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Micrel, Inc.
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 upon the
setting of the SEL0 and SEL1 pins. 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
V SEL0 = V SEL1 = 0V, 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 V CAGC 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 CAGC values for different
SEL0 and SEL1 settings.
MICRF220
Figure 3. RSSI Overshoot and Slow TTGD (9.1ms)
Figure 4 shows the behavior with a larger capacitor on
CAGC pin (2.2 μ F), V SEL1 = 0V, and V SEL0 = V DD . In this
case, V CAGC does not undershoot (RSSI does not
overshoot), and TTGD is relatively short at 1ms.
V SEL1
0V
0V
V DD
V DD
V SEL0
0V
V DD
0V
V DD
CAGC value
4.7 μ F
2.2 μ F
1 μ F
0.47 μ F
Table 3. Minimum Suggested CAGC Values
Figure 3 illustrates what occurs if CAGC capacitance is too
small for a given SEL1, SEL0 setting. Here, V SEL1 = 0V,
V SEL0 = V DD , the capacitance on CAGC pin is 0.47 μ F, 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 μ A 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 4. Proper TTGD (1ms) with Sufficient CAGC
Reference Oscillator
The reference oscillator in the MICRF220 (Figure 5) uses a
basic Pierce crystal oscillator configuration with MOS
transconductor to provide negative resistance. Though the
MICRF220 has built-in load capacitors for the crystal
oscillator, the external load capacitors are still required for
tuning it to the right frequency. RO1 and RO2 are external
pins of the MICRF220 to connect the crystal to the
reference oscillator.
August 2010
10
M9999-082610-A
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