LA1245
Using the automatic search-stop signal
Signal Meter-driving output circuit is equivalent to Figure. 1, signal meter driving output (abbreviated as V ) is
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narrowed in band width and can be used as an automatic search-stop signal when a narrow band series resonator is
connected to pin 15. V
can be adjusted with R
and R
both in wide band and narrow band since R
is
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inversely proportional to V , while R
is proportional to V . R
is related to the Q of narrow band signal
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meter. When the resistance of R
is increased, the Q will be damped and the band width increased. On the other
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hand, R
used as the output impeadance of V and affects the cut-off frequency and time constant of low pass
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filter for V and the meter drive impedance. The time constant τ and the cut-off frequency fc can be expressed as
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follows :
τ = (C +C +C ) (R //R )
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1
115
S
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in
f =
c
2πτ
A semi-fixed resistor is recommended to be used as R to cope with the fluctuation of V . Refer to Figure. 3 for
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the value of the semi-fixed resistor since this depends upon V and R . Figure. 3 shows the lowest limit of the
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semi-fixed resistor in relation to R
with the parameter of V set point, and the value of the semi-fixed resistor
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will be equal to or greater than that shown in Figure. 3. For example, when V =5V and R =240Ω, R becomes
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28kΩ. Thus, the value of the semi-fixed resistor is determined to be about 30kΩ. When the value of V is too large,
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it is limited and saturated to the source voltage so it is recommended to follow the condition of V ≤V –2(V).
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CC
When a narrow band serial resonator is used, include the resonant impedance to determine the value of R
.
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No.737-3/11
SANYO [ SANYO SEMICON DEVICE ]
