RFL 9780 RFL Electronics Inc.
April 8, 2003 2-18 (973) 334-3100
The signal is then fed to the RF Interface Module. The RF Interface front end consists of attenuators
and gain sections to normalize the received level to the design level. This allows a wide dynamic range
of signals to be accepted by the unit. Once the signal level is normalized, it is passed through a
bandpass filter. The filter has a bandwidth of approximately 10 KHz with a programmable center
frequency. This reduces the extraneous noise content and energy to be processed. See Section 11 for
more information about the RF Interface Module.
The signal is then sent into the IF/BF Module. This module shifts the received carrier from the
customer specified center frequency down to the 9780’s baseband frequency of 4 KHz. There are three
mixers on this module. There is a very sharp cutoff crystal bandpass filter located between the first and
second mixer. The bandwidth of this filter is approximately 4 KHz; further reducing noise and energy
levels. See Section 12 for more information about the IF/BF Module.
The 4 KHz signal out of the IF/BF is then sent to the CLI Module. The CLI contains a narrow-band
(bandpass) filter. The bandwidth of the filter is selected based upon the frequency shifts used in the
system, 200, 500, and 1000 Hz filters are available. The filter is generally chosen to be as narrow as
practical while still passing the received signals.
This filtered signal is then sent to the LM/SL Module and is further processed on the CLI Module. On
the CLI Module the level of the carrier is extracted. This level signal is then compared to a user
selectable threshold to alert the user to a low received level. The level is also sent into a log- amplifier
circuit to convert the level to a dB measurement. This dB measurement is displayed on a front panel
display and also made available for external meters. See Section 13 for more information about the
CLI Module.
The filtered signal sent into the LM/SL Module follows two paths. The level of the carrier is again
extracted and sent through a corona filter. The corona filter is a triple notch filter. The notches are
tuned for the grid frequency (50/60 Hz) and it’s harmonics. The signal is then used by the logic
functions to detect noise and loss of carrier.
The signal is also fed through a limiter followed by a discriminator which shifts signals at the baseband
frequency by 90°. The output of the discriminator is also limited and then combined with the output of
the first limiter. The combined signal is a rectangle-wave whose duty cycle varies depending upon the
frequency of the received signal. (The module allows one of the two limited signals to be inverted,
which reverses the phase relationship.) The average value of the rectangle-wave is extracted and used
to determine the frequency of the received signal. This is then used to generate a trip or guard signal.
See Section 14 for more information about the LM/SL Module.
2.9.3 LOGIC FUNCTIONS
The Logic Module provides features related to both the transmit and receive functions. For the transmit
path, the user has two keying inputs, how they are interpreted in 2F and 3F systems is configurable by
the user. The Logic Module is also responsible for telling the Transmitter Module which frequency and
power level to transmit.
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