MICRF008BM TR Micrel Inc, MICRF008BM TR Datasheet - Page 12

• Manufacturer Part Number: MICRF008BM TR
• Manufacturer: Micrel Inc

Specifications of MICRF008BM TR

• Operating Band Frequency: 300 to 440MHz
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Product Depth (mm): 3.94mm
• Product Height (mm): 1.48mm
• Operating Supply Voltage (min): 4.75V
• Operating Supply Voltage (typ): 5V
• Operating Supply Voltage (max): 5.5V
• Lead Free Status / Rohs Status: Not Compliant

SEL0 Pin – Setting the Demodulator Bandwidth
The SEL0 pin sets the demodulator bandwidth. When
the pin is connected to ground the demodulator
bandwidth is set to its minimum. When the pin is left
floating (internal pull-up) or connected to VDD, the
demodulator bandwidth is set to its maximum. The
demodulator bandwidth is a function of the RF frequency
used. See Table 7 for the most common frequency used
versus the demodulator bandwidth.
To determine what is the demodulator bandwidth
required for the application, the absolute minimum pulse
width used in the encoder transmitted signal needs to be
found. The data pattern has several different types of
pulse width depending what data format is being used
(NRZ, Manchester, PWM, etc). The minimum pulsewidth
is the shortest pulse in the output of the encoder being
sent to the transmitter input measured during one bit
time when the signal is high. See Figure 9 below for
clarification.
Then, the demodulator bandwidth required is calculated
by:
Demod
Micrel, Inc.
August 2008
SEL0
0
1
BW
1.6kHz
3.6kHz
bit period = 1/data rate
Table 7. Demodulator Bandwidth
=
Preamble
315
shortest
Figure 9. Data Pattern
RF Carrier Frequency (MHz)
0.65
pulse
2.0kHz
4.0kHz
390
[ ]
Dead Time
Hz
2.2kHz
4.4kHz
418
pulsewidth
Shortest
Manchester
Data 50%
433.92
2.4kHz
4.8kHz
12
Data Squelching
Data squelching can be achieved by software or
hardware. The software squelch is preferred over
hardware squelch for it is more flexible, reliable, lower
cost over manufacturing, etc. However, it takes time and
resources to develop any software. If one needs a
squelch function, a resistor can be used in the C
R1 and R2 are in the test circuit for this purpose. Either
one of them can be used or both. If only one resistor is
used, the value of the resistor should be chosen
according to the amount of squelch needed and if the
DO pin needs to be sitting high or low without an RF
income signal. Values from 10MΩ and below can be
used. When both resistors are used, a more accurate
control can be achieved for the squelch function.
The drawbacks using these resistors are: loss in receiver
sensitivity, reduced demodulator bandwidth, and the C
capacitor equation will not work. As a result, C
capacitor will have to be determined empirically. Another
way to implement a hardware squelch is to make R1
equal to 10MΩ to 6.8MΩ, and use a low-pass filter after
the DO pin and a comparator circuit. The low-pass filter
can be a capacitor, which is C6 in the Test Circuit. The
reference voltage in the comparator can be set close to
VDD/2 and the other input connected to the DO pin.
Power Supply Filter
A decoupling capacitor right at the VDD pin is strongly
recommended as it is in the test circuit, which has 0.1μF.
Not shown in the Test Circuit Schematic is an EMI
Inductor, L1 in the silkscreen layer. It has the purpose to
further filter an income noise on the VDD line, generally
present in switching power supplies. It is good practice
to separate RF ground from digital ground and also VDD
RF from other VDD lines
M9999-080108
MICRF008
TH
pin.
TH
TH