MICRF219A datasheet(11/23 Pages) MICREL

11 / 23 page

Micrel, Inc.

MICRF219A

July 2011

M9999-071811-A

RadioTech@micrel.com

The ability of the chip to track to a signal that

DECREASED in strength is much slower, since only

1.5μA 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).

CAGC value

4.7μF

2.2μF

1μF

or (408) 944-0800

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 0.47μF, 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 1.5μA 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 (2.2μF), D[4:3] = 01. In this case, VCAGC 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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