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Micrel
Single Crystal Operation for Dual
Frequency Operation
When using a single crystal, the IF_BW function may
be used to switch between two operating frequencies.
Bandwidth will scale directly with operating frequency
(equation 3). Higher operating frequency will have the
wider IF bandwidth.
Given one operating frequency, the other frequency
can be determined.:
Freq2 Narrow Bandwidth ? Freq1 Wide Bandwidth * (384 ? 1.198) (4)
(384 - 2.178)
MICRF218
Slicer and Slicing Level
The signal prior to slicer is still linear demodulated
AM. Data slicer converts this signal into digital “1”s
and “0”s by comparing with the threshold voltage built
up on the CTH capacitor. This threshold is
determined by detecting the positive and negative
peaks of the data signal and storing the mean value.
Slicing threshold is at 50%. After the slicer, the signal
is now digital OOK data.
During long periods of “0”s or no data period,
threshold voltage on the CTH capacitor may be very
low. Large random noise spikes during this time may
cause erroneous “1”s at DO pin.
Freq2 Wide Bandwidth ? Freq1 Narrow Bandwidth *
(384 ? 2.178) (5)
(384 - 1.198)
AGC Comparator
The AGC comparator monitors the signal amplitude
from the output of the programmable low-pass filter.
OOK Demodulator
The following section discusses the Demodulator
which is comprised of Detector, Programmable Low
Pass Filter, Slicer, and AGC comparator.
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
programmable low-pass filter further enhances the
baseband information through the use of SEL0 and
SEL1. There are four programmable low-pass filter
BW settings for 433.92MHz operation, see Table 1.
Low pass filter BW will vary with RF Operating
Frequency. Filter BW values can be easily calculated
by direct scaling. See equation below for filter BW
calculation:
When the output signal is less than 750mV, the
threshold 1.5μA current is sourced into the external
CAGC capacitor. When the output signal is greater
than 750mV, a 15μA current sink discharges the
CAGC capacitor. The voltage developed on the
CAGC capacitor acts to adjust the gain of the mixer
and the IF amplifier to compensate for RF input signal
level variation.
Reference Control
There are two components in Reference and Control
sub-block: 1) Reference Oscillator and 2) Control
Logic through parallel Inputs: SEL0, SEL1, SHDN and
IF_BW.
Reference Oscillator
BW Operating Freq ? B W @433.92MHz *
(Operating Freq)
433.92
(6)
It is very important to choose the filter setting that
best fits the intended data rate to minimize data
distortion.
Demod BW is set at 13000Hz @ 433.92MHz as
default (assuming both SEL0 and SEL1 pins are
floating). The low pass filter can be hardware set by
external pins SEL0 and SEL1.
SEL0
0
1
0
SEL1
0
0
1
Demod BW (@ 434MHz)
1625Hz
3250Hz
6500Hz
1
1 13000Hz - default
Table 1. Demodulation BW Selection
Figure 6. Reference Oscillator Circuit
The reference oscillator in the MICRF218 (Figure 6)
November 2011
12
M9999-111111
(408) 944-0800
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