Keysight 478A Thermistor
Mount
Operating and
Service Manual
Notices
CAUTION
WARNING
Copyright Notice
© Keysight Technologies 2008 - 2020
No part of this manual may be repro-
duced in any form or by any means
(including electronic storage and
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Manual Part Number
00478-90021
Edition
Edition 3, January 23, 2020
Printed in:
Printed in Malaysia
Published by:
Keysight Technologies
Bayan Lepas Free Industrial Zone,
11900 Penang, Malaysia
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The hardware and/or software
described in this document are furnished under a license and may be
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the terms of such license.
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product and for other Keysight products may be downloaded from the
Web. Go to http://www.keysight.com/
go/conformity. You can then search by
product number to find the latest Declaration of Conformity.
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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 U.S.
government acquires commercial computer software under the same terms
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embodied in its End User License
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find/sweula. The license set forth in the
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related to commercial computer software or commercial computer software
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respect to any technical data as
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227.7102, the U.S. government
acquires no greater than Limited Rights
as defined in FAR 27.401 or DFAR
227.7103-5 (c), as applicable in any
technical data.
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
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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
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SHALL NOT BE LIABLE FOR ERRORS
OR FOR INCIDENTAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE FURNISHING, USE, OR
PERFORMANCE OF THIS DOCUMENT
OR OF 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.
Safety Information
A CAUTION notice denotes a hazard. It
calls attention to an operating procedure, practice, or the like that, if not
correctly performed or adhered to,
could result in damage to the product
or loss of important data. Do not proceed beyond a CAUTION notice until
the indicated conditions are fully
understood and met.
A WARNING notice denotes a hazard. 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.
2 Keysight 478A 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.
Warranty Information
This Keysight Technologies instrument product is warranted against defects in
material and workmanship for a period of one year from date of shipment. During
the warranty period, Keysight Technologies will at its option, either repair or
replace products which prove to be defective. For warranty service or repair, this
product must be returned to a service facility designated by Keysight
Technologies. Buyer shall prepay shipping charges to Keysight Technologies and
Keysight Technologies shall pay shipping charges, duties, and taxes for products
returned to Keysight Technologies from another country. Keysight Technologies
warrants that its software and firmware designated by Keysight Technologies for
use with an instrument will execute its programming instructions when properly
installed on that instrument. Keysight Technologies does not warrant that the
operation of the instrument, or firmware will be uninterrupted or error free.
Limitation of Warranty
The foregoing warranty shall not apply to defects resulting from improper or
inadequate maintenance by Buyer, Buyer-supplied software or interfacing,
unauthorized modification or misuse, operation outside of the environmental
specifications for the product, or improper site preparation or maintenance. NO
OTHER WARRANTY IS EXPRESSED OR IMPLIED. KEYSIGHT TECHNOLOGIES
SPECIFICALLY DISCLAIMS THE IMPLIED WARRANTIES OF MERCHANTABILITY
AND FITNESS FOR A PARTICULAR PURPOSE.
Keysight 478A Operating and Service Manual 3
Exclusive Remedies
CAUTION
THE REMEDIES PROVIDED HEREIN ARE BUYER’S SOLE AND EXCLUSIVE
REMEDIES. KEYSIGHT TECHNOLOGIES SHALL NOT BE LIABLE FOR ANY DIRECT,
INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, WHETHER
BASED ON CONTRACT, TORT, OR ANY OTHER LEGAL THEORY.
Safety Considerations
Read the information below before using this instrument.
The following general safety precautions must be observed during all phases of
operation, service, and repair of this instrument. Failure to comply with these
precautions or with specific warnings elsewhere in this manual violates safety
standards for design, manufacture, and intended use of the instrument. Keysight
Technologies assumes no liability for the customer’s failure to comply with these
requirements.
– This product is designed for use in Installation Category II and Pollution
Degree 2.
– This instrument has been designed and tested in accordance with IEC
Publication 348, Safety Requirements for Electronic Measuring
Apparatus, and has been supplied in a safe condition. The instruction
documentation contains information and warnings which must be
followed by the user to ensure safe operation and to maintain the
instrument in a safe condition.
– “CSA” The CSA mark is a registered trademark of the Canadian Standards
Association.
– Notice for Germany: Noise Declaration LpA < 70 dB am Arbeitsplatz
(operator position) normaler Betrieb (normal position) nach DIN 45635 T.
19 (per ISO 7779).
4 Keysight 478A Operating and Service Manual
Waste Electrical and Electronic Equipment (WEEE) Directive
This instrument complies with the WEEE Directive 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.
To return this unwanted instrument, contact your nearest Keysight Service Center,
or visit http://about.keysight.com/en/companyinfo/environment/takeback.shtml
for more information.
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/478a
(product-specific information and support, software and
documentation updates)
– www.keysight.com/find/assist
(worldwide contact information for repair and service)
Keysight 478A Operating and Service Manual 5
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6 Keysight 478A Operating and Service Manual
Table of Contents
Certification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Warranty Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Limitation of Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Exclusive Remedies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Safety Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Waste Electrical and Electronic Equipment (WEEE) Directive . . . . . . . . 5
Product category: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Sales and Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
1 General Information
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Instruments Covered by Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Incoming Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
2Operation
Biasing Thermistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Maximum Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Average Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Pulse Energy and Peak Power for 478A/432 Combination . . . . . . .19
Pulse Energy and Peak Power for 478A/431 Combination . . . . . . .20
Zero Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
RF Power Turned Off for Zero-Set . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Thermistor Mount Disconnected for Zero Set . . . . . . . . . . . . . . . . . .23
Thermoelectric Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
Thermoelectric Effect Error Correction for 478A / 431
Thermoelectric Error for 478A / 432 Combination . . . . . . . . . . . . . .27
Circuit Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28
431 Power Meter Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Combination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Keysight 478A Operating and Service Manual 7
432 Power Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3 Specifications and Characteristics
4Maintenance
Mechanical Shock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Check on Thermistor Match . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Thermistor Assembly Replacement Procedure . . . . . . . . . . . . . . . . 39
Removal Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Installation Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Recalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
AAppendix A
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
How to Use This Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Manual Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Change A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Change B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
8 Keysight 478A Operating and Service Manual
List of Figures
Figure 1-1 Keysight Model 478A Thermistor Mount . . . . . . . . . . .14
Figure 2-1 Maximum Power Meter Reading vs PRF for Pulses
Figure 2-2 Maximum Power Meter Reading vs Duty Cycle for
Figure 2-3 Maximum Power Meter Reading vs Square and Sine-
Figure 2-4 Source Impedance Shunting One RF Thermistor
Figure 2-5 Schematic Diagram of a Model 478A Thermistor
Figure 4-1 Schematic Diagram of a Model 478A Thermistor
Figure 4-2 Check on Model 478A Thermistor Resistance Match . 35
Figure 4-3 Thermistor Compensation . . . . . . . . . . . . . . . . . . . . . .37
Figure 4-4 Model 478A Thermistor Mount Assembly . . . . . . . . . . 41
Figure 4-5 Keysight Model 478A Printed Circuit Assy Wiring . . . .42
Figure A-1 Schematic Diagram of a Model 478A Thermistor
Figure A-2 Schematic Diagram of a Model 478A Thermistor
Figure A-3 Check on Model 478A Thermistor Resistance Match
Figure A-4 Source Impedance Shunting One RF Thermistor
Figure A-5 Schematic Diagram of a Model 478A Thermistor
Figure A-6 Schematic Diagram of a Model 478A Thermistor
Figure A-7 Thermistor Compensation (P/O Change B) . . . . . . . . .53
Shorter than 250 μs . . . . . . . . . . . . . . . . . . . . . . . .19
Pulses Longer than 250 ms . . . . . . . . . . . . . . . . . . .20
Wave Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Mount . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Mount when Connected to a 431 Power Meter . . .25
Mount when Connected to a 432 Power Meter . . .34
Mount when Connected to a 431 Power Meter.
(P/O Change A) . . . . . . . . . . . . . . . . . . . . . . . . . . . .47
Mount when Connected to a 432 Power Meter.
(P/O Change A) . . . . . . . . . . . . . . . . . . . . . . . . . . . .48
(P/O Change A) . . . . . . . . . . . . . . . . . . . . . . . . . . . .49
Mount (P/O Change B) . . . . . . . . . . . . . . . . . . . . . .50
Mount when Connected to a 431 Power Meter.
(P/O Change B) . . . . . . . . . . . . . . . . . . . . . . . . . . . .51
Mount when Connected to a 432 Power Meter.
(P/O Change B) . . . . . . . . . . . . . . . . . . . . . . . . . . . .52
Keysight 478A Operating and Service Manual 9
Figure A-8 Model 478A Thermistor Mount Assembly
(P/O Change B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Figure A-9 Keysight Model 478A Printed Circuit Assembly Wiring
(P/O Change B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
10 Keysight 478A Operating and Service Manual
List of Tables
Table 2-1 Methods of Switching Off RF Output of Various Keysight
Signal Generators . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Table 4-1 Replaceable Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . .39
Table A-1 Manual Changes by Serial Number . . . . . . . . . . . . . . .46
Keysight 478A Operating and Service Manual 11
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12 Keysight 478A Operating and Service Manual
Keysight 478A Thermistor Mount
Operating and Service Manual
1 General Information
Introduction 15
13
1 General Information
Figure 1-1 Keysight Model 478A Thermistor Mount
14 Keysight 478A Operating and Service Manual
Introduction
General Information 1
The Keysight model 478A coaxial thermistor mount is designed for use with
Keysight model 431 and 432 power meters to measure microwave power from 1
μw to 10 mw. 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, and at
selected frequencies across the operating range; the results are marked on the
label of the instrument (see “Mount Calibration Data” in Chapter 3, "Specifications
and Characteristics"). The model 478A can be used over the 10-HMZ to 10-GHz
frequency range. Throughout the range, the mount terminates the coaxial input in
a 50 Ω 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 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.
Instruments Covered by Manual
Instruments with serial numbers 79401 and below are documented directly by this
manual. Some instruments with serial numbers lower than 79401 may be
documented in Appendix A; other instruments with serial numbers higher than
79401 may be documented in a MANUAL CHANGES supplement.
Incoming Inspection
Inspect the model 478A upon receipt for mechanical damage. Also check it
electrically; if the mount was subjected to severe mechanical shock during
shipment, the match between thermistors may be affected. To check the
thermistor match, proceed as described in “Check on Thermistor Match” on
page 34.
If any damage is found, inform the carrier and your nearest Keysight Sales and
Service Office immediately.
Keysight 478A Operating and Service Manual 15
1 General Information
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16 Keysight 478A Operating and Service Manual
Keysight 478A Thermistor Mount
Operating and Service Manual
2 Operation
Biasing Thermistors 18
17
2Operation
CAUTION
CAUTION
Biasing Thermistors
Mechanical Shock: Do not drop or subject to severe mechanical shock. Shock
may destroy the match between thermistors and increase susceptibility to drift.
Before connecting the model 487A to the 431 or 432 power meters, set the
MOUNT RES switch to 200 W position. CONNECTING A 200 W MOUNT TO A
POWER METER SET FOR A 100 W MOUNT CAN RESULT IN THERMISTOR
DAMAGE.
Maximum Input
The model 478A/431 and 478A/432 combinations respond to the average RF
power applied. The maximum signal applied to the thermistor mount should not
exceed the limitations for
–average power
– pulse energy
– peak pulse power
Excessive input can permanently damage the model 478A by altering the match
between RF and compensation thermistors (resulting in excessive drift or zero
shift) or cause error in indicated power.
Average Power
The 478A/431 and 478A/432 combinations can measure average power up to 10
mW. To measure power in excess of 10 mW, insert a calibrated directional coupler
such as one of the Keysight Technologies model 770 series or 790 series between
the mount and the source. UNDER NO CIRCUMSTANCES APPLY MORE THAN 30
mW AVERAGE POWER TO THE MOUNT.
18 Keysight 478A Operating and Service Manual
Pulse Energy and Peak Power for 478A/432 Combination
For pulses shorter than 250 μs, energy per pulse must not exceed 10 W-μs and
peak power should never exceed 200 watts. (For example, a 40 mW, 250 μs pulse
contains 10 W-μs of energy.) Pulses longer than 250 μs are allowed to contain
more energy but peak power must not exceed 200 W. Figure 2-1 and Figure 2-2
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 487A/431 combination.)
Operation 2
Figure 2-1 Maximum Power Meter Reading vs PRF for Pulses Shorter than
250 μs
Keysight 478A Operating and Service Manual 19
2Operation
Pulse Energy and Peak Power for 478A/431 Combination
The limitations of this combination are basically the same as the 487A/432 with
the exception that at pulse repetition rates less than 1 kHz, energy per pulse must
not exceed 5 W-μs and peak power must not exceed 10 mW. These limits are also
interpreted in Figure 2-1 and Figure 2-2.
Square-wave modulation is a special case of pulse modulation, and maximum
power-meter reading versus square-wave frequency is illustrated in Figure 2-3.
This figure also holds for sine-wave modulation.
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 read ing.
Figure 2-2 Maximum Power Meter Reading vs Duty Cycle for Pulses Longer
than 250 μs
20 Keysight 478A Operating and Service Manual
Operation 2
CAUTION
Figure 2-3 Maximum Power Meter Reading vs Square and Sine-Wave
Frequency
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 Keysight model
8741A/8716A modulators and 8614A/8616A signal generators, however, are not
subject to this consideration because output filtering prevents transmission of
modulating signals.
Thermistors are inherently temeperature-sensitive devices. A cold thermistor
mount connected to a warm piece of equipment, or vice versa, produces
rapid shift. 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.
Keysight 478A Operating and Service Manual 21
2Operation
Zero Set
It is necessarily to electrically zero-set the model 431/432 power meter before
making a power measurement. To preserve the same zero reference throughout
the measurement, maintain the same thermal environment when RF power is
applied. Two recommended set-ups for 431 zero-setup are presented in “RF
Power Turned Off for Zero-Set” on page 22 and “Thermistor Mount Disconnected
for Zero Set” on page 23. The recommended set-up for zero-set in the 432 is
shown in “478A/432” on page 25.
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. The methods used to switch off RF output in Keysight signal
generators are listed in Table 2-1. After allowing time for the mount to stabilize
thoroughly, follow zero-set procedures in the 431 or 432 power meter manual and
then turn on the RF source.
Tab le 2-1 Methods of Switching Off RF Output of Various Keysight Signal
Generators
Keysight Generator Frequency Range
Model 606 50 kHz to 65 MHz
Model 8614A / 8616A
Model 614 / 616
Model 618 / 620
Model 608
Model 682 / 687
Model 612 450 to 1230 MHz
Model 690, 8690 Series 1 to 20 GHz Set LINE to STANDBY
800 to 2400 MHz / 1800 to 4500 MHz
800 to 2100 MHz / 1800 to 4200 MHz
3.8 to 7.6 GHz / 7 to 11 GHz
10 to 480 MHz / 10 to 420 MHz
1 to 2 GHz / 12.4 to 18.0 GHz
Procedure to Switch Off RF
Output
Increase the generator output
attenuation 30 or more dB
Release RF push button
Set modulation selector to OFF
Set modulation selector to OFF
Set MOD SELECTOR to PULSE,
but do not apply modulation
signal to modulation input
terminal
Set MOD SELECTOR to PULSE
2, but do not apply modulation
signal to modulation input
terminal
22 Keysight 478A Operating and Service Manual
Thermistor Mount Disconnected for Zero Set
NOTE
478A/431
When is it inconvenient to turn off the RF power in the RF system, connect the
model 478A mount to the RF system and set RANGE on the model 431 power
meter for an appropriate midscale reading. When the reading no longer drifts,
disconnect the mount from the source, terminate the mount, if necessary, as
described in “Thermistor Mount Disconnected for Zero Set” on page 23, and then
the zero-set the power meter. Immediately reconnect the mount to the RF source
for the power measurement.
In the proximity of a high RF field, shield the disconnected thermistor mount
from possible stray RF pick-up during the zero-set.
Operation 2
Figure 2-4 Source Impedance Shunting One RF Thermistor Mount
Keysight 478A Operating and Service Manual 23
2Operation
With the model 478A mount connected to the RF system, the source impedance
shunts one of the RF thermistors (see Figure 2-4); when the model 478A mount is
disconnected, the source impedance is removed. Unless source impedance is
high, this variation in impedance affects the RF bridge 10-kHz feedback loop in
the power meter, and the zero-level setting obtained with the source
disconnected is no longer zero for the measurement. This error can be eliminated
by terminating the mount in an impedance which approximately matches the
generator impedance at 10 kHz; the termination should be connected while the
mount is disconnected (see “Thermistor Mount Disconnected for Zero Set” on
page 23) from the source. For example, if the impedance presented by the RF
system to 10 kHz is low (1 kΩ or less) terminate the thermistor mount in a 50 Ω
resistor or a short. On the other hand, if the impedance of the RF system at 10 kHz
is high (100 kΩ or more) leave the thermistor mount unterminated during
zero-set.
Note that some 10-kHz bias signal is coupled into the RF transmission system by
C2 (Figure 2-5). If the Rf source output impedance at 10 kHz is 15 kΩ or greater,
10-kHz bias voltage is typically 1.3 V RMS and could equal 1.5 V RMS. For an RF
source output impedance of 50 Ω at 10 kHz, bias signal voltage is typically 5 mV
RMS.
The presence of this 10-kHz bias signal may affect solid state RF sources and RF
voltmeter measurements. To minimize or eliminate these effects, use an
additional blocking capacitor at the model 478A or a high pass filter at the RF
source output.
24 Keysight 478A Operating and Service Manual
Operation 2
478A/432
When it is inconvenient to turn the RF power off while using the 478A/432
combination simply remove the mount from the source, and using the COARSE
and FINE ZERO, zero the 432.
Figure 2-5 Schematic Diagram of a Model 478A Thermistor Mount when
Connected to a 431 Power Meter
Keysight 478A Operating and Service Manual 25
2Operation
Thermoelectric Effect
Mount calibration uncertainties given in Chapter 3, "Specifications and
Characteristics" include inaccuracies caused by thermoelectric effect error.
Calibration Factor uncertainty of ±1.5% and Effective Efficiency uncertainty of
±2.5% can be maintained on the three lowest power ranges of the model 431
series power meters by correcting for the measurement error introduced by
thermoelectric effect. An error connection procedure is given in “Thermoelectric
Effect Error Correction for 478A / 431 Combination” on page 27.
A mild thermocouple exists at each point of contact where the connecting wires to
join the thermistor elements. Each thermocouple creates a DC voltage. Thus, two
thermocouple voltages of opposite relative polarity are formed, one at each
junction to each thermistor element.
Ideally, each thermocouple voltage would be equal in magnitude so that they
cancel with no resultant effect on the accuracy of power measurement. In practice
however, each point of contact does not have identical thermocouple
characteristics, and in addition, the temperatures at each junction may not be the
same. These differences cause an incomplete cancellation of the thermoelectric
voltages, resulting in a voltage that causes a thermoelectric effect error. The
magnitude of the error is important when making DC substitution measurements
on the 0.1 mW, 0.03 mW and 0.01 mW ranges with one of the model 431 series
power meters. On other ranges, the effect is negligible. Maximum error introduced
by thermoelectric effect is about 0.3 μw and is typically 0.1 μw on the 0.01 mW
range.
26 Keysight 478A Operating and Service Manual
Operation 2
Power
P
1P2
+
2
--------------=
Thermoelectric Effect Error Correction for 478A / 431 Combination
Use the following technique to correct for thermoelectric effect error.
1 Measure power
2 Connect an Keysight model 8402 power meter calibration to the power meter
DC CALIBRATION AND SUBSTITUTION connector.
3 Zero and null power meter.
4 By DC substitution (refer to procedure in 431 Manual), duplicate power
measurement made in step 1. Calculate and record substituted power as P
5 Reverse connection polarity between the calibrator and power meter.
6 Re-zero and re-null power meter, if necessary.
7 By DC substitution, duplicate lower measurement made in step 1. Calculate
and record substituted power as P
8 Calculate arithmetic mean of the two substitution powers P1 and P2.
This mean power includes a correction for thermoelectric effect error.
2.
.
1
Thermoelectric Error for 478A / 432 Combination
The thermoelectric errors present in the 431 are minimized in the 432 since the
thermoelectric voltage is negligible compared with the DC voltage used to bias
the thermistor bridges.
Keysight 478A Operating and Service Manual 27
2Operation
Circuit Description
Two matched series thermistors are mounted on a common thermal conducting
block, represented by the shaded rectangle in Figure 2-5. One pair, marked “D”
for detection, is mounted between the end of a coaxial cable and cylindrical
cavity. These thermistors are exposed to incoming RF power which heats them,
lowering their resistance. The other pair, marked “C” for compensation and
situated immediately outside the cavity, is completely shielded from RF. With the
478A attached to 431 power meter, the detection thermistors are part of the
metering circuit.
With the 478A attached to the 432 power meter, the detection thermistors are
part of the RF bridge and the compensation thermistors are part of the
compensation bridge. Since the two pairs of thermistors show the same thermal
environment, any change in temperature which affects the RF bridge
simultaneously affects the mastering bridge; this allows the power meter circuit to
compensate for changes in temperature and thus minimize drift.
During 431/478A operation, sufficient amounts of DC and 10 kHz bias currents
are supplied from the 431 power meter to heat the thermistors until their
resistances are reduced to approximately 200 Ω per series pair. Capacitor C1
offers high impedance to 10 kHz, but is practically a short of RF. This causes “D”
to appear series connected to 10 kHZ, but parallel connected to RF. In this
manner, “D” appears to the audio bridge of of the 431 power meter as a 200 Ω
resistance, but terminates the coaxial cable in 50 Ω. Capacitor C2 blocks any DC
and audio power that may be present in the incoming signal, and passes only RF
power.
During 432 operation (see Figure 4-1) sufficient amounts of DC current are
supplied from 432 power meter to heat the thermistors until their resistances are
reduced to approximately 200 Ω per series pair. Capacitor C1 is practically a short
to RF. This causes “D” to appear series connected to the DC bridge, parallel
connected to RF. In this manner, “D” appears to the DC bridge in the 432 power
meter as a 200 Ω resistance that terminates the coaxial cable into 50 Ω. Capacitor
C2 blocks any DC and audio power that may be present in the incoming signal
and passes only RF power.
28 Keysight 478A Operating and Service Manual
431 Power Meter Detection
Under normal operation, the total power supplied to heat thermistor pair “D” (see
Figure 2-5) consists of:
–RF signal
– 10 kHz bias
– Heat from the same environment
As “D” and “C” are matched thermally, the total amounts of heat applied to
reduce their series resistance equally must be equal.
432 Power Detection
Under normal operation, the total power applied to heat thermistor pair “D” (see
Figure 4-1) consists of:
–RF signal
– Heat from the environment
The total power supplied to heat thermistor “C” consists of:
–DC bias
– Heat from the same environment
Operation 2
As “D” and “C” are matched thermally, the total amounts of heat applied to
reduce their series resistance equally must be equal.
Keysight 478A Operating and Service Manual 29
2Operation
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30 Keysight 478A Operating and Service Manual
Keysight 478A Thermistor Mount
Operating and Service Manual
3 Specifications and
Characteristics
For the specifications and characteristics of the 478A Thermistor Mount, refer to
the datasheet at http://literature.cdn.keysight.com/litweb/pdf/5990-5740EN.pdf
31
3 Specifications and Characteristics
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32 Keysight 478A Operating and Service Manual
Keysight 478A Thermistor Mount
Operating and Service Manual
4 Maintenance
Mechanical Shock 34
33
4 Maintenance
Mechanical Shock
The model 478A is a precision instrument. Avoid dropping or other mechanical
shocks. Such shocks can destroy the match between the thermistors.
Check on Thermistor Match
Match between the thermistors may be checked by comparing the thermistor
resistances under simulated operating conditions. Equipment required is
indicated in Figure 4-1. Make connection to the connector at the rear of the
thermistor mount; pins are shown in Figure 4-1. Note that the small battery in
series with the Keysight model 3440 digital voltmeter is connected in opposition
to the power supply. The value of this bucking voltage should be such that
voltmeter resolution down to 0.001 volt is obtained. Take readings with switch S
connected to pin 1 and then to pin 3. Thermistor match is satisfactory if the two
readings do not differ by more than 0.030 volt. Non-operating mounts with
readings as high as 0.150 volt can probably be repaired as outlined in the
succeeding paragraphs.
Figure 4-1 Schematic Diagram of a Model 478A Thermistor Mount when
Connected to a 432 Power Meter
34 Keysight 478A Operating and Service Manual
Maintenance 4
WARNING
Under no conditions should the mount be required to carry a current higher
than 14 mA.
Keysight 478A Operating and Service Manual 35
Figure 4-2 Check on Model 478A Thermistor Resistance Match
4 Maintenance
NOTE
Repair
Exceeding the cw or pulse power limit of the model 478A thermistor mount may
result in damage such that the mount will no longer zero on the model 431 or 432
power meter.
Before adjusting the mount in any way, make sure that the mount is the cause of
the problem. An open or short indication, using the checks in “Check on
Thermistor Match” on page 34, or “Repair” on page 36, means that the mount is
not repairable by the procedures outlined in the following paragraphs. However,
the mount may be nonoperative but still repairable. Test for this by using the
procedures in Chapter 4, “Check on Thermistor Match,” starting on page 34, or by
connecting the mount to a model 431 or 432 power meter and a cable which is
known to be good. A faulty cable will not have continuity through the respective
connector pins or may have poor contact in the mount connector. Poor contact
will show as an intermittancy or a great deal of noise (visible on 431 or 432 meter)
when the cable is gently flexed near the connector end.
The folliwing procedures are set down for the 431/478A combination. The same
procedures can be used for the 432 except for the ZERO/VERNIER control. Those
controls on the 431 are replaced by the COARSE ZERO and FINE ZERO control on
the 432.
The major difference between the 478A/431 and 478A/432 combination is that
the 478A/432 combination does not use 10 kHz as a biasing factor in the
thermistor. All biasing factors in the 432 are from DC sources therefore, lowering
the thermoelectric effect.
To troubleshoot a damaged mount, proceed as follows:
1 Connect Mount to model 431.
2 Set:
MOUNT RES: 200 W
RANGE: 10 mW
POWER: ON
3 Rotate ZERO from one limit to the other.
36 Keysight 478A Operating and Service Manual
Maintenance 4
If the meter remains pegged upscale, the thermistor elements have been
damaged. However, it may be possible to recompensate the thermistors (refer to
“Repair” on page 36) and return the mount to operation: otherwise they must be
replaced. In either case, the Effective Efficiency and Calibration Factor data on the
nameplate are no longer valid (see “Recalibration” on page 43).
Figure 4-3 Thermistor Compensation
If meter remains pegged downscale, measure resistance between pins 1 and 2,
and pins 3 and 4. The resistance should measure between 1000 and 5000 Ω. An
open or shorted reading indicates the need for replacement of the thermistors.
If the resistance reading is satisfactory, it may be possible to recompensate the
mount and return it to service. The drift with temperature changes will be higher
because of the damage to the thermistors, but it will be possible to zero the meter
and to make measurements. The Effective Efficiency and Calibration Factor
indicated on the label will no longer be valid (see “Recalibration” on page 43).
There are two adjusting screws inside the instrument which permit
recompensation within limits. Most instruments with serial lower than 7663 do not
have the adjusting screws at the time of manufacture, but are modified if the
instrument was sent in for repair after March 1964.
Keysight 478A Operating and Service Manual 37
4 Maintenance
CAUTION
NOTE
Refer to Figure 4-3 and proceed as follows:
1 Remove the three screws (A).
2 Slide instrument out of its cover.
3 Plug cover into model 431.
4 Set:
MOUNT RES: 200 W
MOWER: ON
If meter is pegged downscale:
1 Set RANGE to 10 mW.
2 Set ZERO and VERNIER to mid-range.
3 Turn screws (B) clockwise, 1 8 turn alternatively.
If there is a sudden jump in meter indication when ad vancing either screw,
back off 1 8 turn, and do not advance that screw further. Check resistance as
in “Repair” on page 36. If either screws bottoms, do not apply force.
Thermistor replacement (see “Thermistor Assembly Replacement
Procedure” on page 39) is indicated.
4 When meter pointer rises, trim to zero with each adjusting screw.
5 Replace cover and three screws (A). The instrument is now operative.
Make sure the three screws in the body of the thermistor mount are tight when
the mount is put together; this insures quiet operation.
38 Keysight 478A Operating and Service Manual
If the meter is pegged upscale:
NOTE
1 Set ZERO and VERNIER to mid-range.
2 Set RANGE to highest position which will not peg the meter.
3 Turn one of the screws (B) counterclockwise to obtain a meter reading half that
observed in step 2.
4 Turn the other screw (B) counterclockwise to zero the meter. If it is impossible
to zero the meter, replace the thermistors (see “Thermistor Assembly
Replacement Procedure” on page 39).
5 Replace cover and three screws (A). The instrument is now operative.
Thermistor Assembly Replacement Procedure
After replacement of the thermistor assembly, the Effective Efficiency and
Calibration Factor indicated on the label of the mount are no longer valid (see
“Recalibration” on page 43).
The procedure consists of removing the damaged thermistor assembly, the
printed circuit assembly and replacing them with pretested assemblies included in
Thermistor Assembly Replacement Kit, Keysight Stock No. 00478-60002 (see
Table 4-1).
Maintenance 4
Table 4 -1 Replaceable Parts
Keysight Part Number Qty Description
0520-0136 3 Screw, 2-56 x 5/8 , RDHD
0520-0173 3 Screw, 2-56 x 3/16, PNHD
1251-0153 1 Receptacle Connector
1251-1281 1 Nut, Knurled
1460-1547 1 Spring, Compressor
2190-0014 3 Internal Lock Washer No.2
2190-0036 1 Washer Lock, 818 ID
08491-2009 1 Sliding Contact
Keysight 478A Operating and Service Manual 39
4 Maintenance
NOTE
Tab le 4-1 Replaceable Parts (continued)
Keysight Part Number Qty Description
00478-60002 1
00478-60003 1 Connector Assembly
00478-60001 1 Terminal Shield
478A-54A 3 Insulator Bushing
Thermistor Assembly (includes 478A-65A and
components)
The replacement assemblies are pretested at the factory. However, since the
operation of the thermistor mount depends on proper installation of the assembly,
it may be desirable to check the SWR and efficiency following replacement. The
efficiency may be checked by comparing against a known mount. The SWR is
checked at 9 GHz and 10 GHz. SWR should be approximately equal at 9 and 10
GHz, and should be 1.5 or less.
In the field, replaceable thermistor assembly connection of the RF thermistors to
the Type N center conductor , SWR will be about 2.0 at 10 GHz. The bellows
may be lengthened slightly with a pair of tweezers.
The following special tools may be required for the completion of this procedure:
– One small screwdriver, 0.070 tip, suitable for removing a 00-90 x 1/8 screw.
– One pair of tweezers.
40 Keysight 478A Operating and Service Manual
Removal Procedure
CAUTION
1 Remove three 2-56 x 3/16 screws holding terminal shield (Figure 4-5). Move
terminal shield aside.
Maintenance 4
Figure 4-4 Model 478A Thermistor Mount Assembly
Do not break wires connecting printed circuit assembly to receptacle
connector.
2 Loosen locknut and remove 5/16-32 setscrew from thermistor assembly.
3 Disconnect the three wires between the printed circuit assembly and the
receptacle connector from the printed circuit assembly.
4 In early thermistor assemblies there is a 00-90 x 1/8 screw used to connect the
RF Thermistors to the Type N center conductor. Remove this screw, if present,
using small screwdriver and tweezers.
5 Remove the three 2-56 x 5/8 inch screws holding printed circuit and
thermistor assemblies to the RF connector assembly.
Keysight 478A Operating and Service Manual 41
4 Maintenance
NOTE
Installation Procedure
6 Remove printed circuit and thermistor assemblies.
7 Remove the three insulator bushings from thermistor assembly. This
completes the removal of the damaged assembly.
1 Referring to Figure 4-5, connect the three wires from receptacle connector to
printed circuit assembly.
For strain relief, the wires should go through the holes indicated and connect
from the bottom of the printed circuit assembly.
Figure 4-5 Keysight Model 478A Printed Circuit Assy Wiring
2 Install the three insulator bushings in the thermistor assembly.
3 Pass the red wire from the thermistor assembly through the hole indicated in
Figure 4-5. Do not connect to printed circuit assembly at this time.
42 Keysight 478A Operating and Service Manual
Maintenance 4
NOTE
CAUTION
4 Mount thermistor and printed circuit assemblies on RF connector assembly.
Use three 2-56 x 5/8 inch screws and lockwashers. Screws must be tightened
firmly to insure proper bellows contact with the Type N center conductor.
Printed circuit assembly must be positioned so it does not cover compensating
screws.
5 Connect red and white wires from thermistor assembly to printed circuit
assembly.
6 Connect thermistor mount to a Keysight model 431. Check for proper null and
zero.
7 If desired, check SWR and efficiency. The Mount SWR has been adjusted at
the factory to be about equal at 9 GHz and 10 GHz and less than 1.5. The
adjustment is made with the 5/16-23 setscrew which is secured by a locknut.
The setscrew should not be moved unless SWR is being calibrated. Efficiency
may be checked by comparing to a known good mount.
To prevent pulling wires out of terminal connector, secure terminal shield
with one 2-56 x 3/16 inch screw while making checks.
8 When any testing or recalibration is completed, secure terminal shield with
three 2-56 x 3/16 inch screws. This completes the Installation Procedure.
Recalibration
If recalibration of the mount is desired, the instrument may be sent to the factor
for repair and recalibration. Any Keysight Technologies Sales and Service Office
will arrange for such repair.
Keysight 478A Operating and Service Manual 43
4 Maintenance
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44 Keysight 478A Operating and Service Manual
Keysight 478A Thermistor Mount
Operating and Service Manual
A Appendix A
Introduction 46
Manual Changes 47
45
A Appendix A
Introduction
How to Use This Appendix
This appendix contains manual change instructions for backdating the manual for
thermistor mounts with serial numbers lower than 79401.
To adapt the manual to your instrument, refer to Tab le A-1. Make all of the
manual changes listed opposite your instrument’s serial number. The manual
changes should be performed in the sequence shown in the table. For example,
Change B should be done before Change A.
If your instrument’s serial number is higher than 79401 or not listed in Ta ble A-1,
it may be documented in a separate MANUAL CHANGES supplement.
Tab le A-1 Manual Changes by Serial Number
Serial Number Make Manual Changes
65600 and below B, A
65601 through 73300 B
46 Keysight 478A Operating and Service Manual
Manual Changes
Change A
Replace Figure 2-5, “Schematic Diagram of a Model 478A Thermistor Mount
when Connected to a 431 Power Meter,” on page 25 with Figure A-1.
Appendix A A
Figure A-1 Schematic Diagram of a Model 478A Thermistor Mount when
Connected to a 431 Power Meter. (P/O Change A)
Keysight 478A Operating and Service Manual 47
A Appendix A
Replace Figure 4-1, “Schematic Diagram of a Model 478A Thermistor Mount
when Connected to a 432 Power Meter,” on page 34 with Figure A-2.
Figure A-2 Schematic Diagram of a Model 478A Thermistor Mount when
Connected to a 432 Power Meter. (P/O Change A)
48 Keysight 478A Operating and Service Manual
Appendix A A
Replace Figure 4-2, “Check on Model 478A Thermistor Resistance Match,” on
page 35 with Figure A-3.
Figure A-3 Check on Model 478A Thermistor Resistance Match (P/O
Change A)
Keysight 478A Operating and Service Manual 49
A Appendix A
Change B
Replace Figure 2-4, “Source Impedance Shunting One RF Thermistor Mount,” on
page 23 with Figure A-4.
Figure A-4 Source Impedance Shunting One RF Thermistor Mount (P/O
Change B)
50 Keysight 478A Operating and Service Manual
Appendix A A
Replace Figure 2-5, “Schematic Diagram of a Model 478A Thermistor Mount
when Connected to a 431 Power Meter,” on page 25 with Figure A-5.
Figure A-5 Schematic Diagram of a Model 478A Thermistor Mount when
Connected to a 431 Power Meter. (P/O Change B)
Keysight 478A Operating and Service Manual 51
A Appendix A
Replace Figure 4-1, “Schematic Diagram of a Model 478A Thermistor Mount
when Connected to a 432 Power Meter,” on page 34 with Figure A-6.
Figure A-6 Schematic Diagram of a Model 478A Thermistor Mount when
Connected to a 432 Power Meter. (P/O Change B)
52 Keysight 478A Operating and Service Manual
Appendix A A
Replace Figure 4-3, “Thermistor Compensation,” on page 37 with Figure A-7.
Figure A-7 Thermistor Compensation (P/O Change B)
Replace Figure 4-4, “Model 478A Thermistor Mount Assembly,” on page 41 with
Figure A-8.
Figure A-8 Model 478A Thermistor Mount Assembly (P/O Change B)
Keysight 478A Operating and Service Manual 53
A Appendix A
Replace Figure 4-5, “Keysight Model 478A Printed Circuit Assy Wiring,” on
page 42 with Figure A-9.
Figure A-9 Keysight Model 478A Printed Circuit Assembly Wiring (P/O
Change B)
54 Keysight 478A Operating and Service Manual
This information is subject to change
without notice. Always refer to the
Keysight website for the latest
revision.
© Keysight Technologies 2008 - 2020
Edition 3, January 23, 2020
Printed in Malaysia
*00478-90021*
00478-90021
www.keysight.com
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