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TK10931VTL

NARROW BAND FM/AM IF SYSTEM

Toko America Inc  

TK10931V

CIRCUIT DESCRIPTION (CONT.)

(3) FM-IF Limiter Amplifier, RSSI

The IF limiter amplifier is composed of 6 differential gain

stages. The total gain of the IF limiter amplifier is 80 dB at

455 kHz. The output signal of the IF limiter amplifier is

provided at Pin 10 through the emitter-follower output

stage. The IF limiter amplifier output level is 0.5 VP-P. The

operating current of the emitter-follower at the IF limiter

amplifier is 200 µA.

If the capacitive load is heavy, the negative half cycle of the

output waveform may be distorted. This can be improved

by connecting an external resistor between Pin 10 and

GND to increase the operating current. The increased

operating current by an external resistor is calculated as

follows (see Figure 7):

The increased operating current Ie(mA) = (VCC - 1.0)/

Re(kΩ)

VCC

Current Output

RSSI OUT

I-V Converting Resistor

FIGURE 8: RSSI OUTPUT EQUIVALENT CIRCUIT

VCC

VCC

VCC

QB

The RSSI output is a current output. It is converted to a

voltage by an external resistor connected between Pin 15

and GND. The time constant of the RSSI output is

determined by the product of the external converting

resistance and the parallel capacitance. When the time

constant is longer, the RSSI output response is slower.

Determine the external resistance and capacitance by the

application. The slope of the RSSI curve characteristics

can be changed by changing the external resistance. In

this case, the maximum range of converted RSSI output

voltage is from GND level to about VCC-0.2 V (the supply

voltage minus the collector saturation voltage of the output

transistor). In addition, the temperature characteristic of

the RSSI output voltage can be changed by changing the

temperature characteristic of the external resistor.

Normally, the temperature characteristic of the RSSI

output voltage is very stable when using a carbon resistor

or metal film resistor with a temperature characteristic of 0

to 200 ppm/°C.

VCC

100

200 µA

IF OUT

Re

Ie

The emitter-follower operating

current can be increased by

external resistor Re when the

capacitive load is heavy and

the waveform distortion will be

reduced.

FIGURE 7

FIGURE 9: FM DETECTOR EQUIVALENT CIRCUIT

(4) FM Detector

The FM detector is included in the Quadrature FM detector

using a Gilbert multiplier. The phase shifter is connected

between Pin 10 (IF limiter output) and Pin 11 (detector

input), with any available phase shifter. A phase shifter can

be used such as the LC resonance circuit, the ceramic

discriminator, and the delay line. Figure 11 shows the

internal equivalent circuit of the detector. The signal from

the phase shifter is applied to the multiplier (in the dotted

line) through emitter-follower stage QA. Note that Pin 11

must have the bias voltage impressed from an external

source, because Pin 11 is connected with the base of QA

only. Because the base of QB of the opposite side is

connected with the supply voltage, Pin 11 has to be biased

with the equivalent voltage. Using an LC resonance circuit

is not a problem (see Figure 10), but attention must be paid

when using a ceramic discriminator. If the base voltages

are different, the DC voltage of the multiplier does not

balance. This alters the DC zero point or worsens the

distortion of the demodulation output. The Pin 11 input

level should be saturated at the multiplier, if this level is

lower, it is easy to disperse the detector output. Therefore,

to be in stable operation, the Pin 16 input level should be

higher than 100 mVP-P. Figure 10 shows examples of

phase shifters.

Page 20

January 2001 TOKO, Inc.

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