Keysight 8478B, Руководство по эксплуатации и обслуживанию

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Keysight 8478B Thermistor Mount 19
Operation
Drift Precaution
In the discussions above, the primary consideration is maximum power or
energy. However; for modulation frequencies less than 100 Hz, the low
repetition frequency itself causes errors in indicated power. These errors may
be as large as two percent regardless of range or reading.
When RF is switched by pulse-gating (coaxial solid state switches),
consideration must be given to the RF energy contained in the switching pulse
itself. This energy must be added to actual RF pulse power when estimating
the RF power dissipated in the thermistor mount. PIN diode modulators of
Model 8741A/8716A Modulators and 8614A/8616A Signal Generators,
however, are not subject to this consideration because output filtering
prevents transmission of modulating signals.
Drift Precaution
Thermistors are inherently temperature sensitive devices. A cold thermistor
mount connected to a warm piece of equipment, or vice versa, produces rapid
drift. FOR MINIMUM DRIFT on sensitive ranges, make sure that the mount and
the equipment connected to it are at nearly the same temperature before
making a measurement.
Let the power sensor warm up at least 30 min before perform COARSE and
FINE ZERO.
Operation with 431A/B/C or 430 Power Meters
If the 8478B is used with a 430 Power Meter, a Model 11528A Adapter is
required.
Because the mount is unbalanced, a large amount of 10kHz audio bias signal
may be coupled from the power meter to the RF source output. The RF source
10kHz output impedance forms a parallel circuit shunt across one of the
detection thermistor elements. If the RF source presents a 10kHz impedance of
15k ohms or greater, the audio bias voltage appearing at the RF input
connection is typically 1.3 Vrms. For an RF source 10kHz impedance of 50
ohms, the audio bias voltage is greatly reduced to typically 5 mVrms: The
presence of a large 10 kHz audio bias voltage at the RF source output may
affect solid state RF sources and RF voltmeter measurements. To minimize or
eliminate these effects, use a high-pass filter at the RF source output.
A variation in 10kHz impedance at the RF input connection affects the power
meter RF detection bridge circuit. This causes a shift in the power meter zero
setting. Refer to the following section for the proper power meter zero
procedure.