RFL 9780 RFL Electronics Inc.
April 8, 2003 13-4 (973) 334-3100
NOTE
THE ACTIVE FILTER FORMED BY U2 AND U3 IS 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.
13.3.2 SIGNAL LEVEL MONITOR
The signal passed by the narrowband filter is fed to operational amplifier U1A, which serves as a
buffer. The output of this buffer is passed through edge connector terminal A13 to the RFL 97 PLC
LM/SL module for further processing. It is also fed to a precision rectifier and averager formed from
operational amplifiers U1B and U1C, and their associated components.
The output of the rectifier is amplified by operational amplifier U4A, and then applied to operational
amplifier U4B, which serves as a comparator. CD potentiometer R58 sets the gain for U4A which is set
so that LO LEV indicator DS1 lights when the signal level falls below the desired limit.
Besides driving DS1, U4B also drives FET Q2, an open-drain FET that keeps the station alarm relay
energized unless the input signal fails.
13.3.3 CARRIER LEVEL INDICATOR
The digital panel meter (DPM1) is used to provide a visual indication of any signal level deviations
from normal, expressed in dB. Dual transistor Q1 and operational amplifiers U5B and U5A form a
logarithmic amplifier to convert the received signal strength to dB. R37 is used to temperature
compensate the log amp.
Jumper J2 and resistors R92, 93 and 94 are used to scale down the input to the logarithmic amplifier by
0dB, 3dB, or 10dB. This allows for a 0dB reading when receiving:
1. A full power carrier (typically from a 10W transmitter),
2. 30% of full power carrier (systems with a 3.33:1 trip boost, typically a 3W guard transmitter), or
3. 10% of full power carrier (systems with a 10:1 trip boost, typically a 1W guard transmitter).
R95 is an optional potentiometer that allows for differing trip boost levels.
The output of the log amp is a 0 to –5 Vdc signal. Op amp U1D is used to invert, scale, and offset this
signal to convert it to a ±1.0 Vdc signal (corresponding to ±10 dB). This signal is scaled down to
±100mVdc to drive the front panel meter DPM1.
When an external ±1.0 Vdc meter is used, jumper J1 must be in the “A” position. The ±1.0 Vdc signal
out of U1D will then be buffered by U7B and sent out to pin C16. The external meter is referenced to
pin C17. Plus or minus 1.0 Vdc corresponds to ±10.0 dB (0 volts represents 0 dB).
When an external 0 to -100 µA meter is used, jumper J1 must be in the “B” position. This sends the
output of the log amplifier through R42 and out on pin C16. The external meter return is connected to
pin C17. Zero µA corresponds to –10.0 dB, -50 µA to 0 dB, and -100 µA to +10 dB.
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