Micrel
MICRF506BML/YML
Baudrate: The baud rate given is bit/sec
Switched Capacitor Filter
A6..A0 D7
D6
D5
D4
D3
D2
D1
D0
0001000 ‘1’ ScClk_X2 ‘0’ ScClk4 ScClk3 ScClk2 ScClk1 ScClk0
RSSI
A6..A0 D7
D6 D5
D4
D3 D2 D1 D0
0000001 Modulation1 Modulation0 ‘0’ ‘0’ RSSI_en LD_en PF_FC1 PF_FC0
The main channel filter is a switched-capacitor
implementation of a six-pole elliptic low pass filter.
The elliptic filter minimized the total capacitance
required for a given selectivity and dynamic range.
The cut-off frequency of the switched-capacitor filter
is adjustable by changing the clock frequency.
RSSI
33kohm, 1nF, 125kbps, BW=200kHz, Vdd=2.5V
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
-125
-115
-105
-95
-85
-75
-65
-55
-45
-35
-25
Pin [dBm]
RSSI [V]
The clock frequency is designed to be 20 times the
cut-off frequency. The clock frequency is derived
from the reference crystal oscillator. A
programmable 6-bit divider divides the frequency of
the crystal oscillator. To generate the correct non-
overlapping clock-phases needed by the filter this
frequency is then divided by 4. The cut-off frequency
of the filter is given by:
Figure 13. RSSI Voltage
f
CUT
=
f
XCO
40
⋅
ScClk
C10
1nF
R2
33k
Pin 14
RSSI
RSSI
f
CUT
: Filter cutoff frequency
f
XCO
: Crystal oscillator frequency
ScClk: Switched capacitor filter clock, bits
ScClk5-0
For instance, for a crystal frequency of 16MHz and if
the 6 bit divider divides the input frequency by 4 the
cut-off frequency of the SC filter is 16MHz/(40 x 4) =
100kHz. 1
st
order RC low pass filters are connected
to the output of the SC filter-to-filter the clock
frequency.
Figure 14. RSSI Network
A Typical plot of the RSSI voltage as function of
input power is shown in Figure 13. The RSSI has a
dynamic range of about 50dB from about -110dBm
to -60dBm input power.
The lowest cutoff frequency in the pre- and the main
channel filter must be set so that the received signal
is passed with no attenuation, which is frequency
deviation plus modulation. If there are any frequency
offset between the transmitter and the receiver, this
must also be taken into consideration. A formula for
the receiver bandwidth can be summarized as
follows:
The RSSI can be used as a signal presence
indicator. When a RF signal is received, the RSSI
output increases. This could be used to wake up
circuitry that is normally in a sleep mode
configuration to conserve battery life.
Another application for which the RSSI could be
used is to determine if transmit power can be
reduced in a system. If the RSSI detects a strong
signal, if could tell the transmitter to reduce the
transmit power to reduce current consumption.
f
BW
=
+
f
OFFSET
+
f
DEV
+
Baudrate / 2
where
f
BW
: Needed receiver bandwidth, fcut above
should not be smaller than f
BW
[Hz]
f
OFFSET
: Total frequency offset between
receiver and transmitter [Hz]
f
DEV
: Single-sided frequency deviation, see
chapter Modulator on how to calculate [Hz]
July 2006
21
M9999-092904
+1 408-944-0800
