11

µPC2533

2. Operation of Each Block

2.1

FR Amplifier Circuit Block

Fig. 2-1 RF Ampliier Circuit

VCC

LPF

L4

L5

C6

33

C3

C4

C5

C7

30

35

36

34

+

C2

C1

L2

L1

Q2

Q1

L3

C9

+

R2

R1

C8

MIX1

RF AGC

LO/DXNote

4

From MIX2

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.