Comtech EF Data LTT1450G - Manual (Page 27)

LTT1450G Ku-Band Loop Test Translator Maintenance
TM063 - Rev. 1 Page 4-3
an average DC value by putting the voltmeter on AC and making sure that the voltmeter reads zero. An
AC measurement other than zero indicates a sweeping or oscillating PLL. If you suspect sweeping, look
at a harmonic on the output with a spectrum analyzer to verify.
Look for the presence of 5.000 MHz at the REF OUT jack at the rear of the translator. If the fault has
been isolated, the faulty module should be repaired or replaced by a qualified person. Contact the
Radyne Microwave Field service office to arrange for repair.
4.2.2 Transfer Characteristics Faults
Faults with the ability of the translator to translate the input signal within the limits of acceptable
conversion loss over the specified range of attenuation are more difficult to troubleshoot.
Converter modules contain no serviceable components and must be returned to the factory for repair. A
change in gate voltage could render a converter inoperative. Data sheets shipped with the converter
indicate what the gate voltages were when the unit was shipped. It may be possible to return the unit to
operation by returning the gate voltage to the proper value. This adjustment should not be attempted by
an unqualified person. Typical gate voltages range from -1 to -3 Vdc.
4.3 Attenuator DAC and Calibration
Attenuation error in all bands may indicate a DAC problem. DAC span and zero is typically set at 0.5 V
(zero) to 9.0 Vdc full scale. Zero and full scale can be jumpered (see AS/1030) or controlled remotely via
the serial port. Adjusting the DAC zero or span will require complete calibration of all bands in the
translator. This is a time-consuming task when performed manually. When a full-calibration is planned,
adjustment of the DAC should be done first.
See the section on serial protocol and performance testing to gain insight into the calibration procedure.