TK10931VTL データシートの表示（PDF） - Toko America Inc

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TK10931VTL

NARROW BAND FM/AM IF SYSTEM

Toko America Inc  

TK10931V

CIRCUIT DESCRIPTION (CONT.)

VCC

X'tal

RE

C1

C3

L

C2

200 µA

FIGURE 4: EXAMPLE FOR OVERTONE OSCILLATION

+j

FA FB

Frequency

-j

FIGURE 5

F(osc) is the fundamental frequency of the crystal

resonator.

3 x F(osc) is the 3rd overtone frequency of the crystal

resonator.

Moreover, it is established that the series value of the

equivalent capacitance of the circuit is in the dotted line of

Figure 4 and capacitance C1 is the load capacitance of the

crystal resonator.

When the operating current of the oscillator is insufficient,

increase Gm of the oscillator by the external resistor RE. In

this case, the increased operating current (Ie) of the

oscillator is calculated by the following:

Ie(mA) = VCC(V) - 0.7

RE(KΩ)

(2) RF AGC Output

For manufacturers with equipment requiring external RF

amplifiers, etc., the RF-AGC output terminal is readily

available. Because this AGC output is picked out from the

first stage of the FM-IF portion, the AGC output is not

affected by the AM switch when stopping the AM-IF

operation. Therefore, when receiving an FM signal, its

output contributes to improving strong input characteris-

tics.

The constants of the circuit in the dotted line of Figure 4

establish that the condition of oscillation is met for the 3rd

overtone frequency only.

Figure 5 shows the characteristics of the impedance of the

two terminals versus frequency of this circuit.

The condition of oscillation is the capacitance at the

overtone frequency and the inductance at near-fundamen-

tal frequency. Therefore, when it is established that the

fundamental frequency is included between FA and FB and

that the overtone frequency is above FB, the condition of 3

times overtone is:

F(osc) > FA

3 x F(osc) > FB,

FA =

1

2π LC3

FB = FA 1 + C2

C3

Its output is implemented as a current output from the

collector of the PNP-transistor. Therefore, its current is

converted into a voltage with an external resistor.

A composite example is shown as Figure 6. A receiver

circuit example in CB radios is shown at “CB Radio

Application” of the Application Information section. The

example in Figure 6 controls the gain by changing the bias

voltage of the RF-AMP with the output of transistor QA1

that is driven with the RF-AGC output of Pin 16.

VCC

RF-AMP

VCC

VCC

QA1

AGC Line

FIGURE 6: EXAMPLE FOR EXTERNAL AGC

AMPLIFIER

January 2001 TOKO, Inc.

Page 19

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