GW Instek GRF-1300A - Manual (Page 50)

GRF-1300A User Manual and Teaching Materials
Figure 4-1. PLL
circuit structure
Above: PD is the phase-locked loop phase detector, LF is the loop
filter and VCO stands for voltage-controlled oscillator.
The purity of the output signal from the VCO is directly
related to the phase noise. The lower the distortion of the output
signal, the lower the harmonic components and noise contained
in the output signal.
Phase noise is usually specified in dBc/Hz at a given
frequency offset value, where dBc is dB in relation to the center
frequency. The phase noise of an oscillator is normalized to the
noise generated in a bandwidth of 1Hz. The phase noise is
usually calculated using the formula below, where f
m
is the
frequency of a single sideband from the carrier and P
ssB
is the
measured sideband power:
L(f
m
) = (P
ssB
– P
0
) – log B + 2.5
where,
B = 1.2RBW (RBW is the resolution bandwidth)
Figure 4-2. Phase
noise definition
m
f
0
P
ssB
P
As the oscillator is a non-linear component, it will produce
higher-harmonic content. Harmonic distortion is also an
important factor for RF signals. In general we use a filter to filter
this out.
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