User's Manual

1.3.2.5.1 DME Interrogator CCA Block Diagram Theory
Refer to Figure 1-11. The Interrogator (RF board) has all the necessary circuitry to modulate the
interrogations from the Monitor and demodulate the replies from the RTCs.
For DME systems, the ANTENNA_RF signal comes from the monitor port on the DME antenna. For dual
DME systems, the monitor signal is split and run to each Monitor. For TACAN systems, this signal comes
from a monitor antenna placed near the TACAN antenna. It is attenuated by a fixed 16dB attenuator and
then by a digital attenuator controlled by the monitor DSP.
The digital attenuation stage setting depends on an initial calibration, which varies over antenna cable
lengths and types. After the attenuators, the ANTENNA_RF signal is detected and the shapes of the pulses
are sent as the ANT_DET_REPLY signal to be processed.
The STANDBY_RF signal only applies for dual equipment. It is derived from a 20dB or 30dB attenuator
that is split for each Monitor, then is attenuated by 16dB and demodulated similar to the ANTENNA_RF
signal.
The ANT_REFLECTED signal comes from the transmitter 30dB reflected coupler. For dual equipment it
is split before routing to each Monitor. After an attenuation of 26dB, this signal is sent to a 45dB precision
temperature-compensated logarithmic detector. It is used to shutdown the transmitter if the Monitor detects
too much reverse power; which could be caused by an open or shorted antenna cable.
The signal FWD/~RFL~_CTRL is controlled by the PLD 1 to switch the source of the logarithmic detector
between the reflected and the forward coupler. An additional 10dB attenuation to both signals is applied
before the logarithmic detector for High Power DME and TACAN systems; therefore the measurements are
equal for low and high power equipment. The ~10DB_CTRL signal activates this additional attenuation
when the Monitor is in a high power configuration.
The ANT_FORWARD signal is a bi-directional signal that comes from the transmitter 30dB forward
coupler. For dual equipment it is split/combined from each Monitor. As an input, it is used to monitor the
transmitter output power and calculate the antenna VSWR. To accomplish this, the signal runs through a
circulator and then through a 6dB splitter. One of the split signals is attenuated by 20dB and then can be
multiplexed to the 45dB logarithmic detector. The other split signal is attenuated by 10dB and then has an
additional 10dB attenuation stage, which is only active in an HP DME or TACAN system. Finally, it is
detected and the signal is sent in a differential pair to the RTC for feedback control of composite
transmitted pulse shape for TACAN mode.
The Interrogator module has its own synthesizer for on and off-channel measurements. The reference
frequency comes from the digital Monitor board and the accuracy of the synthesizer signal is ±5 parts-per-
million (ppm). The synthesizer signal is buffered and sent to the Monitor’s front panel for external
measurements. Also it is divided by 2
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to be measured by the Monitor’s DSP.
The interrogation pulse shapes generated by the interrogator in the PLD 1 are modulated with the
synthesizer signal and then run through three digital attenuation stages. The three attenuators are also
controlled by the interrogator. These digital attenuators have a 95dB dynamic range. The resulting signal is
multiplexed to the antenna after a low pass filter and the combiner or to the logarithmic detector after a
10dB attenuator; calibrating the power of the Monitor transmitter and consequently adjusting the
interrogation attenuation to be equal over temperature.
This calibration is done periodically and is controlled by the PLD 1 interrogator using the INT_CAL signal.
The monitor interrogation is demodulated again and sent to the digital board through the MON_INT signal
to be used by the interrogator as previously explained.