
Signal sources 237
-12
dB/octave
ω
increasing ωι
\ -1.0
/ ω increasing
ω,/ΙΟ I log/
Η
h
(c)
Figure 9.19 PLL with second-order active loop
filter.
ω
increasing ^
(b)
To
zero frequency
where the loop filter is an integrator as in Figure
9.19c; this is an example of a high-gain second-
order loop. With the 90° phase lag of the active
loop filter added to that of the phase detector,
there is no phase margin whatever at the unity gain
frequency; as Figure 9.19b shows, we are heading
for disaster (or at least instability) at ωι. By
reducing the slope of the roll-off in Figure 9.19a to
6 dB per octave before the frequency reaches ω,
(dashed line),