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MC13110A Datasheet(PDF) 30 Page - Motorola, Inc |
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MC13110A Datasheet(HTML) 30 Page - Motorola, Inc |
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30 / 68 page  MC13110A/B MC13111A/B 30 MOTOROLA ANALOG IC DEVICE DATA RSSI and Carrier Detect The Received Signal Strength Indicator (RSSI) indicates the strength of the IF level. The output is proportional to the logarithm of the IF input signal magnitude. RSSI dynamic range is typically 80 dB. A 187 k Ω resistor to ground is provided internally to the IC. This internal resistor converts the RSSI current to a voltage level at the “RSSI” pin. To improve the RSSI accuracy over temperature an internal compensated reference is used. Figure 56 shows the RSSI versus RF input. The slope of the curve is 16.5 mV/dB. The Carrier Detect Output (CD Out) is an open–collector transistor output. An external pull–up resistor of 100 k Ω will be required to bias this device. To form a carrier detect filter a capacitor needs to be connected from the RSSI pin to ground. The carrier detect threshold is programmable through the MPU interface (see “Carrier Detect Threshold Programming” in the serial interface section). The range can be scaled by connecting additional external resistance from the RSSI pin to ground in parallel with the capacitor. From Figure 57, the affect of an external resistor at RSSI on the carrier detect level can be noticed. Since there is hysteresis in the carrier detect comparator, one trip level can be found when the input signal is increased while the another one can be found when the signal is decreased. Figure 58 represents the RSSI ripple in relation to the RF input for different filtering capacitors at RSSI. Clearly, the higher the capacitor, the less the ripple. However, at low carrier detect thresholds, the ripple might supersede the hysteresis of the carrier detect. The carrier detect output may appear to be unstable. Using a large capacitor will help to stabilize the RSSI level, but RSSI charge time will be affected. Figure 59 shows this relationship. The user must decide on a compromise between the RSSI ripple and RSSI start–up time. Choose a 0.01 µf capacitor as a starting point. For low carrier detect threshold settings, a 0.047 µf capacitor is recommended. –60 RSSI AND CARRIER DETECT 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 –120 –100 –80 –60 –40 –20 0 Mix1 In, RF INPUT (dBm) Figure 56. Typical RSSI Voltage Level versus RF Input –90 –80 –70 –50 –40 –30 –20 0 100 1000 RRSSI, LOAD RESISTANCE (kΩ) Figure 57. Carrier Detect Threshold versus External RSSI Resistor –10 Decreasing Signal Limiter Input Mixer 1 Input Increasing Signal Decreasing Signal Increasing Signal 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 11 –120 –110 –100 –90 –80 –70 –60 Mix1 In, RF INPUT (dBm) Figure 58. RSSI Ripple versus RF Input Level for Different RSSI Capacitors 0 5.0 10 15 20 25 35 0.01 0.10 CRSSI, LOAD CAPACITANCE (µF) Figure 59. RSSI Charge Time versus Capacitor Value 30 8.0 9.0 10 10 nF 22 nF 33 nF 47 nF 100 nF 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 |
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