XY graph and map mode

[Kaspar]

Hello,

  I am using a XY graph and using the map mode for the Y axis to change the Yaxis to a logarithmic mode. The min and max values of the y axis are set to -50 and -7. The y axis of the XY graph is display as 0.01 (min) 10 max. Why doesn't the XY graph accpet the main and max limits that I sent to it?

[johnsold]

What is the logarithm of a negative number?

You can certainly set the mapping mode to be logarithmic. I have a progam running right now which does it.  But if you try to force a negative value for the scale while it is in log mode, it will not resond.

Lynn

[Kaspar]

Hello,

   I need to have the Y axis in log mode with negative numbers because I am trying to duplicate a screenshot of a spectrum analyzer that is taking measurements that range from -7 to -20.

[johnsold]

Are those values in decibels?

Lynn

[Kaspar]

Hello,

  Yes the values are are in dbm, but as far as Labview graph go, the units hould be irelevant. I am trying to duplicate the screen on a HP8562E spectruum analyzer that alsway has a 10 by 10 grid to display waveforms.

[johnsold]

It can be done, bu it may not be trivial, depending on how automated you want it to be.

First, create a duplicate scale.  See the LV Help page titled Adding Multiple X- and Y-Scales on Graphs or Charts.

Next, calculate the values in dBm from the range in the data or the range of the instrument.

Use property nodes to set the properties of one scale to display the dBm values and hide the other scale.

Here is a little example which shows what I tried.  Some of the things in there may not be needed.  This is not optimized.

Lynn

[Hornless.Rhino]

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@Kaspar wrote:

Hello,

  Yes the values are are in dbm, but as far as Labview graph go, the units hould be irelevant. I am trying to duplicate the screen on a HP8562E spectruum analyzer that alsway has a 10 by 10 grid to display waveforms.

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If you're just trying to make it match the screen you could"

1) negate the vale and then

2) make the axis go from big to small rather than small to big,

so as Y increased (which would be increasing -ve in the actual value) to would move down the y axis. (then just make sure the axis is labelled (-ve) or something.