Tektronix CMC251 User manual

User Manual
CMC251
1.3 GHz Frequency Counter
070-8527-03

Copyright Tektronix, Inc. 1991. All rights reserved.
Tektronix products are covered by U.S. and foreign patents, issued and
pending. Information in this publication supercedes that in all previously
published material. Specifications and price change privileges reserved.
Tektronix, Inc., P.O. Box 1000, Wilsonville, OR 97070–1000
TEKTRONIX and TEK are registered trademarks of Tektronix, Inc.

WARRANTY
Tektronix warrants that this product will be free from defects in materials and
workmanship for a period of one (1) year from the date of shipment. If any such product
proves defective during this warranty period, Tektronix, at its option, either will repair the
defective product without charge for parts and labor, or will provide a replacement in
exchange for the defective product.
In order to obtain service under this warranty, Customer must notify Tektronix of the defect
before the expiration of the warranty period and make suitable arrangements for the
performance of service. Customer shall be responsible for packaging and shipping the
defective product to the service center designated by Tektronix, with shipping charges
prepaid. Tektronix shall pay for the return of the product to Customer if the shipment is to
a location within the country in which the Tektronix service center is located. Customer
shall be responsible for paying all shipping charges, duties, taxes, and any other charges for
products returned to any other locations.
This warranty shall not apply to any defect, failure or damage caused by improper use or
improper or inadequate maintenance and care. Tektronix shall not be obligated to furnish
service under this warranty a) to repair damage resulting from attempts by personnel other
than Tektronix representatives to install, repair or service the product; b) to repair damage
resulting from improper use or connection to incompatible equipment; or c) to service a
product that has been modified or integrated with other products when the effect of such
modification or integration increases the time or difficulty of servicing the product.
THIS WARRANTY IS GIVEN BY TEKTRONIX WITH RESPECT TO THIS
PRODUCT IN LIEU OF ANY OTHER WARRANTIES, EXPRESSED OR
IMPLIED. TEKTRONIX AND ITS VENDORS DISCLAIM ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
PURPOSE. TEKTRONIX’ RESPONSIBILITY TO REPAIR OR REPLACE
DEFECTIVE PRODUCTS IS THE SOLE AND EXCLUSIVE REMEDY
PROVIDED TO THE CUSTOMER FOR BREACH OF THIS WARRANTY.
TEKTRONIX AND ITS VENDORS WILL NOT BE LIABLE FOR ANY
INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
IRRESPECTIVE OF WHETHER TEKTRONIX OR THE VENDOR HAS
ADVANCE NOTICE OF THE POSSIBILITY OF SUCH DAMAGES.

CMC251 User Manual i
Table of Contents
General Safety Summary iii. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Getting Started 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preparing the Frequency Counter for Use 2. . . . . . . . . . . . . . . . .
Front Panel 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Rear Panel 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reference 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Making Frequency Measurements 9. . . . . . . . . . . . . . . . . . . . . . .
Frequencies Below 10 MHz 9. . . . . . . . . . . . . . . . . . . . . . . . .
Frequencies Below 100 MHz 10. . . . . . . . . . . . . . . . . . . . . . . .
Frequencies Between 80 MHz and 1.3 GHz 11. . . . . . . . . . . .
Making Period Measurements 11. . . . . . . . . . . . . . . . . . . . . . . . . .
Making Pulse-Width Measurements 13. . . . . . . . . . . . . . . . . . . . .
Remote Control of Pulse-Width Measurements 14. . . . . . . . .
Counting Events 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Remote Control of TOTAL Mode 16. . . . . . . . . . . . . . . . . . . .
Checking Instrument Operation 17. . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A: Specifications 19. . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix B: Maintenance 25. . . . . . . . . . . . . . . . . . . . . . . . . . .
Cleaning 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preparing for Shipment 25. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Troubleshooting 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
No Display with Power On 26. . . . . . . . . . . . . . . . . . . . . . . . .
Appendix C: Replaceable Parts 27. . . . . . . . . . . . . . . . . . . . . . .
Standard Accessories 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Optional Accessories 27. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table of Contents
ii CMC251 User Manual
List of Tables
Table 1: A or B Channel Input 9. . . . . . . . . . . . . . . . . . . . . . .
Table 2: Selecting Features For Channel A
(1 Hz to 10 MHz) 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Table 3: Selecting Features For Channel A
(10 MHz to 100 MHz) 10. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Table 4: Gate Time Versus Resolution for Channel A 11. . . . .
Table 5: Gate Time Versus Resolution for Channel B 11. . . . .
Table 6: Period Measurements 12. . . . . . . . . . . . . . . . . . . . . . . .
Table 7: Resolution For Period Measurements 12. . . . . . . . . . .
Table 8: Pulse Width Measurements 13. . . . . . . . . . . . . . . . . . .
Table 9: Resolution for Pulse Measurements 14. . . . . . . . . . . .
Table 10: Warranted Characteristics 19. . . . . . . . . . . . . . . . . .
Table 11: Certifications and Compliances 23. . . . . . . . . . . . . .
Table 12: Typical Characteristics 23. . . . . . . . . . . . . . . . . . . . .
Table 13: Standard Accessories 27. . . . . . . . . . . . . . . . . . . . . . .
Table 14: Optional Accessories 27. . . . . . . . . . . . . . . . . . . . . . .
Table 15: Accessory Power Cords 28. . . . . . . . . . . . . . . . . . . . .

CMC251 User Manual iii
General Safety Summary
Review the following safety precautions to avoid injury and prevent
damage to this product or any products connected to it.
Injury Precautions
Use Proper Power Cord
To avoid fire hazard, use only the power cord specified for this
product.
Avoid Electric Overload
To avoid electric shock or fire hazard, do not apply a voltage to a
terminal that is outside the range specified for that terminal.
Ground the Product
This product is grounded through the grounding conductor of the
power cord. To avoid electric shock, the grounding conductor must
be connected to earth ground. Before making connections to the
input or output terminals of the product, ensure that the product is
properly grounded.
Do Not Operate Without Covers
To avoid electric shock or fire hazard, do not operate this product
with covers or panels removed.
Use Proper Fuse
To avoid fire hazard, use only the fuse type and rating specified for
this product.

General Safety Summary
iv CMC251 User Manual
Do Not Operate in Wet/DampConditions
To avoid electric shock, do not operate this product in wet or damp
conditions.
Do Not Operate in Explosive Atmosphere
To avoid injury or fire hazard, do not operate this product in an
explosive atmosphere.
Product Damage Precautions
Use Proper Voltage Setting
Before applying power, ensure that the line selector is in the proper
position for the power source being used.
Provide Proper Ventilation
To prevent product overheating, provide proper ventilation.
Do Not Operate With SuspectedFailures
If you suspect there is damage to this product, have it inspected by
qualified service personnel.
Safety Terms and Symbols
Terms in This Manual
These terms may appear in this manual:
WARNING. Warning statements identify conditions or practices that
could result in injury or loss of life.

General Safety Summary
CMC251 User Manual v
CAUTION. Caution statements identify conditions or practices that
could result in damage to this product or other property.
Terms on the Product
These terms may appear on the product:
DANGER indicates an injury hazard immediately accessible as you
read the marking.
WARNING indicates an injury hazard not immediately accessible as
you read the marking.
CAUTION indicates a hazard to property including the product.
Symbols on the Product
The following symbols may appear on the product:
DANGER
High Voltage ProtectiveGround (Earth)Terminal ATTENTION Referto
Manual
Double
Insulated
Certifications and Compliances
CSA Certified Power Cords
CSA Certification includes the products and power cords appropriate
for use in the North America power network. All other power cords
supplied are approved for the country of use.

General Safety Summary
vi CMC251 User Manual

CMC251 User Manual 1
Getting Started
The Tektronix CMC251 Frequency Counter can make the following
frequency-related measurements:
HFrequency measurements up to 1.3 GHz
HPeriod measurements at rates up to 25 MHz
HPeriod averaging of up to 1000 events
HPulse measurements up to 0.25 s
HEvent counting
The 10 MHz internal time base (using a highly stabilized crystal
oscillator) provides a single-shot resolution of 100 ns. Measurement
results are displayed using an eight digit display, automatically
placing the decimal appropriately.
The CMC251 Frequency Counter has a locking, multiposition handle
that folds under the instrument. The instrument is delivered with a
115 V power cord, an installed fuse for 115 V operation, and this
manual.

Getting Started
2CMC251 User Manual
Preparing the Frequency Counter for Use
Check the following items prior to operating the frequency counter
for the first time (see Figure 1 for locations of items 1 through 3):
1 23
Figure 1: Line Voltage Selectors, Power Input, and Fuse Locations
CAUTION. To prevent damage to the instrument, set the line voltage
selectors to the proper voltage setting and install the correct line
voltage fuse before operating the equipment.
1. Set the line voltage selectors to the input line voltage. These
selectors connect internal wiring for various line voltages. This
product is intended to operate from a power source that does not
supply more than 250 VRMS between the supply conductors or
between either supply conductor and ground. For line voltage
ranges, refer to Appendix A: Specifications on page 19.
WARNING. To prevent electrical shock, unplug the power cord and
disconnect the signal input cable from any signal source before
checking or replacing the fuse.
2. Check that the correct line fuse is installed. The line fuse
provides protection if the equipment malfunctions or an overload
occurs. Refer to Appendix C: Replaceable Parts on page 27 for
fuse part numbers.

Getting Started
CMC251 User Manual 3
WARNING. To prevent electrical shock, connect the power cord to a
properly grounded power source. The outside (ground) of this
connector is connected through the equipment to the power source
ground. Do not remove the ground lug from the power cord for any
reason.
3. Connect the input power cord. Use only the power cords specified
for this equipment. Refer to Appendix C: Replaceable Parts on
page 27 for power cord part numbers.
Front Panel
Figure 2 shows the front panel controls, connectors, and indicators
with brief descriptions of the items following the figure.
17
12345
16 15 14 13 12 11 10 9 8 7 6
Figure 2: Front Panel Controls, Connectors, and Indicators
CAUTION. To avoid the risk of fire or possible damage to the
CMC251, be sure that the equipment generating the signal is
properly grounded before you make any connections to the inputs.
1. CHANNEL B INPUT Button. Use this input connector to
measure signals with a repetition rate from 80 MHz to 1.3 GHz.

Getting Started
4CMC251 User Manual
2. OVERRANGE Indicator. Lights when frequencies above the
specified limits are applied to either channel.
3. Display. The display area provides measurement information on
the digital display. Single LEDs indicate the units and conditions
of the display.
4. kHzs IndicatorLights when a frequency mode is selected and
the PRESCALE button is in its out position, indicating that the
reading is in kilohertz. When the period or pulse mode is
selected, the LED lights to indicate the reading is in
microseconds.
5. MHz Indicator. Lights when a frequency mode is selected and the
PRESCALE button is pressed in, indicating that the reading is in
megahertz.
6. GATE Indicator. Lights while the frequency counter is making a
measurement. The LED extinguishes when updating the display.
7. Gate Resolution Indicators. These LEDs light to indicate the
present gate resolution settings (X1, X10, X100, or X1000).
8. POWER Switch. Powers the instrument on and off.
9. GATE Button. Determines the display’s degree of resolution by
setting the gate time (the amount of time the counter uses to
derive its displayed value). The selected resolution is indicated by
the lighted LED. Each press of the button changes the degree of
resolution to the next level.
10.FUNC Button. Determines the operating mode of the CMC251
Frequency Counter.
11. HOLD Button. Stops or locks the display when pressed in.
Toggling the button to its out position allows the measurement to
continue. When using the TOTAL mode, pressing the HOLD
button freezes the display but event counting continues.
12.RESET Button. Sets the display to all zeros; new measurements
will be displayed after the button is released.

Getting Started
CMC251 User Manual 5
13.PRESCALE Button. Divides the frequency of the signal applied
to CHANNEL A INPUT by a factor of ten and sets the measure-
ment units indicator to either MHz or kHzs. When pushed in,
frequencies up to 100 MHz or periods up to 0.04 s can be
measured. In the out position, frequencies up to 10 MHz or
periods up to 0.4 s can be measured.
14.Mode Indicators. These LEDs light to indicate the present mode
settings. One of the following modes is indicated:
HFREQ A—The Frequency A mode measures the repetition
rate of the signal applied to the CHANNEL A INPUT. In this
mode, measurements from 1 Hz to 100 MHz can be made.
HFREQ B—The Frequency B mode measures the repetition
rate of the signal applied to the CHANNEL B INPUT. In this
mode, measurements from 80 MHz to 1.3 GHz can be made.
HPERIOD A—The Period A mode measures the time it takes
for one cycle of the signal applied to CHANNEL A INPUT to
be completed.
HPULSE A—The Pulse A mode measures the width of the
positive pulses of the signal applied to CHANNEL A INPUT.
The Pulse mode can also measure the negative pulse of the
input signal when using the external control input connector
located on the rear panel. See Making Pulse-Width Measure-
ments on page 13 for a description of its use.
HTOTAL—The Totalize mode counts signal events applied to
CHANNEL A INPUT, incrementing one count for each
event. The display is updated continuously unless the HOLD
button is pressed. The count resumes when the HOLD button
is released. The Totalize mode can also be controlled
remotely with the use of the external control input connector
located on the rear panel.
HCHECK—The Check mode uses the internal time base as a
signal source to make a general check of the operating
system. The display will indicate a frequency of 10 MHz.

Getting Started
6CMC251 User Manual
15.LOW PASS FILTER Button. Attenuates signals applied to
CHANNEL A INPUT that are above 100 kHz, preventing
high-frequency interference. Select this mode to measure signals
below 100 kHz.
16.INPUT VOLTAGE Button. Allows you to select between two
levels of input signals for the CHANNEL A INPUT. When
pushed in, the maximum allowable peak-to-peak input level is
from 3 V to 42 V (30 Vrms). When in the out position, the
maximum allowable peak-to-peak input level is from 50 mV to
5V.
CAUTION. To avoid the risk of fire or possible damage to the
CMC251, be sure that the equipment generating the signal is
properly grounded before you make any connections to the inputs.
17.CHANNEL A INPUT. Use this connector to measure signals with
a repetition rate from 1 Hz to 100 MHz.
Rear Panel
In addition to the line voltage selectors, power input, and fuse, there
is a BNC connector for an external control input located on the rear
panel
CAUTION. To avoid the risk of fire or possible damage to the
CMC251, be sure the input equipment is properly grounded before
making any connections. The external input connector is grounded to
the power source ground.
The external control input (see Figure 3, item 1) provides remote
control of two instrument functions: TOTAL and PULSE modes.

Getting Started
CMC251 User Manual 7
1
Figure 3: Rear Panel Signal Connections
In Total mode, the TTL logic levels start (TTL high) and stop (TTL
low) the display updates. This performs the same function as the
HOLD button.
In Pulse mode, the TTL logic levels select either negative (TTL low)
or positive (TTL high) pulse width measurement.

Getting Started
8CMC251 User Manual

CMC251 User Manual 9
Reference
This section describes typical setups of the CMC251 Frequency
Counter for each of its measurement modes.
Making Frequency Measurements
Measuring frquencies with the CMC251 Frequency Counter is
simple, although there are some items you need to consider before
applying the signal to the inputs of the frequency counter.
1. Estimate the frequency of the signal source and determine the
voltage level.
This enables you to use the appropriate input channel of the
frequency counter. A good practice is to display the signal on an
oscilloscope and obtain the peak voltage level. Refer to Table 1
and pick the appropriate input channel.
Table
1
:
A
o
r
B Ch
a
nn
el
Input
EstimatedFrequency Input Voltage Proper Input Channel
1 Hz to 100 MHz Less than 42 V peak CHANNEL A INPUT
80 MHz to 1.3 GHz Less than 1 VRMS CHANNEL B INPUT
2. Prepare to make a measurement, using the steps under one these
sections: Frequencies Below 10 MHz, Frequencies Below
100 MHz, or Frequencies Between 80 MHz and 1.3 GHz.
Frequencies Below 10 MHz
1. Press the FUNC button until FREQ A is indicated with a lighted
LED.
2. Refer to Table 2 and set the frequency counter front-panel buttons
to obtain the best measurement for frequencies below 10 MHz.

Reference
10 CMC251 User Manual
Table
2
: S
ele
ct
i
ng F
ea
tu
re
s Fo
r
Ch
a
nn
el
A
(
1
Hz to
10
MHz)
50 mV to 5 V 3 V to 42 V
INPUT VOLTAGE LO (button out) HI (button in)
LOW PASS FILTER ON (button in) ON (button in)
PRESCALE Button Out Button Out
3. Connect the signal to the CHANNEL A INPUT connector.
4. Press the GATE button to select the desired display resolution.
See Tables 4 and 5 for resolution-versus-gate settings.
Frequencies Below 100 MHz
1. Press the FUNC button until FREQ A is indicated with a lighted
LED.
2. Refer to Table 3 and set the frequency counter front-panel buttons
to obtain the best measurement for frequencies between 10 MHz
and 100 MHz.
Table
3
: S
ele
ct
i
ng F
ea
tu
re
s Fo
r
Ch
a
nn
el
A
(
10
MHz to
100
MHz)
50 mV to 5 V 3 V to 42 V
INPUT VOLTAGE LO (button out) HI (button in)
LOW PASS FILTER OFF (button out) OFF (button out)
PRESCALE Button In Button In
3. Connect the signal to the CHANNEL A INPUT input connector.
4. Press the GATE button to select the desired display resolution.
See Tables 4 and 5 for resolution versus gate settings.