RF83 datasheet(14/22 Pages) ETC2

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RF83//RF83C/RF83L/RF83CL ASK/OOK Receiver V2.0

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13.1.4. Step 4: Selecting the CAGC Capacitor

The signal path has AGC (automatic gain control) to increase input dynamic range. The attack time

constant of the AGC is set externally by the value of the CAGC capacitor connected to the CAGC pin of

the device. To maximize system range, it is important to keep the AGC control voltage ripple low,

preferably under 10mVpp once the control voltage has attained its quiescent value. For this reason capacitor

values of at 4.7µF are recommended.

The AGC control voltage is carefully managed on-chip to allow duty-cycle operation of the RF83/RF83C.

When the device is placed into shutdown mode (SHUT pin pulled high), the AGC capacitor floats to retain

the voltage. When operation is resumed, only the voltage droop due to capacitor leakage must be

replenished. A relatively low-leakage capacitor is recommended when the devices are used in duty-cycled

operation.

To further enhance duty-cycled operation, the AGC push and pull currents are boosted for approximately

10ms immediately after the device is taken out of shutdown. This compensates for AGC capacitor voltage

droop and reduces the time to restore the correct AGC voltage. The current is boosted by a factor of 45.

13.1.1.7. Selecting CAGC Capacitor in Continuous Mode

A CAGC capacitor in the range of 1µF to 4.7µF is typically recommended. The value of the CAGC should

be selected to minimize the ripple on the AGC control voltage by using a sufficiently large capacitor.

However if the capacitor is too large the AGC may react too slowly to incoming signals. AGC settling time

from a completely discharged (zero-volt) state is given approximately by Equation 6:

(6) ∆t = 1.333CAGC − 0.44

Where: CAGC is in µF, and ∆t is in seconds.

13.1.1.8. Selecting CAGC Capacitor in Duty-Cycle Mode

Voltage droop across the CAGC capacitor during shutdown should be replenished as quickly as possible

after the IC is enabled. As mentioned above, the RF83/RF83C boosts the push-pull current by a factor of

45 immediately after start-up. This fixed time period is based on the reference oscillator frequency fT. The

time is 10.9ms for fT = 6.00MHz, and varies inversely with fT. The value of CAGC capacitor and the

duration of the shutdown time period should be selected such that the droop can be replenished within this

10ms period.

Polarity of the droop is unknown, meaning the AGC voltage could droop up or down. Worst-case from a

recovery standpoint is downward droop, since the AGC pull-up current is 1/10th magnitude of the pulldown

current. The downward droop is replenished according to the Equation 7:

(7)

Where:

∆V

CAGC

∆t

I = AGC pullup current for the initial 10ms (67.5µA)

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