RF83 Datasheet, PDF (14/22 Pages) List of Unclassifed Manufacturers – 300~440MHz ASK/OOK Receiver

RF83//RF83C/RF83L/RF83CL ASK/OOK Receiver V2.0
13.1.4. Step 4: Selecting the C_AGC 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 C_AGC 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 C_AGC Capacitor in Continuous Mode
A C_AGC capacitor in the range of 1µF to 4.7µF is typically recommended. The value of the C_AGC 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 = 1.333C_AGC − 0.44
Where: C_AGC is in µF, and ∆t is in seconds.
13.1.1.8. Selecting C_AGC Capacitor in Duty-Cycle Mode
Voltage droop across the C_AGC 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 f_T. The
time is 10.9ms for f_T = 6.00MHz, and varies inversely with f_T. The value of C_AGC 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:
I
∆V
(7)
=
C_AGC
∆t
Where:
I = AGC pullup current for the initial 10ms (67.5µA)
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