SA1140 Datasheet, PDF (4/6 Pages) Silan Microelectronics Joint-stock – FM RADIO IC FOR DIGITAL TUNING SYSTEM

SA1140
(Continued)
Pin No.
9
10
11
12
13
14
15
16
Pin Name
IFOUT
GND2
QUADIN
VCC
VREG
MUTEdrive
S-meter
AGC
Description
Out of IF
Gnd2
In of quadrature det
Vcc
Regulative voltage
Connected to p6 through resister
Signal meter
Automatic gain control
FUNCTION DESCRIPTION
1．Muting Characteristic
The muting operation is performed by an AF preamplifier, the gain of which varies continuously with control
current, and a muting drive output circuit which supplies the control current.
The gain of the AF preamplifier decreases with increasing gain control current applied to pin 6. However, the
gain does not decrease further when the control current reaches approximately 120 µA or greater. The lower limit
of the gain under this condition depends upon a resistor connected between pin 5 and GND, and the higher the
resistance the lower the gain (the higher the attenuation). Thus the maximum muting attenuation will be set by
connecting the resistor between pin 5 and GND.
2．The muting drive output comes in three types:
1) Hole detector output which develops a voltage when C/N (carrier-to-noise ratio) lowers under weak signal
input conditions.
2) A reversed output of the signal strength indicating output (output at pin 15)
3) A bandwidth limited muting drive output which develops a voltage when the AFC output becomes higher
than ±VBE during tuning-off operation.
All these outputs are led to an OR circuit and the processed output is developed at pin 14.
3．The general method to adjust the muting circuit of the SA1140 is: to set the signal input level required to
actuate the muting circuit with the R16-G, to adjust the slope of the curve for the muting attenuation vs. antenna
signal input with the R15-G, and to adjust the maximum muting attenuation (determined by setting the noise level
at no signal) with the R5-G. The slope of the curve for the muting attenuation vs. antenna signal input level can
also be adjusted by the resistor connected between pins 14 and 6 in addition to R15-G, however, selecting a
resistor too high does not allow the muting control current flowing into pin 6 to reach 120 µA even though the
maximum muting drive output (V14) is applied, namely the muting attenuation does not reach its maximum value.
Accordingly a recommended value of the resistor between pins 14 and 6 is about 22 kΩ.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
REV:1.0 2004.10.27
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