Adjustment Of Oscillator Signal Pulse Width; Oscillator Signal Pulse Width Vs Resistor Value (Estimated Values) - Texas Instruments 2000 Series Reference Manual

The RFM has an built-in feature to allow setting of the pulse width of the transmitter signal coming from
the oscillator. This enables the generated field strength to be reduced from 50% down to 0%.
For this purpose a pulse width setting resistor may be inserted between J4 pins 3 and 4 on the RFM.
Inserting a smaller resistance value decreases the pulse width and thus also the field strength. As default,
no resistor is connected, thus selecting the maximum pulse width of 50% and the maximum field strength.
By connecting a shorting bridge, the smallest pulse width of approximately 0% is selected.
Table C-1 provides an overview of oscillator signal pulse width and corresponding field strength reduction
when different oscillator signal pulse widths are selected by connecting different resistor values.
When using pulse widths smaller than 50%, the RFM transmitter power stage
works less efficiently. This leads to an increased power dissipation and thus to
a higher temperature of the transmitter power stage. Ensure that the antenna
resonance voltage does not exceed 200 Vp when the selected oscillator signal
pulse width setting is smaller than 25%.
Note: The pulse width for an oscillator signal pulse width setting of 5% and smaller is extremely
short. The pulse response of the RFM transmitter power stage to this short pulse is
different for each unit. In order to have reproducible field strength values for different
RFMs, it is not recommended to use the smallest pulse width setting.
SCBU022 – November 2002
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Adjustment of Oscillator Signal Pulse Width

Table C-1. Oscillator Signal Pulse Width vs Resistor
Value (Estimated Values)
Oscillator Signal
Resistor value
(kΩ)
open
151
59
17
10
shorted
Field Strength
Pulse Width Reduction
(%) (dB)
50 0
37 -3
25 -6
12 -12
6 -18
∞
0
CAUTION
Adjustment of Oscillator Signal Pulse Width
Appendix C
SCBU022 – November 2002
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