Comtech EF Data LTT1450G - Manual (Page 26)

Maintenance LTT1450G Ku-Band Loop Test Translator
4-2 TM063 - Rev. 1
With the attenuator set to zero, increase the power to +5 dBm and make sure that the converter signal
also increases by 10 dB. Compression on all bands is tested to provide 1 dB compression of greater than
+7 dBm.
4.1.3 Attenuator Calibration Testing
Next the attenuator calibration can be tested. The spectrum analyzer resolution bandwidth should be
adjusted to provide dynamic range on the display to be 6-10 dB grater than the maximum attenuation of
the translator. Next the vertical scale on the spectrum analyzer should be adjusted to 1 dB/ division. With
the attenuator set to zero and the amplitude on the analyzer set to the top line on the display, set the
relative offset on the analyzer to indicate 0 dB relative. The attenuator can now be stepped 1 dB at a
time and every five dB the reference level can be adjusted 5 dB so that analyzer reads out in relative
attenuation identically to the thumbwheel switches. At the factory, calibration is performed mid-band in
each band. At the high end of the attenuation range the nonlinearity of the attenuator versus frequency is
the greatest. During calibration the attenuator is calibrated mid-band. Nonlinearity from band edge to
band edge should not exceed
±
1 dB. Errors not due to nonlinearity can be removed by performing
attenuator calibration. Calibration of the attenuator can be performed via the terminal for any setting that
does not meet the operators requirements. See the section on serial protocol for performing attenuation
calibration.
4.1.4 Spectral Response Testing
Beyond frequency amplitude response and attenuator calibration, the operator should check the spurious
responses. Spurious responses in-band can be seen by sweeping the input while looking at the output
bandwidth. Out of band spurious signals are usually limited to harmonics of the local oscillator and input
out isolation.
LO harmonics are easier to measure if you calculate integer harmonics of the LO that fall near the band
edges of the selected band. Some of the signals are near -90 dBm and are difficult to spot unless you
know exactly where they are in frequency.
LO harmonics that are visible are a useful way of measuring the performance of the PLO. A synthesized
spectrum analyzer can measure the frequency of a low level spur with accuracy. Also, for
troubleshooting an LO fault, this technique will be useful in verifying the nature of the fault. For example,
an LO that is sweeping or false locked to the wrong frequency can be identified.
4.2 Troubleshooting
4.2.1 LO Fault
LO fault indications mean that one or both of the two phase lock loops is indicating an output voltage
beyond an acceptable limit. Phase lock loop voltage limits for the various oscillators are set in software
to be the following:
Oscillator Model Freq Band Limits
AS/5003B Ku-Band Primary Loop 1 - 7 Vdc
25 MHz Loop 1 - 7 Vdc
These voltages can be measured at the test fault connector located at the rear of the translator. Check
the voltage to determine which loop has failed. If an acceptable DC level is read, make sure that it is not