Noise Considerations; Characteristics Of Radiated And Conducted Noise - Texas Instruments 2000 Series Reference Manual

Noise can have a negative effect on the receive performance of the RFM. There are two different kinds of
noise: radiated and conducted. Their characteristics are shown in
Radiated Noise
Inductive parts; for example, deflection coils, motor
Source
coils.
Path Via magnetic fields.
Disturbs receive function by magnetic interference
Effect
with signal from transponder at the antenna.
Method for detecting and distinguishing between noise types:
The principle of this procedure is to eliminate any conducted noise from the supply and all interface lines.
In order to do this test the RFM must be powered from a battery (for example: 9 V, 20 mA) in order to
eliminate any conducted noise from a power supply. Conducted noise via the interface lines is eliminated
for this test by simply disconnecting all interface lines to the RFM. The measurement criteria for low noise
is the amplitude of the receive signal strength detector of the RFM.
The test pin RSTP at connector J1 pin 10 carries an analog output voltage indicating the receive signal
strength. This voltage should be measured in combination with the antenna RI-ANT-G02. The necessary
set-up for this test is shown in
interface line connected. As the transmitter is switched off in this configuration, a normal battery may be
used. A low noise level is indicated by an RSTP voltage of less than 1.0 VDC when using antenna
RI-ANT-G02.
Note: Both noise types can be either differential or common mode noise. Use common mode
noise filters (for example: a BALUN transformer) to reduce common mode noise and use
selective filters to reduce differential noise.
The following procedure for testing for noise impact should be implemented when the normal set-up for
the RFM and antenna gives bad reading distances, even though the antenna is correctly tuned for
sufficient transponder charge-up.
Try the configuration shown in
more than approximately 1.0 VDC) then the problem is radiated noise.
SCBU022 – November 2002
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Table F-1. Characteristics of Radiated and Conducted Noise
Figure F-1. This configuration operates the RFM from a battery and has no
Figure F-1. If this configuration shows bad noise conditions (RSTP voltage

Noise Considerations

Table F-1.
Conducted Noise
Power units, for example: motors, switched mode power
supplies. Can be seen as voltage spikes or ripple voltage.
Galvanically conducted via all cables (supply and interface)
connected to the RFM.
Leads to malfunction and reduced sensitivity of receiver circuitry
due to, for example, interfered supply voltage. Conducted noise
can also cause radiated noise.
Appendix F
SCBU022 – November 2002
Noise Considerations 45