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Sec. 4–11 Mixers, Up Converters, and Down Converters 271
+
+
v in
RF
port
LO
port
v LO
Local
oscillator
–
–
IF port
–
v 1 (t)
+
(a) A Double-Balanced Mixer Circuit
Load
+
–
v in (t)
v LO (t)
v in (t)
v LO (t)
–
+
v 1 (t)
(b) Equivalent Circuit When v LO (t) Is Positive
v 1 (t)
(c) Equivalent Circuit When v LO (t) Is Negative
+1
s(t)
–1
t
(d) Switching Waveform Due to the Local Oscillator Signal
T 0
Figure 4–11
Analysis of a double-balanced mixer circuit.
signal. This circuit thus acts as a time-varying linear circuit (with respect to the RF input
port), and its analysis is very similar to that used for the analog-switch mixer of Fig. 4–10.
During the portion of the cycle when v LO (t) has a positive voltage, the output voltage is proportional
to +v in (t), as seen from the equivalent circuit shown in Fig. 4–11b. When v LO (t) is
negative, the output voltage is proportional to -v in (t), as seen from the equivalent circuit
shown in Fig. 4–11c. Thus, the output of this double-balanced mixer is
v 1 (t) = Kv in (t) s(t)
(4–69)