1. GENERAL INFORMATION User Manual CNT-90/91/91R/91RAF/90XL

About this Manual

This manual contains directions for use that apply to the Timer/Counter/Analyzers CNT-90 and CNT-91 as well as the Frequency Calibrator/Analyzer CNT-91R and CNT-91R/AF and the Mi­crowave Counter/Analyzer CNT-90XL.

In order to simplify the references, these instruments are further referred to throughout this manual as the ‘9X’, whenever the information applies to all types. Differences are clearly marked.

Examples:

  • CNT-90 means CNT-90 and CNT-90/XL
  • CNT-90/91 means CNT-90 and CNT-91
  • CNT-91(R) means CNT-91, CNT-91R and CNT-91R/AF

Chapter 8, Specifications is divided into four separate sections to increase legibility. Much of the contents is common, so redundant data is the price in this case.

Warranty

The Warranty Statement is part of the folder Important Information that is included with the shipment.

Declaration of Conformity

The complete text with formal statements concerning product identification, manufacturer and standards used for type testing is available on request.

2. Chapter 1: Preparation for Use

2.1. Preface

2.1.1. Introduction

Congratulations on your choice of instrument. It will serve you well and stay ahead of most competition for many years to come, whether in bench-top or rack system use. It gives significantly increased performance compared to traditional Timer/Counters. The ‘9X’ offers the following advantages:

  • 12 digits of frequency resolution per sec­ond and 50 or 100 ps resolution, as a re­sult of high-resolution interpolating recip­rocal counting.
  • A high measurement rate of up to 250k  readings/s to internal memory.
  • Optional oven-controlled timebase oscilla­tors, except the CNT-91R & CNT-91R/AF, which have an ultra-stable rubidium oscillator.
  • CNT-90, CNT-91(R): A variety of RF prescaler options with up­per frequency limits ranging from 3 GHz to 20 GHz.
  • CNT-91R/AF: Special version of CNT-91R with selected Rubidium oscillator, plus 5 Reference frequency outputs covering 100 kHz, 1 MHz, 5 MHz, and 10 MHz. Model CNT-91R/AF has a 3 GHz input C and CNT-91R/AF/20G has a 20 GHz input C as standard
  • CNT-90XL: A number of microwave inputs with upper frequency limits ranging from 27 to 60 GHz.
  • CNT-90(XL): Optional Pulsed RF for RF pulse characterization up to 60 GHz carrier frequency and down to 30 ns pulse width.
  • Optional built-in Li-Ion battery supply realizes instant high-precision measurements in the field and true UPS operation.
  • Integrated high performance GPIB inter­face using SCPI commands.
  • A fast USB interface that replaces the tra­ditional but slower RS-232 serial interface
  • Optional external GPIB-to Ethernet controller that allows connection to LAN .

2.1.1.1. Powerful and Versatile Functions

A unique performance feature in your new instrument is the comprehensive arming possibilities, which allow you to characterize virtually any type of complex signal concerning frequency and time.

For instance, you can insert a delay between the external arming condition and the actual arming of the counter. Read more about Arming in Chapter 5, “Measurement Control”.

In addition to the traditional measurement functions of a timer/counter, these instruments have a multitude of other functions such as phase, duty factor, rise/fall-time and peak voltage. The counter can perform all measurement functions on both main inputs (A & B). Most measurement functions can be armed, either via one of the main inputs or via a separate arming channel (E).

By using the built-in mathematics and statis­tics functions, the instrument can process the measurement results on your benchtop, with­out the need for a controller. Math functions include inversion, scaling and offset. Statistics functions include Max, Min and Mean as well as Standard and Allan Deviation on sample sizes up to 2*109.

2.1.1.2. No Mistakes

You will soon find that your instrument is more or less self-explanatory with an intuitive user interface. A menu tree with few levels makes the timer/counter easy to operate. The large backlit graphic LCD is the center of in­formation and can show you several signal pa­rameters at the same time as well as setting status and operator messages.

Statistics based on measurement samples can easily be presented as histograms or trend plots in addition to standard numerical mea­surement results like max, min, mean and standard deviation.

The AUTO function triggers automatically on any input waveform. A bus-learn mode sim­plifies GPIB programming. With bus-learn mode, manual counter settings can be trans­ferred to the controller for later reprogramming. There is no need to learn code and syn­tax for each individual counter setting if you are an occasional GPIB bus user.

2.1.2. Design Innovations

2.1.2.1. State of the Art Technology Gives Durable Use

These counters are designed for quality and durability. The design is highly integrated. The digital counting circuitry consists of just one custom-developed FPGA and a 32-bit microcontroller. The high integration and low component count reduces power consumption and results in an MTBF of 30,000 hours. Modern surface-mount technology ensures high production quality. A rugged mechanical construction, including a metal cabinet that withstands mechanical shocks and protects against EMI, is also a valuable feature.

2.1.2.2. High Resolution

The use of reciprocal interpolating counting in this new counter results in excellent relative resolution: 12 digits/s for all frequencies.

The measurement is synchronized with the in­put cycles instead of the timebase. Simulta­neously with the normal “digital” counting, the counter makes analog measurements of the time between the start/stop trigger events and the next following clock pulse. This is done in four identical circuits by charging an integrating capacitor with a constant current, starting at the trigger event. Charging is stopped at the leading edge of the first follow­ing clock pulse. The stored charge in the inte­grating capacitor represents the time differ­ence between the start trigger event and the leading edge of the first following clock pulse. A similar charge integration is made for the stop trigger event.

When the “digital” part of the measurement is ready, the stored charges in the capacitors are measured by means of Analog/Digital Converters.

The counter’s microprocessor calculates the result after completing all measurements, i.e. the digital time measurement and the analog interpolation measurements.

The result is that the basic “digital resolution” of + 1 clock pulse (10 ns) is reduced to 100 ps for the CNT-90 and 50 ps for the CNT-91(R).

Since the measurement is synchronized with the input signal, the resolution for frequency measurements is very high and independent of frequency.

CNT-91/91R features gap-free back-to-back frequency measurements, ensuring no missing periods The counters have 14 display digits to ensure that the display itself does not restrict the res­olution.

2.1.3. Remote Control

This instrument is programmable via two in­terfaces, GPIB and USB.

The GPIB interface offers full general func­tionality and compliance with the latest stan­dards in use, the IEEE 488.2 1987 for HW and the SCPI 1999 for SW.

In addition to this ‘native’ mode of operation there is also a second mode that emulates the Agilent 53131/132 command set for easy ex­change of instruments in operational ATE systems. The USB interface is mainly intended for the lab environment in conjunction with the op­tional TimeView™ analysis software. The communication protocol is a proprietary ver­sion of SCPI.

2.1.3.1. Fast GPIB Bus

These counters are not only extremely power­ful and versatile bench-top instruments, they also feature extraordinary bus properties.

The bus transfer rate is up to 4000 triggered measurements/s in CNT-91(R). Array mea­surements to the internal memory can reach 250 k measurements/s.

This very high measurement rate makes new measurements possible. For example, you can perform jitter analysis on several tens of thousands of pulse width measurements and cap­ture them in less than a second.

An extensive Programmer’s Handbook helps you understand SCPI and counter program­ming.

The counter is easy to use in GPIB environ­ments. A built-in bus-learn mode enables you to make all counter settings manually and transfer them to the controller. The response can later be used to reprogram the counter to the same settings. This eliminates the need for the occasional user to learn all individual pro­gramming codes.

Complete (manually set) counter settings can also be stored in 20 internal memory locations and can easily be recalled on a later occasion. Ten of them can be user protected.

2.2. Safety

2.2.1. Introduction

Even though we know that you are eager to get going, we urge you to take a few minutes to read through this part of the introductory chapter carefully before plugging the line con­nector into the wall outlet.

This instrument has been designed and tested for Measurement Category I, Pollution Degree 2, in accordance with EN/IEC 61010-1:2001 and CAN/CSA-C22.2 No. 61010-1-04 (in­cluding approval). It has been supplied in a safe condition. Study this manual thoroughly to acquire ade­quate knowledge of the instrument, especially the section on Safety Precautions hereafter and the section on Installation on page 1-7.

2.2.2. Safety Precautions

All equipment that can be connected to line power is a potential danger to life. Handling restrictions imposed on such equipment should be observed.

To ensure the correct and safe operation of the instrument, it is essential that you follow gen­erally accepted safety procedures in addition to the safety precautions specified in this manual.

These units are designed for indoor use only.

The instrument is designed to be used by trained personnel only. Removing the cover for repair, maintenance, and adjustment of the instrument must be done by qualified personnel who are aware of the hazards involved.

The warranty commitments are rendered void if unauthorized access to the interior of the instrument has taken place during the given warranty period.

2.2.2.1. Caution and Warning Statements

CAUTION: Shows where incorrect procedures can cause damage to, or destruction of equipment or other property.

WARNING: Shows a potential danger that requires correct procedures or practices to prevent personal injury.

2.2.2.2. Symbols

Shows where the protective ground terminal is connected inside the instrument. Never remove or loosen this screw

This symbol is used for identifying the functional ground of an I/O signal. It is always connected to the instrument chassis.

Indicates that the operator should consult the manual.

One such symbol is printed on the instrument, below the A and B inputs. It points out that the damage level for the input voltage decreases from 350 Vp to 12Vrms when you switch the input impedance from 1 MΩ to 50  Ω.

2.2.2.3. If in Doubt about Safety

Whenever you suspect that it is unsafe to use the instrument, you must make it inoperative by doing the following:

  • Disconnect the line cord
  • Clearly mark the instrument to prevent its further operation
Fig. 1‑1 Do not overlook the safety instructions!
  • Inform your Pendulum Instruments representative.

For example, the instrument is likely to be un­safe if it is visibly damaged.

2.2.2.4. Disposal of Hazardous Materials

CNT-90 & CNT-90XL only

If your instrument was ordered with a built-in battery supply (Option 23/90), it contains 12 Li-Ion cells arranged as a fixed battery pack with internal protection circuitry.

Even though this type of cell does not cause environmental damage in the same way as NiCd, for instance, you should dispose of a worn-out battery pack at an authorized recy­cling station or return it to Spectracom.

Individual cells cannot be replaced.

2.3. Unpacking

Check that the shipment is complete and that no damage has occurred during transportation. If the contents are incomplete or damaged, file a claim with the carrier immediately. Also notify your local Spectracom sales or service organization in case repair or replacement may be required.

2.3.1. Check List

The shipment should contain the following:

  • Counter/Timer/Analyzer CNT-90/91 or Frequency Calibrator/Analyzer CNT-91R or CNT-91R/AF or Microwave Coun­ter/Analyzer CNT-90XL
  • Line cord
  • Brochure with Important Information
  • Certificate of Calibration
  • Options you ordered should be installed. See Identification below.
  • CD including the following documentation in PDF:
    • Getting Started Manual
    • User’s Manual
    • Programmer’s Handbook

2.3.2. Identification

The type plate on the rear panel shows type number and serial number. See illustrations on page 2-5 and 2-6. Installed options are listed under the menu User Options – About, where you can also find information on firmware version and calibration date. See page 2-15. The CNT-91R/AF version is identified by a unique identification marking, or UID. This permanent tag contains a barcode and allows customers to track easily their inventory and property.

2.3.3. Installation

2.3.3.1. Supply Voltage

Setting

The Counter may be connected to any AC supply with a voltage rating of 90 to 265 Vrms, 45 to 440 Hz. The counter automatically adjusts itself to the input line voltage.

Fuse

The secondary supply voltages are electroni­cally protected against overload or short cir­cuit. The primary line voltage side is protected by a fuse located on the power supply unit. The fuse rating covers the full voltage range. Consequently, there is no need for the user to replace the fuse under any operating condi­tions, nor is it accessible from the outside.

CAUTION: If this fuse is blown, it is likely that the power supply is badly damaged. Do not replace the fuse. Send the counter to the local Service Center.

Removing the cover for repair, maintenance and adjustment must be done by qualified and trained personnel only, who are fully aware of the hazards involved.

The warranty commitments are rendered void if unauthorized access to the interior of the instrument has taken place during the given warranty period.

2.3.3.2. Battery Supply

CNT-90 & CNT-90XL only

It is possible to run the counter from an op­tional battery supply, Option 23/90. You must charge the battery before use or stor­age. The counter charges the battery automatically when connected to line power or an external DC source, whether the instrument is in standby or turned on. See the specifications for charging time in different modes of operation.

2.3.3.3. Grounding

Grounding faults in the line voltage supply will make any instrument connected to it dan­gerous. Before connecting any unit to the power line, you must make sure that the pro­tective ground functions correctly. Only then can a unit be connected to the power line and only by using a three-wire line cord. No other method of grounding is permitted. Extension cords must always have a protective ground conductor.

CAUTION: If a unit is moved from a cold to a warm environment, con­densation may cause a shock hazard. Ensure, therefore, that the grounding requirements are strictly met.

WARNING: Never interrupt the grounding cord. Any interruption of the protective ground connection inside or outside the instrument or disconnection of the protective ground terminal is likely to make the instrument dangerous.

2.3.3.4. Orientation and Cooling

The counter can be operated in any position desired. Make sure that the air flow through the ventilation slots at the top, and side panels is not obstructed. Leave 5 centimeters (2 inches) of space around the counter.

2.3.3.5. Fold-Down Support

For bench-top use, a fold-down support is available for use underneath the counter. This support can also be used as a handle to carry the instrument.

Fig. 1-2 Fold-down support for comfortable bench-top use.

2.3.3.6. Rackmount Adapter

Fig. 1-3 Dimensions for rackmounting hardware.

If you have ordered a 19-inch rack-mount kit for your instrument, it has to be assembled af­ter delivery of the instrument. The rackmount kit consists of the following:

  • 2 brackets, (short, left; long, right)
  • 4 screws, M5 x 8
  • 4 screws, M6 x 8

WARNING: Do not perform any internal service or adjustment of this instrument unless you are qualified to do so. Before you remove the cover, disconnect mains cord and wait for one minute. Capacitors inside the instrument can hold their charge even if the instru­ment has been separated from all voltage sources.

Fig. 1-4 Fitting the rack mount brackets on the counter.

2.3.3.7. Assembling the Rackmount Kit (Option 22/05)

1-7
  • Turn the devices upside down
  • Remove the rubber feet in the plastic stand
  • Loosen the screws underneath the rubber feet
  • Remove the plastic stands
  • Remove the four decorative plugs that cover the screw holes on the right and left side of the front panel.

Use the following steps to complete the side by side rack mount installation for your products. If necessary, refer to the item numbers in the following diagram for additional detail.

  • Determine where you would like each unit positioned (i.e., on the right or left side)
  • If plugs exist on the mounting holes on the front left and right side of product cover, remove and discard them
  • Using screwdriver, screw the rack ear (Item #2) into place using the supplied 10mm screws (Item #5)
  • Pinch the hinge pins together to separate the right and left hinge halves (Items #3 and 4)
  • Attach hinge halves to the unit with hinge facing towards the front (as displayed in diagram)
  • Using a screwdriver, remove the existing rear brackets on the back of each unit
  • Using existing machine screws removed in previous steps, attach the rear brackets supplied with the mounting kit (Item #1)
  • Pinch the hinge pins together into the stored position. Align the hinge halves together between the two units, and swing together side by side. The hinge pins should snap into place securing the front of the two units together
  • Take the supplied Hex Spacer (Item #7) and place between middle rear brackets, and secure using the supplied 8mm screws (Item #6)
  • Assembly is now ready for installation into standard 19” rack
1-8

3. Chapter 2: Using the Controls

3.1. Basic Controls

A more elaborate description of the front and rear panels including the user interface with its menu system follows after this introductory survey, the purpose of which is to make you familiar with the layout of the instrument. See also the appendix.

3.2. Secondary Controls

3.2.1. Connectors & Indicators

3.2.2. Rear Panel

Pulse Output [CNT-91(R) only]: User definable to serve as output for built-in pulse gener­ator, gate indicator or alarm.

Optional Main Input Connectors (not with Option 23/90): The front panel inputs can be moved to the rear panel by means of an optional cable kit. Note that the input capacitance will be higher.

Type Plate: Indicates instrument type and serial number.

Fan: A temp. sensor controls the speed of the fan. Normal bench-top use means low speed, whereas rack-mount­ing and/or options may result in higher speed.

Protective Ground Terminal: This is where the protective ground wire is connected inside the in­strument. Never tamper with this screw!

Line Power Inlet: AC 90-265 Vrms, 45-440 Hz, no range switching needed.

Reference Output: 10 MHz derived from the internal or, if present, the external reference.

External Reference Input: Can be automatically selected if a signal is present and approved as timebase source, see Chapter 9.

External Arming Input: See page 5-7.

GPIB Connector: Address set via User Options Menu.

Ext. DC Connector: Part of Option 23/90 for CNT-90(XL). Range: 12-18 V Note the polarity.

USB Connector: Universal Serial Bus (USB) for data communication with PC.

3.2.3. Rear Panel (CNT-91R/AF)

Pulse Output [CNT-91(R) only]: User definable to serve as output for built-in pulse gener­ator, gate indicator or alarm.

Additional output frequencies Connectors: These connectors provide additional out­put frequencies which are, from left to right, 100kHz, 1MHz, 5MHz and 10MHz.

Type Plate: Indicates instrument type and serial number.

Fan: A temp. sensor controls the speed of the fan. Normal bench-top use means low speed, whereas rack-mount­ing and/or options may result in higher speed.

Protective Ground Terminal: This is where the protective ground wire is connected inside the in­strument. Never tamper with this screw!

Line Power Inlet: AC 90-265 Vrms, 45-440 Hz, no range switching needed.

Reference Output: 10 MHz derived from the internal or, if present, the external reference.

External Reference Input: Can be automatically selected if a signal is present and approved as timebase source, see Chapter 9.

External Arming Input: See page 5-7.

GPIB Connector: Address set via User Options Menu.

USB Connector: Universal Serial Bus (USB) for data communication with PC.

3.3. Description of Keys

3.3.1. Power

The ON/OFF key is a toggling secondary power switch. Part of the instrument is always ON as long as power is applied, and this standby condition is indicated by a red LED above the key. This indicator is consequently not lit while the instrument is in operation.

CNT-91R and CNT-91R/AF only

While the rubidium oscillator is warming up, an open padlock symbol labeled RB is flash­ing at the top right corner of the display, indi­cating that the control loop is not locked. Normal time to lock is about 5 min. Do not start measuring until the unlock symbol disappears.

New Message Box

Information exchange between the rubidium oscillator and the CPU takes place over a serial bus. Any malfunction in the UART-con-trolled communication link will be reported in a pop-up message box on the display.

CNT-90(XL) w. Option 23/90

The User Interface Screens have two indica­tors near the upper right corner of the display. One is a power supply status indicator, and the other is a battery charging level indicator.

The status indicator shows:

  • a fixed battery symbol when the internal battery is the active power source
  • a charging battery symbol when the in­ternal battery is being charged
  • a power plug symbol when the mains is the active power source
  • a power plug symbol on top of a battery symbol when the instrument has been prepared for UPS operation and charging is not going on

The charging level indicator shows:

  • the relative charging level in percent

3.3.2. Select Function

This hard key is marked MEAS FUNC.

When you depress it, one of the menus below will open.

Fig. 2‑1 CNT-90: Select measurement function.
Fig. 2-2    CNT-90XL: Select measurement function.
Fig. 2-3    CNT-91(R): Select measurement function.

The current selection is indicated by text in­version that is also indicating the cursor posi­tion. Select the measurement function you want by depressing the corresponding softkey right below the display.

Alternatively, you can move the cursor to the wanted position with the RIGHT/LEFT arrow keys. Confirm by pressing ENTER.

A new menu will appear where the contents depend on the function. If you for instance have selected Frequency, you can then select between Frequency, Frequency Ratio and Frequency Burst. Finally you have to decide which input channel(s) to use.

3.3.3. Autoset/Preset

By depressing this key once after selecting the wanted measurement function and input channel, you will most probably get a measure­ment result. The AUTOSET system ensures that the trigger levels are set optimally for each combination of measurement function and input signal amplitude, provided rela­tively normal signal waveforms are applied. If Manual Trigger has been selected before pressing the AUTOSET key, the system will make the necessary adjustments once (Auto Once) and then return to its inactive condition.

AUTOSET performs the following functions:

  • Set automatic trigger levels
  • Switch attenuators to 1x
  • Turn on the display
  • Set Auto Trig Low Freq to
    • 100 Hz, if fin >100 Hz, or to
    • fin, if 10<fin<100 Hz, or to
    • 10 Hz, if fin <10 Hz

A higher value means faster settling time.

By depressing this key twice within two sec­onds, you will enter the Preset mode, and a more extensive automatic setting will take place. In addition to the functions above, the following functions will be performed:

  • Set Meas Time to 200 ms
  • Switch off Hold-Off
  • Set HOLD/RUN to RUN
  • Switch off MATH/LIM
  • Switch off Analog and Digital Filters
  • Set Timebase Ref to Auto
  • Switch off Arming

Default Settings

An even more comprehensive preset function can be performed by recalling the factory de­fault settings. See page 2-16.

3.3.4. Move Cursor

There are four arrow keys for moving the cur­sor, normally marked by text inversion, around the menu trees in two dimensions.

3.3.5. Display Contrast

When no cursor is visible (no active menu se­lected), the UP/DOWN arrows are used for adjusting the LCD display contrast ratio.

3.3.6. Enter

The key marked ENTER enables you to con­firm a choice without leaving your menu position.

3.3.7. Save & Exit

This hard key is marked EXIT/OK. You will confirm your selection by depressing it, and at the same time you will leave the current menu level for the next higher level.

3.3.8. Don't Save & Exit

This hard key is marked CANCEL. By de­pressing it you will enter the preceding menu level without confirming any selections made at the current level. If the instrument is in REMOTE mode, this key is used for returning to LOCAL mode, unless LOCAL LOCKOUT has been programmed.

3.3.9. Presentation Modes

VALUE

Fig. 2-4    Main and aux. parameters.

Value mode gives single line numerical pre­sentation of individual results, where the main parameter is displayed in large characters with full resolution together with a number of auxiliary parameters in small characters with limited resolution.

Fig. 2-5    Limits presentation.

If Limit Behavior is set to Alarm and Limit Mode is set to Range you can visualize the de­viation of your measurements in relation to the set limits. The numerical readout is now com­bined with a traditional analog pointer-type in­strument, where the current value is represented by a “smiley”. The limits are presented as numerical values below the main parameter, and their positions are marked with vertical bars labelled LL (lower limit) and UL (upper limit) on the autoscaled graph.

If one of the limits has been exceeded, the limit indicator at the top of the display will be flashing. In case the current measurement is out of the visible graph area, it is indicated by means of a left or a right arrowhead.

STAT/PLOT

If you want to treat a number of measurements with statistical methods, this is the key to operate. There are three display modes available by toggling the key:

  • Numerical
  • Histogram
  • Trend Plot

Numerical

Fig. 2-6    Statistics presented numerically.

In this mode the statistical information is dis­played as numerical data containing the fol­lowing elements:

  • Mean: mean value
  • Max: maximum value
  • Min: minimum value
  • P-P: peak-to-peak deviation
  • Adev: Allan deviation
  • Std: Standard deviation

Histogram

Fig. 2-7    Statistics presented as a histo­gram.

The bins in the histogram are always autoscaled based on the measured data. Limits, if enabled, and center of graph are shown as vertical dotted lines. Data outside the limits are not used for autoscaling but are replaced by an arrow indicating the direction where non-displayed values have been recorded.

Trend Plot

Fig. 2-8    Running trend plot.

This mode is used for observing periodic fluc­tuations or possible trends. Each plot terminates (if HOLD is activated) or restarts (if RUN is activated) after the set number of samples. The trend plot is always autoscaled based on the measured data, starting with 0 at restart. Limits are shown as horizontal lines if enabled.

Remote

When the instrument is controlled from the GPIB bus or the USB bus, the operating mode changes to Remote, indicated by the label REM on the display. All front panel keys ex­cept CANCEL are then disabled. See also page 2-8 for more information on this key.

3.3.10. Entering Numeric Values

Sometimes you may want to enter constants and limits in a value input menu, for instance one of those that you can reach when you press the MATH/LIMIT key.

You may also want to select a value that is not in the list of fixed values available by pressing the UP/DOWN arrow keys. One example is Meas Time under SETTINGS.

A similar situation arises when the desired value is too far away to reach conveniently by incrementing or decrementing the original value with the UP/DOWN arrow keys. One example is the Trig Lvl setting as part of the INPUT A (B) settings.

Whenever it is possible to enter numeric val­ues, the keys marked with 0-9;. (decimal point) and ± (stands for Change Sign)take on their alternative numeric meaning.

It is often convenient to enter values using the scientific format. For that purpose, the rightmost softkey is marked EE (stands for Enter Exponent), making it easy to switch be­tween the mantissa and the exponent. Press EXIT/OK to store the new value or CANCEL to keep the old one.

3.3.11. Hard Menu Keys

These keys are mainly used for opening fixed menus from which further selections can be made by means of the softkeys or the cur­sor/select keys.

Input A (B)

Fig. 2-9    Input settings menu.

By depressing this key, the bottom part of the display will show the settings for Input A (B).

The active settings are in bold characters and can be changed by depressing the correspond­ing softkey below the display. You can also move the cursor, indicated by text inversion, to the desired position with the RIGHT/LEFT arrow keys and then change the active setting with the ENTER key.

The selections that can be made using this menu are:

  • Trigger Slope: positive or negative, indi­cated by corresponding symbols
  • Coupling: AC or DC
  • Impedance: 50 W or 1 MW
  • Attenuation: 1x or 10x
  • Trigger:1 Manual or Auto
  • Trigger Level:2 numerical input via front panel keyboard. If Auto Trigger is active, you can change the default trigger level manually as a percentage of the amplitude.
  • Filter:3 On or Off

Notes: Always Auto when measuring risetime or falltime

The absolute level can either be adjusted using the up/down arrow keys or by pressing ENTER to reach the numerical input menu.

Pressing the corresponding softkey or ENTER opens the Filter Settings menu. See Fig. 2-10. You can select a fixed 100 kHz analog filter or an adjustable digital filter. The equivalent cutoff frequency is set via the value input menu that opens if you select Digital LP Frequency from the menu.

Fig. 2-10    Selecting analog or digital filter.

Input B

The settings under Input B are equal to those under Input A.

Settings

This key accesses a host of menus that affect the measurement. The figure above is valid after changing the default measuring time to 10 ms.

Fig. 2-11     The main settings menu.

Meas Time

Fig. 2-12    Submenu for entering measuring time.

This value input menu is active if you select a frequency function. Longer measuring time means fewer measurements per second and gives higher resolution.

Burst

Fig. 2-13    Entering burst parameters.

This settings menu is active if the selected measurement function is BURST – a special case of FREQUENCY – and facilitates mea­surements on pulse-modulated signals. Both the carrier frequency and the modulating fre­quency – the pulse repetition frequency (PRF) – can be measured, often without the support of an external arming signal.

Arm

Fig. 2-14     CNT-90 & CNT-90XL: Setting arming conditions.
Fig. 2-15    CNT-91(R): Setting arming con­ditions.

Arming is the general term used for the means to control the actual start/stop of a measurement. The normal free-running mode is inhib­ited and triggering takes place when certain pretrigger conditions are fulfilled.

The signal or signals used for initiating the arming can be applied to three channels (A, B, E), and the start channel can be different from the stop channel. All conditions can be set via this menu.

NOTE:   Stop Delay can only be used for realizing the function Timed Totalize in the CNT-91(R).

Trigger Hold-Off

Fig. 2-16    The trigger hold-off submenu.

A value input menu is opened where you can set the delay during which the stop trigger conditions are ignored after the measurement

start. A typical use is to clean up signals gen­erated by bouncing relay contacts.

Statistics

Fig. 2-17    Entering statistics parameters.

In this menu you can do the following:

  • Set the number of samples used for calculation of various statistical measures.
  • Set the number of bins in the histogram view.
  • Pacing

The delay between measurements, called pacing, can be set to ON or OFF, and the time can be set within the range 2 ms – 500 s.

Timebase Reference

Fig. 2-18    Selecting timebase reference source.

Here you can decide if the counter is to use an Internal or an External timebase. A third al­ternative is Auto. Then the external timebase will be selected if a valid signal is present at the reference input. The EXT REF indicator at the upper right corner of the display shows that the instrument is using an external timebase reference.

Miscellaneous

Fig. 2-19    CNT-90: The ‘Misc’ submenu.
Fig. 2-20    CNT-90XL: The ‘Misc’ submenu.
Fig. 2-21     CNT-91(R): The ‘Misc’ submenu.

The options in this menu are:

  • Smart Measure with submenus:
    • Smart Time Interval (valid only if the selected measurement function is Time Interval) The counter decides by means of timestamping which measurement channel precedes the other.
    • Smart Frequency (valid only If the selected measurement function is Frequency or Period Average) By means of continuous timestamping and regression analysis, the resolution is increased for measuring times between 0.2 s and 100 s.
  • Input C Acquisition (CNT-90XL only) Auto means that the whole specified fre­quency range is scanned for valid input signals.

Fig. 2-22   CNT-90XL: The ‘Input C Acqui­sition’ submenu.

Manual means that a narrow band around the manually entered center fre­quency is monitored for valid input sig­nals. This mode is compulsory when measuring burst signals but is also rec­ommended for FM signals, when the ap­proximate frequency is known. An additional feature is that the measure­ment results are presented much faster, as the acquisition process is skipped.

NOTE:   Signal frequencies outside the manual capture range may cause erroneous results. In order to draw the operator’s attention to this eventuality, the sign “M.ACQ” is visible in the upper right corner of the display.

  • Auto Trig Low Freq: In a value input menu you can set the lower frequency limit for automatic trig­gering and voltage measurements within the range 1 Hz – 100 kHz. A higher limit means faster settling time and consequently faster measurements.
  • Timeout: From this submenu you can activate/deactivate the timeout function and set the maximum time the instrument will wait for a pending measurement to finish before outputting a zero result. The range is 10 ms to 1000 s.
  • Interpolatator Calibration: By switching off the interpolator calibra­tion, you can increase the measurement speed at the expense of accuracy.
  • TIE (CNT-91 only): From a submenu you can either let the counter choose the reference frequency automatically (Auto) or enter it manually.

Math/Limit

Fig. 2-23    Selecting Math or Limits pa­rameters.

You enter a menu where you can choose be­tween inputting data for the Mathematics or the Limits postprocessing unit.

Fig. 2-24    The Math submenu.

The Math branch is used for modifying the measurement result mathematically before presentation on the display. Thus you can make the counter show directly what you want without tedious recalculations, e.g. revolutions/min instead of Hz.

The Limits branch is used for setting numerical limits and selecting the way the instrument will report the measurement results in relation to them.

Let us explore the Math submenu by pressing the corresponding softkey below the display.

Fig. 2-25    Selecting Math formula for postprocessing.

The display tells you that the Math function is not active, so press the Math Off key once to open the formula selection menu.

Select one of the five different formulas, where K, L and M are constants that the user can set to any value. X stands for the current non-modified measurement result.

Fig. 2-26    Selecting formula constants.

Each of the softkeys below the constant labels opens a value input menu like the one below.

Fig. 2-27    Entering numeric values for constants.

Use the numeric input keys to enter the man­tissa and the exponent, and use the EE key to toggle between the input fields. The key marked X0 is used for entering the display reading as the value of the constant.

The Limit submenu is treated in a similar way, and its features are explored beginning on page 6-6.

User Options

Fig. 2-28    CNT-90: The User Options menu.
Fig. 2-29   CNT-90XL & CNT-90 with Option 23/90: The User Options menu.
Fig 2-30    CNT-91(R): The User Options menu.

From this menu you can reach a number of submenus that do not directly affect the measurement. You can choose between a number of modes by pressing the corresponding softkey.

Save/Recall Menu

Fig. 2-31     The menu appearance after pressing Save/Recall.

Twenty complete front panel setups can be stored in non-volatile memory. Access to the first ten memory positions is prohibited when Setup Protect is ON. Switching OFF Setup Protect releases all ten memory positions si­multaneously.

The different setups can be individually la­beled to make it easier for the operator to re­member the application.

Fig. 2-32   The memory management menu after pressing Setup.

The following can be done:

  • Save current setup
Fig. 2-33    Selecting memory position for saving a measurement setup.

Browse through the available memory positions by using the RIGHT/LEFT arrow keys. For faster browsing, press the key Next to skip to the next memory bank. Press the softkey below the num­ber (1-20) where you want to save the setting.

  • Recall setup
Fig. 2-34    Selecting memory position for recalling a measurement setup.

Select the memory position from which you want to retrieve the contents in the same way as under Save current setup above. You can also choose Default to restore the preprogrammed factory set­tings. See the table on page 2-19 for a complete list of these settings.

  • Modify labels

Select a memory position to which you want to assign a label. See the descrip­tions under Save/Recall setup above. Now you can enter alphanumeric char­acters from the front panel. See the fig­ure below.

The seven softkeys below the display are used for entering letters and digits in the same way as you write SMS messages on a cell phone.

  • Setup protection

Toggle the softkey to switch between the ON/OFF modes. When ON is ac­tive, the memory positions 1-10 are all protected against accidental overwriting.

Fig. 2-35    Entering alphanumeric charac­ters.

Dataset Menu

Fig. 2-36     The memory management menu after pressing Dataset.

This feature is available in statistics mode only, and if HOLD has been pressed prior to initiating a measurement with RESTART. Up to 8 different datasets can be saved in FLASH memory, each containing up to 32000 sam­ples. If the pending measurement has more than 32000 samples, only the last 32000 will be saved. A default label will be assigned to the dataset. It can be changed in a similar way as the setup labels. See Modify labels above.

  • Save: Select a memory position, accept or change the name, and press OK.
  • Recall: Select a memory position and press OK.
  • Total Reset: The safety screen below will appear. Pressing OK will restore all factory set­tings and erase all user information.

Fig. 2-37    The Total Reset safety screen.

Calibrate Menu

This menu entry is accessible only for calibra­tion purposes and is password-protected.

Interface Menu

Fig. 2-38 Selecting active bus interface. Bus Type

Select the active bus interface. The alterna­tives are GPIB and USB. If you select GPIB, you are also supposed to select the GPIB Mode and the GPIB Address. See the next two paragraphs.

GPIB Mode

There are two command systems to choose from.

  • Native: The SCPI command set used in this mode fully exploits all the features of this instru­ment series.
  • Compatible: The SCPI command set used in this mode is adapted to be compatible with Agilent 53131/132/181.

GPIB Address

Value input menu for setting the GPIB ad­dress.

Test

A general self-test is always performed every time you power-up the instrument, but you can order a specific test from this menu at any time.

Fig. 2-39    Self-test menu.

Press Test Mode to open the menu with avail­able choices.

Fig. 2-40    Selecting a specific test.

Select one of them and press Start Test to run it.

Digits Blank

Jittery measurement results can be made easier for an operator to read by masking one or more of the LSDs on the display.

Place the cursor at the submenu Digits Blank and increment/decrement the number by means of the UP/DOWN arrow keys, or press the soft key beneath the submenu and enter the desired number between 0 and 13 from the keyboard. The blanked digits will be represented by dashes on the display. The default value for the number of blanked digits is 0.

Misc (CNT-90XL & CNT-90 with Op­tion 23/90)

The CNT-90XL without Option 23/90 has a single submenu called Units. By pressing this softkey you get to the submenu Power. Press Power and then select dBm or W as the unit of measurement, when either of the functions Frequency C or Power C is selected from the MEAS FUNC menu.

The CNT-90 with Option 23/90 has a single submenu called Use Battery in Standby. By toggling this softkey you can decide if the in­ternal OCXO will remain powered or not when you turn off the instrument in battery operation mode.

The CNT-90XL with Option 23/90 has a combination of the two submenus mentioned above. See the figure below.

Fig. 2-41     The ‘Misc’ submenu for CNT-90XL with battery option.

Output [CNT-91(R) only]

The rear panel pulse output can be used for three different purposes:

  • pulse generator
  • gate indicator
  • alarm

Press the softkey Output to open the submenu below.

Fig. 2-42 Selecting output mode and pulse parameters.

Off is the default mode and inhibits all activity on the output connector.

The pulse generator parameters Period and Width can be entered by first pressing the corresponding softkeys, then setting the numerical values as usual. By placing the cursor over the parameter, you can also set the values directly in 1-2-5 steps with the UP/DOWN arrow keys.

Press Output Mode to enter the mode selection menu below:

  • Gate Open indicates to external equip­ment when a measurement is in progress.
  • Pulse Generator activates a continuous pulse train having the parameters en­tered in the previous menu.
  • Alarm can be set to be active low or ac­tive high. The MATH/LIM menu is used for setting up the behavior and the nu­merical limits that trigger the alarm.
Fig 2-43    Output Mode selection menu.

The amplitude is fixed at TTL levels into 50 Ω irrespective of the output mode.

About

  • Here you can find information on:
  • model
  • serial number
  • instrument firmware version
  • timebase option & calibration date
    • The CNT-91R reports “Rubidium” in this field.
  • RF input option
    • The CNT-90XL reports the upper frequency limit.

Hold/Run

This key serves the purpose of manual arm­ing. A pending measurement will be finished and the result will remain on the display until a new measurement is triggered by pressing the RESTART key.

Restart

Often this key is operated in conjunction with the HOLD/RUN key (see above), but it can also be used in free-running mode, especially when long measuring times are being used, e.g. to initiate a new measurement after a change in the input signal. RESTART will not affect any front panel settings.

3.4. Default Settings

See page 2-16 to see how the following prepro­grammed settings are recalled by a few key­strokes.

PARAMETERVALUE/SETTING
Input A & B
Trigger LevelAUTO
Trigger SlopePOS
Impedance1 MW
Attenuator1x
CouplingAC
FilterOFF
Arming
StartOFF
Start SlopePOS
Start Arm Delay0
StopOFF
Stop SlopePOS
Hold-Off
Hold-Off StateOFF
Hold-Off Time200 ms
Time-Out
Time-Out StateOFF
Time-Out Time100 ms
Statistics
StatisticsOFF
No. of Samples100
No. of Bins20
Pacing StateOFF
Pacing Time20 ms
Mathematics
MathematicsOFF
Math ConstantsK=1, L=0, M=1
Limits
Limit StateOFF
Limit ModeRANGE
Lower Limit0
Upper Limit0
Burst
Sync Delay400 ms
Start Delay0
Meas. Time200 ms
Freq. Limit400 MHz
Miscellaneous
FunctionFREQA
Smart FrequencyAUTO
Smart Time IntervalOFF
Meas. Time200 ms
Auto Trig Low Freq100 Hz
Timebase ReferenceAUTO
Blank Digits0
Interpolator calibrationON
Output (CNT-91(R))OFF

4. Chapter 3: Input Signal Conditioning

4.1. Input Amplifier

The input amplifiers are used for adapting the widely varying signals in the ambient world to the measuring logic of the timer/counter.

These amplifiers have many controls, and it is essential to understand how these controls work together and affect the signal.

The block diagram below shows the order in which the different controls are connected. It is not a complete technical diagram but in­tended to help understanding the controls.

The menus from which you can adjust the set­tings for the two main measurement channels are reached by pressing INPUT A respectively INPUT B. See Figure 3-2. The active choices are shown in boldface on the bottom line.

Fig. 3-1 Block diagram of the signal conditioning
Fig. 3-2 Input settings menu.

4.1.1. Impedance

The input impedance can be set to 1 MΩ or 50 Ω by toggling the corresponding softkey.

CAUTION: Switching the impedance to 50 Ω when the input voltage is above 12 Vrms may cause perma­nent damage to the input circuitry.

4.1.2. Attenuation

The input signal’s amplitude can be attenuated by 1 or 10 by toggling the softkey marked 1x/10x. Use attenuation whenever the input signal ex­ceeds the dynamic input voltage range ±5 V or else when attenuation can reduce the influence of noise and interference. See the section deal­ing with these matters at the end of this chap­ter.

4.1.3. Coupling

Switch between AC coupling and DC cou­pling by toggling the softkey AC/DC.

Fig. 3-3 AC coupling a symmetrical signal.

Use the AC coupling feature to eliminate un­wanted DC signal components. Always use AC coupling when the AC signal is superim­posed on a DC voltage that is higher than the trigger level setting range. However, we rec­ommend AC coupling in many other measure­ment situations as well.

When you measure symmetrical signals, such as sine and square/triangle waves, AC cou­pling filters out all DC components. This means that a 0 V trigger level is always cen­tered around the middle of the signal where triggering is most stable.

Fig. 3-4 Missing trigger events due to AC coupling of signal with varying duty cycle.

Signals with changing duty cycle or with a very low or high duty cycle do require DC coupling. Fig. 3-4 shows how pulses can be missed, while Fig. 3-5shows that triggering does not occur at all because the signal ampli­tude and the hysteresis band are not centered.

NOTE: For explanation of the hysteresis band, see page 4-3.

Fig. 3-5 No triggering due to AC coupling of signal with low duty cycle.

4.1.4. Filter

If you cannot obtain a stable reading, the sig­nal-to-noise ratio (often designated S/N or SNR) might be too low, probably less than 6 to 10 dB. Then you should use a filter. Certain conditions call for special solutions like highpass, bandpass or notch filters, but usu­ally the unwanted noise signals have higher frequency than the signal you are interested in. In that case you can utilize the built-in lowpass filters. There are both analog and dig­ital filters, and they can also work together.

Fig. 3-6 The menu choices after selecting FILTER.

Analog Lowpass Filter

The counter has analog LP filters of RC type, one in each of the channels A and B, with a cutoff frequency of approximately 100 kHz, and a signal rejection of 20 dB at 1 MHz.

Accurate frequency measurements of noisy LF signals (up to 200 kHz) can be made when the noise components have significantly higher frequencies than the fundamental signal.

Digital Lowpass Filter

The digital LP filter utilizes the Hold-Off function described below.

With trigger Hold-Off it is possible to insert a deadtime in the input trigger circuit. This means that the input of the counter ignores all hysteresis band crossings by the input signal during a preset time after the first trigger event.

When you set the Hold-Off time to approx. 75% of the cycle time of the signal, erroneous triggering is inhibited around the point where the input signal returns through the hysteresis band. When the signal reaches the trigger point of the next cycle, the set Hold-Off time has elapsed and a new and correct trigger will be initiated. Instead of letting you calculate a suitable Hold-Off time, the counter will do the job for you by converting the filter cutoff frequency you enter via the value input menu below to an equivalent Hold-Off time.

Fig. 3-7 Value input menu for setting the cutoff frequency of the digital filter.

You should be aware of a few limitations to be able to use the digital filter feature effectively and unambiguously. First you must have a rough idea of the frequency to be measured. A cutoff frequency that is too low might give a perfectly stable reading that is too low. In such a case, triggering occurs only on every 2nd, 3rd or 4th cycle. A cutoff frequency that is too

high (>2 times the input frequency) also leads to a stable reading. Here one noise pulse is counted for each half-cycle.

Use an oscilloscope for verification if you are in doubt about the frequency and waveform of your input signal. The cutoff frequency setting range is very wide: 1 Hz – 50 MHz

Fig. 3-8 Digital LP filter operates in the measuring logic, not in the input amplifier.

4.1.5. Man/Auto

Toggle between manual and automatic trigger­ing with this softkey. When Auto is active the counter automatically measures the peak-to-peak levels of the input signal and sets the trigger level to 50% of that value. The attenuation is also set automatically.

At rise/fall time measurements the trigger lev­els are automatically set to 10% and 90% of the peak values.

When Manual is active the trigger level is set in the value input menu designated Trig. See below. The current value can be read on the display before entering the menu.

Speed

The Auto-function measures amplitude and calculates trigger level rapidly, but if you aim at higher measurement speed without having to sacrifice the benefits of automatic trigger­ing, then use the Auto Trig Low Freq func­tion to set the lower frequency limit for volt­age measurement.

If you know that the signal you are interested in always has a frequency higher than a cer­tain value flow , then you can enter this value from a value input menu. The range for flow is 1 Hz to 100 kHz, and the default value is 100 Hz. The higher value, the faster measure­ment speed due to more rapid trigger level voltage detection.

Even faster measurement speed can be reached by setting the trigger levels manually. See Trig below.

Follow the instructions here to change the low-frequency limit:

  • Press SETTINGS->Misc->Auto Trig Low Freq.
  • Use the UP/DOWN arrow keys or the nu­meric input keys to change the low fre­quency limit to be used during the trigger level calculation, (default 100 Hz).
  • Confirm your choice and leave the SET­TINGS menu by pressing EXIT/OK three times.

4.1.6. Trig

Value input menu for entering the trigger level manually.

Use the UP/DOWN arrow keys or the nu­meric input keys to set the trigger level. A blinking underscore indicates the cursor po­sition where the next digit will appear. The LEFT arrow key is used for correction, i.e. deleting the position preceding the current cursor position.

Fig. 3-9 Value input menu for setting the trigger level.

NOTE:  It is probably easier to make small ad­justments around a fixed value by us­ing the arrow keys for incrementation or decrementation. Keep the keys de­pressed for faster response

NOTE:   Switching over from AUTO to MAN Trig­ger Level is automatic if you enter a trigger level manually.

Auto Once

Converting “Auto” to “Fixed”

The trigger levels used by the auto trigger can be frozen and turned into fixed trigger levels simply by toggling the MAN/AUTO key. The current calculated trigger level that is visible on the display under Trig will be the new fixed manual level. Subsequent measurements will be considerably faster since the signal levels are no longer monitored by the instrument. You should not use this method if the signal levels are unstable.

NOTE: You can use auto trigger on one input and fixed trigger levels on the other.

4.2. How to Reduce or Ignore Noise and Interference

Sensitive counter input circuits are of course also sensitive to noise. By matching the signal amplitude to the counter’s input sensitivity, you reduce the risk of erroneous counts from noise and interference. These could otherwise ruin a measurement.

Fig. 3-10 Narrow hysteresis gives errone¬ous triggering on noisy signals.
Fig. 3-11 Wide trigger hysteresis gives correct triggering.

To ensure reliable measuring results, the coun­ter has the following functions to reduce or eliminate the effect of noise:

  • 10x input attenuator
  • Continuously variable trigger level
  • Continuously variable hysteresis for some functions
  • Analog low-pass noise suppression filter
  • Digital low-pass filter (Trigger Hold-Off)

To make reliable measurements possible on very noisy signals, you may use several of the above features simultaneously. Optimizing the input amplitude and the trigger level, using the attenuator and the trigger con­trol, is independent of input frequency and useful over the entire frequency range. LP fil­ters, on the other hand, function selectively over a limited frequency range.

4.2.1. Trigger Hysteresis

The signal needs to cross the 20 mV input hysteresis band before triggering occurs. This hysteresis prevents the input from self-oscil­lating and reduces its sensitivity to noise. Other names for trigger hysteresis are “trigger sensitivity” and “noise immunity”. They ex­plain the various characteristics of the hyster­esis.

Fig. 3-12 Erroneous counts when noise passes hysteresis window.

Fig. 3-10 and Fig. 3-12 show how spurious signals can cause the input signal to cross the trigger or hysteresis window more than once per input cycle and give erroneous counts.

Fig. 3-13 Trigger uncertainty due to noise.

Fig. 3-13 shows that less noise still affects the trigger point by advancing or delaying it, but it does not cause erroneous counts. This trig­ger uncertainty is of particular importance when measuring low frequency signals, since the signal slew rate (in V/s) is low for LF sig­nals. To reduce the trigger uncertainty, it is de­sirable to cross the hysteresis band as fast as possible.

Fig. 3-14 Low amplitude delays the trig¬ger point

Fig. 3-14 shows that a high amplitude signal passes the hysteresis faster than a low ampli­tude signal. For low frequency measurements where the trigger uncertainty is of importance, do not attenuate the signal too much, and set the sensitivity of the counter high.

In practice however, trigger errors caused by erroneous counts (Fig. 3-10 and Fig. 3-12) are much more important and require just the op­posite measures to be taken.

To avoid erroneous counting caused by spuri­ous signals, you need to avoid excessive input signal amplitudes. This is particularly valid when measuring on high impedance circuitry and when using 1 MW input impedance. Under these conditions, the cables easily pick up noise.

External attenuation and the internal 10x attenuator reduce the signal amplitude, includ­ing the noise, while the internal sensitivity control in the counter reduces the counter’s sensitivity, including sensitivity to noise. Re­duce excessive signal amplitudes with the 10x attenuator, or with an external coaxial attenuator, or a 10:1 probe.

4.2.2. How to use Trigger Level Setting

For most frequency measurements, the optimal triggering is obtained by positioning the mean trigger level at mid amplitude, using either a narrow or a wide hysteresis band, de­pending on the signal characteristics.

Fig. 3-15 Timing error due to slew rate.

When measuring LF sine wave signals with little noise, you may want to measure with a high sensitivity (narrow hysteresis band) to re­duce the trigger uncertainty. Triggering at or close to the middle of the signal leads to the smallest trigger (timing) error since the signal slope is steepest at the sine wave center, see Fig. 3-15.

When you have to avoid erroneous counts due to noisy signals, see Fig. 3-12, expanding the hysteresis window gives the best result if you still center the window around the middle of the input signal. The input signal excursions beyond the hysteresis band should be equally large.

Auto Trigger

For normal frequency measurements, i.e. without arming, the Auto Trigger function changes to Auto (Wide) Hysteresis, thus wid­ening the hysteresis window to lie between 70 % and. 30 % of the peak-to-peak ampli­tude. This is done with a successive approxi­mation method, by which the signal’s MIN. and MAX. levels are identified, i.e., the levels where triggering just stops. After this MIN./MAX. probing, the counter sets the trig­ger levels to the calculated values. The default relative trigger levels are indicated by 70 % on Input A and 30 % on Input B. These values can be manually adjusted between 50 % and 100 % on Input A and between 0 % and 50 % on Input B. The signal, however, is only ap­plied to one channel.

Before each frequency measurement the coun­ter repeats this signal probing to identify new MIN/MAX values. A prerequisite to enable AUTO triggering is therefore that the input signal is repetitive, i.e., >100 Hz (default). Another condition is that the signal amplitude does not change significantly after the mea­surement has started.

NOTE:   AUTO trigger limits the maximum measuring rate when an automatic test system makes many measurements per second. Here you can increase the measuring rate by switching off this probing if the signal amplitude is constant. One single command and the AUTO trigger function determines the trigger level once and enters it as a fixed trigger level.

Manual Trigger

Switching to Man Trig also means Narrow Hysteresis at the last Auto Level. Pressing AUTOSET once starts a single automatic trigger level calculation (Auto Once). This cal­culated value, 50 % of the peak-to-peak am­plitude, will be the new fixed trigger level, from which you can make manual adjustments if need be.

Harmonic Distortion

As rule of thumb, stable readings are free from noise or interference.

However, stable readings are not necessarily correct; harmonic distortion can cause errone­ous yet stable readings. Sine wave signals with much harmonic distor­tion, see Fig. 3-17, can be measured correctly by shifting the trigger point to a suitable level or by using continuously variable sensitivity, see Fig. 3-16. You can also use Trigger Hold-Off, in case the measurement result is not in line with your expectations.

Fig. 3-16 Variable sensitivity.
Fig. 3-17 Harmonic distortion.

5. Chapter 4: Measuring Functions