2001 Oct 04 18
Philips Semiconductors Product specification
HITAG core module hardware HTCM400
V
o
≈ 2.5 V
V
L
=L×ω×I
a
where ω =2×π×f (f = 125 kHz)
The maximum value for V
L
reads 32 V (p-p), safeguarding
against damage to the input level of the read/write device:
• With V
L
< 32 V (p-p) the resistance R
e
can be omitted
• With V
L
> 32 V (p-p) you have to calculate and insert R
e
according to the following formula:
and
R
e
≥ L ×ω×0.078 − 22 − R
s
11.8 Procedure for practical antenna design
The procedure how to design a HITAG proximity antenna
is described in the previous sections. The main steps are
the following:
• The desired inductance for the antenna coil can be
chosen in a range between 350 and 500 µH; e.g.
L = 420 µH
• The number of windings N can be calculated with the
following formula:
For L = 420 µH:
N=
Remark: the factor K (see Section 11.2) normally is
much smaller than and therefore can be left out.
• Now the antenna can be built up with the desired
dimensions (circumference a) with the calculated
number of turns.
Remark: the antenna coil must be changed afterwards
because with the mechanical dimensions the electrical
specifications are changing too. That means the number
of turns, the shape, arrangement of the coil windings
and antenna supply cable must be in their final form.
Remark: metal influences the electrical characteristics
of the antenna very much. That is why all future tasks
have to be done with the antenna in its final environment
if metal will be in the antenna’s neighbourhood (distance
of the metal shorter than the maximum antenna
diameter)
• Measurement of the inductance L of the antenna is
described in Section 11.3
• Determination of the serial capacitor C
S
is described in
Section 11.5
Remark: the capacitance of the antenna supply cable
can be measured or found out in the data sheet of the
cable (e.g. C
p
= 180 pF/m)
• Now the antenna has to be tuned according to
Section 11.5. The tuning is acceptable if the resonant
frequency is within a range of 125 ± 4 kHz
• The serial resistance R
s
of the antenna is the impedance
of the tuned antenna and is an ohmic resistance at the
resonance frequency (f = 125 kHz). It can be calculated
as shown in Section 11.6
• To get a satisfactory reading distance the quality factor
of the antenna coil (for non-metal environment) should
be approximately Q = 40. The quality factor of a coil is
calculated as follows:
• By knowing R
S
and the dropping resistor (R1 = 22 kΩ)it
is possible to calculate the current I
a
and the antenna
voltage V
L
.
It is very important to calculate the antenna voltage
before connecting the antenna to the HTCM400 to avoid
damage. Is the calculated value of V
L
higher
than 32 V (p-p) a resistor R
e
has to be integrated to
protect the module output circuit. The resistor has to be
placed as shown in Section 11.7
• After checking the antenna voltage connect your
antenna to the HTCM400 and measure the read/write
distances with your transponders. If the read/write
distances do not fulfil your expectations, the following
points should be considered:
– The size of the antenna and the size of the
transponder have to be in a defined ratio (between 3
and 1). That means, if you increase the antenna over
a certain size, the maximum read/write distances will
decrease by the use of the same transponder
– The optimal shape of the antenna coil is a circle. The
performance of a square shaped coil is much better
than that of a rectangular shaped coil (with the same
circumference)
– To get better read/write distances the quality factor of
the antenna coil should be increased, but it must not
be higher than Q = 40. This can be reached by the
following measures:
I
a
V
o
R1 R
s
R
e
++
----------------------------------
=
R
e
L ω×
V
o
V
L(max)
------------------
R1– R
s
–×=
N
LnH()
2a× ln×
a
D
----
K–×
--------------------------------------------------
1.9
=
420000
2a× ln
a
D
----
K–×
--------------------------------------------
1.9
633
aln×
a
D
----
1.9
---------------------------------
=
a
D
----
Q
ω L×
R
s
-------------
2 π× f× L×
R
s
------------------------------
==
