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MICRF219A Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MICRF219A Datasheet(HTML) 11 Page - Micrel Semiconductor |
11 / 25 page Micrel, Inc. MICRF219A August 12, 2015 11 Revision 3.0 RadioTech@micrel.com or (408) 944-0800 The ability of the chip to track to a signal that DECREASED in strength is much slower, since only 1.5 μA is available to charge CAGC to increase the gain. When designing a transmitter that communicates with the MICRF219A, ensure that the power level remains constant throughout the transmit burst. The value of CAGC impacts the time to good data (TTGD), which is defined as the time when signal is first applied, to when the pulse width at DO is within 10% of the steady state value. The optimal value of CAGC depends on the setting of the D4 and D3 bits. A smaller CAGC value does NOT always result in a shorter TTGD. This is due to the loop dynamics, the fast discharge current being 600µA, and the charge current being only 1.5µA. For example, if D4 = D3 = 0, the low pass filter bandwidth is set to a minimum and CAGC capacitance is too small, TTGD will be longer than if CAGC capacitance is properly chosen. This is because when RF signal first appears, the fast discharge period will reduce VCAGC very fast, lowering the gain of the mixer and IF amplifier. But since the low pass filter bandwidth is low, it takes too long for the AGC comparator to see a reduced level of the audio signal, so it can not stop the discharge current. This causes an undershoot in CAGC voltage and a corresponding overshoot in RSSI voltage. Once CAGC undershoots, it takes a long time for it to charge back up because the current available is only 1.5µA. Table 3 lists the recommended minimum CAGC values for different D[4:3] settings to insure that the voltage on CAGC does not undershoot. The recommendation also takes into account the behavior in auto-polling. If CAGC is too small, the chip can have a tendency to false wake up (DO releases even when there is no input signal). D4 D3 CAGC value 0 0 4.7 μF 0 1 2.2 μF 1 0 1 μF 1 1 1 μF Table 3. Minimum Suggested CAGC Values Figure 3 illustrates what occurs if CAGC capacitance is too small for a given D[4:3] setting. Here, D[4:3] = 01, the capacitance on CAGC pin is 0.47 μF, and the RF input level is stepped from no signal to −100dBm. RSSI voltage is shown instead of CAGC voltage because RSSI is a buffered version of CAGC (with an inversion and amplification). Probing CAGC directly can affect the loop dynamics through resistive loading from a scope probe, especially in the state where only 1.5 μA is available, whereas probing RSSI does not. When RF signal is first applied, RSSI voltage overshoots due to the fast discharge current on CAGC, and the loop is too slow to stop this fast discharge current in time. Since the voltage on CAGC is too low, the audio signal level is lower than the slicing threshold (voltage on CTH), and DO pin is low. Once the fast discharge current stops, only the small 1.5µA charge current is available in settling the AGC loop to the correct level, causing the recovery from CAGC undershoot/RSSI overshoot condition to be slow. As a result, TTGD is about 9.1ms. Figure 3. RSSI Overshoot and Slow TTGD (9.1ms) Figure 4 shows the behavior with a larger capacitor on CAGC pin (2.2 μF), D[4:3] = 01. In this case, V CAGC does not undershoot (RSSI does not overshoot), and TTGD is relatively short at 1ms. Figure 4. Proper TTGD (1ms) with Sufficient CAGC |
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Similar Description - MICRF219A_15 |
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