How Can I Calibrate the Amplitude and Offset of my Pulse Generator?

Contents

Introduction

Avtech pulse generators with the "-B" suffix (i.e., with IEEE-488.2 GPIB control) with firmware revision 1.96 or higher have user-adjustable amplitude and offset calibration. (Use the "*idn?" command to determine your firmware revision number.)

For brevity, the details below address amplitude calibration only. Offset calibration is similar, with the following command changes:

diag:ampl:cal:point
changes to
diag:offset:cal:point
diag:ampl:cal:point?
changes to
diag:offset:cal:point?
volt?
changes to
volt:low?
curr?
changes to
curr:low?

 

Step 1 - Attach a Test Load and Connect an Oscilloscope

Make sure that the instrument is off. Attach an appropriate test load to the output of the pulse generator. Consult the instruction manual that was supplied with the instrument (not the programming manual) for information on an appropriate test load. (The test load is typically 50 Ohms for voltage pulsers, and a few Ohms for current pulsers, but this must be confirmed with the instruction manual or the end-user.) The test load should be rated to withstand the maximum possible output amplitude (in Volts and/or Amps) of the instrument, and have a sufficiently high power rating (in Watts) to withstand both the peak and average power output.

Connect an oscilloscope probe to the test load, to observe the output waveform.

 
Step 2 - Set Timing Parameters

The next step is to turn on the instrument, and set the frequency and pulse width, by sending commands similar to these:

*rst
freq 10 Hz
pulse:width 1 us

The actual values of frequency and pulse width used depend on the capabilities of the instrument, and can vary over a wide range. However, as a general rule, they should be chosen so that the output duty cycle is low. This is a safety precaution, since the internal protection circuits are partly over-ridden during the calibration process.


Step 3 - Determine The Number of Calibration Points

To start adjusting the amplitude calibration, send the query:

diag:ampl:cal:point? max

and note the returned number, which shall be referred to as "N". (N will be between 2 and 100). This is the number of calibration points that must be checked.

Step 4A - Setting the Calibration Points for Current-Output Pulsers

This section deals with current-output pulsers, i.e., instruments whose output amplitude is measured in Amps (such as the AV-106, AV-107, AV-108, AV-155, and AV-156 families of instruments). Skip this section if it is not appropriate for your instrument.

The calibration sequence consists of a repeated sequence of the following commands:

diag:ampl:cal:point <calibration_point_number> go
output on
curr?
diag:ampl:cal:point <calibration_point_number> <new_calibration_value>

where <calibration_point_number> is replaced with 1, 2, 3.... N in sequence, until all of the calibration points have been examined.

The first command in the loop,

diag:ampl:cal:point <calibration_point_number> go

switches the instrument to amplitude calibration mode, and sets the amplitude to one of the calibration points. The "output on" command ensures that the output is active. The nominal value of this calibration point is returned by the next command:

curr?

If the nominal value is zero, the last command in the loop may be skipped. Calibration points with zero value are not adjustable. If the nominal value is non-zero, the user should use an oscilloscope to determine the actual amplitude observed at the calibration testpoint. Compare the measured value to the nominal value, and confirm that the difference between the two is less than 30%. If it is, update the calibration using the last command in the sequence:

diag:ampl:cal:point <calibration_point_number> <new_calibration_value>

Otherwise, examine your setup - a deviation greater that 30% may signify an improper connection, or an instrument failure.

Here is an example procedure:

Initialize instrument
3
diag:ampl:cal:point? max
Assume this query returns the value "3". Step 4A will thus be repeated 3 times.
4A
diag:ampl:cal:point 1 go
output on
curr?
Assume that calibration point "1" has a nominal value of zero. Zero values are non-adjustable.
4A
diag:ampl:cal:point 2 go
output on
curr?
diag:ampl:cal:point 2 1.43
Assume that calibration point "2" has a non-zero nominal value, of 1.3 Amps, and that the observed measured value is actually 1.43 Amps.
4A
diag:ampl:cal:point 4 go
output on
curr?
diag:ampl:cal:point 4 5.1
Assume that calibration point "3" has a non-zero nominal value, of 5.25 Amps, and that the observed measured value is actually 5.10 Amps.
5 *rst Reset the instrument

Step 4B - Setting the Calibration Points for Voltage-Output Pulsers With Fixed Output Impedance

This section deals with voltage-output pulsers (i.e., instruments whose output amplitude is measured in volts), that do not have an adjustable output impedance, such as the AVR and AVO families of instruments. (Models with adjustable output impedance will display "Zout:2Ω" or "Zout:50Ω" on the front-panel main menu.) Skip this section if it is not appropriate for your instrument.

The calibration sequence consists of a repeated sequence of the following commands:

diag:ampl:cal:point <calibration_point_number> go
output on
volt?
diag:ampl:cal:point <calibration_point_number> <new_calibration_value>

where <calibration_point_number> is replaced with 1, 2, 3.... N in sequence, until all of the calibration points have been examined.

The first command in the loop,

diag:ampl:cal:point <calibration_point_number> go

switches the instrument to amplitude calibration mode, and sets the amplitude to one of the calibration points. The "output on" command ensures that the output is active. The nominal value of this calibration point is returned by the next command:

volt?

If the nominal value is zero, the last command in the loop may be skipped. Calibration points with zero value are not adjustable. If the nominal value is non-zero, the user should use an oscilloscope to determine the actual amplitude observed at the calibration testpoint. Compare the measured value to the nominal value, and confirm that the difference between the two is less than 30%. If it is, update the calibration using the last command in the sequence:

diag:ampl:cal:point <calibration_point_number> <new_calibration_value>

Otherwise, examine your setup - a deviation greater that 30% may signify an improper connection, or an instrument failure.

Here is an example procedure:

Initialize instrument
3
diag:ampl:cal:point? max
Assume this query returns the value "3". Step 4B will thus be repeated 3 times.
4B
diag:ampl:cal:point 1 go
output on
volt?
Assume that calibration point "1" has a nominal value of zero. Zero values are non-adjustable.
4B
diag:ampl:cal:point 2 go
output on
volt?
diag:ampl:cal:point 2 1.43
Assume that calibration point "2" has a non-zero nominal value, of 1.3 Volts, and that the observed measured value is actually 1.43 Volts.
4B
diag:ampl:cal:point 4 go
output on
volt?
diag:ampl:cal:point 4 5.1
Assume that calibration point "3" has a non-zero nominal value, of 5.25 Volts, and that the observed measured value is actually 5.1 Volts.
5 *rst Reset the instrument

Step 4C - Setting the Calibration Points for Voltage-Output Pulsers With Switchable Output Impedance

This section deals with voltage-output pulsers (i.e., instruments whose output amplitude is measured in volts), that do have an adjustable output impedance, such as the AV-1011 and AV-1015 families of instruments. (Models with adjustable output impedance will display "Zout:2Ω" or "Zout:50Ω" on the front-panel main menu.) Skip this section if it is not appropriate for your instrument.

For calibrating these instruments, a test load of 50W must be used.

One range of calibration points in reserved for operation in the "output:impedance min" mode. This range starts at calibration point "N", with a non-zero nominal value, and extends to the first point in decreasing order that has a nominal value of zero. For this reason, before starting the calibration loop, the

output:impedance min

command must be sent. This is repeated for the following loops until the "volt?", described below, has returned a value of zero. After this occurs, the

output:impedance max

command is sent (once), and the loop continues down to calibration point 1.

The main calibration loop consists of a repeated sequence of the following commands:

diag:ampl:cal:point <calibration_point_number> go
output on
volt?
diag:ampl:cal:point <calibration_point_number> <new_calibration_value>

where <calibration_point_number> is replaced with N, N-1, .... 3, 2, 1 in decreasing sequence, until all of the calibration points have been examined.

The first command in the loop,

diag:ampl:cal:point <calibration_point_number> go

switches the instrument to amplitude calibration mode, and sets the amplitude to one of the calibration points. The "output on" command ensures that the output is active. The nominal value of this calibration point is returned by the next command:

volt?

If the nominal value is zero, the last command in the loop may be skipped. Calibration points with zero value are not adjustable. However, note when a zero occurs - as described above, the output impedance must be switched after the first zero.

If the nominal value is non-zero, the user should use an oscilloscope to determine the actual amplitude observed at the calibration testpoint. If the "output:impedance max" command has not been executed, send the measured value directly:

diag:ampl:cal:point <calibration_point_number> <measured_value>

If the "output:impedance max" command has been executed, multiply the measured value by a factor of two (to correct for the resister-divider effect of the output impedance and the load), and then update the calibration:

diag:ampl:cal:point <calibration_point_number> <2 x measured_value>

Here is an example procedure:

Initialize instrument.
3
diag:ampl:cal:point? max
Assume this query returns the value "9". Step 4C will thus be repeated 9 times.
4C
output:impedance min
The first batch of calibration points is for the minimum output impedance mode.
4C
diag:ampl:cal:point 9 go
output on
volt?
diag:ampl:cal:point 9 107
Start with the last calibration point, and work in reverse order. Assume that the observed measured value is 107 Volts.
4C
diag:ampl:cal:point 8 go
output on
volt?
diag:ampl:cal:point 8 33.3
Assume that the observed measured value is 33.3 Volts.
4C
diag:ampl:cal:point 7 go
output on
volt?
Assume that point 7 has a nominal value of zero (as reported by "volt?"). Zero points are not adjustable, so the measured value is not used.
4C
output:impedance max
The last point had a nominal value of zero, so it terminates the range of points used for the minimum output impedance mode. The output impedance must now be switched to its maximum value.
4C
diag:ampl:cal:point 6 go
output on
volt?
diag:ampl:cal:point 6 106
Assume that the observed measured value is 53 Volts. Double this to 106V (which should agree approximately with the nominal setting returned by "volt?"), and update the calibration.
4C
diag:ampl:cal:point 5 go
output on
volt?
diag:ampl:cal:point 5 32
Assume that the observed measured value is 16 Volts. Double this to 32V, and update the calibration.
4C
diag:ampl:cal:point 4 go
output on
volt?
Assume that point 4 has a nominal value of zero (as reported by volt?). Zero points are not adjustable.
4C
diag:ampl:cal:point 3 go
output on
volt?
diag:ampl:cal:point 3 20.2
Assume that the observed measured value is 10.1 Volts. Double this to 20.2V, and update the calibration.
4C
diag:ampl:cal:point 2 go
output on
volt?
diag:ampl:cal:point 2 4.2
Assume that the observed measured value is 2.1 Volts. Double this to 4.2V, and update the calibration.
4C
diag:ampl:cal:point 1 go
output on
volt?
Assume that point 1 has a nominal value of zero (as reported by volt?). Zero points are not adjustable.
5 *rst Reset the instrument

Step 5 - Reset The Instrument

After completing the calibration, the instrument must be turned off, or reset using the command:

*rst

General Notes

When updating the calibration values, do not use units in the command string (e.g. "Amps", or "V").

Some dual-polarity instruments use the same set of calibration points for positive and negative outputs, while other have half of the points devoted to positive values, and half to negative values.

The instrument will report an error (in standard IEEE-488.2 fashion) and ignore the command if the calibration change for any point is excessive, or if the change would prevent operation at the minimum or maximum settings.

Dual-Channel Instruments

Some Avtech models are available with two output channels, with independent control of amplitude (and offset, if present). In this case, the above procedures are valid for the Channel 1. The procedure then needs to be repeated for Channel 2, taking care to add the channel suffix "2" to the calibration commands, like so:

*rst
freq 10 Hz
pulse:width2 1 us
diag:ampl:cal:point2? max
diag:ampl:cal:point2 1 go
output on
volt2
etc....

The added channel suffixes are shown in bold.

Other Calibration Procedures

The procedure for updating timing calibration is described in Technical Brief 4 - How Can I Calibrate the Timebase of my Pulse Generator?

Some current-output pulse generators (such as the AV-155 and AV-156 families of products) have additional calibration procedures described in their manuals for adjusting the output impedance of the current source.

Some product manuals also describe an amplitude procedure using commands like "diag:ampl:cal 5.1" (i.e., without the ":point" subcommand). This command scales all of the calibration points by a common amount. It does not offer point-by-point calibration. This command is now obsolete, and it is replaced by the procedures described in this document. It is, however, maintained in the firmware for compatibility.


Calibration Intervals

If you are required by corporate policy to calibrate your instruments on a regular schedule, Avtech suggests 1 year as an appropriate time between calibration checks. The instrument should first be checked to see if the calibration has slipped beyond an acceptable range. If it is within an acceptable range, do not perform the above procedure.

The above process should only be performed when required, since it is possible for an end-user to defeat the numerous built-in safeguards and de-calibrate the instrument to a point where it requires adjustment at the factory.

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