MICRF230 datasheet(12/20 Pages) MICREL

12 / 20 page

Micrel, Inc.

MICRF230

MICRF230, ensure that the power level remains constant

throughout the transmit burst.

The value of AGC impacts the period between the TTGD,

which is defined as the time when signal is first applied,

to the pulse width at DO, 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 SEL0 = SEL1 = 0, the

low pass filter bandwidth is set to a minimum and the

AGC capacitance is too small. The 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. 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,

and cannot stop the discharge current. This causes an

undershoot

AGC

voltage

and

corresponding

overshoot in RSSI voltage. Once the AGC undershoots, it

takes a long time for it to charge back up because the

current available is only 1.5µA.

Table 2 lists the recommended minimum AGC values for

different SEL0 and SEL1 settings to insure that the

voltage on AGC does not undershoot.

Table 2. Minimum Suggested AGC Values

SEL0

SEL1

AGC value

4.7

μF

2.2

μF

Figure 4 illustrates what occurs if AGC is too small for a

given bandwidth setting. In this instance, SEL0 = 1, SEL1

= 0, AGC = 0.47

μF, and the RF input level is stepped

from no signal to

−100dBm. RSSI voltage is shown in

place 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 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 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 caps are used for AGC to minimize capacitor

leakage current which may affect the performance of the

AGC.

Figure 4. RSSI Overshoot and Slow TTGD (9.1ms)

Figure 5 shows the behavior with a larger capacitor on

AGC pin (2.2

μF), SEL0:SEL1 = 10. In this case, V

AGC

does not undershoot (RSSI does not overshoot), and

TTGD is relatively short at 1ms.

Figure 5. Proper TTGD (1ms) with Sufficient AGC

Reference Oscillator

The reference oscillator in the MICRF230, shown in

Figure

uses

basic

Pierce

crystal

oscillator

April 15, 2015

Revision 2.0

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