Threshold Level Adjustment - Texas Instruments 2000 Series Reference Manual

The RFM has a built-in receive signal field strength detector with the output signal RXSS- and an on-board
potentiometer (R409) to adjust the threshold level of field strength detection. The digital output RXSS- is
used for wireless synchronization of two or more reading units. This is necessary to ensure that if more
than one reading unit is in an area, they do not interfere with each other. The Control Module software
monitors the RXSS- signal to detect whether other reading units are transmitting. The Control Module can
operate the transmitter of the RFM such that the reading units either transmit simultaneously or
alternately. In this way the read cycles of each of the reading units occur at the same time or at secure
different times. Depending on the antenna type used and the local noise level, the RXSS- threshold level
has to be adjusted. This needs to be done after the antenna has been tuned to resonance. It is
recommended to use a small screwdriver to adjust the RXSS- threshold level. The R409 potentiometer is
located on the upper side of the RFM board near connector switch SW1. Turning the potentiometer all the
way clockwise (right-hand stop), results in minimum threshold sensitivity, i.e. the RXSS- signal will be
activated at high receive field strength. This is the default position and can be used for standard gate
antennas. It may be necessary to increase the sensitivity when using ferrite core antennas. If there is high
noise level in the area, it is necessary to adjust the RXSS- threshold level.
Adjust the RXSS- threshold level as follows:
1. Turn the RXSS- threshold level potentiometer fully counter-clockwise (left-hand stop).
2. Deactivate the transmitter by jumpering pin 1 to pin 3 of connector J2.
3. Ensure that no other reading units are transmitting, by connecting pin 1 to pin 3 of connector J2
(jumper) of all other RFMs in the area.
4. Monitor the voltage at RXSS- output pin with a voltmeter or an oscilloscope.
5. Turn the RXSS- threshold level adjustment potentiometer on the RFM clockwise, until the RXSS-
output is just statically inactive. "Statically" means no voltage spikes present on the RXSS- signal.
'Inactive' means that the receive signal strength is below the RXSS- threshold level and not triggering
RXSS- (the RXSS- output voltage remains > 4 V).
6. Remove all jumpers connected to J2.
SCBU022 – November 2002
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Threshold Level Adjustment

Appendix D
SCBU022 – November 2002
Threshold Level Adjustment 41