MICRF004 データシートの表示（PDF） - Micrel
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MICRF004 Datasheet PDF : 16 Pages
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MICRF004/RF044
cludes all signals within the sweep range. This same range
reduction also occurs with superregenerative receivers as
their RF bandwidth is also generally 2% to 3% around the
nominal transmit frequency. Any superregenerative receiver
application can instead use a MICRF004 in sweep mode.
IF Bandpass Filter
Rolloff response of the IF Filter is 5th order, while the
demodulator data filter exhibits a 2nd order response. The
multiplication factor between the reference oscillator fre-
quency fT and the internal local oscillator (LO) is 32.5× for
fixed mode, and 32.25× for sweep mode (that is, for fT =
6.00MHz in fixed mode, fLO = 6.00MHz × 32.5 = 195.0MHz).
Bandwidth
The inputs SEL0 and SEL1 control the demodulator filter
bandwidth in four binary steps (550Hz to 4400Hz in sweep,
1100Hz to 8800Hz in fixed mode). Bandwidth must be
selected according to the application. See “Applications
Information” for the bandwidth programming table.
Slicing Level
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. Since
the effective resistance of RSC is 124kΩ, the CTH connection
can be considered a low-pass RC filter with source imped-
ance of 124kΩ.
Slicing level time constant values vary somewhat with de-
coder type, data pattern, and data rate, but typical values
range from 5ms to 50ms. Optimization of the value of CTH is
required to maximize range.
Automatic Gain Control
The signal path has AGC (automatic gain control) 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 (that is, the attack time
constant is 1/10th of the decay time constant), and this ratio
cannot be changed by the user. However, the attack time
constant is set externally by choosing a value for CAGC.
The AGC control voltage is carefully managed on-chip to
allow duty-cycle operation of the MICRF004 in excess of
100:1. 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
on the capacitor due to leakage must be replenished, there-
fore a relatively low-leakage capacitor is recommended for
duty-cycled 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-cycled operation of the IC, the AGC
push and pull currents are increased for a fixed time immedi-
Micrel
ately after the device is taken out of shutdown mode (turned-
on). This compensates for AGC capacitor voltage droop
while the IC is in shutdown mode, reduces the time to restore
the correct AGC voltage, and therefore extends maximum
achievable duty ratios. Push-pull currents are increased by
45 times their nominal values. The fixed time period is based
on the reference oscillator frequency fT, 10.9ms for fT =
6.00MHz, and varies inversely as fT varies.
Reference Oscillator
All timing and tuning operations on the MICRF004 are de-
rived from the internal Colpitts reference oscillator. Timing
and tuning is controlled through the REFOSC pin in one of
three ways:
1. Connect a ceramic resonator
2. Connect a crystal
3. Drive this pin with an external timing signal
The third approach is attractive for lowering system cost
further if an accurate reference signal exists elsewhere in the
system, for example, a reference clock from a crystal- or
ceramic-resonator-controlled microprocessor. An externally
applied signal should be ac-coupled and resistively-attenu-
ated, 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 receiver
as set by the SWEN pin.
Wake-Up Function
The wake-up circuit is available for reducing power consump-
tion of the overall wireless system. WAKEB is an output logic
signal, which goes active low when the IC detects a constant
RF carrier “header” in the demodulated output signal. This
output may be used to enable external circuits, such as a data
decoder or microprocessor, when there is a detection of an
incoming RF signal. The wake-up function is unavailable
when the IC is in shutdown mode.
The wake-up function consists of a resettable counter, based
on an internal 23.4kHz clock (created from a 6.0MHz refer-
ence frequency). When this constant carrier is detected,
without interruption for 128 clock cycles of 25kHz or 5.12ms,
WAKEB will transition low and stay low until data begins. This
approach is utilized over others because constant tones in
excess of 5ms are rare, resulting in few false detections, and
this technique does not require the introduction of a signal
path offset which impacts achievable range.
Shutdown Function
The shutdown function is controlled by a logic state applied
to the SHUT pin. When VSHUT is high, the device goes into
low-power standby mode, consuming less than 1µA. This pin
is pulled high internally. It must be externally pulled low to
enable the receiver.
MICRF004
8
February 9, 2000
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