mc13150ftb Lansdale Semiconductor, Inc., mc13150ftb Datasheet - Page 7

• Manufacturer Part Number: mc13150ftb
• Description: Narrowband Fm Coilless Detector If Subsystem
• Manufacturer: Lansdale Semiconductor, Inc.

LANSDALE Semiconductor, Inc.
COILLESS DETECTOR
The quadrature detector is similar to a PLL. There is an inter-
nal oscillator running at the IF frequency and two detector
outputs. One is used to deliver the audio signal and the other
one is filtered and used to tune the oscillator.
The oscillator frequency is set by and external resistor at the
F adj pin. Figure 9 shows the control current required for a
particular frequency; Figure 10 shows the pin voltage at that
current. From this the value of RF is chosen. For example,
455 kHz would require a current of around 50 µA. The pin
voltage (Pin 16 in the 32 pin QFP package) is around 655mV
giving a resistor of 13.1 kΩ. Choosing 12 kΩ as the nearest
standard value gives a current of approximately 55 µA. The
5.0 µA difference can be taken up by the tuning resistor, R T .
The best nominal frequency for the AFTout pin (Pin 17)
would be half supply. A supply voltage of 3.0 Vdc suggests a
resistor value of (1.5 – 0.655) V/5.0 µA = 169 kΩ. Choosing
150 kΩ would give a tuning current of 3/150 kΩ = 20 µA.
From Figure 9 this would give a tuning range of roughly 10
kHz/µA or ± 100 kHz which should be adequate.
The bandwidth can be adjusted with the help of Figure 11.
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10 –3
10 –4
10 –5
10 –6
10 –7
2.3
V CC = 3.0 Vdc
T A = 25 C
Figure 12. BW adj Current
versus BW adj Voltage
BW adj VOLTAGE (Vdc)
2.5
2.7
www.lansdale.com
For example, 1.0 µA would give a band width of ± 13 kHz.
The voltage across the bandwidth resistor, RB from Figure 12
is V CC – 2.44 Vdc = 0.56 Vdc for V CC = 3.0 Vdc, so R B =
0.56V/1.0 µA = 560 kΩ. Actually the locking range will be
±13 kHz while the audio bandwidth wil be approximately
±8.4 kHz due to an internal filter capacitor. This is verified in
Figure 13. For some applications it may be desireable that the
audio bandwidth is increased; this is done by reducing R B .
Reducing R B widens the detector bandwidth and improves
the distortion at high input levels at the expense of 12 dB
SINAD sensitivity. The low frequency 3.0dB point is set by
the tuning circuit such that the product
So, for example, 150 kΩ and 1.0 µF give a 3.0 dB point of
4.5 kHz. The recovered audio is set by R L to give roughly
50mV per kHz deviation per 100 k of resistance. The dc
level can be shifted by R S from the nominal 0.68 V by the
following equation:
Thus R S = R L sets the output at 2 x 0.68 = 1.36 V; R L =
2R S sets the output at 3 x .068 = 2.0V.
–10
–20
–30
–40
–50
10
Detector DC Output = ((R L + R S )/R S ) 0.68 Vdc
0
0.1
V CC = 3.0 Vdc
T A = 25 C
f RF = 50 MHz
f LO = 50.455 MHz
LO Level =–10 dBm
No IF Bandpass Filters
f dev = ±4.0 kHz
Figure 13. Demodulator Output
R T C T = 0.68/f 3dB .
versus Frequency
1.0
f, FREQUENCY (kHz)
R B = 1.0 M
10
R B = 560 k
100
ML13150
Issue A