Micrel, Inc.
MICRF002/RF022
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.
Step 3: Selecting the CTH Capacitor
Extraction of the dc value of the demodulated signal
for purposes of logic-level data slicing is accomplished
using the external threshold capacitor CTH and the on-
chip switched capacitor “resistor” RSC, shown in the
block diagram.
Slicing level time constant values vary somewhat with
decoder type, data pattern, and data rate, but typically
values range from 5ms to 50ms. Optimization of the
value of CTH is required to maximize range.
Selecting CAGC Capacitor in Continuous Mode
A CAGC capacitor in the range of 0.47µF 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:
Selecting Capacitor CTH
The first step in the process is selection of a data-slicing-
level time constant. This selection is strongly dependent on
system issues including system decode response time and
data code structure (that is, existence of data preamble,
etc.). This issue is covered in more detail in Application
Note 22.
(6)
∆t = 1.333CAGC − 0.44
where:
CAGC sin in µF, and ∆t is in seconds.
The effective resistance of RSC is listed in the electrical
characteristics table as 145kΩ at 315MHz, this value scales
linearly with frequency. Source impedance of the CTH pin at
other frequencies is given by Equation 4, where fT is in
MHz:
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 MICRF002 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.
4.8970
(4)
RSC = 145kΩ
fT
τ of 5x the bit-rate is recommended. Assuming that a slicing
level time constant τ has been established, capacitor CTH
may be computed using Equation 5:
τ
(5)
CTH =
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:
RSC
A standard ±20% X7R ceramic capacitor is generally
sufficient. Refer to Application Hint 42 for CTH and CAGC
selection examples.
Step 4: Selecting the CAGC Capacitor
I
∆V
∆t
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 least 0.47µF are
recommended.
(7)
=
CAGC
where:
I = AGC pullup current for the initial 10ms (67.5µA)
AGC = AGC capacitor value
C
∆t = droop recovery time
∆V = droop voltage
For example, if user desires ∆t = 10ms and chooses a
4.7µF CAGC, then the allowable droop is about 144mV.
Using the same equation with 200nA worst case pin
leakage and assuming 1µA of capacitor leakage in the
same direction, the maximum allowable ∆t (shutdown time)
is about 0.56s for droop recovery in 10ms.
The AGC control voltage is carefully managed on-chip to
allow duty-cycle operation of the MICRF002. 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 dutycycled operation.
The ratio of decay-to-attack time-constant is fixed at 10:1
(that is, the attack time constant is 1/10th of the decay time
constant). Generally the design value of 10:1 is adequate
M9999-070808
July 2008
9
MICREL [ MICREL SEMICONDUCTOR ]
