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MC13150FTA Arkusz danych(PDF) 7 Page - LANSDALE Semiconductor Inc. |
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MC13150FTA Arkusz danych(HTML) 7 Page - LANSDALE Semiconductor Inc. |
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7 / 20 page  LANSDALE Semiconductor, Inc. ML13150 Figure 12. BWadj Current versus BWadj Voltage BWadj VOLTAGE (Vdc) Figure 13. Demodulator Output versus Frequency 2.3 2.5 2.7 f, FREQUENCY (kHz) 0.1 1.0 10 100 10–3 10–4 10–5 10–6 10–7 VCC = 3.0 Vdc TA = 25 C 10 0 –10 –20 –30 –50 –40 VCC = 3.0 Vdc TA = 25 C fRF = 50 MHz fLO = 50.455 MHz LO Level =–10 dBm No IF Bandpass Filters fdev = ±4.0 kHz RB = 560 k RB = 1.0 M 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 k Ω. 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 k Ω 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. 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 k Ω 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 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. www.lansdale.com Page 7 of 20 Issue A |
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