Micrel, Inc. MICRF002/RF022
July 2008
9
M9999-070808
Step 3: Selecting the C
TH
Capacitor
Extraction of the dc value of the demodulated signal
for purposes of logic-level data slicing is accomplished
using the external threshold capacitor C
TH
and the on-
chip switched capacitor “resistor” R
SC
, 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 C
TH
is required to maximize range.
Selecting Capacitor C
TH
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.
The effective resistance of R
SC
is listed in the electrical
characteristics table as 145kΩ at 315MHz, this value scales
linearly with frequency. Source impedance of the C
TH
pin at
other frequencies is given by Equation 4, where f
T
is in
MHz:
(4)
T
SC
f
4.8970
145kΩR =
τ of 5x the bit-rate is recommended. Assuming that a slicing
level time constant τ has been established, capacitor C
TH
may be computed using Equation 5:
(5)
SC
TH
R
τ
C =
A standard ±20% X7R ceramic capacitor is generally
sufficient. Refer to Application Hint 42 for C
TH
and CAGC
selection examples.
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 C
AGC
pin of the device. To maximize
system range, it is important to keep the AGC control
voltage ripple low, preferably under 10mV
PP
once the
control voltage has attained its quiescent value. For this
reason capacitor values of at least 0.47µF are
recommended.
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.
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.
Selecting C
AGC
Capacitor in Continuous Mode
A C
AGC
capacitor in the range of 0.47µF 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) 0.441.333C∆t
AGC
−
where:
C
AGC
sin in µF, and ∆t is in seconds.
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 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 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:
(7)
∆t
∆V
C
I
AGC
=
where:
I = AGC pullup current for the initial 10ms (67.5µA)
C
AGC
= AGC capacitor value
∆t = droop recovery time
∆V = droop voltage
For example, if user desires ∆t = 10ms and chooses a
4.7µF C
AGC
, 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 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
