TK14588 TOKO, TK14588 Datasheet - Page 15

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

FM Detector:
The FM detector is included in the quadrature FM detector using a Gilbert multiplier.
It is suitable for high speed data communication because the demodulation bandwidth is over 1 MHz.
The phase shifter is connected between Pin 14 (IF limiter output) and Pin 13 (input detector). Any available phase shifter
can be used: a LC resonance circuit, a ceramic discriminator, a delay line, etc.
Figure 4 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. When
the phase shifter is connected between Pin 14 and Pin 13, note that the bias voltage to Pin 13 should be provided from
an external source because Pin 13 is only connected to the base of QA.
Because the base of QB (at the opposite side) is connected with the supply voltage, Pin 13 has to be biased with the
equivalent voltage.
Using an LC resonance circuit is not a problem (see Figure 5). However, when using a ceramic discriminator, it is necessary
to pay attention to bias. If there is a difference of the base voltages, the DC voltages of the multiplier do not balance. It
alters the DC zero point or worsens the distortion of demodulation output.
The Pin 13 input level should be saturated at the multiplier; if this level is lower, it is easy to disperse the modulation output.
Therefore, to have stable operation, Pin 13 should be higher than 100 mV
The following figures show examples of the phase shifter.
January 2000 TOKO, Inc.
CIRCUIT DESCRIPTION (CONT.)
LC resonance circuit
V CC
FIGURE 5 - EXAMPLES OF PHASE SHIFTER
V CC
ceramic discriminator
QA
V CC
FIGURE 4
V CC
V CC
QB
multiplier core circuit
P-P
.
Rz is the characteristic impedance
Rz
delay line
Delay
Line
Rz
V CC
TK14588
Page 15