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A frequency divider, also called a clock divider or scaler or prescaler, is a circuit that takes an input signal of a
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is e ...
, f_, and generates an output signal of a frequency: : f_ = \frac where n is an integer.
Phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
frequency synthesizer A frequency synthesizer is an electronic circuit that generates a range of frequencies from a single reference frequency. Frequency synthesizers are used in many modern devices such as radio receivers, televisions, mobile telephones, radiotelephone ...
s make use of frequency dividers to generate a frequency that is a multiple of a reference frequency. Frequency dividers can be implemented for both analog and digital applications.


Analog

Analog frequency dividers are less common and used only at very high frequencies. Digital dividers implemented in modern IC technologies can work up to tens of GHz.


Regenerative

A regenerative frequency divider, also known as a Miller frequency divider, mixes the input signal with the feedback signal from the mixer. The feedback signal is f_/2. This produces sum and difference frequencies f_/2, 3f_/2 at the output of the mixer. A low pass filter removes the higher frequency and the f_/2 frequency is amplified and fed back into mixer.


Injection-locked

A free-running
oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
which has a small amount of a higher-frequency signal fed to it will tend to oscillate in step with the input signal. Such frequency dividers were essential in the development of
television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, ...
. It operates similarly to an
injection locked oscillator Injection locking and injection pulling are the frequency effects that can occur when a harmonic oscillator is disturbed by a second oscillator operating at a nearby frequency. When the coupling is strong enough and the frequencies near enough, th ...
. In an injection locked frequency divider, the frequency of the input signal is a multiple (or fraction) of the free-running frequency of the oscillator. While these frequency dividers tend to be lower power than broadband static (or flip-flop based) frequency dividers, the drawback is their low locking range. The ILFD locking range is inversely proportional to the quality factor (Q) of the oscillator tank. In integrated circuit designs, this makes an ILFD sensitive to process variations. Care must be taken to ensure the tuning range of the driving circuit (for example, a voltage-controlled oscillator) must fall within the input locking range of the ILFD.


Digital

For power-of-2 integer division, a simple binary counter can be used, clocked by the input signal. The least-significant output bit alternates at 1/2 the rate of the input clock, the next bit at 1/4 the rate, the third bit at 1/8 the rate, etc. An arrangement of flipflops is a classic method for integer-n division. Such division is frequency and phase coherent to the source over environmental variations including temperature. The easiest configuration is a series where each flip-flop is a divide-by-2. For a series of three of these, such system would be a divide-by-8. By adding additional logic gates to the chain of flip flops, other division ratios can be obtained. Integrated circuit logic families can provide a single chip solution for some common division ratios. Another popular circuit to divide a digital signal by an even integer multiple is a
Johnson counter A ring counter is a type of counter composed of flip-flops connected into a shift register, with the output of the last flip-flop fed to the input of the first, making a "circular" or "ring" structure. There are two types of ring counters: * A s ...
. This is a type of
shift register A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one loca ...
network that is clocked by the input signal. The last register's complemented output is fed back to the first register's input. The output signal is derived from one or more of the register outputs. For example, a divide-by-6 divider can be constructed with a 3-register Johnson counter. The six valid values of the counter are 000, 100, 110, 111, 011, and 001. This pattern repeats each time the network is clocked by the input signal. The output of each register is a f/6 square wave with 120° of phase shift between registers. Additional registers can be added to provide additional integer divisors.


Mixed signal

(''Classification:'' ''
asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal with ...
sequential logic In automata theory, sequential logic is a type of logic circuit whose output depends on the present value of its input signals and on the sequence of past inputs, the input history. This is in contrast to ''combinational logic'', whose output i ...
'')
An arrangement of D flip-flops is a classic method for integer-n division. Such division is frequency and phase coherent to the source over environmental variations including temperature. The easiest configuration is a series where each D flip-flop is a divide-by-2. For a series of three of these, such system would be a divide-by-8. More complicated configurations have been found that generate odd factors such as a divide-by-5. Standard, classic logic chips that implement this or similar frequency division functions include the 7456, 7457, 74292, and 74294. (see list of 7400 series and list of 4000 series logic chips)


Fractional-n synthesis

A fractional-n frequency synthesizer can be constructed using two integer dividers, a divide-by-n and a divide-by-(n + 1) frequency divider. With a modulus controller, n is toggled between the two values so that the VCO alternates between one locked frequency and the other. The VCO stabilizes at a frequency that is the time average of the two locked frequencies. By varying the percentage of time the frequency divider spends at the two divider values, the frequency of the locked VCO can be selected with very fine granularity.


Delta-sigma

If the sequence of divide by n and divide by (n + 1) is periodic, spurious signals appear at the VCO output in addition to the desired frequency. Delta-sigma fractional-n dividers overcome this problem by randomizing the selection of n and (n + 1), while maintaining the time-averaged ratios.


See also

{{Portal, Electronics *
Phase-locked loop A phase-locked loop or phase lock loop (PLL) is a control system that generates an output signal whose phase is related to the phase of an input signal. There are several different types; the simplest is an electronic circuit consisting of a ...
*
Prescaler A prescaler is an electronic counting circuit used to reduce a high frequency electrical signal to a lower frequency by integer division. The prescaler takes the basic timer clock frequency (which may be the CPU clock frequency or may be some high ...
* Pulse-swallowing counter and pulse-swallowing divider


References


External links


Delta-sigma fractional-n synthesizers

A Study of High Frequency Regenerative Frequency Dividers
Electronic circuits