User's Manual

1.3.2.4 Receiver Transmitter Controller Theory
The Receiver Transmitter Controller (RTC) is an integral part of the DME dedicated to receiving aircraft
interrogations and controlling the transmitter replies. All of the receiver hardware is contained on the RTC
assembly except for the pre-selector filter that is tuned to the station frequency.
The RTC Assembly (030805-0001) consists of two circuit card assemblies (CCA). First is the RTC CCA
(012168-1001) that slides into the card cage assembly and plugs into the motherboard for power and signal
connections. The Receiver RF CCA (012180-0001) is a second circuit card that plugs into the side of the
RTC CCA. Power and signal lines come from the stacked circuit card connectors while the receiver RF
signal enters through a SMA connector. The RF signal is routed from the backplane connector on the RTC
CCA to the Receiver RF CCA using conformable RF cable. The Receiver RF CCA is housed in a
completely shielded enclosure consisting of a backing plate, fence, and cover.
1.3.2.4.1 CCA, Receiver Transmitter Controller Block Diagram Theory
This section describes the details of the RTC CCA. Throughout this section refer to Figure 1-8 and
012169-9001 schematic.
A Digital Signal Processor (DSP) and two Field Programmable Gate Arrays (FPGA) comprise the heart of
the RTC CCA. All of the timing critical functions of pulse reception and transmitter control are located in
the FPGA hardware. Non critical tasks such as identification control, squitter/transmitter rate control, and
transmitter pulse shaping are handled by the RTC software in the DSP. The DSP uses its speed to help the
hardware handle tasks such as short and long distance echo suppression, decoder correlation, and CW
desensitization.
Control of the transmitter is accomplished by the RTC CCA. LVDS hardware on the board provides
differential gate pulses that enable the RF modulators and apply the synthesizer RF output to the transistor
amplifiers. Additionally the RTC CCA provides differential Gaussian shaped pulses used to control the RF
modulator amplitudes. With each pulse transmitted the RTC CCA samples the detected RF output and
determines what, if any, errors exist in the pulse shape parameters. The RTC CCA then modifies the output
waveform before transmission of the next pulse. This N-1 pulse shaping algorithm is used to ensure the
transmitter meets critical spectral requirements. Pulse shaping is one of the transmitter control loops in the
DME/TACAN equipment and does not require any user intervention.
Each RTC CCA has an associated low power amplifier and synthesizer. For high power DME operation
the LP amp is followed by a HP amplifier. In this case the RTC CCA drives the LP amp with a trapezoidal
shape pulse and performs the final pulse shaping in the high power amplifier. When operating as a
TACAN, up to five HP amps are summed together with a RF combiner. In this case the RTC CCA shapes
each HP amp individually and monitors the composite detected RF envelop for pulse corrections using the
forward power detector input signal.
In addition to driving the RF modulators, the RTC CCA controls the transmitter pulse pair spacing and
transmission rate. The transmission rate is monitored in the FPGA hardware and randomly spaced pulse
pairs are generated if necessary in order to meet a minimum transmission rate. When overloaded by too
many aircraft interrogations, the RTC CCA limits the transmitter rate by reducing the receiver sensitivity.
Functions also accomplished by the RTC CCA include the decoder that correlates each received pulse with
previous pulses to find a pair that meets the decoder aperture for the DME operation mode. Received pulse
widths are monitored such that narrow radar pulses are rejected and wide out of tolerance pulses are
rejected. The receiver also provides continuous wave (CW) interference rejection and suppression of
transmission due to interrogation echoes.
Communications between the RTC CCA and the RMS is accomplished with a serial data link. Transmitter
and receiver configurations are received from the RMS while operating data and maintenance alert signals
are sent to the RMS for display on the PMDT. A serial data link between the RTC CCA and the Monitor
CCA is provided so the RTC software can control the transmitter delay as well as provide a transmitter soft