MICRF102BM TR Micrel Inc, MICRF102BM TR Datasheet - Page 7

• Manufacturer Part Number: MICRF102BM TR
• Manufacturer: Micrel Inc

Specifications of MICRF102BM TR

• 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

Applications Information
Design Process
The MICRF102 transmitter design process is as follows:
Where:
Note 1. The total inductance, however, will be a little greater
than the L
added to the calculated value. The L
proximated way to calculate the inductance of the antenna.
The inductance value will vary however, depending on PCB
material, thickness, ground plane, etc. The most precise way
to measure is to use a RF network analyzer.
Where:
Where:
Repeat this calculation until C
they can be found as regular 5% commercial values.
Note 2. Ideally, the antenna size should not be larger than
the one shown in Figure 7. The bigger the antenna area,
the higher the loaded Q in the antenna circuit will be. This
will make it more diffi cult to match the parallel and series
December 2006
4.1) Ideally for the MICRF102 the series and paral-
4.2) Start with a parallel capacitor value and plug in
1) Set the transmit frequency by providing the cor-
2) Ensure antenna resonance at the transmit fre-
3) Calculate the total capacitance using the follow-
4) Calculate the parallel and series capacitors,
L
Length is the total antenna length in mm.
d is the trace width in mm.
k is a frequency correction factor.
L
henries.
C
π = 3.1416.
f = carrier frequency in hertz.
L
C
MICRF102) in farads.
C
C
rect reference oscillator frequency.
quency by:
ing equation.
which will resonate the antenna.
lel capacitors should have the same value or as
close as possible.
the following equation.
ANT
ANT
ANT
C
T
C
VAR
P
S
S
T
total capacitance in farads.
is the parallel capacitor in farads.
is the series capacitor in farads.
ANT
=
=
= 0.2 × Length × ln(Length/d - 1.6) × 10
is the approximate antenna inductance in
inductance of the antenna in henries.
is the center varactor capacitance (5pF for the
(
⎛
⎜
⎝
4
calculated due to parasitics. A 2nH should be
C
1
×
T
π π
−
2
(
×
C
1
f
1
2
VAR
×
1
L
+
ANT
C
P
S
)
)
⎞
⎟
⎠
and C
ANT
P
are very close and
formula is an ap-
-9
× k
7
capacitors. Another point to take into consideration is the
total AC rms current going through the internal varactor in
the MICRF102. This current should not exceed 16mA rms.
The parallel capacitor will absorb part of this current if the
antenna dimensions are appropriate and not exaggerated
larger than the one shown here.
Note 3. A strong indication that the right capacitor values
have been selected is the mean current with a 1kHz signal
in the ASK pin. Refer to the “Electrical Characteristics” for
the current values.
Note 4. For much smaller antennas, place a blocking capaci-
tor for the series capacitance (around 100pF to 220pF) and
use the following formula for the parallel capacitance C
C
no dc current fl ows from one antenna pin to the other.
Reference Oscillator Selection
An external reference oscillator is required to set the transmit
frequency. The transmit frequency will be 32 times the refer-
ence oscillator frequency.
Crystals or a signal generator can be used. Correct reference
oscillator selection is critical to ensure operation. Crystals
must be selected with an ESR of 20Ω or less. If a signal
generator is used, the input amplitude must be greater than
200 mV
Antenna Considerations
The MICRF102 is designed specifi cally to drive a loop antenna.
It has a differential output designed to drive an inductive load.
The output stage of the MICRF102 includes a varactor that
is automatically tuned to the inductance of the antenna to
ensure resonance at the transmit frequency.
A high-Q loop antenna should be accurately designed to set
the center frequency of the resonant circuit at the desired
transmit frequency. Any deviation from the desired frequency
will reduce the transmitted power. The loop itself is an induc-
tive element. The inductance of a typical PCB-trace antenna
is determined by the size of the loop, the width of the antenna
traces, PCB thickness and location of the ground plane.
The tolerance of the inductance is set by the manufacturing
tolerances and will vary depending upon how the PCB is
manufactured.
The MICRF102 features automatic tuning. The MICRF102
automatically tunes itself to the antenna, eliminating the need
for manual tuning in production. It also dynamically adapts
to changes in impedance in operation and compensates for
the hand-effect.
Automatic Antenna Tuning
The output stage of the MICRF102 consists of a variable
capacitor (varactor) with a nominal value of 5.0pF tunable
over a range of 3pF to 7pF. The MICRF102 monitors the
phase of the signal on the output of the power amplifi er and
automatically tunes the resonant circuit by setting the varactor
value at the correct capacitance to achieve resonance.
In the simplest implementation, the inductance of the loop
antenna should be chosen such that the nominal value is
P
5) Set PC pin to the desired transmit power.
+ C
VAR
f
TX
PP
=
. The blocking capacitor is needed to ensure that
and less than 500 mV
32
×
f
REFOSC
PP
.
MICRF102
T
=