Small question for the SDS experts
These instruments allow the user to set the horizontal reference strategy (Utility > Menu > Reference position). It is described in the manual on page 337.
- You can choose between a "Fixed Delay' (simply called 'Delay' on the screen of the device). Then the reference point is simply in the middle of the screen, and when the horizontal timebase scale is changed, the waveform expands/contracts around the center of the display.
- Then there is "Fixed Position" (simply called 'Position' on the screen of the device). Here the user can select a reference position from 0% (outer left of the screen) to 100% (outer right of the screen). Then the waveform expands/contract from that chosen position.
This is a welcome function indeed.
Now the question... In the current FW 1.3.9R6, when ‘position’ is selected (i.e., the button turns blue), the position entry field disappears, and if you select “delay”, the horizontal entry field re-appears.
Could it be that the labels on these two buttons are accidentally switched in the user interface?
(Also, I find the terminology position vs. delay used kind of confusing. It would be more intuitive calling this "Centre Position" and "Adjustable" Position", or simply a single parameter that is default at 50% and can be changed. Just my two cents.)
This setting alters the function of the horizontal position knob, and everything is labelled correctly.
If we use the “fixed delay” strategy, then the trigger delay with respect to the reference position is set by the horizontal position knob. The reference position on the other hand can be set to anything between 0 and 100 % of the display width in the Horizontal Ref. Pos. field, where 50 % is the default value.
It means that the delay remains constant and because of this, the trigger position changes with timebase and can get shifted far outside the visible screen quite easily. That’s fine if you want to observe a certain region of the waveform that has a certain delay with respect to the trigger point – this is the reference position, that remains constant, regardless of the timebase.
For example, if you have a trigger delay set to 1 µs and the reference position to 20 % in constant delay mode, the trigger point will be at 10 % of the screen width at 1 µs/div and you get the part of the waveform at the reference position at 20 %, that is exactly 1 µs after the trigger event. Now if you change the timebase to e.g. 100 ns/div, the trigger point is far outside the visible screen area, but you still see a zoomed version of that 1 µs delayed part of the waveform at the reference position, which you had set to 20 % of the screen width.
If we use the “fixed position” strategy, then the trigger position is always equal to the reference position and both can be adjusted simultaneously using the horizontal position knob. The only problem is the hollow triangle that remains at 50 % of the screen width, suggesting there is anything of importance there – actually, there is not.
It means that the reference = trigger position remains constant with various timebase settings. That’s fine if you want to observe the trigger point in a waveform regardless of the timebase setting.
For example, if you set the horizontal position to 20 % in constant position mode, the trigger point will be at 20 % of the screen width at any timebase setting.
You can ask yourself why we need the two different modes – after all, “constant delay” already does everything we need. Well, trigger delay is a concept dating back to the times where memoryless storage scopes (from Tek) were a thing and we needed a set of crouches to overcome the limitations going with it. Nowadays, with deep memory and well implemented zoom modes, trigger delay is not nearly as important as it used to be, so we might prefer just setting the trigger position with the position knob quickly, instead of digging into a menu.