TK14588V 데이터 시트보기 (PDF) - Toko America Inc

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TK14588V

FM IF IC

Toko America 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 and the capacitor C connected between Pin

O

O

11 and Pin 12 (see Figure 6).

In other words, the slope of the S-curve characteristic can be established optionally with resistor RO without changing the

constant of the phase shifter. The demodulated bandwidth can be established optionally by the time constant of this

external resistor RO and capacitor CO inside of a bandwidth of the IF-filter and phase shifter. Figure 7 shows an example

of this characteristic.

The -3 dB frequency Fc is calculated by the following:

Vref

I to V convertor

Fc = 1

2 π C0R0

io

R0

Demodulated

Output Current

C0

Demodulated

Output Voltage

VOUT

The S-curve output voltage is calculated by the following

as centering around the internal reference voltage Vref:

VOUT = Vref ± io X R0

Where Vref = 1.4 V, maximum of current io = ±100 µA

FIGURE 6 - INTERNAL EQUIVALENT CIRCUIT OF DETECTOR OUTPUT STAGE

2

0 dB = 35.2 mVrms

0

C=∞

-2

C=

10 pF

-4

-6

C = 330 pF

-8

-10 C = 1000 pF

C=

47 pF

-12

1 3 10 30 100 300 1000

MODULATING FREQUENCY fm (Hz)

Operating Condition:

Measured by the standard test circuit.

Parallel resistor to phase shift coil = 1 kΩ.

fIN = 10.7 MHz, modulation = ±100 kHz.

External capacitance C = 0~1000 pF.

O

FIGURE 7 - EXAMPLE: BANDWIDTH OF DEMODULATION VS. TIME CONSTANT 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.

Page 16

January 2000 TOKO, Inc.

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