MICRF102 View Datasheet(PDF) - Micrel
Part Name
Description
Manufacturer
MICRF102 Datasheet PDF : 12 Pages
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MICRF102
Applications Information
Design Process
The MICRF102 transmitter design process is as follows:
1) Set the transmit frequency by providing the cor-
rect reference oscillator frequency.
2) Ensure antenna resonance at the transmit fre-
quency by:
LANT = 0.2 × Length × ln(Length/d - 1.6) × 10-9 × k
Where:
Length is the total antenna length in mm.
d is the trace width in mm.
k is a frequency correction factor.
LANT is the approximate antenna inductance in
henries.
Note 1. The total inductance, however, will be a little greater
than the LANT calculated due to parasitics. A 2nH should be
added to the calculated value. The LANT formula is an ap-
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.
3) Calculate the total capacitance using the follow-
ing equation.
( ) CT =
1
4 × π2 × f2 × LANT
Where:
CT total capacitance in farads.
π = 3.1416.
f = carrier frequency in hertz.
LANT inductance of the antenna in henries.
4) Calculate the parallel and series capacitors,
which will resonate the antenna.
4.1) Ideally for the MICRF102 the series and paral-
lel capacitors should have the same value or as
close as possible.
4.2) Start with a parallel capacitor value and plug in
the following equation.
CS =
1
⎛1
⎝⎜ CT
−
1⎞
( CVAR + CP )⎠⎟
Where:
CVAR is the center varactor capacitance (5pF for the
MICRF102) in farads.
CP is the parallel capacitor in farads.
CS is the series capacitor in farads.
Repeat this calculation until CS and CP are very close and
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 difficult to match the parallel and series
Micrel
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 CT =
CP + CVAR. The blocking capacitor is needed to ensure that
no dc current flows from one antenna pin to the other.
5) Set PC pin to the desired transmit power.
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.
fTX= 32 × fREFOSC
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 mVPP and less than 500 mVPP.
Antenna Considerations
The MICRF102 is designed specifically 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 amplifier 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
December 2006
7
MICRF102
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