Texas Instruments RFM003B Manuale Utente
29
November ’00
Chapter 5. Installation and Use
5.6
Antenna Tuning
In order to get all the available field strength from the antenna, it must be tuned to
resonance. To do this, the following set-up and equipment are required.
resonance. To do this, the following set-up and equipment are required.
5.6.1
To Switch-ON the Transmitter
The reading/writing unit or test set up should be able to switch the RF module’s trans-
mitter to a fast repeating reading mode or continuous wave mode. If a continuous
wave mode is applied, care must be taken not to exceed the duty cycle limitation as
given in section 4.3. Time when the transmitter is switched on but during which the
current consumption is less than I_VSPcont, does not add to the duty cycle.
mitter to a fast repeating reading mode or continuous wave mode. If a continuous
wave mode is applied, care must be taken not to exceed the duty cycle limitation as
given in section 4.3. Time when the transmitter is switched on but during which the
current consumption is less than I_VSPcont, does not add to the duty cycle.
5.6.2
To Tune the Antenna Circuit
The built-in tuning capacitors C_AT1 to C_AT6 which are available at ST2 should be
used to tune the resonance frequency of the antenna circuit to the output frequency
of the transmitter. The most simple method of tuning is to use a “Switch Box” con-
nected to the antenna tuning connector ST2, this will allow the capacity of the anten-
na resonant circuit to be increased by connecting the built-in tuning capacitors to the
ground.
used to tune the resonance frequency of the antenna circuit to the output frequency
of the transmitter. The most simple method of tuning is to use a “Switch Box” con-
nected to the antenna tuning connector ST2, this will allow the capacity of the anten-
na resonant circuit to be increased by connecting the built-in tuning capacitors to the
ground.
The “Switch Box” in its simplest version is an array of six well isolated, high voltage
proof ON/OFF switches, which are connected to the antenna tuning connector ST2
(Antenna Tuning Switch Box RI-ACC-ATS1).
proof ON/OFF switches, which are connected to the antenna tuning connector ST2
(Antenna Tuning Switch Box RI-ACC-ATS1).
If you have to tune a lot of Mini-RFMs (for example on a production run) it would be
helpful to build a computer (or electronically) controlled tuning box which allows you
to start the tuning procedure and then set the jumpers according to the result of the
tuning. The cable from the “switch box” to the connector ST2 should be as short as
possible (less that 20 cm) in order to get the same tuning results as with the jumpers.
helpful to build a computer (or electronically) controlled tuning box which allows you
to start the tuning procedure and then set the jumpers according to the result of the
tuning. The cable from the “switch box” to the connector ST2 should be as short as
possible (less that 20 cm) in order to get the same tuning results as with the jumpers.
5.6.3
To Detect the Correct Tuning
There are two basic methods to detect the correct tuning point, they are:
One method is to tune the antenna circuit to maximum current consumption of
the transmitter power stage by inserting a current meter into the VSP-line
which is an easy way for production. The lines of the meter should be as short
as possible, or they must be RF decoupled close to the RF module.
which is an easy way for production. The lines of the meter should be as short
as possible, or they must be RF decoupled close to the RF module.
The second method is to tune the antenna circuit to the maximum of the mag-
netic field strength generated by the antenna.