µPC2533
2. Operation of Each Block
2.1 FR Amplifier Circuit Block
VCC
R1
C8
L3 R2
Q1
+
L1
Q2
C9
L2
C1
C2
Fig. 2-1 RF Ampliier Circuit
LPF
L4 L5
C6
33
C3 C4 C5
30
C7
35
36
34
+
MIX1
RF AGC
From MIX2
LO/DXNote
4
Note LO : 3 V or higher
DX : 1 V or lower
In the AM band, the capacitance of a car radio antenna depends on its length, diameter, cable length, etc. Therefore,
J-FET is used in the µPC2533 to raise RF input impedance.
Since the µPC2533 raises the first IF (intermediate frequency) to 10.71 MHz, there is no need for a tuning circuit
between the RF amplifier circuit and MIX1. Instead, it employs an LPF (about 6 MHz) consisting of L4, L5 and C3
to C5 between the RF amplifier circuit and MIX1 in order to cut image frequency (21.4 MHz or higher). Because this
allows a wide-band RF amplifier circuit to be configured without using a tuning circuit, frequency sensitivity deviation
can be minimized to a high degree.
The AGC circuit consists of RF AGC1 by the PIN diode connected to the FET gate and RF AGC2 by the cascade
transistor Q1. Use a low-noise transistor even with low current for the cascade transistor Q1 (if a high-noise one is
used, the S/N ratio deteriorates).
Remark Set bias voltage for cascade transistor Q1 to VC > VB.
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