RFL 9780 RFL Electronics Inc.
April 8, 2003 14-5 (973) 334-3100
14.3.6 SCALING AND BIAS CIRCUITS
Operational amplifier U3D adds the output of the low pass filter to a bias voltage, and then amplifies
the result. The output of the low pass filter is biased at approximately 2.5 volts when receiving a 4 kHz
signal. To remove this bias, a negative voltage of the same value is added to the signal. This bias
voltage is set using potentiometer R36 and is buffered by U3C. U3D then sums the two signals and
provides a gain which is determined by the Scale potentiometer R29.
14.3.7 SLICER CIRCUITS
The slicer circuits determine whether the incoming signal is trip or guard. Comparator U4B and its
associated components form the trip slicer. Its slice level is set by TRIP SLICE potentiometer R5. The
slice level can be varied from approximately +28 to +4.8 volts, as measured at test point TP6. The
output of the trip slicer is buffered by U1C and combined with the block signal before it is passed to
edge connector terminal A14. This signal is also used to control the trip LED (DS1).
GUARD SLICE potentiometer R49 sets the slice level for the guard slicer, which is formed from
Comparator U4A and its associated components. The slice level can be varied from –2.8 to –4.8 Volts,
as measured at test point TP5. U1D buffers the output of the guard slicer before it is combined with the
block signal and is passed to edge connector terminal A13. This signal is also used to control the guard
LED (DS2).
14.3.8 AM ENVELOPE DETECTOR
The 4-kHz signal from the narrowband filter on the RFL 97 PLC CLI module is also fed to the AM
envelope detector, which is comprised of quad operational amplifier U10 and its associated components.
U10D serves as an input amplifier, U10C and U10B form a precision rectifier, and U10A is an active low-
pass filter. The output of the low-pass filter is sent into a notch filter consisting of U11, U12 and U13. These
filters are set to remove frequency components at 60, 120 and 180 Hz. These notch filters reduce the effects
of corona discharge on system operation.
NOTE
THESE NOTCH FILTERS ARE TIGHTLY TUNED AT RFL. NONE OF THE PARTS
WITHIN THIS CIRCUIT ARE FIELD REPLACEABLE. ANY COMPONENT
CHANGES OR ADJUSTMENTS MADE TO THIS CIRCUIT WILL DEGRADE
SYSTEM PERFORMANCE.
The output of the notch filter is an envelope which follows the average amplitude of the input
signal. It is passed out of connector pin A17 which then goes to the Logic Module for signal
analysis.
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