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BW Operating Freq = BW @315MHz × ?
?
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 base band information. The programmable
low-pass filter further enhances the base band
information. There are four programmable low-pass filter
BW settings: 1180Hz, 2360Hz, 4270Hz, 9400Hz for
315MHz operation. Low pass filter BW will vary with RF
Operating Frequency. Filter BW values can be easily
calculated by direct scaling. See the equation below for
the filter BW calculation:
? Operating Freq (MHz) ?
? 315 ?
It is very important to choose filter setting that best fits
MICRF213
1.5μA current is then 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.
Reference Oscillator
V BIAS
the intended data rate as this will minimize data
distortion.
Demod BW is set at 9700Hz @ 315MHz as default
(assuming both SEL0 and SEL1 pins are floating). The
R 1
R 2
C 0
low pass filter can be hardware set by external pins
SEL0 and SEL1.
RO1
1
Startup
C C1
M1
gm
I BIAS
SEL0
0
SEL1
0
Demod BW (@ 315MHz)
1180Hz
Circuit
C C2
1
0
0
1
2360Hz
4270Hz
M2
M3
1
1
9400Hz
- default
RO2
C 1
Table 1. Demodulation BW Selection
Slicer, Slicing Level and Squelch
M4
Normally on
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
default is 50%. After the slicer, the signal becomes
digital OOK data.
During long periods of “0”s or no data period at all,
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. Squelch pin when pull
down low will suppress these errors.
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 threshold,
Figure 2. Reference Oscillator Circuit
The reference oscillator in the MICRF213 (reference
Figure 2) uses a basic Colpitts crystal oscillator
configuration with a MOS transconductor to provide
negative resistance. All capacitors shown in the figure
are integrated inside MICRF213. R01 and R02 are
external pins of MICRF213. The user only need connect
the reference oscillation crystal.
Reference oscillator crystal frequency can be calculated
thus as:
F REFOSC = F RF /(32 + 1.1/12)
For 315MHz, F REFOSC = 9.81563 MHz.
To operate the MICRF213 with minimum offset, crystal
frequencies should be specified with 10pF loading
capacitance.
May 2007
7
M9999-052307-A
(408) 944-0800
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