Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
1998-11-19
2001-06-12
Pham, Chi (Department: 2631)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C375S376000, C327S156000, C327S147000, C329S325000
Reexamination Certificate
active
06246738
ABSTRACT:
TECHNICAL FIELD
This application pertains to a phase detector which controls a phase locked loop by modulating a synchronization clock reference signal with a modulation signal having modulation frequencies outside the narrow loop bandwidth of the phase locked loop to provide very fine interval resolution and minimize clock wander.
BACKGROUND
The phenomenon of “clock wander” in synchronous wide area networks (WAN) can cause errors in transmission of digital signals over such networks. Error-free signal transmission over such networks requires that data enter a network node at the same rate as data emerges from the node. If this condition is not satisfied at all network nodes by implementation of an appropriate network synchronization scheme, then some data can be lost.
In the prior art, network synchronization has been implemented by means of “building integrated timing supply” (BITS) clocks, or by Digital Signal 1 (“DS1”, formerly “T1”) signals consisting of framed all ones. As specified in ITU-T G.703, DS1 signals comprise a 1.544 Mbps time division multiplexed bit stream. More recently, Optical Carrier level N (“OC-N”) signals have been used synchronize multiple WAN clocks. OC-N signals are optical signals, with “N” being a multiplier (i.e. N=1, 3, 12, 48, 192, . . . ) applied to a basic 51.84 Mbps optical signal. Ideally, all network clocks are synchronized to a primary reference source (PRS). This is typically achieved via phase locked loop (PLL) methodology.
The performance characteristics of frequency locked loop (FLL) methodologies are relatively poor in comparison to those of PLL methodologies. FLLs use a frequency discriminator to detect loop lock, with a feedback signal being provided to maintain loop lock. The frequency discriminator output level is sufficiently high at higher frequency offsets, but at lower frequency offsets the output level can be so low that it will not suffice to maintain proper loop lock, resulting in “wander” of the synchronization frequency produced by the FLL. For example, the frequency offsets between different network clocks are typically a fraction of 1 Hertz, so a frequency discriminator produces no practically useful output. Phase detectors and PLLs are accordingly preferred in network synchronization methodologies.
Synchronization quality can be defined in accordance with any one of several national or international telecommunication or organizational specifications, such as Bellcore, ITU-T, ANSI, etc. The “wander generation” and “wander transfer” parameters of a clock are defined in terms of “maximum time interval error” (MTIE) and “timing deviation” (TDEV), as will now be explained.
“Wander generation” is a measurement used to quantify the amount of “wander” generated by a clock synchronization circuit. It is well known that digital clocks exhibit pulse position modulation errors. That is, the edges of clock pulse signals output by digital clocks deviate from the ideal clock edge positions. The term “jitter” is used to describe short term signal variations, such as pulse position modulation frequencies which exceed 10 Hz. The term “wander” is used to describe longer term variations of significant digital signal properties (e.g., zero level crossings) from their ideal positions in time, and is applied to pulse position modulation frequencies below 10 Hz. Jitter is typically attributable to additive Gaussian noise, whereas wander is typically attributable to slowly varying environmental conditions.
A variety of different sources can introduce low frequency modulation resulting in wander. These include changes in operating temperature and 1/ƒ noise. An oscillator locked to a wander-free reference clock in a narrow bandwidth PLL will exhibit wander above the frequencies at which the open loop gain is below 0 dB. In digital network synchronization, very narrow bandwidth (below 0.1 Hz) PLLs are used to filter accumulated wander from the PRS to generate a timing reference signal. Such clocks must satisfy short term stability requirements and limit the amount of wander which they generate. For example, the Bellcore and ANSI standards stratify clocks according to their long term stability, and limit their short term stability.
“Wander transfer” is a measurement used to quantify the amount of wander that is transferred from a “wandering” input reference to an outgoing OC-N signal. The synchronization reference circuit containing the PLL should filter wander above 0.3 Hz according to the ITU-T G.813 standard.
Optical signal telecommunication systems are characterized by various sources of wander besides those mentioned above. For example, changes in operating temperature cause wander by varying the light propagation characteristics of the optical signal-carrying fiber. A light pulse propagates through a long fiber with a given velocity. If the fiber's index of refraction changes due to temperature variation, the signal propagation velocity through the fiber is also changed. This is mathematically equivalent to a change in the physical length of the fiber (i.e. as though the distance between the signal transmitter and receiver had changed, relative to the transmitter-receiver distance at a different fiber temperature). Consequently, data transmitted into one end of the fiber at one frequency is received at the opposite end of the fiber at a different frequency. The output frequency of the laser which produces the optical signal is also temperature dependent. Further, the propagation delay of light pulses transmitted through the fiber varies as a function of signal wavelength, producing a further frequency differential effect.
As previously explained, jitter is defined in terms of frequencies exceeding 10 Hz, whereas wander is defined in terms of frequencies below 10 Hz. Consequently, jitter and wander are specified in different ways. Jitter is specified in terms of UI (unit interval) RMS or Peak-to-Peak values, but may also be specified in units of time (e.g. nanoseconds) or phase (e.g. degrees). Wander is specified in terms of maximum time interval error (MTIE) and time deviation (TDEV).
MTIE describes the frequency offset of a clock from its ideal frequency, and the phase changes of the clock, over an “observation” period. MTIE is specified in units of nanoseconds of peak-to-peak wander. TDEV describes the spectral content of the clock and is specified in units of RMS nanoseconds of wander.
In order to determine the MTIE and TDEV parameters of a clock, one must measure the clock's time interval error (TIE). TIE is the time difference between an ideal clock and the observed clock, as will now be explained with reference to
FIG. 1
in which &Dgr;&tgr;1, &Dgr;&tgr;2, . . . represent TIE.
The observation time interval, &tgr;, can be set to any convenient value (it is independent of clock frequency). At the end of the observation time interval a measure is made of how much the observed imperfect clock has deviated from the ideal clock (the phase difference). Longer observation times should produce larger values of &Dgr;&tgr;. The MTIE for a particular observation interval is simply the maximum TIE that results from an infinite number of TIE measurements, each performed after particular observation intervals.
By definition, the maximum wander frequency is 10 Hz, so it is possible to completely describe the wander signal up to 10 Hz if the TIE is sampled at twice this rate (the Nyquist rate of 20 Hz; although a higher rate is preferably used to avoid any aliasing). The TIE can be measured and recorded at 50 ms intervals, then by setting different “sliding windows” equal to different observation intervals, the maximum TIE values can be determined by grouping the measured values into the sliding windows. A constant frequency offset will produce the same MTIE value for a given observation period; MTIE will increase linearly with increasing observation time. MTIE is a good metric of frequency offsets between an observed clock and an ideal clock, and also provides a record of any phase transients (i.e. one-shot phase jumps).
MTIE
Acimovic Predrag
Huscroft Charles Kevin
Oyen Wiggs Green & Mutala
Pham Chi
PMC-Sierra Ltd.
Tran Khai
LandOfFree
Phase modulated reduction of clock wander in synchronous... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Phase modulated reduction of clock wander in synchronous..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Phase modulated reduction of clock wander in synchronous... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2479310