Time and Frequency Sync Applications
There are many applications in the field of time-frequency synchronization. Indeed, many applications require a reference time in order to date measurements or events in a safe manner, or to synchronize equipment or events. Below are explained why synchronisation in the areas of energy, radar and telecommunications. There are still many other areas such as finance/trading or audio/video.
There 3 different steps in the Production and Distribution of Electricity :
- Production: will all different types of power plants
- Transport: The transport network is equivalent to highways and national roads
- Distribution: The distribution network is equivalent to departmental roads : the transmission of electricity to different consumers.
To switch from one network to another, the transformer stations act as exchangers. Thanks to transformer stations, the High Voltage (90,000 or 63,000 volts) is lowered to Medium Voltage (20,000 volts) or Low Voltage (400 or 230 volts).
In France, 757 779 transformers connect the 617.642 km of MV lines to the 697 206 km of LV lines. Electricity is transported to about 33 million consumers (individuals, professionals, industry, local authorities …), whose needs are very varied. Consumption therefore varies continuously throughout the day and year.
The electricity produced by the power stations is not stored. Also, in order to adjust production very precisely to demand, the network relies on dispatchings managing distribution of electricity. Consumption forecasts define the theoretical requirements and adjustments are made continuously during the day. In France, there are :
- 1 national dispatching which manages the interconnection network at 400 000 volts and connexion with foreign countries
- 7 regional dispatchers, which are responsible for regional networks
The daily load graph: RTE, Electricity Transmission Network, a subsidiary of EDF, is in charge of the electricity network in France. Thanks to past measurements of electricity consumption and weather forecasts, RTE determines daily a consumption forecast of the day in order to be able to adjust production to demand as much as possible in real time.
These transmission and distribution networks require very precise monitoring and control using SCADA and a whole set of Intelligent Electronic Devices (IEDs). A considerable set of measurements as well as the control of deviation of the 50HHz frequency deviation are performed. All these measurements and control systems require a perfectly synchronized network. Synchronization takes place in the control and monitoring stations of the network (clock distributed in transformer stations).
The synchronization system is also used for special needs, for example for circuit breaker control and the opening time of the contacts.
A radar transmits a signal to a target (mobile or fixed), and measures the round trip propagation time of this signal to calculate the distance to which the target is located. This measurement of time must be very precise to achieve an accurate measurement of the distance.
The signal is output at a frequency F0. The stability of the frequency (Df / F0) and the phase of the signal are crucial and determine the accuracy of the distance measurement.
There is a direct relationship between the stability of the frequency and the accuracy of the measured distance (D).
To maintain distance errors less than 2m in the range (1 to 4000km), the stability of the frequency must be DF / F0 <= 4 / c x Tp, where Tp is the propagation time (= 2D / c)
The higher the distance, the better the accuracy: at 4000Km, DF / f0 = 5.10-7
The propagation time Tp = 27ms for 4000Km.
Similarly, there is a direct relationship between the phase and the accuracy of the measured distance (D).
A phase jump of 100ns generates an error of 20m at 1500Km. To give an order of magnitude, if we take for example a Meinberg OCXO-HQ oscillator that derives ± 22us over 24 hours, ie ± 0.25ns per second: 100 seconds of signal loss will generate 25 ns of phase jump after re-synchronization.
The voice and image data require real-time synchronous transmission (otherwise the image is chopped, the sound is inaudible, etc.). It is therefore imperative that a reference clock signal be distributed to all equipment of all radio and antenna stations.
The voice is an analog signal, and a bandwidth of 3.1 kHz (300 Hz to 3.4 kHz) is sufficient but must be respected by all the nodes along the network. This signal is in most cases converted into a digital signal. An analog signal can be converted into a sampled signal without loss of information as long as the sampling frequency is at least 2 times the maximum frequency of the signal (Shannon’s theorem). For the voice signal at a maximum frequency of 4 kHz, sampling is therefore done at 8 kHz, ie every 125 us. The samples are then quantified by an 8-bit code (Pulse Code Modulation = PCM) which therefore generates a bit rate of 64 kbit / s.
Transmission links are costly and offer a capacity of bandwidth (Hz) or a bit rate (kbit / s) greater than that required by a single communication; The latter can therefore be used by several communications simultaneously by means of so-called multiplexing techniques. This notion is often associated with that of the so-called multiple access method of access of the different users to a common resource, for example TDM / Time Division Multiplexing (TDMA) These are different concepts: multiplexing concerns the technique of sharing resources among several users.
One of the most widely used TDM multiplexes in Europe is the 64kbit / s 32-bit synchronous multiplex called the 2Mbit / s “system” or E1 (E for European). Basically, it is done in the following way. At the input, there are 32 sources of 64 kbit / s each delivering 8 bits (one TS) every 125 us. The multiplex therefore outputs a signal whose frame comprises 32 TS of 8 bits and has a duration of 125 μs, which gives a bit rate of 2 Mbit / s. The signal (the frame) is called isochronous because all the moments have a constant duration and follow each other at the rhythm of a clock. In the frame, TSs are numbered from 0 to 31. Two TSs are used to synchronize and manage the system (transport of the sync), which means that the E1 allows 30 useful channels to be carried, for example 30 telephone calls simultaneously.
In order to ensure the transport of the clock, an E1 / T1 sync system comprises:
A PRC (Primary Reference Clock): master clock that must comply with the ITU G.811 standard in terms of accuracy (10-11 ppm). A cesium clock is often used.
SSU (Synchronization Unit): the clock signal of the PRC degrades as it passes through the network equipments (switch, routers etc …), accumulating a jitter / wander more and more important. The signal must therefore be regenerated about every 15-20 network nodes. SSUs must comply with ITU G.812 in terms of accuracy (10-9 ppm)