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MICRF220 Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MICRF220 Datasheet(HTML) 9 Page - Micrel Semiconductor |
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9 / 18 page  Micrel, Inc. MICRF220 August 2010 9 M9999-082610-A Detector and Programmable Low-Pass Filter The demodulation starts with the detector removing the carrier from the IF signal. Post detection, the signal becomes baseband information. The low-pass filter further enhances the baseband signal. There are four selectable low-pass filter BW settings; 1625Hz, 3250Hz, 6500Hz, and 13000Hz for 433.92MHz operation. The low-pass filter BW is directly proportional to the crystal reference frequency, and hence RF Operating Frequency. Filter BW values can be easily calculated by direct scaling. Equation 5 illustrates filter Demod BW calculation: BWOperating Freq = BW@433.92MHz × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ 433.92 (MHz) Freq Operating Eq. 5 It is very important to choose the baseband bandwidth setting suitable for the data rate to minimize bit error rate. Use the operating curves that show BER vs. bit rates for different SEL1, SEL0 settings as a guide. This low-pass filter -3dB corner, or the demodulation BW, is set at 13000Hz @ 433.92MHz as default (assuming both SEL0 and SEL1 pins are floating, internal pull-up resistors set the voltage to VDD). The low-pass filter can be hardware set by external pins SEL0 and SEL1. Table 2 demonstrates the scaling for 315MHz RF frequency: VSEL1 VSEL0 Low-Pass Filter BW Maximum Encoded Bit Rate GND GND 1625Hz 2.5kbps GND VDD 3250Hz 5kbps VDD GND 6500Hz 10kbps VDD VDD 13000Hz 20kbps Table 1. Low-Pass Filter Selection @ 434MHz RF Input VSEL1 VSEL0 Low-Pass Filter BW Maximum Encoded Bit Rate GND GND 1170Hz 1.8kbps GND VDD 2350Hz 3.6kbps VDD GND 4700Hz 7.2kbps VDD VDD 9400Hz 14.4kbps Table 2. Low-Pass Filter Selection @ 315MHz RF Input Slicer and CTH The signal prior to the slicer, labeled “Audio Signal” in Figure 1, is still baseband analog signal. The data slicer converts the analog signal into ones and zeros based upon 50% of the slicing threshold voltage built up in the CTH capacitor. After the slicer, the signal is demodulated OOK digital data. When there is only thermal noise at ANT pin, the voltage level on CTH pin is about 650mV. This voltage starts to drop when there is RF signal present. When the RF signal level is greater than −100dBm, the voltage is about 400mV. The value of the capacitor from CTH pin to GND is not critical to the sensitivity of MICRF220, although it should be large enough to provide a stable slicing level for the comparator. The value used in the evaluation board of 0.1μF is good for all bit rates from 500bps to 20kbps. CTH Hold Mode If the internal demodulated signal (DO ′ in Figure 1) is at logic LOW for more than about 4msec, the chip automatically enters CTH hold mode, which holds the voltage on CTH pin constant even without RF input signal. This is useful in a transmission gap, or “deadtime”, used in many encoding schemes. When the signal reappears, CTH voltage does not need to re-settle, improving the time to output with no pulse width distortion, or time to good data (TTGD). AGC Loop and CAGC The AGC comparator monitors the signal amplitude from the output of the programmable low-pass filter. The AGC loop in the chip regulates the signal at this point to be at a constant level when the input RF signal is within the AGC loop dynamic range (about −115dBm to −40dBm). When the chip first turns on, the fast charge feature charges the CAGC node up with 120µA typical current. When the voltage on CAGC increases, the gains of the mixer and IF amplifier go up, increasing the amplitude of the audio signal (as labeled in Figure 1), even with only thermal noise at the LNA input. The fast-charge current is disabled when the audio signal crosses the slicing threshold, causing DO’ to go high, for the first time. When an RF signal is applied, a fast attack period ensues, when 600µA current discharges the CAGC node to reduce the gain to a proper level. Once the loop reaches equilibrium, the fast attack current is disabled, leaving only 15µA to discharge CAGC or 1.5µA to charge CAGC. The fast attack current is enabled only when the RF signal increases faster than the ability of the AGC loop to track it. |
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