Transmitter; Pulse Width Examples - Texas Instruments 2000 Series Reference Manual

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The data input and output lines, which are connected to a data processing unit, are low-power Schottky
TTL and HCMOS logic compatible.
The functions of the RFM are described in
1.2

Transmitter

The transmitter power stage is supplied with power via two separate supply lines VSP and GNDP.
Because of the high current requirements for the transmitter power stage, these supply lines are
separated from the logic section supply lines and have two pins per line.
The ground pins for the logic section and the transmitter are not connected internally in order to avoid
possible problems with a high resistivity of GNDP pins and in order to increase flexibility when using long
supply lines. Pins GND and GNDP must be connected to each other externally. For more details, refer to
Section 3.1, Power Supply Connection.
The regulated transmitter power stage supply may vary between +7 V and +24 V. The supply lines VSP
and VSL should be connected together when the supply voltage is +7 V or more. For details refer to
Chapter 2, Specifications.
Note: The RFM has an in-built temperature protection circuit which sharply limits the transmitter
power stage output if an over-current situation or an over-temperature environment
causes the temperature to exceed the allowed limits. After the device is switched off and
has time to recover (when the temperature drops again or the over-current situation is
otherwise rectified) the unit reverts to normal operation when it is switched on again. Such
an occurrence is an indication that the RFM is not being operated within specification.
The transmit frequency (134.2 kHz) from the oscillator is fed to the pulse width modulator (PWM). By
changing the value of a resistor, the PWM can set the pulse width ratio between 0% and 50%. For an
example of two different oscillator signal pulse widths see
pulse width ratio decreases the generated transmit (charge-up) field strength.
It is therefore possible to adjust the generated field strength by selecting different pulse width ratios. For
more information about setting the field strength, refer to
The RFM must not be operated in continuous transmit mode when operated at
full power output. For details please refer to
using pulse widths smaller than 50%, the RFM transmitter power stage works in
a less efficient way. This leads to an increased power dissipation and thus to
higher temperature increase of the transmitter power stage, so ensure that
more cooling is provided.
Note: If the RFM is going to be physically located within the antenna field, it may be necessary
to shield the module.
SCBU022 – November 2002
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Section
Pulse width of 50%
Figure 1-2. Pulse Width Examples
CAUTION
1.2.
Figure 1-2. Decreasing the 134.2 kHz frequency
Appendix B, Field Strength Adjustment.
Pulse width of 12.5%
Chapter 2, Specifications. When
Transmitter
Product Description 9