V CAL knob on a scope - what is it used for?

[Philbywhizz]

Hi there,

I recently scored a Hitachi V1656 Scope and I have been playing around with it on how it works (and having lots of fun in the process!)  I now have 2 probes and have calibrated them to the scope.

However while playing around, one particular control (well 2 if you count for each channel) stumps me as to what is is used for? It is the 'knob' within a 'knob' on the Vertical Volts/Div (not the outer dial, I know what this one does). The inner dark grey dial. (I can take a closer photo if you don't know what I'm talking about).

All the tutorials I've read online all say to make sure that this knob is locked to the CAL position, and you don't hear about its use since. What is the purpose of this knob if it is always at CAL? It seems to 'shrink' the screen (any signal projected above the center of the screen shifts down and anything below the center of the screen shifts up) when I turn it out of the CAL position so the scales change.

I'm speculating that it is a manual calibration where you can calibrate up one channel to compare it with another? I've probed the 'probe adjust' signal on both channels at the same time and they line up so I'm guessing that you only use it when one channel goes out of whack?

Are there any examples online on how this control is used? It's a bit hard to google an example for this one.

[RobertHolcombe]

From - http://www.tpub.com/neets/book16/71e.htm

The vertical attenuator control (VOLTS/DIV in figure 6-19 ) provides a means of adjusting the input signal level to the amplifiers by steps. These steps are sequenced from low to high deflection factors. The potentiometer control (VAR in figure 6-19) provides a means of fine, or variable, control between steps. This control may be mounted separately, or it may be mounted on the attenuator control. When the control is mounted separately, it is often marked as FINE GAIN or simply GAIN. When mounted on the attenuator control, it is usually marked VARIABLE or VAR.

The variable control adds attenuation to the step that is selected. Since accurately calibrating a potentiometer is difficult, the variable control is either left unmarked or the front panel is marked off in some convenient units, such as 1-10 and 1-100. The attenuator control, however, can be accurately calibrated. To do this, you turn off the variable control to remove it from the attenuator circuit. This position is usually marked CAL (calibrate) on the panel, or an associated light indicates if the VAR control is on or off. In figure 6-19, the light called UNCAL indicates the VAR control is in the uncalibrated position.

In the case of your particular scope the VAR control is either off (CAL position), at which point the vertical attenuation is accurate as per the VOLT/DIV selection for the relevant channel, or allows variable attenuation up to 2.5X as you turn the VAR knob anticlockwise.

There are probably other applications but yes it does seem to be most useful for comparing two waveforms as you mentioned

[dfmischler]

This knob changes the vertical response in an uncalibrated way.  Hardly anyone uses it often because the graticule won't correspond to anything (e.g. the knob says 5 volts per division, but that is only when the V CAL knob is in the CAL position).  It might be useful for fine tuning the vertical trace for measuring rise and fall times using the 10% and 90% marks.  Play with it for yourself and see.

[IconicPCB]

Also good for setting up display to measure 3dB frequency response in case the sig generator does not have adjustable output
At a lower inband frequency adjust screen display for 7 divisions..and go hunt half power voltage/frequency

[Philbywhizz]

Thanks everyone.

It is starting to make some sense to me now - although it doesn't seem to be a function that I would use very often so I'll just leave it in the CAL position for 99% of the time.

From - http://www.tpub.com/neets/book16/71e.htm

That's a handy reference page (and the other documents up a level) that I hadn't come across some of this info before - thanks!

[tautech]

Thanks everyone.

It is starting to make some sense to me now - although it doesn't seem to be a function that I would use very often so I'll just leave it in the CAL position for 99% of the time.

As you should with a CRO. 

There have been several threads over the years where members have reported amplitude errors only then to pointed out that their variable V/div pot wasn't in the Cal position.
Pretty normal newbie mistake....most of us have at some stage been guilty of it. 

DSO's on the other hand have a fine V/div setting that accomplishes the same result as the V/div Cal on a CRO, that is providing amplitude adjustment between the coarse attenuation steps.

This fine adjustment is normally accessed by pushing the V/div selector and a further push toggles back to the default coarse adjustment.

[danadak]

Can also be used to G match two probes, so that when you are doing differential measurements,
you can maximize CMR.

Regards, Dana.

[vk6zgo]

In the past,the v/cm & time/cm controls were often used in the "uncal" position.

In many situations,the percentage difference in two voltage levels is more important than the absolute voltages.

For example,the nominal voltage between peak white & peak sync of a PAL analog TV signal is 1.0volt.
Between peak sync & blanking will be 300mv (30% of the overall voltage),& between blanking & peak white ,700mv (70%).

All this is cool,if you are looking at a signal which is exactly at nominal voltage,so it will fit perfectly between the appropriate graticule lines,but the signal might be a bit out,(but still within limits),say 0.995v,or 1.005 volts. 
For the former case,peak sync to blanking will be 298.5mv,& blanking to peak white will be 696.5mv,while for the latter,the values will be 301.5mv & 703.5mv,respectively.

OK,pretty close,but they don't quite fit the graticule lines.
Easy fix is to adjust the v/cm until they do fit,& you can read out the values as percentages.

With NTSC,a special IEEE graticule is used,as black level is slightly elevated above blanking,unlike PAL,so you need to fit the two voltage extremes into the graticule,& determine if they have the correct relationships.

Special graticules were used widely in analog TV, for the above & many other tests.
These were both physical & projected,with the latter preferred,as they don't have parallax problems.

Two  factors arose:
(1)The graticules were a standard size,& not all analog 'scope faces were,so the 'scope display size needed to be adjusted to suit the graticule.
(2)Some tests required the amplitude and apparent duration of only that part of the test signal which was of immediate interest to be "tweaked" to fit into the graticule.(There were normally reference points provided in the graticule).

With the advent of DSOs,the need to be able to continuously adjust v/cm & time/cm has diminished in importance,as cursors can easily provide percentage readings,& special graticules have been supplanted by masks.