Clock generator - an electronic circuit producing a clock signal for synchronizing the operation of digital circuits. Such a signal can have any shape: both simple rectangular and more complex. The main elements of the generator are a resonant circuit and an amplifier.
Clock signals
In electronics, especially in synchronous digital networks, a clock signal is a signal with a constant frequency, two stable states (upper and lower), designed to coordinate the operation of digital circuits.
Clock signals are created by clock generators. The most common form of clock is the meander (signal with a duty cycle of 50%). Duty cycle - the ratio of the duration to the period of the pulse. In other words, this is part of the period during which the signal is active.
Circuits using clock signals can become active during a rising edge, falling edge, or, in the case of a doubled data rate, rising and falling edges.
The principle of the formation of a clock signal
The source of clock oscillations is a quartz crystal located in a tin case. When voltage is applied to the quartz plate, it begins to perform mechanical vibrations. Under the influence of the piezoelectric effect, EMF is induced on the electrodes of the crystal. Fluctuations of electric current follow the generator, which, in fact, converts them into pulses.
Computer Clock
In a computer, the generator is responsible for the synchronous operation of all its devices: processor, RAM, data buses. The work of the processor can be compared with the work of a watch. Execution of instructions by the central processor is carried out for a certain number of clock cycles. The watch also functions in the same way. Beats in a mechanical watch are determined by the oscillations of the pendulum.
Processor performance directly depends on the clock frequency. The higher the clock frequency, the more instructions the processor is able to execute in a certain period of time. One command or instruction can be executed by the processor in a part of a clock cycle or in several hundred clock cycles. The general tendency of the modern development of computer technology is to reduce the number of clock cycles allocated for one simple instruction.
Overclocking
Of particular interest is the processor clock for overclockers. Overclockers include specialists in the field of computer technology and just amateurs who want to increase the productivity of their equipment. Overclocking is currently available even to simple users. To change the settings of computer components, sometimes just logging into the BIOS.
First of all, it is necessary to answer the question: due to what will productivity increase? Everything is very simple here. Manufacturers of computer components to increase the reliability of their components lay technological stock in them. It is this supply that attracts fans to squeeze the most out of their computer.
One way to accelerate a computer is to replace the quartz resonator with a crystal having a higher frequency. Or, for example, you can remove additional elements in the form of frequency dividers from the generator circuit.
In modern computers, generators are usually implemented on a single integrated circuit. The clock frequency and processor multiplier, as noted above, can be changed directly from the BIOS.
Novice overclockers often wonder how to determine the clock model. Software cannot do this. It remains only to open the system unit and look for the generator visually.
On the other hand, the model of the motherboard (AIDA64, Everest and others) is determined programmatically. Then, for this model, detailed instructions are searched, and in it it will be quite possible to find information about the name of the generator. And how to find out the default clock frequency and the value after overclocking for the clock generator? This information can also be obtained from the instructions for the motherboard.
Main elements
A quartz piezoelectric exciter often acts as a resonant circuit of a generator. At the same time, simpler circuits of a parallel resonant circuit and an RC circuit (a circuit consisting of a capacitor and a resistor) can be used.
The generator may have additional circuits for changing the main signal. So the 8088 processor uses only two-thirds of the clock cycle. This requires a clock in the generator. And an integrated logic circuit to transform the duty cycle.
As the shape of the output clock signal becomes more complicated, a mixer, divider or frequency multiplier can be used in the clock generator circuit. The frequency mixer generates a signal whose frequency is equal to the sum or difference of the two frequencies of the input signals.
Phase Locked Loop
Many devices use phase locked loop (PLL) to compare the phase of the signal from the output of the generator with the phase of the frequency and adjust the frequency of the generator so that the phase values match.
The figure shows a phase locked loop (PLL). The phase comparison device (comparator) has 2 inputs and 1 output. As input signals, the signal from the master oscillator (the signal at the input of the PLL circuit) and the signal from the output of the voltage-controlled oscillator (VCO) are used. The comparator compares the phases of the two signals and generates an error signal, which follows the low-pass filter (LPF), and from it to the VCO, controlling its frequency.
Types of Clock Generators
1. General-purpose generators
General purpose generators typically use PLLs to generate output signals from a common input frequency. They use simple inexpensive quartz crystals to obtain the reference frequency. From the reference frequency signal, they generate output clock signals with a low level of signal edge jitter.
2. Programmable generators
Allows you to change the coefficient used by the divider or multiplier. Thanks to this, you can choose any of the many output frequencies without changing the hardware.
Application of clock generators in SONET networks
This is a clock used by service provider networks often in the form of an integrated signal source (BITS) for the central office.
Digital switching systems and some transmission systems (such as SONET Synchronous Digital Hierarchy Systems) depend on reliable, high-quality synchronization. To ensure this state, most service providers use synchronization distribution schemes between offices and implement the BITS concept to provide intra-office synchronization.
The synchronization input signals come to the input of the clock generator, and the synchronization output signals follow from the output. DS-1 or CC synchronization signals (composite signals) can act as input reference signals, and DS-1 or CC signals can also be output signals.
The composition of the generator:
- a synchronization input interface receiving DS-1 or CC input signals;
- a clock generation circuit that generates clocks used by the distribution circuit by the output signal distribution circuit;
- an output synchronization signal distribution circuit generating a plurality of DS-1 and CC signals;
- a performance control circuit designed to control input clock synchronization parameters;
- alarm interface connected to the central office's alarm management system;
- service interface intended for use by local service personnel and communicating with remote service systems.