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Comtech EF Data LTT1450G User Manual

Page 14

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Installation

LTT1450G Ku-Band Loop Test Translator

Page 2-2

TM063 - Rev. 1

2.4

Receive Signal Injection

The translated receive signal is available at the RX signal output port. This signal will be the transmit
input signal translated by the LO frequency represented mathematically as TX-LO=RX. With the
attenuator set to 00, the Rx signal will be 15 dB lower in power.

Alternately, the RX signal can be viewed on a spectrum analyzer with an additional 15 dB of attenuation
at the Rx monitor SMA connector on the front panel.

Next, you will want to set the power level of the receive signal for injection into the receive hardware. To
inject the signal directly into a downconverter, a small amount of additional attenuation may be required.
Typical downconverter input saturations may run as high as -35 dBm. With a TX input level of -10 dBm
and 20 dB of attenuation, an output of -45 dBm can be attained. This will allow direct interface with the
downconverter.

If you wish to include an LNA in the receive path, additional attenuation of 50 to 70 dB will be required.
Using an LNA in the receive path is an excellent method of raising the noise floor of the receive signal
without having to inject external broadband noise. In order to test the BER performance of a system, it
becomes necessary to degrade the signal-to-noise ratio of the receive signal to a range of 5 to 15 dB
Eb/No. The noise figure of the LNA will guarantee that when the receive signal, when attenuated to
within 5 to 15 dB, the noise floor (No) of the LNA will yield a usable signal-to-noise ratio.

Please note that as a rule, the output of the translator should be attenuated to reach the desired power
level.

NOTE: Always attenuate the RX output path versus attenuating the TX input signal beyond -25
dBm to achieve the desired signal power.

2.5

Hardware Description

The Model LTT1450G Translator is comprised of a phase-locked oscillator source and a converter
module. The phase-locked oscillator subsystem provides the local oscillator frequencies for the converter
module. The converter module is a complete microwave integrated assembly that contains the filters,
mixer, amplifier and voltage-controlled attenuator.

The microcontroller module (AS1030) is an 8-bit Intel 80C32 microprocessor-based subsystem. The
controller contains various power supplies that provide positive and negative voltages used throughout
the translator, a 12-bit digital-to-analog converter, an 8-channel, 8-bit analog-to-digital converter, 24 bits
of parallel input and output, and a serial interface.

The A/D allows monitoring of various phase-lock loop voltages throughout the system. The 12-bit D/A
provides the precise currents for attenuation control. A nonvolatile RAM on the microcontroller board
stores calibration data for the different look-up tables. The parallel I/O provides the interface to the front
panel and to the LO selector module.

A 10.000 MHz ovenized oscillator assembly located at the rear of the chassis provides the internal
reference to each of the phase lock oscillator modules. This assembly also provides an interface for
external references to be applied to the system. An external reference, when applied to the system, is
exclusively OR’d with the internal 10.0 MHz oscillator. The microcontroller will detect the presence of an
external reference and will turn the internal reference off.

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