September 2002
7
MICRF102
MICRF102
Micrel
Applications Information
Design Process
The MICRF102 transmitter design process is as follows:
1). Set the transmit frequency by providing the
correct reference oscillator frequency
2). Ensure antenna resonance at the transmit
frequency by:
L
ANT
= 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.
L
ANT
is the approximate antenna inductance in
henries.
Note 1. The total inductance however will be a little greater
than the L
ANT
calculated due to parasitics. A 2nH should be
added to the calculated value. The L
ANT
formula is an
approximated 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.
C
f
L
T
ANT
=
×
× ×
(
)
1
4
2
2
ππ
Where:
C
T
total capacitance in farads.
π
= 3.1416.
f = carrier frequency in hertz.
L
ANT
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 parallel
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.
C
C
C
C
S
T
VAR
P
=
−
+
(
)
1
1
1
Where:
C
VAR
is the center varactor capacitance (5pF for the
MICRF102) in farads.
C
P
is the parallel capacitor in farads.
C
S
is the series capacitor in farads.
Repeat this calculation until C
S
and C
P
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 here. The bigger the antenna area, the higher the
loaded Q in the antenna circuit will be. This will make more
difficult to match the parallel and series 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
T
= C
P
+ C
VAR
. 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
reference oscillator frequency.
f
f
TX
REFOSC
=
×
32
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 20 Ohms or less. If a signal
generator is used, the input amplitude must be greater than
200 mV
P-P
and less than 500 mV
P-P
.
Antenna Considerations
The MICRF102 is designed specifically to drive a loop an-
tenna. It has a differential output designed to drive an induc-
tive 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
inductive 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 manufac-
turing tolerances and will vary depending how the PCB is
manufactured.
The MICRF102 features automatic tuning. The MICRF102
automatically tunes itself to the antenna, eradicating 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 from 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.
