MICRF229 datasheet(12/23 Pages) MICREL

12 / 23 page

Micrel, Inc.

MICRF229

April 15, 2015

Revision 1.0

AGC Loop

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 AGC node up with 120µA typical current.

When the voltage on AGC increases, the gains of the

mixer and IF amplifier go up, increasing the amplitude of

the audio signal (as labeled in the Functional Diagram),

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 AGC 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 AGC or 1.5µA to charge

AGC. The fast attack current is enabled only when the

RF signal increases faster than the ability of the AGC

loop to track it.

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 AGC to increase the gain. When designing a

transmitter that communicates with the MICRF229,

ensure that the power level remains constant throughout

the transmit burst.

The value of AGC 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 AGC depends on the

setting of the D4 and D3 bits.

A smaller AGC 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 AGC

capacitance is too small, TTGD will be longer than if AGC

capacitance is properly chosen. This is because when

the RF signal first appears, the fast discharge period will

reduce VAGC 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 cannot stop the

discharge current. This causes an undershoot in AGC

voltage and a corresponding overshoot in RSSI voltage.

Once AGC undershoots, it takes a long time for it to

charge back up because the current available is only

1.5µA.

Table 4 lists the recommended minimum AGC values for

different D[4:3] settings to insure that the voltage on AGC

does not undershoot. The recommendation also takes

into account the behavior in auto-polling. If AGC is too

small, the chip can have a tendency to false wake up (DO

releases even when there is no input signal).

Table 4. Minimum Suggested AGC Values

AGC value

4.7

μF

2.2

μF

Figure 3 illustrates what occurs if AGC is too small for a

given D[4:3] setting. Here, D[4:3] = 01, AGC = 0.47

μF,

and the RF input level is stepped from no signal to

−100dBm.

RSSI voltage is shown instead of AGC

voltage because RSSI is a buffered version of AGC (with

an inversion and amplification). Probing AGC 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 the RF

signal is first applied, RSSI voltage overshoots due to the

fast discharge current on AGC, and the loop is too slow

to stop this fast discharge current in time. Since the

voltage on AGC is too low, the audio signal level is lower

than the slicing threshold (voltage on CTH), and DO 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 AGC

undershoot/RSSI overshoot condition to be slow. As a

result, TTGD is about 9.1ms. It is recommended that

Tantalum caps or high voltage ceramic cap is used for

AGC to minimize capacitor leakage current which may

affect the performance of the AGC.

Figure 3. RSSI Overshoot and Slow TTGD (9.1ms)

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