.
In the state of the prior art, increasing of the frequency acquisition range and decreasing of phase noise in the steady state are antagonistic constraints, and in a large number of applications it is difficult or even impossible to find a happy medium without recourse to acquisition aiding techniques.
Frequency sweeping is a known acquisition aiding technique which consists, during the acquisition process, in sweeping the nominal frequency of the local oscillator over the uncertainty interval of the frequency of the received signal, by means of an external signal which is inhibited as soon as the acquisition is achieved. The timing recovering circuit is then locked onto the frequency of the received signal. However, it is a well known fact to those skilled in the art that frequency sweeping has the disadvantage of slowing down the acquisition process, especially in a transmission chain comprising several timing recovering circuits. The present invention was developed to avoid the preceding disadvantages, according to the prior art.