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frequency synthesizer


A frequency synthesizer is a circuit that generates one or more output signals whose frequency bears a prescribed relation to the frequency of an input reference signal. Such circuits are used as accurate frequency sources in a wide variety of electronic apparatus. Frequency synthesizers are widely used in modern radio communication systems. Such devices typically make use of a single quartz-controlled reference oscillator combined with a phase-locked loop to provide a multitude of output frequencies traceable to the highly stable reference from the oscillator. A phase looked-loop (PLL) frequency synthesizer uses a feedback loop for synchronization in frequency and phase with a certain reference frequency signal and generates a signal of a target frequency by multiplying the reference frequency signal or by combining the reference frequency signal with another reference frequency signal. A PLL frequency synthesizer comprises a reference signal generator, a voltage controlled oscillator (VCO), a variable frequency divider, a phase frequency comparator, low pass filter, and a control voltage supplying circuit. The reference signal generator generates a reference signal with a reference frequency. The phase comparator detects a phase difference between an output frequency of the reference oscillator and an output frequency of the voltage controlled oscillator. A signal corresponding with this phase difference is supplied as a control voltage to the voltage controlled oscillator through the low pass filter. The phase difference signal causes the output signal from the VCO to change in frequency so as to minimise the phase difference between the frequency divided signal and the reference signal.

Because the synthesizer is a closed-loop system, its loop
bandwidth, which describes the rate of system response
and indicates whether it is stable, is an important
parameter and about 1/20 of the reference frequency. On
the other hand, phase noise optimization requires a certain
bandwidth with respect to the given charge pump and
VCO circuits. For every different output frequency, the
loop bandwidth must be constant to minimize phase noise
variation and guarantee loop stability over a wideband
frequency range. Usually open-loop cut-off frequency is
selected as the loop bandwidth by designers, however, for
third or higher order loop there is no intuitive expression
about it. Loop gain is a good indication of low-pass corner
frequency for the open-loop transfer function and is
defined as bandwidth.

PLL TERMS

Output Power – The output power of a synthesizer, typically
expressed in dBm, is measured into a 50 ohm load. The
output power varies with frequency and temperature, and is
always combined with the specification for output power
tolerance.

Step Size – The step size frequency in conventional PLL
circuits is the same as the reference comparison frequency
at the phase detector. It also signifies the change of the output
frequency for a step increment or decrement step. In most
applications, the step size is dictated by the channel spacing
of the radio system.

Settling Time – The maximum amount of time required for
the output frequency to reach a stable state. The reference
and output frequencies reach frequency and phase lock
conditions. The major factors that affect settling time are
register VCO tuning speed, register loading time, loop filter
bandwidth and loop filter order.

Spurious Reference Frequency Sideband – Spurious signals
that result from unwanted FM modulation in the control lines.
The most likely cause is phase detector pulse components
that leak through the loop filter.

Non-Harmonic Spurious – Spurious sidebands appearing at
the output not related to the reference frequency. Modulating
signals of any kind at the VCO control line will cause spurious
sidebands at the output. Non-Harmonic Sideband spurs
usually result from noisy power supply/data lines. Proper AC
bypassing and supply decoupling are critical for rejecting nonharmonic
sideband spurious. Additionally, good grounding
techniques must be exercised.

Harmonic Output Power– The harmonic content is
measured relative to the output power. Typical harmonic
suppression is 10 to 20 dB. In narrow band applications, this
parameter can be improved.

Phase Noise – affects the system performance in such areas
as multiple signal selectivity and signal to noise ratio. The
phase noise inside the loop bandwidth is of a synthesizer is
mainly controlled by the quality of the reference input signal
source, loop filter, step size, and noise of the associated
circuitry, including the phase detector noise floor. The VCO
noise outside the loop is unaffected by the loop parameters.





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