Keysight 8478B Service manual
Operating and
Service Manual
Keysight 8478B
Thermistor Mount
Notices
© Keysight Technologies
1981-2018
No part of this manual may be
reproduced in any form or by any
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and retrieval or translation into a
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agreement and written consent from
Keysight Technologies, Inc. as
governed by United States and
international copyright laws.
Manual Part Number
08478-90015
Edition
Edition 3, March 2018
Printed in:
Printed in Malaysia
Published by:
Keysight Technologies
Bayan Lepas Free Industrial Zone,
11900 Penang, Malaysia.
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http://www.keysight.com/go/confor
mity. You can then search by product
number to find the latest Declaration
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The Software is “commercial
computer software,” as defined by
Federal Acquisition Regulation
(“FAR”) 2.101. Pursuant to FAR
12.212 and 27.405-3 and
Department of Defense FAR
Supplement (“DFARS”) 227.7202, the
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under the same terms by which the
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provides the Software to U.S.
government customers under its
standard commercial license, which
is embodied in its End User License
Agreement (EULA), a copy of which
can be found at
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Warranty
THE MATERIAL CONTAINED IN THIS
DOCUMENT IS PROVIDED “AS IS,”
AND IS SUBJECT TO BEING
CHANGED, WITHOUT NOTICE, IN
FUTURE EDITIONS. FURTHER, TO
THE MAXIMUM EXTENT PERMITTED
BY APPLICABLE LAW, KEYSIGHT
DISCLAIMS ALL WARRANTIES,
EITHER EXPRESS OR IMPLIED WITH
REGARD TO THIS MANUAL AND
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HEREIN, INCLUDING BUT NOT
LIMITED TO THE IMPLIED
WARRANTIES OF
MERCHANTABILITY AND FITNESS
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FOR ERRORS OR FOR INCIDENTAL
OR CONSEQUENTIAL DAMAGES IN
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PERFORMANCE OF THIS
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WITH WARRANTY TERMS
COVERING THE MATERIAL IN THIS
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THESE TERMS, THE WARRANTY
TERMS IN THE SEPARATE
AGREEMENT WILL CONTROL.
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calls attention to an operating
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to, could result in damage to the
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It calls attention to an operating
procedure, practice, or the like that,
if not correctly performed or adhered
to, could result in personal injury or
death. Do not proceed beyond a
WARNING notice until the indicated
conditions are fully understood and
met.
3
Keysight 8478B Thermistor Mount
Operating and Service Manual
Certification
Keysight Technologies certifies that this product met its published
specifications at the time of shipment. Keysight further certifies that its
calibration measurements are traceable to the United States National Institute
of Standard and Technology (formerly National Bureau of Standards), to the
extent allowed by that organization’s calibration facility, and to the calibration
facilities of other International Standards Organization members.
General Warranty
The material contained in this document is provided “as is,” and is subject to
being changed, without notice, in future editions. Further, to the maximum
extent permitted by applicable law, Keysight disclaims all warranties, either
express or implied with regard to this manual and any information contained
herein, including but not limited to the implied warranties of merchantability
and fitness for a particular purpose. Keysight shall not be liable for errors or for
incidental or consequential damages in connection with the furnishing, use, or
performance of this document or any information contained herein. Should
Keysight and the user have a separate written agreement with warranty terms
covering the material in this document that conflict with these terms, the
warranty terms in the separate agreement shall control. Duration and
conditions of warranty for this product may be superseded when the product is
integrated into (becomes a part of) other Keysight products. During the
warranty period, Keysight will, at its option, either repair or replace products
which prove to be defective. The warranty period begins on the date of delivery
or on the date of installation if installed by Keysight.
Warranty Service
For warranty service or repair, this product must be returned to a service facility
designated by Keysight. For products returned to Keysight for warranty service,
the Buyer shall prepay shipping charges to Keysight and Keysight shall pay
shipping charges to return the product to the Buyer. However, the Buyer shall
pay all shipping charges, duties, and taxes for products returned to Keysight
from another country.
4 Keysight 8478B Thermistor Mount
Certification
Limitation of Warranty
-
Limitation of Warranty
The foregoing warranty shall not apply to defects resulting from improper or
inadequate maintenance by the Buyer, Buyer-supplied products or interfacing,
unauthorized modification or misuse, operation outside of the environmental
specifications for the product, or improper site preparation or maintenance.
The design and implementation of any circuit on this product is the sole
responsibility of the Buyer. Keysight does not warrant the Buyer’s circuitry or
malfunctions of Keysight products that result from the Buyer’s circuitry. In
addition, Keysight does not warrant any damage that occurs as a result of the
Buyer’s circuit or any defects that result from Buyer-supplied products.
To the extent allowed by local law, Keysight makes no other warranty,
expressed or implied, whether written or oral with respect to this product and
specifically disclaims any implied warranty or condition of merchantability,
fitness for a particular purpose, or satisfactory quality.
Exclusive Remedies
To the extent allowed by local law, the remedies provided herein are the
Buyer’s sole and exclusive remedies. Keysight shall not be liable for any direct,
indirect, special, incidental, or consequential damages (including lost profit or
data), whether based on warranty, contract, tort, or any other legal theory.
Keysight 8478B Thermistor Mount 5
Certification
Waste Electrical and Electronic Equipment (WEEE) Directive 2002/96/EC
Waste Electrical and Electronic Equipment (WEEE) Directive 2002/96/EC
This instrument complies with the WEEE Directive (2002/96/EC) marking
requirement. This affixed product label indicates that you must not discard this
electrical or electronic product in domestic household waste.
Product category
With reference to the equipment types in the WEEE directive Annex 1, this
instrument is classified as a “Monitoring and Control Instrument” product.
The affixed product label is as shown below.
Do not dispose in domestic household waste.
Sales and Technical Support
To contact Keysight for sales and technical support, refer to the support links
on the following Keysight websites:
•www.keysight.com/find/powersensors
(product-specific information and support, software and documentation
updates)
•www.keysight.com/find/assist
(worldwide contact information for repair and service)
6 Keysight 8478B Thermistor Mount
Certification
Sales and Technical Support
-
Contents
Keysight 8478B Operating and Service Manual 7
Table of Contents
Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
General Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Warranty Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Limitation of Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Exclusive Remedies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Waste Electrical and Electronic Equipment (WEEE) Directive 2002/96/EC . . . . . . . . . . . . . . . 5
Product category . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Sales and Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1. General Information
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Instruments Covered By Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Incoming Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2. Operation
Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Mechanical Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Biasing Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Maximum Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Average Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Pulse Energy and Peak Power for 8478B/432 Combination. . . . . . . . . . . . . . . . . . . . . . . 17
Pulse Energy and Peak Power for 8478B/431 Combination. . . . . . . . . . . . . . . . . . . . . . . 17
Drift Precaution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Operation with 431A/B/C or 430 Power Meters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Zero Set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
RF Power Turned Off for Zero-Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Unbalanced Operation with Model 431A/B/C and 430 Power Meters. . . . . . . . . . . . . . . 20
Balanced Operation with Model 431C Power Meter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8478B/432 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Mount Calibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Calibration Factor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Effective Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Calibration Data Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Thermoelectric Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Thermoelectric Effect Error Correction for 8478B/431 Combination . . . . . . . . . . . . . . . . 23
Thermoelectric Error for 8478B/432 Combination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8 Keysight 8478B Operating and Service Manual
Contents
3. Operating Principles
General Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
Operation with 432 Power Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
Operation with 431 Power Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
432 Power Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
431 Power Meter Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
4. Maintenance
Mechanical Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
Performance Tests. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
SWR Measurement, 8GHz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32
SWR Measurement, 15 and 18GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
Thermistor Resistive Match Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Troubleshooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
Thermistor Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
Repair and Recalibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
A.Manual Changes
CHANGE A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43
CHANGE B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46
CHANGE C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47
10 Keysight 8478B Thermistor Mount
General Information
Introduction
1-
Introduction
The Keysight Model 8478B Coaxial Thermistor Mount is designed for use with
Keysight Model 431 and 432 Power Meters to measure microwave power from
1μW to 10mW. Design of the mount minimizes adverse effects from
environmental temperature changes during measurement. For increased
measurement accuracy, Effective Efficiency and Calibration Factor are
measured for each mount at selected frequencies across the operating range;
the results are marked on the label of the instrument (see “Mount Calibration”
on page 21). The Model 8478B can be used over the 10MHz to 18GHz
frequency range. Throughout the range, the mount terminates the coaxial
input in a 50-ohm impedance, and has a SWR of not more than 1.75 without
external tuning.
Each mount contains two series pairs of thermistors, which are matched to
cancel the effects of drift with ambient temperature change. Thermal stability
is accomplished by mounting the leads of all four thermistors on a common
thermal conductor to ensure a common thermal environment. This conductor
is thermally insulated from the main body of the mount so the thermal noise or
shocks applied externally to the mount, such as those from handling the mount
manually, cannot significantly penetrate to disturb the thermistor. This thermal
immunity enables the thermistors to be used in the measurement of microwave
power down to the microwatt region.
The 8478B operates directly with 431C and 432 Power Meters. Model 11527A
adapter is used for operation with 431A/B Power Meters. Model 11528A is
used for operation with 430 Power Meters.
Specifications
The specifications are listed in “Specifications” on page 15. These
specifications are the performance standards against which the Thermistor
Mount/Power Meter operation is compared.
For operation with the 431 Power Meter, add the following uncertainties to
those indicated in Table 2-1: for 0.3mW range, ±0.3%; for 0.1mW range,
±0.7%; for 0.03mW range, ±1.5%; for 0.01mW range, ±2.8%. The power range
in which the 431/8478B operates is 1μW to 10mW.
Instruments Covered By Manual
This manual applies directly to instruments with serial numbers prefixed
2106A. With the backdating found in Appendix A, the manual also applies to
instruments with serial number prefixed 2030A and below.
Keysight 8478B Thermistor Mount 11
General Information
Incoming Inspection
Incoming Inspection
Inspect the Model 8478B upon receipt for mechanical damage. Also check it
electrically; if the mount was subjected to severe mechanical shock during
shipment, the match between the thermistors may be affected. To check
thermistor match, proceed as described in “Performance Tests” on page 32.
If any damage is found, inform the carrier and your nearest Keysight sales and
Service office immediately.
12 Keysight 8478B Thermistor Mount
General Information
Incoming Inspection
1-
13
Keysight 8478B Thermistor Mount
Operating and Service Manual
2 Operation
Precautions 14
Specifications 15
Drift Precaution 19
Zero Set 20
Mount Calibration 21
Calibration Data Application 22
Thermoelectric Effect 23
This chapter contains information about specifications and operation of the
Keysight 8478B Thermistor Mount.
14 Keysight 8478B Thermistor Mount
Operation
Precautions
2-
Precautions
Mechanical Shock
Biasing Thermistors
DO NOT SUBJECT THE MOUNT TO MECHANICAL SHOCK. Shock may
destroy the match between thermistors and increase susceptibility to drift.
Before connecting the 8478B to a 431C or 432-series Power Meter, set the
MOUNT RES switch to the 200Ωposition. For Mounts with serial numbers
prefixed below 1833A, set the MOUNT RES switch to 200ΩBAL (431) or
200Ω(432).
When using the 8478B with 431A/B or 430 Power Meter, use Model
11527A or 11528A Adapter respectively and set MOUNT RES switch to
200Ωposition. Connecting a 200-ohm mount to a power meter set for a
100-ohm mount can result in thermistor damage.
Keysight 8478B Thermistor Mount 15
Operation
Specifications
Specifications
Frequency Range: 10MHz to 18GHz
Table 2-1 Uncertainty of Calibration Factor Data
Frequency (GHz) Sum of Uncertainty1
1. Includes uncertainty of reference standard and transfer uncertainties.
Directly traceable to NBS.
Probable Uncertainty2
2. Square root of the sum of the squares of the individual uncertainties
(RSS).
2.0
3.0
4.0
5.0
6.0
±2.60%
±2.60%
±2.70%
±2.70%
±2.70%
±1.60%
±1.60%
±1.60%
±1.60%
±1.60%
7.0
8.0
9.0
10.0
11.0
±2.70%
±3.10%
±3.30%
±3.40%
±3.60%
±1.60%
±1.80%
±1.80%
±2.00%
±2.00%
12.4
13.0
14.0
15.0
16.0
±3.70%
±3.70%
±4.00%
±4.00%
±4.40%
±2.20%
±2.20%
±2.20%
±2.50%
±2.50%
17.0
18.0
±5.20%
±5.10%
±3.20%
±3.20%
Input Impedance: 50 ohms
Operating Resistance: 200 ohms, unbalanced
Power Range with Model 432: 1μW to 10mW
Maximum Peak Power: 200W
Maximum Average Power: 30mW
Maximum Energy per Pulse: 10W-μsforaPRF≥1 k H z ;
5W-μs for a PRF <1kHz
16 Keysight 8478B Thermistor Mount
Operation
Specifications
2-
Figure 2-1 SWR Limits
Maximum
Reflection
Coefficient:
10 to 30MHz: 0.273(1.75SWR, 11.3 dB return loss)
30MHz to 100MHz: 0.15 (1.35 SWR, 16.5 dB return loss)
100MHz to 1GHz: 0.048 (1.1 SWR, 26.4 dB return loss)
1 to 12.4GHz: 0.15 (1.35 SWR, 16.5 dB return loss)
12.4 to 18GHz: 0.230 (1.6 SWR, 12.8 dB return loss)
Elements: Thermally balanced thermistor assembly. Thermistor
assembly is field adjustable so that full zero-set capability can
be restored in the event of inadvertent overload.
RF Connector: Stainless steel type N male (APC-7 also available- see Option
11 below).
Output Connector: Mates with power meter cable (operates directly with 432).
Weight: Net 140g (5 oz).
Option 11: 8478B Thermistor Mount supplied with APC-7 RF connector.
Keysight 8478B Thermistor Mount 17
Operation
Specifications
Maximum Input
Average Power
Pulse Energy and Peak Power for 8478B/432 Combination
Pulse Energy and Peak Power for 8478B/431 Combination
The limitations of this combination are basically the same as the 8478B/432
with the exception that at pulse repetition rates less than 1kHz, energy per
pulse must not exceed 5 W-µ and peak power must not exceed 10mW. These
limits are also interpreted in Figure 2-2 and Figure 2-3.
The Model 8478B/431 and 8478B/432 combinations respond to the
average RF power applied. The maximum signal applied to the thermistor
mount should not exceed the limitations for 1) average power, 2) pulse
energy, and 3) peak pulse power. Excessive input can permanently
damage the Model 8478B by altering the match between the RF and
compensation thermistors (resulting in excessive drift or zero shift) or
cause error in indicated power.
UNDER NO CIRCUMSTANCES APPLY MORE THAN 30mW AVERAGE
POWER TO THE MOUNT. The 8478B/431 and 8478B/432 combinations
can measure average power up to 10mW (ranges 1µ W to 10mW). To
measure power in excess of 10mW, insert a calibrated directional coupler
such as one of the Model 790 series or one of the 8491 series coaxial
attenuators between the mount and the source.
For pulses shorter than 250µs, energy per pulse must not exceed 10 W-µs
and peak power should never exceed 200 watts. (For example, a 40mW,
250µs pulse contains 10 W-µs of energy.) Pulses longer than 250µs are
allowed to contain more energy but peak power must not exceed 200
watts. Figure 2-2 and Figure 2-3 interpret these limits in graphical form.
(For lack of space, the mount nameplate lists only a 5 W-µs limit, a rating
which applies to the 8478B/432A combination.)
18 Keysight 8478B Thermistor Mount
Operation
Specifications
2-
Figure 2-2 Maximum Power Meter Reading vs PRF for Pulses Shorter than 250µs
Figure 2-3 Maximum Power Meter Reading vs Duty Cycle for Pulses Longer than 250µs
Square-wave modulation is a special case of pulse modulation, and maximum
power meter reading versus square-wave frequency is illustrated in Figure 2-4.
This figure also holds for sinewave modulation.
Figure 2-4 Maximum Power Meter Reading vs Square and Sine-Wave Frequency
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.
20 Keysight 8478B Thermistor Mount
Operation
Zero Set
2-
Zero Set
It is necessary to electrically zero-set the Power Meter before making a power
measurement. To preserve the same zero reference through the measurement,
maintain the same thermal environment when RF power is applied.
Recommended setups for 430 and 431 zero-settings are presented in
“Unbalanced Operation with Model 431A/B/C and 430 Power Meters” and
“Balanced Operation with Model 431C Power Meter”. The recommended
setup for zero-set in the 432 is shown in “8478B/432”.
RF Power Turned Off for Zero-Set
There is minimum zero drift when the zero is set with the RF system connected
to the thermistor mount and the RF power switch off or greatly attenuated by
the generator attenuator. After allowing time for the mount to stabilize
thoroughly, follow zero-set procedures in the Power Meter manual.
Unbalanced Operation with Model 431A/B/C and 430 Power Meters
When the RF source presents a high 10kHz output impedance of 100k ohms or
greater, the power meter may be zeroed with the 8478B disconnected from the
RF source and unterminated as explained in the previous section. If the RF
source presents a low 10kHz impedance of 1k ohm or less, a zero setting made
with the 8478B disconnected from the RF source and unterminated will not be
the proper zero setting for power measurement. This error can be eliminated by
terminating the 8478B in an impedance that approximates the RF source
impedance at 10kHz while zeroing the power meter. For example, if the RF
source 10kHz impedance is 50 ohms, terminate the RF input connection to the
8478B with a 50 ohm resistor.
Balanced Operation with Model 431C Power Meter
When using a balanced mount, if it is inconvenient to turn off RF power in the
RF source, connect the 8478B to the RF source and set 431C Power Meter
RANGE switch to obtain an approximate midscale reading. When the reading
does not drift, disconnect the 8478B from the source, zero the power meter,
and immediately reconnect the mount to the RF source for the power
measurement.
8478B/432
When it is inconvenient to turn the RF power off while using the 8478B/ 432
combination simply remove the mount from the source, and using the COARSE
and FINE ZERO, zero the 432.
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