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4.1.3 Adjusting The Calibration Data

Several controls < and > are provided to adjust each of three calibration points for each of

the channels. These calibration points must be adjusted for each of the nine settings of the input

attenuator. In total then there are 2x9x3 = 54 settings to be made. To be sure that everything is

adjusted properly it helps to be systematic about it. The following is a suggested approach.

Set the front end to DC coupling and 50 ohms and be sure that no signal is applied. The

Bit Mag setting must be set to 1. Set the display to show one channel at a time and complete all

of these steps for that channel before moving on to the other channel. For the channel being

calibrated, start with the 100 mV per division setting and work upwards. For each setting

proceed as follows.

1) Click Zero Offset to bring the trace to the middle of the gradicule. If properly adjusted,

the trace should appear exactly in the middle. If it does not, use the Fixed Offset controls

for that channel to move the trace to the middle of the gradicule.

2) Move the offset towards the top of the screen. If properly adjusted, the trace should be at

the sample location as the offset arrow. If it is not, use the Offset Gain control to bring

the trace into agreement with the icon. Move the offset towards the bottom of the screen

and repeat. It may take a few tries to reach a good compromise on offset gain over all

locations on the screen. It will never be perfect at all locations, but should be quite close.

Return the attenuator to the 100 mV setting and apply an appropriate amplitude signal to the

channel under test from a calibrated source. It may be helpful to apply a signal which covers an

integer number of divisions and preferably covers half or more of the full scale range. At higher

ranges, this may not always be possible. A sine wave is generally the best choice for examining

amplitude. For each setting of the attenuator proceed as follows, adjusting the amplitude of the

known signal as needed.

1) Adjust the Gain control so that the amplitude displayed on the gradicule matches the

amplitude of the known signal.

2) Change the frequency of the incoming signal to an order of magnitude higher or lower

than the current setting without changing its amplitude. Change the Sec/Div control

to view and verify the signal amplitude across at least two frequencies well within the

instrument bandwidth. Good choices for calibration signals are 1 MHz and 10 MHz.

This approach will allow multiple viewings of the same information on different settings

of the instrument, making it more likely to converge on a good calibration setting.

When all calibrations have been made, use the Save Calibration button to write the new data to

the instrument non-volatile memory. To protect tedious calibration work from interruption and

data loss, it may be advisable to perform this step at several points during the process. Success

of the calibration process should be reviewed at all settings after the process is complete. This

concludes instrument calibration.

DDC TR122 User Manual

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