la1862m Sanyo Semiconductor Corporation, la1862m Datasheet - Page 13

• Manufacturer Part Number: la1862m
• Description: Single-chip Tuner System For Car Stereo
• Manufacturer: Sanyo Semiconductor Corporation

IF Block
Obtaining Stable Characteristics
1.
2.
3.
4.
S-meter Output V
1.
2.
3.
4.
5.
Soft Muting
1.
2.
3.
Band Muting
1.
2.
3.
4.
Pin 36 is the IF input; pin 35, the IF bypass. Connect a
330
Position the capacitor between thr IF bypass (pin 35) and
the ground pattern so as to maximize AM rejection.
Provide separate ground pattern islands for the IF input
and detection circuits, as sharing the same island reduces
stability.
Position the IF input and detector coil as far apart as
possible, as proximity reduces stability and introduces beat
noise in the output.
Pin 1 is the field strength indicator (or ‘‘S-meter’’) output. Its current-driven circuit uses an external resistance to adjust the
slopes of the I/O characteristics curves.
The S-meter output is internally connected to the soft muting and IF buffer blocks for use as a control signal for the soft
muting drive and IF count buffer.
The point at which the input produces an S-meter output depends on the front end (FE) and interstage amplifier gains.
Too large a front end gain produces floating S-meter output even when there is no input. Either use a smaller load resistance
on pin 1 or reduce the front end gain so that the output with no load does not exceed 0.5 V.
The S-meter output circuit has a dynamic range of approximately 80 dB, but this is limited by the front end noise component
and broadband AGC circuit.
The soft muting circuit operates in response to the S-meter output voltage. The amount of muting is related to the pin 5
output voltage.
There are two mechanisms for adjusting the soft muting
I/O characteristic curve:
a. Start point for muting: Resistance attached to pin 4
b. Attenuation for muting: Resistance attached to pin 32
*Note that the resistance attached to pin 1 also affects the
The soft muting circuit automatically varies the amount of
muting in response to the IF input. In the absence of front
end broadband AGC effects, the time constant of the RC
circuit between pins 1 and 5 determines the response.
Band muting uses the detector’s S-curve. The band-width depends on the resistance between pins 29 and 33. Select this value
to match the needs of the destination market.
Keep in mind that changing the detector coil or tuning capacity Q changes the slope of the S-curve and hence the bandwidth.
The attenuation muting depends on the resistance connected to pin 32.
The muting transient response depends on the resistance between pins 29 and 33, the capacitance at pin 29, and the time
constant for the RC circuit at pin 5.
curve.
resistor between them for ceramic filter matching.
SM
LA1862M
Discharging
current
Charging
current
Unit (resistance:
•
•
Time constant for charging
re
Time constant for discharging
R
(Muting drive output)
5
C
C
5
5
=
=
, capacitance: F)
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