Giga-tronics 8003, Руководство по эксплуатации

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8003 Precision Scalar Analyzer
2-102 Manual 20791, Rev. C, June 2001
2.13 Accurate Range Measurements
Two of the key features of the 8003 Precision Scalar Analyzer are its broad 90 dB dynamic range and its
exceptional -70 dBm sensitivity. Dynamic range and sensitivity are often key issues in component
measurements. They determine the maximum insertion loss measurable for switches, attenuators, and
filters. They determine how well input return loss can be measured on a high gain amplifier with low
input levels. They also help when sweeper power is reduced through the use of splitters, switches,
matching attenuators, or long lengths of cable.
It makes sense that wide dynamic range measurements start with getting the most power available from
the source. But the art is in the low level measurement made at the other end of the device under test.
At very low signal levels, noise dominates the measurement. As a user, you will detect this noise in two
forms - broadband thermal and circuit noise, and long term drift. Eliminating, or at least controlling this
noise means attention to three factors - temperature stability, measurement mode, and averaging. If you
are making measurements below -40 dBm, attention to these three factors will improve your
measurement accuracy.
2.13.1 Temperature Stability
One of the best ways to control long term drift is to make sure that the analyzer and sensor have been
connected and powered up long enough to stabilize in temperature. In general, if the instruments are at
room temperature, you should allow at least 15 minutes after turn-on before attempting an accurate low
level measurement. Temperature stability will continue to improve until 1 or 2 hours after turn-on.
When calibrating or connecting the DUT, try to minimize handling the sensors or bridges. Although
temperature corrected and insulated, you may still see residual effects of a long hand grasp of the sensor -
especially when measuring below -55 dBm.
2.13.2 Measurement Modes
Low level measurements can be made in three modes: the CW mode and two SWEPT modes (ac and dc
detection). The mode chosen will impact the characteristics of any low level measurements. You may
wish to choose different modes for different test requirements.
2.13.2.1 CW Mode
The 8003 Analyzer makes its best low level measurements in the CW mode. To get to CW mode, either
press
[MEAS]
and select the [ ALL CW ] softkey, or press
[DEFINE]
and the [ CW/SWEPT ] softkey for
your desired channel. Unlike the two swept modes, CW mode uses narrow bandwidth filtering to reduce
noise much more efficiently. This means that the CW mode will give a stable reading much more
quickly at low levels than either swept mode (of course, only at a single frequency). You can use this
characteristic to either use less averaging (hence, less time) to get a stable reading, or more stable digits
of resolution for the same measurement time.
To make the best low level measurements in the CW mode you should frequently re-zero the 8003
Analyzer. This removes the effect of drift from the measurement. Press
[CAL]
and select the ZERO
SENSOR softkey. Then press the [ SENSORXZERO ] softkey, where X is the sensor letter (A, B, or C). If
you are using a GPIB-supported sweeper, the 8003 Analyzer will automatically turn the RF off during
zeroing. The sweeper must have less than -80 dBm of power output during RF off. Please be aware that
many sweepers do not adequately suppress their signal during RF OFF if their output level is set above
-20 dBm. If in doubt, disconnect the sensor, zero it, and reconnect it to the sweeper to check that the
sweeper level is -70 dBm during RF OFF. Note that the closer you get to -70 dBm, the more apparent
the drift will become, and the more frequently you will need to re-zero the Analyzer. Select the amount
of averaging desired to get a stable reading. To use averaging, press
[DISPLAY]
and select the AVG