Anritsu MS2681A User manual

MS2681A/MS2683A/MS2687A
Spectrum Analyzer
Service Manual
To ensure that this equipment is used safety, important
safety items are explained in the MS2681A/83A/87A
Operation Manual. This document explains important
service items related to service. Read both the
operation manual and this document, and keep both
with the MS2681A/83A/87A.
Second Edition
Measurement Solutions
ANRITSU CORPORATION
Document No.: M-W1754BE-2.0

ii
To prevent the risk of personal injury or loss related to equipment malfunction, Anritsu Corporation uses the following
safety symbols to indicate safety-related information. Insure that you clearly understand the meanings of the
symbols BEFORE using the equipment.
Some or all of the following five symbols may not be used on all Anritsu equipment. In addition, there may be other
labels attached to products which are not shown in the diagrams in this manual.
Symbols used in manual
This indicates a very dangerous procedure that could result in death or serious
injury if not performed properly.
This indicates a hazardous procedure that could result in death or serious injury if
not performed properly.
This indicates a hazardous procedure or danger that could result in light-to-severe
injury, or loss related to equipment malfunction, if proper precautions are not taken.
Safety Symbols Used on Equipment and in Manual
The following safety symbols are used inside or on the equipment near operation locations to provide information
about safety items and operation precautions. Insure that you clearly understand the meanings of the symbols
and take the necessary precautions BEFORE using the equipment.
This indicates a prohibited operation. The prohibited operation is indicated
symbolically in or near the barred circle.
This indicates an obligatory safety precaution. The obligatory operation is
indicated symbolically in or near the circle.
This indicates warning or caution. The contents are indicated symbolically in or
near the triangle.
This indicates a note. The contents are described in the box.
These indicate that the marked part should be recycled.
MS2681A/83A/87A
Spectrum Analyzer
Service Manual
16 June 2000 (First Edition)
12 October 2001 (Second Edition)
Copyright © 2001, ANRITSU CORPORATION.
All rights reserved. No part of this manual may be reproduced without the prior written permission of the
publisher.
The contents of this manual may be changed without prior notice.
Printed in Japan
Safety Symbols
DANGER
WARNING
CAUTION

iii
For Safety
If repair is required, contact the sales representative, branch office, or agent at the telephone number and address
given in this document or in the equipment operation manual.
Although not recommended by Anritsu Corporation, if it is really imperative to open the covers for emergency repair,
take great care not to touch any dangerous parts.
Always request repair by a trained engineer who understands the hazards.
Anritsu Corporation will not accept liability for any injuries sustained as a result of opening the equipment covers.

iv
Trademark
MS, Microsoft, MS-DOS, Windows and Word98 are the registered
trademarks of Microsoft Corporation US in the US and other countries.
IBM and AT are the registered trademarks of International Business
Machines Corporation.
NI-488.2TM is a registered trademark of National Instruments
Corporation.

v
Power Line Fuse Protection
For safety, Anritsu products have either one or two fuses in the AC power lines as
requested by the customer when ordering.
Single fuse: A fuse is inserted in one of the AC power lines.
Double fuse: A fuse is inserted in each of the AC power lines.
Example: An example of the double fuse is shown below:
Fuse Holders

vi

I
About This Manual
This Service Manual for the MS268*A Spectrum Analyzer provides the trouble-
shooting instructions on how to identify and replace the faulty modules. Instruc-
tions on repairing module's internal parts are beyond the scope of this manual.
This service manual is written for both our own and our dealer's service personnel
who have completed a prescribed training course and are aware of the danger of
fire and electrical shock and other accidents.
“MS268*A” in the main chapter means “MS2681A/83A/87A”.

II.
Table of Contents
For Safety...................................................... iii
About This Manual ....................................... I
Section 1 Description of Module Operation 1-1
1.1 Outline........................................................................... 1-2
1.2 List of Replacement Modules........................................ 1-21
1.3 Module Operation.......................................................... 1-23
Section 2 Troubleshooting........................ 2-1
2.1 Troubleshooting Flow.................................................... 2-3
2.2 Mechanical Configuration ............................................. 2-5
2.3 Check after Module Replacement................................. 2-33
Section 3 Adjustment and Calibration...... 3-1
3.1 Service Kit ..................................................................... 3-2
3.2 Required Equipment ..................................................... 3-2
3.3 Sweep Adjustment ........................................................ 3-3
3.4 YTF D/A Adjustment (for MS2683A)............................. 3-5
3.5 YTF D/A Adjustment (for MS2687A)............................. 3-6
3.6 YTF Adjustment (for MS2683A or MA2687A)............... 3-7
3.7 CAL OSC Adjustment ................................................... 3-9
3.8 IF Gain2 Adjustment (for MS2683A or MA2687A)........ 3-10
3.9 Maintenance Mode........................................................ 3-12
3.10 Frequency Characteristics Compensation .................... 3-13
3.11 Internal Calibration ........................................................ 3-17
3.12 Installing Software......................................................... 3-19
3.13 Mechanical Switch Counter Setting .............................. 3-20

Section 1 Description of Module Operation
1-1
1.1 Outline ....................................................................... 1-2
1.1.1 Outline for MS2681A ................................................. 1-2
1.1.2 Outline for MS2683A ................................................. 1-4
1.1.3 Outline for MS2687A ................................................. 1-6
1.2 List of Replacement Modules .................................... 1-21
1.2.1 List of Replacement Modules for MS2681A .............. 1-21
1.2.2 List of Replacement Modules for MS2683A .............. 1-21
1.2.3 List of Replacement Modules for MS2687A .............. 1-22
1.3 Module Operation ...................................................... 1-23

Section 1 Description of Module Operation
1-2
1.1 Outline
1.1.1 Outline for MS2681A
Please refer to Fig. 1-1 Overall Block Diagram.
The MS2681A is a superheterodyne system scanning-type spectrum analyzer.
For RF input after passing through the 8 GHz STEP ATTENUATOR frequency of 9
kHz to 3.0 GHz the signal pass through the Coaxial LPF and then to A10 3 GHz
CONVERTER.
In the A10 3 GHz CONVERTER, RF input signal gets mixed with first Local
Oscillator signal to generate a 4060.69 or 4066 MHz first IF signal by first mixer.
After passing several filters and amplifier, First IF signal gets mixed with second
Local Oscillator signal (4000 MHz) to generate a 60.7 or 66 MHz second IF signal
by second mixer, and fed to A06 IF.
The first Local signal generated at YTO (YIG tuned oscillator) is frequency-swept by
scan signal from A05 SCAN/AD after phase-lock to reference signal (its frequency is
26 to 32 MHz) generated on A08 LOCAL02 at the center frequency of its sweeping
range, in normal sweep condition.
The YTO output is passed through an amplifier, and then divided into two paths with
a directional coupler. One of divided signal is fed to sampler circuit, sampling
signal (its frequency is 194 to 206 MHz) generated on A08 LOCAL02 is frequency
multiplied, and mixed with YTO output to generate sampler IF signal with a
frequency of 26 to 32 MHz.
The 4-GHz second local signal is phase-locked to 100-MHz VCXO signal, of which
the frequency is also phase-locked to 10-MHz crystal oscillator.
In the instrument, high accuracy 625-kHz/50-MHz signals are present for calibration.
These signals are generated by dividing the reference signal, and the CAL ATT
varies power level of 625-kHz signal with 1-dB steps.
Internal calibration operation being carried out, these calibration signal is fed to RF
signal-route through the switch in 8 GHz STEP ATTENUATOR.
66 MHz second IF signal is selected when resolution band width (RBW) setting is 1,
3, 5, 10, or 20 MHz. For other settings, 60.7 MHz is selected.

1.1 Outline
1-3
At A06 IF unit, the incoming signal is divided in two paths. The main route leads to
image rejection filters; while second, highly attenuated feeler path signal (only 66
MHz) is fed to A05 SCAN/AD. Second route is not used in spectrum analyzer
mode.
The main signal after passing through image rejection filters is switched to two
different signal routes according to RBW setting. One is 66-MHz wide RBW route
(RBW ≥1 MHz), the other is 60.7-MHz narrow RBW route (RBW <1 MHz). At
narrow RBW route, 60.7 MHz second IF signal is down converted to 10.7 MHz.
This 10.7/66 MHz signal is fed to A05 SCAN/AD through various Resolution Width
setting circuits.
In the A05 SCAN/AD, RBW processed 10.7/66-MHz signal passes through 10.7/66-
MHz logarithmic amplifiers and then to linear detectors. This linear detected signal
is passed through smoothing filters called Video Band Width Filters (VBW).
This smoothed signal is then passed through Positive or Negative peak detection
circuits and output is converted to digital signal by high-resolution Analog to Digital
Converter (ADC) or high-speed ADC.
In the A07 MEAS DSP, the digital signal from the A05 SCAN/AD is processed by
DSP. A07 MEAS DSP also generates various commands required for controlling or
setting of all hardware units inside the instrument.
The A03 MAIN CPU controls various interface functions such as reading the Key
Inputs or remote commands received, and various outputs such as prints of various
data.
The A02 FONT BOAD generates the Key and rotary-knob encoder data, drives the
LCD, detects the power switch setting, and controls the power-supply On/Stby
setting.
The INVERTER supplies power for LCD backlight.

Section 1 Description of Module Operation
1-4
1.1.2 Outline for MS2683A
Please refer to Fig. 1-2 Overall Block Diagram.
The MS2683A is a superheterodyne system scanning-type spectrum analyzer.
The RF input signal after passing through the 8 GHz STEP ATTENUATOR is
switched by 1.6 GHz YTF/SW to two different signal routes according to the
measurement frequency range.
For RF input frequency of 9 kHz to 3.2 GHz (Termed as Band 0) the signal pass
through the Coaxial LPF and then to A10 3 GHz CONVERTER.
For RF input frequency of 3.15 to 7.8 GHz (option03: 1.6 to 7.8 GHz) (Bands (1-L),
1–, 1+) the signal goes to a frequency tuned filter (YTF) with 1.6 GHz YTF/SW and
then to the 8 GHz MIXER.
In the 8 GHz MIXER, RF signal gets mixed with first Local Oscillator signal to
generate a 460.7 or 466 MHz First IF signal. And this First IF signal is fed to the
A13 2ND CONVERTER.
In the A10 3 GHz CONVERTER, RF input signal gets mixed with first Local
Oscillator signal to generate a 4060.69 or 4066 MHz first IF signal by first mixer.
After passing several filters and amplifier, First IF signal gets mixed with second
Local Oscillator signal (4000 MHz) to generate a 60.7 or 66 MHz second IF signal
by second mixer, and fed to A06 IF.
The first Local signal generated at YTO (YIG tuned oscillator) is frequency-swept by
scan signal from A05 SCAN/AD after phase-lock to reference signal (its frequency is
26 to 32 MHz) generated on A08 LOCAL02 at the center frequency of its sweeping
range, in normal sweep condition.
The YTO output is passed through an amplifier, and then divided into two paths with
a directional coupler. One of divided signal is fed to sampler circuit, sampling
signal (its frequency is 194 to 206 MHz) generated on A08 LOCAL02 is frequency
multiplied, and mixed with YTO output to generate sampler IF signal with a
frequency of 26 to 32 MHz.
The 4-GHz second local signal is phase-locked to 100-MHz VCXO signal, of which
the frequency is also phase-locked to 10-MHz crystal oscillator.
In the instrument, high accuracy 625-kHz/50-MHz signals are present for calibration.
These signals are generated by dividing the reference signal, and the CAL ATT
varies power level of 625-kHz signal with 1-dB steps.

1.1 Outline
1-5
Internal calibration operation being carried out, these calibration signal is fed to RF
signal-route through the switch in 8 GHz STEP ATTENUATOR.
In the A13 2ND CONVERTER, 460.7 or 466 MHz first IF signal gets mixed with
second Local Oscillator signal (400 MHz) to generate a 60.7 or 66 MHz second IF
signal by second mixer after passing several band-pass filters and amplifiers, and fed
to A06 IF.
66 MHz second IF signal is selected when resolution band width (RBW) setting is 1,
3, 5, 10, or 20 MHz. For other settings, 60.7 MHz is selected.
At A06 IF unit, the incoming signal is divided in two paths. The main route leads to
image rejection filters; while second, highly attenuated feeler path signal (only 66
MHz) is fed to A05 SCAN/AD. Second route is not used in spectrum analyzer
mode.
The main signal after passing through image rejection filters is switched to two
different signal routes according to RBW setting. One is 66-MHz wide RBW route
(RBW ≥1 MHz), the other is 60.7-MHz narrow RBW route (RBW <1 MHz). At
narrow RBW route, 60.7 MHz second IF signal is down converted to 10.7 MHz.
This 10.7/66 MHz signal is fed to A05 SCAN/AD through various Resolution Width
setting circuits.
In the A05 SCAN/AD, RBW processed 10.7/66-MHz signal passes through 10.7/66-
MHz logarithmic amplifiers and then to linear detectors. This linear detected signal
is passed through smoothing filters called Video Band Width Filters (VBW).
This smoothed signal is then passed through Positive or Negative peak detection
circuits and output is converted to digital signal by high-resolution Analog to Digital
Converter (ADC) or high-speed ADC.
In the A07 MEAS DSP, the digital signal from the A05 SCAN/AD is processed by
DSP. A07 MEAS DSP also generates various commands required for controlling or
setting of all hardware units inside the instrument.
The A03 MAIN CPU controls various interface functions such as reading the Key
Inputs or remote commands received, and various outputs such as prints of various
data.
The A02 FONT BOAD generates the Key and rotary-knob encoder data, drives the
LCD, detects the power switch setting, and controls the power-supply On/Stby
setting.
The INVERTER supplies power for LCD backlight.

Section 1 Description of Module Operation
1-6
1.1.3 Outline for MS2687A
Please refer to Fig. 1-3 Overall Block Diagram.
The MS2687A is a superheterodyne system scanning-type spectrum analyzer.
The RF input signal after passing through the SWITCHED ATTENUATOR is
switched by DIPLEXING Band SW to two different signal routes according to the
measurement frequency range.
For RF input frequency of 9 kHz to 3.2 GHz (Termed as Band 0) the signal pass
through the Coaxial LPF and then to A10 3 GHz CONVERTER.
For RF input frequency of 3.15 to 3.0 GHz (Bands 1–, 1+, 2+, 3+, 4+) the signal goes
to a frequency tuned filter (YTF) and then to the H.Mixer.
In the H.Mixer, RF signal gets mixed with first Local Oscillator signal to generate a
460.7 or 466 MHz First IF signal. And this First IF signal is fed to the A13 2ND
CONVERTER.
In the A10 3 GHz CONVERTER, RF input signal gets mixed with first Local
Oscillator signal to generate a 4060.69 or 4066 MHz first IF signal by first mixer.
After passing several filters and amplifier, First IF signal gets mixed with second
Local Oscillator signal (4000 MHz) to generate a 60.7 or 66 MHz second IF signal
by second mixer, and fed to A06 IF.
The first Local signal generated at YTO (YIG tuned oscillator) is frequency-swept by
scan signal from A05 SCAN/AD after phase-lock to reference signal (its frequency is
26 to 32 MHz) generated on A08 LOCAL02 at the center frequency of its sweeping
range, in normal sweep condition.
The YTO output is passed through an amplifier, and then divided into two paths with
a directional coupler. One of divided signal is fed to sampler circuit, sampling
signal (its frequency is 194 to 206 MHz) generated on A08 LOCAL02 is frequency
multiplied, and mixed with YTO output to generate sampler IF signal with a
frequency of 26 to 32 MHz.
“The first Local signal A10 3 GHz CONVERTER through A14 1ST LO/EXT LO
AMP, and is fed to H.Mixer directly or through the Switched Doubler (Option 22
only).

1.1 Outline
1-7
If frequency Band is selected to Ext Band (bands:K,Ka,Q,U,V,E,W), the first Local
signal is switched by A14 1ST LO AMP/EXT LO AMP to 1st Lo Output, and 460.7
or 466 MHz First IF signal from External mixer is fed to the A13 2ND
CONVERTER.”
The 4-GHz second local signal is phase-locked to 100-MHz VCXO signal, of which
the frequency is also phase-locked to 10-MHz crystal oscillator.
In the instrument, high accuracy 625-kHz/50-MHz signals are present for calibration.
These signals are generated by dividing the reference signal, and the CAL ATT
varies power level of 625-kHz signal with 1-dB steps.
Internal calibration operation being carried out, these calibration signal is fed to RF
signal-route through the switch in Switched ATTENUATOR.
In the A13 2ND CONVERTER, 460.7 or 466 MHz first IF signal gets mixed with
second Local Oscillator signal (400 MHz) to generate a 60.7 or 66 MHz second IF
signal by second mixer after passing several band-pass filters and amplifiers, and fed
to A06 IF.
66 MHz second IF signal is selected when resolution band width (RBW) setting is 1,
3, 5, 10, or 20 MHz. For other settings, 60.7 MHz is selected.
At A06 IF unit, the incoming signal is divided in two paths. The main route leads to
image rejection filters; while second, highly attenuated feeler path signal (only 66
MHz) is fed to A05 SCAN/AD. Second route is not used in spectrum analyzer
mode.
The main signal after passing through image rejection filters is switched to two
different signal routes according to RBW setting. One is 66-MHz wide RBW route
(RBW ≥1 MHz), the other is 60.7-MHz narrow RBW route (RBW <1 MHz). At
narrow RBW route, 60.7 MHz second IF signal is down converted to 10.7 MHz.
This 10.7/66 MHz signal is fed to A05 SCAN/AD through various Resolution Width
setting circuits.
In the A05 SCAN/AD, RBW processed 10.7/66-MHz signal passes through 10.7/66-
MHz logarithmic amplifiers and then to linear detectors. This linear detected signal
is passed through smoothing filters called Video Band Width Filters (VBW).
This smoothed signal is then passed through Positive or Negative peak detection
circuits and output is converted to digital signal by high-resolution Analog to Digital
Converter (ADC) or high-speed ADC.

Section 1 Description of Module Operation
1-8
In the A07 MEAS DSP, the digital signal from the A05 SCAN/AD is processed by
DSP. A07 MEAS DSP also generates various commands required for controlling or
setting of all hardware units inside the instrument.
The A03 MAIN CPU controls various interface functions such as reading the Key
Inputs or remote commands received, and various outputs such as prints of various
data.
The A02 FONT BOAD generates the Key and rotary-knob encoder data, drives the
LCD, detects the power switch setting, and controls the power-supply On/Stby
setting.
The INVERTER supplies power for LCD backlight.

1.1 Outline
1-9
Fig. 1-1

Section 1 Description of Module Operation
1-10

1.1 Outline
1-11

Section 1 Description of Module Operation
1-12