MICRF219AAYQS View Datasheet(PDF) - Micrel
Part Name
Description
Manufacturer
MICRF219AAYQS Datasheet PDF : 25 Pages
| |||
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:
× BWOperating Freq = BW@433.92MHz
Operating Freq (MHz)
433.92
Eq. 5
It is very important to select a suitable low-pass filter BW
setting for the required data rate to minimize bit error
rate. Use the operating curves that show BER vs. bit
rates for different D[4:3] 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 connected to
VDD). The low-pass filter can be set by changing register
bits D[4:3]. Table 2 demonstrates the scaling for
315MHz RF frequency:
Low-Pass
Maximum
D[4] D[3] Filter BW Encoded Bit Rate
0
0
1625Hz
2.5kbps
0
1
3250Hz
5kbps
1
0
6500Hz
10kbps
1
1
13000Hz
20kbps
Table 1. Low-Pass Filter Selection @ 434MHz RF input
Low-Pass
Maximum
D[4] D[3] Filter BW Encoded Bit Rate
0
0
1170Hz
1.8kbps
0
1
2350Hz
3.6kbps
1
0
4700Hz
7.2kbps
1
1
9400Hz
14.4kbps
Table 2. Low-Pass Filter Selection @ 315MHz RF input
MICRF219A
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 on
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
critical to the sensitivity of MICRF219A, although it
should be large enough to provide a stable slicing level
for the comparator. The value used in the evaluation
board of 0.1μF is good for all bit rates from 500bps to
20kbps.
CTH Hold Mode
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
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
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 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 increases faster than the ability of
the AGC loop to track it.
August 12, 2015
10
Revision 3.0
RadioTech@micrel.com or (408) 944-0800
Share Link: 