AN178 Philips, AN178 Datasheet - Page 6

• Manufacturer Part Number: AN178
• Description: Modeling the PLL
• Manufacturer: Philips

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DataSheet
Philips Semiconductors
Zero-Order Filter — F(s) = 1
The simplest case is that of the first-order loop where F(s)= 1 (no
filter). The closed-loop transfer function then becomes
This transfer function gives the root locus as a function of the total
loop gain K
1988 Dec
4
U
Modeling the PLL
H(s)
.com
v
and the corresponding frequency response shown in
S
K
V
K
V
Figure 6. Root Locus and Frequency Response Plots
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(20)
6
Figure 6a. The open-loop pole at the origin is due to the integrating
action of the VCO. Note that the frequency response is actually the
amplitude of the difference frequency component versus modulating
frequency when the PLL is used to track a frequency-modulated
input signal. Since there is no low-pass filter in this case, sum
frequency components are also present at the phase comparator
output and must be filtered outside of the loop if the difference
frequency component (demodulated FM) is to be measured.
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Application note
AN178
SL01016