Giga-tronics GT 9000, Operation & Maintenance Manual

Page: 109 / 238

Many spectrum analyzers have a tuned preselector when the frequency is above about 2 GHz.
This reduces the likelihood of analyzer generated spurious signals, but does not eliminate the
possibility. If in doubt, increase the RF attenuation of the analyzer by 10 dB. The signal in
question should be reduced by exactly 10 dB. If not, it is analyzer generated. It is also
important, for frequencies below the range of the preselector, that sufficient analyzer RF
Attenuation be used (typically 30 dB) to avoid the analyzer generating harmonics of the input
signal. The above attenuator shift technique will also allow verification of harmonic levels.
If a spurious signal appears to be out of specification, first check that the fundamental signal
level is at 0 dBm. Next verify the analyzer accuracy by connecting a known amplitude signal
(from the GT 9000, for example) at the spurious frequency.
The digi-dial of the GT 9000 may generate some noise spikes when it is rotated. The acquisition
of the various phase lock loops in the instrument can also cause transients (within the specified
settling time). Both of these effects will disappear when a steady state condition has been
reached.
It is important to identify the particular class of spurious signal as the specifications may be
different for each. If the spurious signal is an exact multiple of the UUT RF output then the
harmonic specification applies. Any other spurious signal must meet the non-harmonically
related specification.
For frequencies above the fundamental range of the spectrum analyzer, either a mixer supplied
with the spectrum analyzer or an external mixer and local oscillator may be used to
downconvert the signal. Care must be taken to identify spurious signals which are inherently
generated by the mixing process.
Calibration and Testing
Manual No. 120AM00250, Rev C, September 1998
4-15