Frequency synthesizer
The third type are routinely used as communication system IC building blocks: indirect digital (PLL) synthesizers including integer-N and fractional-N.[3] The recently emerged TAF-DPS is also a direct approach.
[4][5] Time-Average-Frequency Direct Period Synthesis (TAF-DPS) focuses on frequency generation for clock signals driving integrated circuits.
Starting from a base time unit, TAF-DPS first creates two types of cycles TA and TB.
[12] Prior to widespread use of synthesizers, in order to pick up stations on different frequencies, radio and television receivers relied on manual tuning of a local oscillator, which used a resonant circuit composed of an inductor and capacitor, or sometimes resonant transmission lines, to determine the frequency.
Automatic frequency control (AFC) solves some of the drift problem, but manual retuning was often necessary.
Quartz crystal resonators are many orders of magnitude more stable than LC circuits and when used to control the frequency of the local oscillator offer adequate stability to keep a receiver in tune.
This "brute force" technique is practical when only a handful of frequencies are required, but quickly becomes costly and impractical in many applications.
The choice of approach depends on several factors, such as cost, complexity, frequency step size, switching rate, phase noise, and spurious output.
Coherent techniques generate frequencies derived from a single, stable master oscillator.
[13] The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost.
Synthesizers used in commercial radio receivers are largely based on phase-locked loops or PLLs.
High end receivers and electronic test equipment use more sophisticated techniques, often in combination.
Variable-frequency synthesizers, including DDS, are routinely designed using Modulo-N arithmetic to represent phase.
[19] The error signal is then low pass filtered and used to drive a voltage-controlled oscillator (VCO) which creates an output frequency.
The block diagram below shows the basic elements and arrangement of a PLL based frequency synthesizer.
In addition, it is hard to make a high frequency VCO that operates over a very wide range.
Many radio applications require frequencies that are higher than can be directly input to the digital counter.
To overcome this, the entire counter could be constructed using high-speed logic such as ECL, or more commonly, using a fast initial division stage called a prescaler which reduces the frequency to a manageable level.
Usually the output of a frequency comparator is in the form of short error pulses, but the input of the VCO must be a smooth noise-free DC voltage.
[21] This simple scheme therefore cannot directly handle low frequency (or DC) modulating signals but this is not a problem in the many AC-coupled video and audio FM transmitters that use this method.
