micrf506 Micrel Semiconductor, micrf506 Datasheet - Page 18

• Manufacturer Part Number: micrf506
• Description: 410mhz And 450mhz Ism Band Transceiver
• Manufacturer: Micrel Semiconductor

Switched Capacitor Filter
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.
The clock frequency is designed to be 20 times the
cut-off frequency. The clock frequency is derived
from
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:
For instance, for a crystal frequency of 32MHz and if
the 6 bit divider divides the input frequency by 5 the
cut-off frequency of the SC filter is 32MHz/(80 x 5) =
80MHz. 1
the output of the SC filter-to-filter the clock
frequency.
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:
f
f
f
0001000
A6..A0
CUT
CUT
BW
Micrel
September 2004
= + f
=
=
when ScClk_X2=0 and
when SCClk_X2=1 where:
f
f
ScCLK: Switched capacitor filter clock, bits
ScClk5-0
CUT
XCO
D7
‘1’
the
80
ScClk
40
ScClk
OFFSET
: Filter cutoff frequency
: Crystal oscillator frequency
st
f
f
order RC lowpass filters are connected to
ScClk_X2
XCO
XCO
D6
reference
+ f
DEV
ScClk5
D5
+ Baudrate / 2
ScClk4
D4
crystal
ScClk3
D3
ScClk2
oscillator.
D2
ScClk1
D1
ScClk0
D0
A
18
0000001
A6..A0
RSSI
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 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.
Modulation1
where
f
should not be smaller than f
f
and transmitter (Hz)
f
chapter Modulator on how to calculate (Hz)
Baudrate: The baud rate given is bit/sec
BW
offset
DEV
Pin 14
RSSI
D7
: Needed receiver bandwidth, fcut above
2.2
1.8
1.6
1.4
1.2
0.8
0.6
: Single-sided frequency deviation, see
2
1
: Total frequency offset between receiver
-125
-115
Modulation0
Figure 14. RSSI Network
Figure 13. RSSI Voltage
D6
-105
33kohm, 1nF, 125kbps, BW=200kHz, Vdd=2.5V
-95
R2
33k
D5
‘0’
-85
RSSI
Pin [dBm]
D4
‘0’
-75
-65
RSSI_en
D3
BW
-55
C10
1nF
(Hz)
-45
LD_en
[phone]
M9999-092904
D2
-35
MICRF506
RSSI
PF_FC1
-25
D1
PF_FC0
D0