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
Application Information
Transmitter Compatibility
Generally, the MICRF004 can be operated in sweep mode,
using a low-cost ceramic resonator. Sweep mode works with
LC-, crystal-, or SAW-based transmitters, without any signifi-
cant range difference. In fixed mode a SAW-based or crystal-
controlled transmitter must be used.
Bypass and Output Capacitors
The bypass and output capacitors connected to VSSBB
should have the shortest possible lead lengths. For best
performance, connect VSSRF to VSSBB at the power supply
only (that is, keep VSSBB currents from flowing through the
VSSRF return path).
Crystal or Ceramic Resonator Selection
Do not use resonators with integral capacitors since capaci-
tors are included in the IC.
If operating in fixed mode, a crystal must be used. In sweep
mode, either a crystal or ceramic resonator may be used.
External Timing Signals
Externally applied signals should be ac-coupled and the
amplitude must be limited to approximately 0.5Vpp.
Bandwidth Programming
Bandwidth must be selected accoring to the application.
SEL0
1
0
1
0
SEL1
1
1
0
0
Demodulator Bandwidth
Sweep Mode FIXED Mode
4400Hz
8800Hz
2200Hz
4400Hz
1100Hz
2200Hz
550Hz
1100Hz
Table 1. Bandwidth Selection
Optional BandPass Filter
For applications located in high ambient noise environments,
a fixed value band-pass network may be connected between
the ANT pin and VSSRF to provide additional receive selec-
tivity and input overload protection. A typical filter is included
in Figure 7a.
Squelch
During quiet periods (no signal) the data output (DO pin)
transitions randomly with noise, presenting problems for
some decoders. A simple solution is to introduce a small
offset, or squelch voltage, on the CTH pin so that noise does
not trigger the internal comparator. Usually 20mV to 30mV is
sufficient, and may be introduced by connecting a several-
megohm resistor from the CTH pin to either VSS or VDD,
depending on the desired offset polarity. Since the MICRF004
has receiver AGC, noise at the internal comparator input is
always the same, set by the AGC. The squelch offset require-
ment does not change as the local noise strength changes
from installation to installation. Introducing squelch will re-
duce range modestly. Only introduce an amount of offset
sufficient to quiet the output.
Micrel
Utilizing Wake-Up
To utilize the wake-up function, a burst of RF carrier in excess
of 5.5ms must be received at the start of each data code word
(preferred for best communication reliability) or a single
5.5ms RF carrier tone must be received at the start of the data
pattern. When this constant carrier is detected, without inter-
ruption, WAKEB will transition low and stay low until data
begins.
For designers who wish to use the wakeup function while
squelching the output, a positive squelching offset voltage
must be used. This simply requires that the squelch resistor
be connected to a voltage more positive than the quiescent
voltage on the CTH pin so that the data output is low in
absence of a transmission.
AGC Configuration
By adding resistance from the CAGC pin to VDDBB or
VSSBB in parallel with the AGC 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.
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.
Frequency and Capacitor Selection
Selection of the reference oscillator frequency fT, slicing level
capacitor (CTH), and AGC capacitor (CAGC) are briefly sum-
marized in this section.
Selecting Reference Oscillator Frequency fT
(Fixed Mode)
As with any superheterodyne receiver, the difference be-
tween the internal LO (local oscillator) frequency fLO and the
incoming transmit frequency fTX ideally must equal the IF
center frequency. Equation 1 may be used to compute the
appropriate fLO for a given fTX:
(1)
fLO
=
fTX
±
0.787
fTX
150
Frequencies fTX and fLO are in MHz. Note that two values of
fLO exist for any given fTX, distinguished as “high-side mixing”
and “low-side mixing,” and there is generally no preference of
one over the other.
After choosing one of the two acceptable values of fLO, use
Equation 2 to compute the reference oscillator frequency fT:
(2)
fT
=
fLO
32.5
Frequency fT is in MHz. Connect a crystal of frequency fT to
REFOSC on the MICRF004. Four-decimal-place accuracy
on the frequency is generally adequate. The following table
identifies fT for some common transmit frequencies when the
MICRF004 is operated in fixed mode.
February 9, 2000
11
MICRF004/RF044
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