Fluke PM6685 User manual

Programmable
Frequency Counter
PM6685 & PM6685R
Service Manual

4822 872 20106
First Edition (May 2003)
No part of this manual may be copied without the express permission of the copyright owner.
All product names are trademarks of their respective companies.
Ó2003 Pendulum Instruments AB
All rights reserved. Printed in Sweden.
This is a complementary service manual covering instruments with manufacturing numbers exceeding 840684. The principal
differences are to be found in Chapter 7 and in Chapter 8 due to a major redesign of the main PCB.
Do not dispose of the previous edition, identified by the part number, 4822 872 25012, and the publishing date, June 1996.
You may have to refer to it for information on older instruments as well as options not mentioned here.

Contents
1 Safety Instructions
2 Performance Check
General Information .........................2-2
Recommended Test Equipment ................2-2
Front Panel Controls .........................2-2
Short Form Specification Test ..................2-3
Rear Input/Output ...........................2-4
Measuring Functions .........................2-4
Options ...................................2-5
3 Disassembly
Removing the Cover .........................3-2
Reinstalling the Cover ........................3-2
PM9624 (HF Input) ..........................3-2
PM9626B (GPIB Interface) ....................3-3
PM9691 or PM9692 (Oven Oscillator) ...........3-3
4 Circuit Descriptions
Block Diagram Description ....................4-2
General ..................................4-2
Hardware Functional Description ...............4-4
Front Unit .................................4-4
Main Board ................................4-5
Rear Panel Unit ............................4-13
Optional Units .............................4-14
Software Functional Description ..............4-15
Test Routines .............................4-16
5 Repair
Preventive Maintenance.......................5-2
Calibration .................................5-2
When to Replace the Fan
(PM6685R only ) ............................5-3
Troubleshooting .............................5-4
General ...................................5-4
Safety Inspection and Test After Repair .........5-9
General Directives...........................5-9
6 Calibration Adjustments
Introduction ................................6-2
Preparation ................................6-2
Power Supply ..............................6-2
Input Amplifier ..............................6-3
Reference Oscillators ........................6-4
Other Options ..............................6-6
7 Replacement Parts
Introduction ................................7-2
Mechanical Parts............................7-3
Main Board ................................7-6
Front Board ...............................7-12
GPIB Interface (PM9626B) ...................7-13
8 Drawings & Diagrams
How to read the diagrams.....................8-2
9 Appendix
How to Replace Surface Mounted Devices........9-2
Electrostatic discharge .......................9-3
Glossary ..................................9-4
Power Supply Switchmode Module .............9-5
Circuit Descriptions ..........................9-5
Repair ....................................9-6
Calibration Adjustments ......................9-7
Replacement Parts..........................9-8
PM6685R ..................................9-12
Introduction ...............................9-12
Performance Check ........................9-12
Functional Description.......................9-12
Calibration Adjustments .....................9-13
Replacement Parts .........................9-14

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Chapter 1
Safety Instructions

WARNING: These servicing instructions are for use
by qualified personnel only. To reduce the risk of
electric shock, do not perform any servicing other
than that specified in the Operating Manual unless
you are fully qualified to do so.
Authorized service and calibration of this instrument is available
through your Fluke representative. See address at the end of this
manual.
Read this chapter carefully before you check, adjust, or repair an in-
strument.
Caution and Warning Statements
You will find specific warning and caution statements where neces-
sary throughout the manual.
CAUTION: Indicates where incorrect operating proce-
dures can cause damage to, or destruction of,
equipment or other property.
WARNING: Indicates a potential danger that requires
correct procedures or practices in order to prevent
personal injury.
This Timer/Counter has been designed and tested in accordance with
safety class 1 requirements for Electronic Measuring Apparatus of
IEC (CENELEC) publication EN61010-1, and CSA 22.2
No. 1010-1, and has been supplied in a safe condition.
This manual contains information and warnings that should be fol-
lowed by the user and the service technician to ensure safe operation
and repair in order to keep the instrument in a safe condition.
WARNING: Opening instrument covers or removing
parts, except those to which access can be gained
by hand, is likely to expose high voltages which
can cause death.
The instrument must be disconnected from all voltage sources before
it is opened. Remember that the capacitors inside the instrument re-
tain their charge even if the instrument has been disconnected from
all voltage sources.
Grounding
This instrument is connected to ground via a sealed three-core power
cable, which must be plugged into socket outlets with protective
ground contacts. No other method of grounding is permitted for this
instrument.
The ground symbol on the rear panel indicates where the
protective ground lead is connected inside the instrument.
Never remove or loosen this screw.
When the instrument is brought from a cold to a warm environment,
condensation may cause hazardous conditions. Therefore, ensure
that the grounding requirements are strictly met.
Power extension cables must always have a protective ground con-
ductor.
Indicates that the operator should consult the manual.
WARNING: Any interruption of the protective ground
conductor inside or outside the instrument, or dis-
connection of the protec- tive ground terminal, is
likely to make the instrument dangerous. Do not in-
tentionally disrupt the protective grounding.
Disposal of Hazardous Materials
WARNING: Disposal of lithium batteries requires spe-
cial attention. Do not expose the batteries to heat
or put them under extensive pressure. These mea-
sures may cause the batteries to explode.
A lithium battery is used to power the nonvolatile RAM in this in-
strument. Our world suffers from pollution, so don’t throw batteries
into your wastebasket. Return used batteries to your supplier or to the
Fluke representative in your country.
Line Voltage
The instrument can be powered by any voltage between 90 and
265 VAC without range switching. This makes it suitable for all nom-
inal line voltages between 100 and 240 V.
nReplacing Components in Primary Circuits
Components that are important for the safety of this instrument may
only be replaced by components obtained from your local Fluke
representative. After exchange of the primary circuits, perform the
safety inspection and tests, as described in Chapter 5, “Repair”.
nFuses
This instrument is protected by an ordinary 1.6 A slow blow fuse
mounted inside the instrument. NEVER replace this fuse without
first examining the Power Supply Unit.
1-2 Safety Instructions,

Chapter 2
Performance Check

General Information
WARNING: Before turning on the instrument, ensure
that it has been installed in accordance with the In-
stallation Instructions outlined in Chapter 3 of the
Operators Manual.
This performance procedure is intended to:
–Check the instrument’s specification.
–Be used for incoming inspection to determine the acceptability
of newly purchased instruments and recently recalibrated in-
struments.
–Check the necessity of recalibration after the specified
recalibration intervals.
NOTE: The procedure does not check every facet of the in-
strument’s calibration; rather, it is concerned primarily
with those parts of the instrument which are essential
for determining the function of the instrument.
It is not necessary to remove the cover of the instrument to perform
this procedure.
If the test is started less than 20 minutes after turning on the instru-
ment, results may be out of specification, due to insufficient
warm-up time.
Recommended Test
Equipment
*) Two of the cables must have 10 ns difference in delay, for ex-
ample: 5 ns and 15 ns.
Preparations
Power up your instruments at least 20 minutes be-
fore beginning the tests to let them reach normal
operating temperature. Failure to do so may result
in certain test steps not meeting equipment specifi-
cations.
Front Panel Controls
Power-On Test
At power-on the counter performs an automatic self-test of the fol-
lowing:
–Microprocessor
–RAM
–ROM
–Measuring circuits
–Display
If a GPIB interface is installed, the GPIB address is displayed.
If there are any test failures, an error message is shown.
–Turn on the counter and check that all segments light up on the
display and that no error message appears.
Internal Self-Tests
The different built-in test routines invoked by the power-on test can
also be activated from the front panel as follows:
–Enter the Auxiliary Menu by pressing AUX MENU.
–Select the test submenu by pressing DATA ENTRY up or
down.
–Enter the test menu by pressing the ENTER key.
Selections for internal self-tests are:
1 TEST ALL (Test 2 to 5 in sequence)
2 TEST RO (ROM)
3 TEST RA (RAM)
4 TEST LOGIC (Measuring Logic)
5 TEST DISP (Display Test)
–Use DATA ENTRY up/down to select TEST ALL, then press
ENTER.
–If any fault is detected, an error message appears on the dis-
play and the program halts.
–If no faults are detected, the program returns to measuring
mode.
2-2 Performance Check, General Information
Type of instru-
ment
Required
Specifications
Suggested
Equipment
LF Synthesizer Square;
Sine up to 10 MHz
Power Splitter 50 WPM9584/02
T-piece
Termination 50 WPM9585
Reference oscilla-
tor
10 MHz ±0.1 Hz for
standard oscillator
Fluke counter with
calibrated option
PM9691
10 MHz ±0.01 Hz for
PM9691 & PM9692
Fluke PM6685R or
PM6681R
10 MHz ±0.0001 Hz
for PM6685R
Fluke 910R or Ce-
sium Standard
HF signal genera-
tor
0.5 GHz (no presc.)
3.3 GHz (option 10)
Pulse Generator 125 MHz
Oscilloscope with
probes
350 MHz
BNC cables 5 to 7 cables *
Table 2-1 Recommended Test Equipment.
Ω
Fig. 2-1 Text on the display.

Keyboard Test
The keyboard test verifies that the counter responds when you press
any key. To check the function behind the keys, see the tests further
on in this chapter.
Press the keys as described in the left column and look on the display
for the text, as described in the second column. Some keys change
more text on the display than described here. The display text men-
tioned here is the text mainly associated with the selected key.
NOTE: For the instrument to respond correctly, this test must
be carried out in sequence and you must start with the
preset (power-on) setting.
* The LSD may vary.
** MENU is not disabled by setting DEFAULT; press menu again.
Short Form Specification
Test
Sensitivity and Frequency Range
–Press the PRESET key to set the counter in the default setting.
Then confirm by pressing ENTER.
–Turn off AUTO.
–Select IMP A = 50 Wand maximum sensitivity.
–Connect a signal from a HF generator to a BNC power splitter.
–Connect the power splitter to your counter and an oscilloscope.
–Set input impedance to 50 Won the oscilloscope.
–Adjust the amplitude according to the following table. Read
the level on the oscilloscope. The counter should display the
correct frequency.
Reference Oscillators
X-tal oscillators are affected by a number of external conditions,
such as ambient temperature and supply voltage, but they are also af-
fected by aging. Therefore, it is hard to give limits for the allowed
frequency deviation. You must decide the limits depending on your
application, and recalibrate the oscillator accordingly. See the Pre-
ventive Maintenance in the Repair chapter, Chapter 5.
To check the accuracy of the oscillator you must have a calibrated
reference signal that is at least five times as stable as the oscillator
that you are testing, see the following table.
–Press the PRESET key, then press the ENTER key to set your
counter in the Default setting.
Performance Check, Short Form Specification Test 2-3
Key(s) Display Note Pass
/Fail
STAND-BY Display Off Red LED
beside
the key
On
ON Backlight on
PRESET
ENTER
DEFAULT?
NO SIGNAL
Default
setting
EXT REF EXT REF
Input A
FILTER FILTER
50 W50 W
(2 times)
SENS 
(2 times)
Bar graph:
zzzzzzzz
SENS 
(2 times)
Bar graph:
zzzzzzzzzz
AUTO AUTO TRIG
Other
PRESET
ENTER
DEFAULT?
NO SIGNAL
Default
setting
MEAS TIME 200–3 s
DATA ENTRY 500–3 s
DATA ENTRY 200–3 s
ENTER NO SIGNAL
DISPLAY HOLD HOLD
DISPLAY HOLD
SINGLE SINGLE
FUNCTION DUTY F A
FUNCTION TOT A MAN
FUNCTION DUTY F A
FUNCTION FREQ A
AUX MENU RECALL
MEAS RESTART NO SIGNAL
PRESET
ENTER
DEFAULT?
NO SIGNAL
Default
setting
CHECK 10.000000006Hz* Start
counting
NULL NULL
NULL 10.000000006Hz*
BLANK DIGITS
(3 times)
10.00000___
6Hz*
MENU Displays all avail-
able functions, pro-
cesses and input
controls. Selected
items are blinking.
Table 2-2 Keyboard Test.
Frequency Level Pass/Fail
MHz mVPP mVRMS dBm Input A
1 30 10 –27
25 30 10 –27
50 30 10 –27
150 60 20 –21
200 90 30 –17
250 150 50 -13
300 150 50 -13
Table 2-3 Sensitivity of input A at various frequencies.
Oscillator Max. tempera-
ture dependence
Max. aging
per month
Max. aging
per year
Standard ±100 Hz ±5 Hz ±50 Hz
PM9691 ±0.05 Hz ±0.1 Hz ±0.75 Hz
PM9692 ±0.025 Hz ±0.03 Hz ±0.2
Rubidium ±0.003 Hz ±0.0005 Hz ±0.002 Hz
Table 2-4 Deviation (for PM9691 and PM9692 after a
warm-up period of 48 hours).

–Connect the reference to input A.
–Check the readout against the accuracy requirements of your
application.
nAcceptance Test
As an acceptance test, the following table gives a worst case figure
after a 30 minute warm up time. All deviations that can occur in a
year are added together.
nAcceptance Test, PM6685R
To fully test the accuracy of the PM6685R, a reference signal of ex-
tremely high stability is needed. Examples of such references are
Cesium Atomic references, or transmitted signals from a nationally
or internationally traceable source, like the GPS satellites.
Recommended Test Equipment
Type Stability Model
10 MHz refer-
ence
£1x10-10 910R with satellite contact dur-
ing the last 72 hours.
Test Procedure
–Connect the counter to the line power.
–Check that the UNLOCK indicator turns on, and then turns off
again within 6 minutes after connecting line power.
–Connect the 10 MHz reference signal to input A of the counter.
–Select FREQUENCY A measurement.
–Select 2 s measuring time.
–Check that the displayed frequency is 10.00000000 MHz
±0.05 Hz £10 minutes after connection to line power.
Rear Input/Output
INT REF Output
–Connect an oscilloscope to the 10 MHz output on the rear of
the counter. Use coaxial cable and 50 Wtermination.
–The output voltage is sinusoidal and should be above 2.8 VPP.
EXT REF Input
–Press the PRESET key, then press the ENTER key to set your
counter in the Default setting.
–Apply 10 MHz sine to input A equipped with a T-piece and to
Ext Ref input at the rear, terminated with 50 W. Amplitude on
10 MHz signal; 200 mVRMS, (560 mVPP)
–Press the Ext Ref key.
–The display should show 10.000000006Hz ± 5 LSD.
EXT ARM INPUT
–Press the PRESET key, then press the ENTER key to set your
counter in the Default setting.
–Select 50 Winput impedance.
–Apply 10 MHz 500 mVRMS, (1.4 VPP) sine to input A
–The counter measures and displays 10 MHz.
–Press the AUX MENU key.
–Press the DATA ENTRY UP/DOWN keys until the display
shows ‘Ar. Start’, confirm by pressing the ENTER key.
–Press DATA ENTRY UP/DOWN keys until the display shows
‘POS’, confirm by pressing the ENTER key.
–Press the ENTER key once more.
–The counter does not measure.
–Connect a pulse generator to Ext Arm input.
–Settings for pulse generator: single shot pulse, amplitude TTL
=0-2V
PP, and duration = 10 ns.
–Apply one single pulse to Ext Arm input.
–The counter measures once and shows 10 MHz on the display.
Measuring Functions
Preparation for Check of Measuring Function is as follows:
–Connect a 10 MHz sine wave signal with 2.0 VPP amplitude
via a T-piece to Input A.
–Connect a cable from the T-piece to Input E (Ext Arm) at the
rear.
–Select the measuring function as in the ‘Selected Function’ col-
umn and check that the counter performs the correct measure-
ment by displaying the result as shown under the “Display”
column in the following table.
1) Value depends on the symmetry of the signal.
2) Exact value depends on the input signal.
2-4 Performance Check, Rear Input/Output
Oscillator Frequency readout Suitable refer-
ence
Pass
/Fail
Standard 10.00000000 MHz
±120 Hz
PM9691
PM9691 10.00000000 MHz
±1 Hz PM6685R
PM6681R
PM9692 10.00000000 MHz
±0.25 Hz
Table 2-5 Acceptance test for oscillators.
Selected Function Display Pass/ Fail
PRESET
ENTER
DEFAULT?
10 MHz 2)
IMPA50W10 MHz 2)
Non AUTO 10 MHz 2)
PER A 100 ns 2)
RATIO A/E 1.0000000
PWIDTH A 50 ns 1)
TOT A MAN
DISPLAY HOLD Start counting
DISPLAY HOLD Stop counting
DUTY FACT 0.500000 1)
AUTO 0.500000 1)
Table 2-6 Measuring functions check.