MICRF610TR MICREL [Micrel Semiconductor], MICRF610TR Datasheet - Page 14

• Manufacturer Part Number: MICRF610TR
• Description: 868-870 MHz ISM Band Transceiver Module
• Manufacturer: MICREL [Micrel Semiconductor]

Front End
A low noise amplifier in RF receivers is used to boost the
incoming signal prior to the frequency conversion process.
This is important in order to prevent mixer noise from
dominating the overall front-end noise performance. The
LNA is a two-stage amplifier and has a nominal gain of
approximately 23dB at 868MHz. The front end has a gain
of about 33dB to 35dB. The gain varies by 1-1.5dB over a
2.0V to 2.5V variation in power supply.
The LNA can be bypassed by setting bit LNA_by to ‘1’.
This can be useful for very strong input signal levels. The
front-end gain with the LNA bypassed is about 9-10dB.
The mixers have a gain of about 10dB at 868MHz.
Sallen-Key Filters
Each channel includes a pre-amplifier and a prefilter,
which is a three-pole Sallen-Key lowpass filter. It protects
the following switched-capacitor filter from strong adjacent
channel signals, and it also works as an anti-aliasing filter.
The preamplifier has a gain of 22.23dB. The maximum
output voltage swing is about 1.4Vpp for a 2.25V power
supply. In addition, the IF amplifier also performs offset
cancellation. Gain varies by less than 0.5dB over a 2.0 –
2.5V variation in power supply. The third order Sallen-Key
lowpass filter is programmable to four different cut-off
frequencies according to the table below:
Switched Capacitor Filter
The
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 the
reference crystal oscillator. A programmable 6-bit divider
divides the frequency of the crystal oscillator. The cut-off
frequency of the filter is given by:
f
CUT
Micrel, Inc.
December 2005
0001000
A6..A0
0000000
A6..A0
0000001
A6..A0
=
main
40
PF_FC1
D7
f
⋅
‘1’
LNA_by
XCO
ScClk
0
0
1
1
D7
‘1’
D7
channel
D6
‘1’
D6
‘0’
PA2
D6
ScClk5
D5
‘0’
D5
PA1
PF_FC0
D5
D4
‘0’
filter
0
1
0
1
ScClk4
D4
PA0
D4
RSSI_en
D3
is
Sync_en
ScClk3
D3
D3
LD_en
a
Cut-off Freq. (kHz)
D2
Mode1
ScClk2
D2
switched-capacitor
D2
100
150
230
340
PF_FC1
D1
Mode0
D1
ScClk1
D1
PF_FC0
D0
D0
’1’
ScClk0
D0
14
1
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:
RSSI
A Typical plot of the RSSI voltage as function of input
power is shown in Figure 9. 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.
st
order RC lowpass filters are connected to the output of
0000001
A6..A0
f
f
ScCLK: Switched capacitor filter clock, bits ScClk5-0
where
f
not be smaller than f
f
transmitter (Hz)
f
Baudrate: The baud rate given is bit/sec
CUT
XCO
BW
offset
DEV
: Needed receiver bandwidth, fcut above should
2,5
0,5
: Filter cutoff frequency
: Single-sided frequency deviation
: Crystal oscillator frequency
1,5
: Total frequency offset between receiver and
2
0
1
f
D7
‘1’
BW
-110
=
D6
‘0’
+
-100
Figure 9. RSSI Voltage
f
OFFSET
D5
‘0’
-90
D4
‘0’
+
BW
f
RSSI_en
Pin
Pin (dBm)
-80
DEV
RSSI
(Hz)
D3
(dBm)
+
-70
Baudrate
LD_en
D2
-60
PF_FC1
-50
M9999-120205
/
D1
2
MICRF610
-40
PF_FC0
D0