Frequency divider

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A frequency divider is a circuit that takes an input frequency and produces an output frequency that is that input divided by an integer N. In other words, if you feed in fin, the output is fin/N. These devices are often used in phase-locked loop (PLL) synthesizers to create frequencies that are multiples of a reference frequency. Dividers can be analog or digital. Analog dividers are rare and work at very high frequencies; digital dividers in modern integrated circuits can handle frequencies up to tens of GHz.

Common divider types

- Regenerative (Miller) divider: The input is mixed with a feedback signal. The feedback is fin/2, which creates outputs at fin/2 and 3fin/2. A low-pass filter keeps the lower frequency, which is then fed back. This helps the circuit lock to the input signal. It’s related to injection-locked dividers.

- Injection-locked frequency divider (ILFD): The input is a multiple (or fraction) of the oscillator’s free-running frequency. ILFDs are usually efficient (low power) but have a narrow locking range and can be sensitive to manufacturing variations. The driving oscillator must stay within the ILFD’s locking range.

- Divide-by-two chains (flip-flops): For simple, fixed ratios, a chain of divide-by-two flip-flops clocked by the input signal gives outputs at fin/2, fin/4, fin/8, etc. More complex arrangements of flip-flops can produce other integer divisions and keep the output phase aligned with the input.

- Johnson counter: This is a kind of shift register network that can divide by even numbers. For example, a 3-register Johnson counter can achieve divide-by-6. You can add more registers for more division options. The outputs are square waves with known phase relationships.

- D flip-flop networks: A classic way to build integer-dividers using chains of D flip-flops. They are capable of producing various division factors, including odd numbers with more complex arrangements. Several standard logic families include chips that implement these functions.

Fractional-n frequency synthesis

- A fractional divider uses two integer dividers, such as divide-by N and divide-by N+1. A control toggles the divider between the two, so the final VCO (voltage-controlled oscillator) frequency locks to the average of the two values. By changing how long the divider stays at each value, you can fine-tune the output frequency very precisely.

- If the switching between N and N+1 repeats regularly, extra spurious signals can appear. Delta-sigma fractional-n dividers randomize (or skew) the switching between N and N+1 while keeping the average ratio, reducing spurs and keeping the desired frequency.

In short, frequency dividers are key building blocks for generating a wide range of clock and RF frequencies by dividing a reference signal in simple, reliable ways or with advanced fractional techniques for fine control.