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ML13150 Datasheet PDF : 20 Pages
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LANSDALE Semiconductor, Inc.
ML13150
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
Fadj 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, RT.
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.
For example, 1.0 µA would give a band width of ± 13 kHz.
The voltage across the bandwidth resistor, RB from Figure 12
is VCC – 2.44 Vdc = 0.56 Vdc for VCC = 3.0 Vdc, so RB =
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 RB.
Reducing RB 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
RTCT = 0.68/f3dB.
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 RL to give roughly
50mV per kHz deviation per 100 k of resistance. The dc
level can be shifted by RS from the nominal 0.68 V by the
following equation:
Detector DC Output = ((RL + RS)/RS) 0.68 Vdc
The bandwidth can be adjusted with the help of Figure 11.
Thus RS = RL sets the output at 2 x 0.68 = 1.36 V; RL =
2RS sets the output at 3 x .068 = 2.0V.
Figure 12. BWadj Current
10–3
versus BWadj Voltage
VCC = 3.0 Vdc
TA = 25 C
10–4
10–5
10–6
10–7
2.3
2.5
2.7
BWadj VOLTAGE (Vdc)
Figure 13. Demodulator Output
versus Frequency
10
0
–10
VCC = 3.0 Vdc
–20 TA = 25 C
fRF = 50 MHz
–30
fLO = 50.455 MHz
LO Level =–10 dBm
No IF Bandpass Filters
–40 fdev = ±4.0 kHz
RB = 560 k
RB = 1.0 M
–50
0.1
1.0
10
100
f, FREQUENCY (kHz)
Page 7 of 20
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