TK14588 TOKO, TK14588 Datasheet - Page 16

• Manufacturer Part Number: TK14588
• Description: FM IF IC
• Manufacturer: TOKO

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
11 and Pin 12 (see Figure 6).
In other words, the slope of the S-curve characteristic can be established optionally with resistor R
constant of the phase shifter. The demodulated bandwidth can be established optionally by the time constant of this
external resistor R
of this characteristic.
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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 7 - EXAMPLE: BANDWIDTH OF DEMODULATION VS. TIME CONSTANT CHARACTERISTIC
Output Current
Demodulated
FIGURE 6 - INTERNAL EQUIVALENT CIRCUIT OF DETECTOR OUTPUT STAGE
-10
-12
-2
-4
-6
-8
O
0
2
and capacitor C
1
MODULATING FREQUENCY fm (Hz)
C = 1000 pF
io
0 dB = 35.2 mVrms
3
V ref
10
C = 330 pF
R 0
C 0
30
O
inside of a bandwidth of the IF-filter and phase shifter. Figure 7 shows an example
I to V convertor
100 300
47 pF
C =
C =
Output Voltage
10 pF
Demodulated
C =
V OUT
1000
The S-curve output voltage is calculated by the following
as centering around the internal reference voltage V
The -3 dB frequency Fc is calculated by the following:
Where V
Operating Condition:
ref
Measured by the standard test circuit.
Parallel resistor to phase shift coil = 1 k .
f
External capacitance C
= 1.4 V, maximum of current io = 100 A
IN
= 10.7 MHz, modulation = 100 kHz.
O
and the capacitor C
V
OUT
Fc =
= V
2
ref
1
C
io X R
0
R
January 2000 TOKO, Inc.
0
O
O
0
connected between Pin
= 0~1000 pF.
O
without changing the
ref
: