MDO3014 AFG Fails to output programmed pulse width

[jaime_g]

When programming a pulse width of 1.25s with a period of 12.5s, i.e., 10% duty cycle, the AFG outputs a 1.0s pulse. The MDO3014 scope was returned 2 weeks ago from the factory after warranty repair was done. It is using firmware version 1.26.

The following commands are used:
AFG:FUNC PULS;
AFG:PUL:WID 1.250;
AFG:PERI 12.500;
AFG:OUTP:LOA:IMPED HIGHZ;
AFG:HIGHL 5.0;
AFG:LOWL 0.0;
AFG:OUTP:STATE ON

Similar behavior is observed when programming the pulse width to 750E-3s or 600E-3. It will default to 500E-3s pulses.

A work around was found, it requires the use of a while loop to query the AFG pulse width, comparing it to the programmed pulse width and sending command AFG:PUL:WID 1.250 until the AFG outputs the programmed pulse width. It usually takes 1 try for the AFG to change the pulse width from 1.0s to 1.25s. This solution does not work with 750E-3 or 600E-3 pulse widths, in this case the workaround is to use a 50% duty cycle.

This has affected our high current trip measurement method. We need to pulse a power controller using 10% duty cycle waveform. I would appreciate any help on how to ensure the 10% duty cycle for 600E-3 and 750E-3 pulses.

Could this be an unintended flaw in firmware v1.26?

[jaime_g]

I need to make a revision. The work around for the AFG to generate 750E-3 and 600E-3 pulse widths is changing the duty cycle of the waveform from 10% to 20% NOT 50%.

[Dave W]

Hi jaime_g,

You can resolve some of your issues by re-arranging your commands. Due to the 10-90% duty cycle relative range, there is interactions between the settings so order of command can have an effect. I would recommend re-ordering your commands as follows.

Code: Select all

AFG:PERI 12.500;
AFG:PUL:WID 1.250;
AFG:FUNC PULS;
AFG:OUTP:LOA:IMPED HIGHZ;
AFG:HIGHL 5.0;
AFG:LOWL 0.0;
AFG:OUTP:STATE ON

There is another issue though that this will not resolve. You are trying to set your pulse period to 12.5 seconds. However, the maximum pulse period on the AFG is 10 seconds. The instrument will not do a 12.5 seconds period.

[jaime_g]

Dave,

I tried the command sequence you suggested. There was no change on the outcome. However, i discovered that 10% duty cycle does not function for 600E-3s or 1.25s pulse widths. Instead, using a 20% duty cycle generated the programmed pulse (600E-3s, 720E-3s, 1.25s) correctly on the first try.

The reason I am using a 10% duty cycle is because I cannot figure out how to set the AFG for a one shot pulse. For my test application I need to measure the high current trip time of a power controller. The AFG pulse is used as the stimulus to turn on the DUT. It is also used as reference to measure the time to shut down of the output signal in the event of a current over load or a short (trip time under current overload or short). The current is measured using a TCP0020 current probe.

What would be of heightened interest is the ability of the AFG to generate a one shot pulse. Perhaps Tek could post the command sequence to generate a 1 shot pulse using the ARB option in the AFG.

Regards,

Jaime Arevalo

[Dave W]

Hi jamie_g,

The AFG built-in to the MDO3014 lacks both burst and trigger capability so it cannot be configured to output a single pulse on trigger. Unfortunately this is just a limitation of the built-in AFG.