50 ohm coupling with LT344

[macarpo]

Hello,

I have a problem acquiring a signal of the Lecroy LT344 scope with Labview.

In fact, the only thing that I need is to capture and store the signals just as they are in the scope with a vi, something that can easily be done with the Scope Xplorer of Lecroy.

But I am facing some problems with Labview, with the instrument vi's proposed I only have access to 3 type of couplings of the scope AC, DC and ground, and I cannot choose th DC 50 Ohm termination, that it is available in the scope but not with Labview function. Is there any low level function available to do that? Is there any low level function to capture the functions as they are in the scope screen without using Read Waveform or Fetch Waveform?

Sorry if these are silly questions but I have not found any solution yet and I am quite new in this topic.

Many thanks for your help.

Regards,

I.

[alanjb]

Alan from LeCroy Tech Support here...

What do you mean by "with the instrument vi's proposed "?  Which driver are you using? 

If you are using the "lcltxxxx" driver, this includes a Configure Channel vi that includes a terminal for "vertical coupling" that has choices of AC, DC and Ground.  There is also a vi called Configure Channel Characteristics that has a terminal for "input impedence" with choices of 1Mohm and 50ohm. This is how to setup the scope for 50ohm coupling.

If you are not using the lcltxxx driver, and you find no VI to setup input impedance, then you instead us the VI's "Write" vi to explicitely send in the following string:   C1:CPL D50    This is the command to set the channel 1 coupling to DC 50ohms.

(Note that there is a completely different command and VI for setting the trigger coupling, as opposed to the channel coupling)

As for commands to capture the functions... Fetch Waveform is will retrieve the data; this function sends low-level command to get the data (in binary), and decodes it to give you an array of voltage values.  Is this not working for you? 

Best Regards,

Alan

[macarpo]

Hi Alan,
thank you very very much for your reply, and sorry that I haven't noticed that with Configure Channel Characteristics you could do that.
Yes, I am using lctxxxx as instrument vi's.
I will try with Fetch Waveform, it is working, it is just if I am not wrong again, you have to set the number of samples that you want and, what I would like, is just something that reads the total samples represented in the scope screen. But I imagine that you can read the sample rate of the scope and the time scale setting to calculate that. Am I right?
Thanks again.
cheers,
I.

[Dennis_Knutson]

Selecting the input impedance is different and separate from coupling. The input impedance is selected with the lcltxxxx Configure Chan Characteristics function. Looking at the block diagrams of the examples is a good way to find out how to setup the scope and use the functions. The example lcltxxxx Acq Wfm Edge Triggered is the specific one I'm talking about.

What do you mean by capturing functions in the scope screen? If you are taking measurements (i.e. rise time, fall time, etc.) you can use the Read Waveform Measurement and Fetch Waveform Measurement. If you are talking about how the scope is setup (i.e. vertical range, horizontal timing, etc.), in theory, these are things that you should be setting in your program. A read of all of these settings are not normally included in a driver. They are easy to implement though if you are allowing manual setup and need to retrieve the information. In modern instruments, you use a command similar to the one used to set something but replace the value used for the setting with a question mark. for example, if the command SEL:CH1:COUP:DC will set channel 1 coupling to DC, sending the command SEL:CH1:COUP? and then doing a read will return DC. You can issue any command you want and do a read with lcltxxxx Write Instrument Data and lcltxxxx Read Instrument Data.

[alanjb]

If you find that you need to send commands or queries using the Write Instrument Data and Read Instrument Data vi's, I recommend that you download the remote control manual for the Waverunner (AKA LT series): http://www.lecroy.com/tm/library/manuals/WaveRunnerSeries/RemoteControlManual/WR2_RCM_RevB.pdf

ScopeExplorer is an excellent application for testing out commands and queries, since it includes a terminal window.  See the attached image that shows how to query for the number of points in your trace:  C1:inspect? 'wave_array_count'   (The inspect? query lets you probe the value of descriptor block field...)

Best Regards,

Alan