APGRD001 Microchip Technology, APGRD001 Datasheet - Page 5

• Manufacturer Part Number: APGRD001
• Description: REF DESIGN PKE PIC16F639
• Manufacturer: Microchip Technology
• Type: PKE, RKEr

Specifications of APGRD001

• Frequency: 125kHz
• Processor To Be Evaluated: PIC16F639
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: PIC16F639, PIC16F636, PIC18F2680
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

External LC Resonant Antenna
The PIC16F639 device includes three low-frequency
input channels. The LCX, LCY and LCZ pins are for
external LC resonant antenna circuit connections (for
each LF input channel). The external circuits are con-
nected to the antenna input pins and the LCCOM pin.
LCCOM is a common pin for all external antenna cir-
cuits. A capacitor (1-10 F) between the LCCOM pin
and ground is recommended to provide a stable condi-
tion for the internal detection circuit when it detects
strong input signals.
Although the PIC16F639 has three LC input pins for the
three external antenna attachments, the user can use
only one or two antennas, instead of using all three,
depending on the application. The operating current
consumption is proportional to the number of channels
enabled. Fewer channels enabled results in lower
current
recommended to use all three antennas for hands-free
PKE applications.
THEORY OF LC RESONANT ANTENNA
To detect a low-frequency magnetic field, a tuned loop
antenna is commonly used. In order to maximize the
antenna voltage, the loop antenna must be precisely
tuned to the frequency of interest. For PKE
applications, the antenna should be tuned to the carrier
frequency of the base station. The loop antenna is
made of a coil (inductor) and capacitors that are
forming a parallel LC resonant circuit. The voltage
across the antenna is also maximized by increasing the
surface area of the loop and quality factor (Q) of the
circuit.
The resonant frequency of the LC resonant circuit is
given by Equation 1:
EQUATION 1:
where L is the inductance of the loop and C is the
capacitance.
© 2007 Microchip Technology Inc.
consumption.
f o
=
------------------
2
However,
1
LC
it
is
highly
For a given LC resonant circuit, the received antenna
voltage is approximately given by Equation 2 (refer to
application note AN710, “Antenna Circuit Design for
RFID Applications,” (DS00710) for details):
EQUATION 2:
where:
In Equation 2, the quality factor (Q) is a measure of the
selectivity of the frequency of the interest by the tuned
circuit. Assuming that the capacitor is lossless at
125 kHz, Q of the LC circuit is mostly governed by the
inductor defined by:
EQUATION 3:
where f
value and r is the resistance value of the inductor.
In typical transponder applications, the inductance
value is in the 1-9 mH range. Q of the LC circuit is
greater than 20 for an air-core inductor and about 40 for
a ferrite-core inductor.
The S cos
surface area of the antenna that is defined as an
exposed area of the loop to the incoming magnetic
field. The effective antenna surface area is maximized
when cos
antennas of the base station and the transponder units
are positioned in a face-to-face arrangement. In
practical applications, the user might notice the longest
detection range when the two antennas are facing each
other and the shortest range when they are
orthogonally faced. Figure 5 shows a graphical
demonstration of the antenna orientation problem in
practical applications.
f
Q
f
N
S
o
o
c
f
o
= Carrier frequency of the base station (Hz)
=
= Resonant frequency of LC circuit (Hz)
= Number of turns of coil in the loop
= Surface area of loop in square meters
= Quality factor of LC circuit
= Magnetic field strength (Weber/m
= Angle of arrival of signal
is the tuned frequency, L is the inductance
| f
term in Equation 2 represents an effective
V coil
becomes unity, which occurs when the
c
- f
o
|
Q
L
-------------------NSQB o cos
1
=
+
f c
2
--------------------- -
f
r
f
o
L
AN1024
DS01024B-page 5
2
)