MICRF218_11 MICREL [Micrel Semiconductor], MICRF218_11 Datasheet - Page 12

• Manufacturer Part Number: MICRF218_11
• Description: 3.3V, 315/433MHz Wide-IF Bandwidth ASK Receiver
• Manufacturer: MICREL [Micrel Semiconductor]

November 2011
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.:
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.
signal
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:
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.
BW
Freq2
Freq2
It is very important to choose the filter setting that
Operating
SEL0
Narrow
Wide
0
1
0
1
becomes
Table 1. Demodulation BW Selection
Bandwidth
Bandwidth
Freq
SEL1

0
0
1
1

B

W
Freq1
Freq1
baseband
@433.92MHz
Demod BW (@ 434MHz)
1625Hz
3250Hz
6500Hz
13000Hz
Narrow
Wide
Bandwidth
Bandwidth
*
(Operating
information.
- default
Post detection, the
*
*
433.92
(384
(384
(384
(384


-
Freq)
-
2.178)
1.198)
1.198)
2.178)
The
(4)
(5)
(6)
12
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.
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.
AGC Comparator
The AGC comparator monitors the signal amplitude
from the output of the programmable low-pass filter.
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.
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
The reference oscillator in the MICRF218 (Figure 6)
Figure 6. Reference Oscillator Circuit
The voltage developed on the
This threshold is
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M9999-111111