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Micrel, Inc.
Detector and Programmable Low-Pass Filter
The demodulation starts with the detector removing the
carrier from the IF signal. Post detection, the signal
becomes baseband information. The low-pass filter further
enhances the baseband signal. There are four selectable
low-pass filter BW settings; 1625Hz, 3250Hz, 6500Hz, and
13000Hz for 433.92MHz operation. The low-pass filter BW
is directly proportional to the crystal reference frequency,
and hence RF Operating Frequency. Filter BW values can
be easily calculated by direct scaling. Equation 5 illustrates
filter Demod BW calculation:
MICRF220
Slicer and CTH
The signal prior to the slicer, labeled “Audio Signal” in
Figure 1, is still baseband analog signal. The data slicer
converts the analog signal into ones and zeros based upon
50% of the slicing threshold voltage built up in the CTH
capacitor. After the slicer, the signal is demodulated OOK
digital data. When there is only thermal noise at ANT pin,
the voltage level on CTH pin is about 650mV. This voltage
starts to drop when there is RF signal present. When the
RF signal level is greater than ? 100dBm, the voltage is
about 400mV.
The value of the capacitor from CTH pin to GND is not
?
?
BW Operating Freq = BW @433.92MHz ×
? Operating Freq (MHz) ?
? 433.92 ?
Eq. 5
critical to the sensitivity of MICRF220, although it should be
large enough to provide a stable slicing level for the
comparator. The value used in the evaluation board of
0.1 μ F is good for all bit rates from 500bps to 20kbps.
CTH Hold Mode
It is very important to choose the baseband bandwidth
setting suitable for the data rate to minimize bit error rate.
Use the operating curves that show BER vs. bit rates for
different SEL1, SEL0 settings as a guide.
This low-pass filter -3dB corner, or the demodulation BW,
is set at 13000Hz @ 433.92MHz as default (assuming both
SEL0 and SEL1 pins are floating, internal pull-up resistors
set the voltage to V DD ). The low-pass filter can be hardware
set by external pins SEL0 and SEL1. Table 2 demonstrates
the scaling for 315MHz RF frequency:
If the internal demodulated signal (DO ′ in Figure 1) is at
logic LOW for more than about 4msec, the chip
automatically enters CTH hold mode, which holds the
voltage on CTH pin constant even without RF input signal.
This is useful in a transmission gap, or “deadtime”, used in
many encoding schemes. When the signal reappears, CTH
voltage does not need to re-settle, improving the time to
output with no pulse width distortion, or time to good data
(TTGD).
AGC Loop and CAGC
V SEL1
GND
GND
V DD
V DD
V SEL0
GND
V DD
GND
V DD
Low-Pass
Filter BW
1625Hz
3250Hz
6500Hz
13000Hz
Maximum Encoded
Bit Rate
2.5kbps
5kbps
10kbps
20kbps
The AGC comparator monitors the signal amplitude from
the output of the programmable low-pass filter. The AGC
loop in the chip regulates the signal at this point to be at a
constant level when the input RF signal is within the AGC
loop dynamic range (about ? 115dBm to ? 40dBm).
When the chip first turns on, the fast charge feature
charges the CAGC node up with 120μA typical current.
When the voltage on CAGC increases, the gains of the
Table 1. Low-Pass Filter Selection @ 434MHz RF Input
mixer and IF amplifier go up, increasing the amplitude of
the audio signal (as labeled in Figure 1), even with only
thermal noise at the LNA input. The fast-charge current is
V SEL1
GND
GND
V DD
V DD
V SEL0
GND
V DD
GND
V DD
Low-Pass
Filter BW
1170Hz
2350Hz
4700Hz
9400Hz
Maximum Encoded
Bit Rate
1.8kbps
3.6kbps
7.2kbps
14.4kbps
disabled when the audio signal crosses the slicing
threshold, causing DO’ to go high, for the first time.
When an RF signal is applied, a fast attack period ensues,
when 600μA current discharges the CAGC node to reduce
the gain to a proper level. Once the loop reaches
equilibrium, the fast attack current is disabled, leaving only
15μA to discharge CAGC or 1.5μA to charge CAGC. The
fast attack current is enabled only when the RF signal
Table 2. Low-Pass Filter Selection @ 315MHz RF Input
increases faster than the ability of the AGC loop to track it.
August 2010
9
M9999-082610-A
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