~
~
f
O
V(t)
V
R
(t)
DF
= 90
0
V
O
(t)
LPF
Quadrature
Monitor*
* Or phase-locked loop
V
f
(t)
Low-Noise
Amplifier
Spectrum Analyzer
S
f
(f)
Reference
DUT
4-29
Phase Detector
The device under test (DUT) and a reference source, at the same frequency and in phase quadrature (i.e., 90
0
out of phase), are input to a double-balanced mixer.
Then,
V
0
(t) = V(t) V
R
(t) = Kcos[
(t) -
R
(t) +
/2] + Kcos[2
(
n
+
n
R
)t + …].
The low-pass filter (LPF) eliminates the second cosine term.
Then,
for
R
(t) <<
(t)
<<
/2,
V
(t) = K
(t),
i.e., the phase detector converts phase fluctuations to voltage fluctuations.
