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MICRF002BN 데이터 시트보기 (PDF) - Micrel

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MICRF002BN Datasheet PDF : 13 Pages
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MICRF002
QwikRadio tm
Micrel
AGC Function and the CAGC Capacitor
The signal path has automatic gain control (AGC) to
increase input dynamic range. An external capacitor,
CAGC, must be connected to the CAGC pin of the device.
The ratio of decay-to-attack time-constant is fixed at 10:1
(i.e., the attack time constant is 1/10th the decay time
constant), and this ratio cannot be changed by the user.
However, the attack time constant is selectable by the user
through the value of capacitor CAGC.
[By adding resistance from the CAGC pin to VDDBB or
VSSBB in parallel with the CAGC capacitor, the ratio of
decay-to-attack time-constant may be varied, although the
value of such adjustments must be studied on a per-
application basis. Generally the design value of 10:1 is
adequate for the vast majority of applications.] See
“Application Note TBD”.
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 0.47uF are recommended.
The AGC control voltage is carefully managed on-chip to
allow duty-cycle operation of the MICRF002 in excess of
100:1. When the device is placed into SHUT mode (i.e.,
SHUT pin pulled high), the AGC capacitor is “floated”, to
retain the voltage. When operation is resumed, only the
voltage droop on the capacitor associated with leakage must
be replenished. Thus a relatively low-leakage capacitor is
recommended for duty cycle operation. The actual tolerable
leakage will be application dependent. Clearly, leakage
performance is less critical when the device off-time is low
(milliseconds), and more critical when the off-time is high
(seconds).
To further enhance Duty-cycle operation of the IC, the AGC
push and pull currents are increased for a fixed time
immediately after the device is taken out of Shutdown mode
(i.e., turned-on). This compensates for AGC capacitor
voltage “droop” while the IC is in Shutdown, reduces the
time to reacquire the correct AGC voltage, and thus extends
maximum achievable duty ratios. Push/Pull currents are
increased by 45X their nominal values. The fixed time
period is based on the REFOSC frequency ft, 10.9msec for
ft = 6.00MHz, and varies inversely as ft varies.
Reference Oscillator (REFOSC) and External Timing
Element
All timing and tuning operations on the MICRF002 are
derived from the REFOSC function. This function is a
single-pin Colpitts-type oscillator. The user may handle this
pin in one of three possible ways:
(1) connect a ceramic resonator, or
(2) connect a crystal, or
(3) drive this pin with an external timing signal.
The third approach is attractive for further lowering system
cost if an accurate reference signal exists elsewhere in the
system (e.g., a reference clock from a crystal or ceramic
resonator-based microprocessor). An externally applied
signal should be AC-coupled, and resistively-divided down
(or otherwise limited) to approximately 0.5Vpp. The specific
reference frequency required is related to the system
transmit frequency, and to the operating mode of the device
as set by the SWEN control pin. See “Application Note
TBD” for a discussion of frequency selection and accuracy
requirements.
Wakeup Function
The Wakeup function is made available for the purposes of
further reducing power consumption of the overall wireless
system. WAKEB is an output logic signal, which goes active
when the IC detects a constant tone “header” in the
demodulated output signal. Sense of the signal is active-
low. This output may be used to “wakeup” other external
circuits, like a data decoder or microprocessor, only at times
when there is a reasonable expectation of an incoming RF
signal. The Wakeup function is unavailable when the IC is
in SHUT mode.
The Wakeup function is composed of a resettable counter,
based on an internal 25kHz clock (based on a 6.4MHz
reference frequency). To utilize this function, a constant
tone in excess of about 4msec must be placed at the start of
each data code word, or a single 4msec tone at the start of
the data pattern (clearly the former is preferred to improve
communication reliability). When this constant tone is
detected, without interruption, for 128 clock cycles of 25kHz,
WAKEB will transition low, and stay low until data begins.
This particular approach is utilized over others (1) since
constant tones in excess of 4msec are very rare, leading to
few false-positive indications, and (2) this technique does
not require the introduction of a signal path offset, which
impacts achievable range.
[Note: For designers who wish to use the Wakeup function
while “squelching” the output, a positive squelching offset
voltage must be used. This simply requires the squelch
resistor be taken to a voltage more positive than the
quiescent voltage on pin CTH, so that the data output is low
in absence of a transmission.]
Shutdown Function
The Shutdown function is controlled by the logic state of
SHUT. When SHUT is high, the device goes into low-power
standby mode, consuming less than 1µA. This pin is pulled
high internally, and so must be pulled low to engage the
device.
MICRF002 Frequency and Capacitor Selection
Selection of the REFOSC frequency ft, Slicing Level (CTH)
capacitor, and AGC capacitor are briefly summarized in this
section. Please see Application Note TBD for complete
details.
July 1999
9
MICRF002

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