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參數(shù)資料
| 型號： | TK14588 |
| 廠商： | TOKO Inc. |
| 英文描述： | FM IF IC |
| 中文描述： | 調頻中頻集成電路 |
| 文件頁數(shù)： | 16/20頁 |
| 文件大小： | 233K |
| 代理商： | TK14588 |
Page 16
January 2000 TOKO, Inc.
TK14588
CIRCUIT DESCRIPTION (CONT.)
Establishing Demodulation Characteristics:
Generally, demodulation characteristics of FM detectors are determined by the external phase shifter. However, this
product has a unique function which can optionally establish the demodulation characteristics by the time constant of the
circuit parts after demodulation. The following explains this concept.
Figure 6 shows the internal equivalent circuit of the detector output stage.
The multiplier output current of the detector is converted to a the voltage by the internal OP AMP. The characteristic of
this stage is determined by converting the current to voltage with resistor R
O
and the capacitor C
O
connected between Pin
11 and Pin 12 (see Figure 6).
In other words, the slope of the S-curve characteristic can be established optionally with resistor R
without changing the
constant of the phase shifter. The demodulated bandwidth can be established optionally by the time constant of this
external resistor R
and capacitor C
O
inside of a bandwidth of the IF-filter and phase shifter. Figure 7 shows an example
of this characteristic.
Center Voltage of Detector DC Output:
The center voltage of the detector DC output is determined by the internal reference voltage source. It is impossible to
change this internal reference voltage source, but it is possible to change the center voltage by the following method.
As illustrated in Figure 8, the demodulated output current at Pin 12 is converted to the voltage by an external resistor R1
without using the internal OP AMP.
Figure 9 shows an example of a simple circuit that divides the supply voltage into halves using resistors. Since both circuits
have a high output impedance, an external buffer amplifier should be connected.
FIGURE 6 - INTERNAL EQUIVALENT CIRCUIT OF DETECTOR OUTPUT STAGE
The -3 dB frequency Fc is calculated by the following:
The S-curve output voltage is calculated by the following
as centering around the internal reference voltage V
ref
:
V
= V
±
io X R
Where V
ref
= 1.4 V, maximum of current io =
±
100
μ
A
Fc =
1
2
π
C
0
R
0
R0
C0
Demodulated
Output Current
Demodulated
Output Voltage
VOUT
Vref
I to V convertor
io
FIGURE 7 - EXAMPLE: BANDWIDTH OF DEMODULATION VS. TIME CONSTANT CHARACTERISTIC
V
2
MODULATING FREQUENCY fm (Hz)
1 3 10 30 100 300 1000
-12
-6
0
-2
-10
-8
-4
0 dB = 35.2 mVrms
C =
∞
C = 1000 pF
C = 330 pF
C =
47 pF
C =
10 pF
Operating Condition:
Measured by the standard test circuit.
Parallel resistor to phase shift coil = 1 k
.
f
= 10.7 MHz, modulation =
±
100 kHz.
External capacitance C
O
= 0~1000 pF.
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