USB 6341 counter output

[Leonid_Pascar]

Hi,

I'm trying to use the output counter  of USB 6341 for generating a square wave with a frequency between100 KHz to 1Mhz.

I used 100Mhz and 20 Mhz time bases and the 10Mhz ref clock as the source clock.

I checked the output signal in the oscilloscope.The performance was good for frequencies close to 100KHz.However, the output signal was distorted for frequencies higher than 300 Khz  

Is there a way of achieving a non distorted waveform for this frequency range?

Thanks,

Leonid

[M.J]

Hi Leonid,

Could you provide more information on the type of distortions you are observing? Maybe also screenshots of the output around 100KHz and higher than 300KHz.

[Leonid_Pascar]

Hi MJ and thanks for the reply.

I observed that the ratio between rising time and the period time is increasing for higher frequencies ,which is logical. However,I didn't expect that it would be a significant  distortion because of the internal clock and time bases of the device(up to 100Mhz)

The attached drawing(I'm currently not at work so I can't attach the oscilloscope screenshots) emphasizes the problem.

Thanks,

Leonid

[M.J]

Hi Leonid,

Could you try a few troubleshooting steps:

1. Generate that 1MHz signal and use another counter on your NI 6341 to read the signal. This would help verify is the distortion source is definitely your NI 6341.

2. It would be best to get screenshots of the oscilloscope to better analyze the observed behavior.

3. What type of connections do you have between your NI 6341 and your oscilloscope? What is the path of the signal? I ask because I would like to see if it could be path induced distortion.

[Leonid_Pascar]

Hi MJ,

Screenshots of the Ossiloscope attached (300Khz 1Mhz and Path)

Thanks,

Leonid

[M.J]

Hi Leonid,

It seems like you are observing an underdamped response. I am leaning towards the path introducing the distortion but need a few more details to better understand how to move forward.

1. Could you use Ossiloscope probes with shorter wire? Do you see any sort of improvement? Screenshot?

2. Could you also please read the frequency of the signal internally. You can use the Read Pulse Width and frequency (Continuous).vi in the LabVIEW example finder and select the input terminal to be the counter you are using to output the signal (example: /Dev1/Ctr1internalOutput). Note you may need to set the enum to "include advance terminals" within the I/O name filtering setting. I ask for this step to assure that the read frequency of the output signal is correct to the NI standards with the NI USB 6341.

[LSS]

I am late to the party, but shouldn't the input impedance on the scope channel be set to 1Mohm?

[Andy-C]

A low input impedance on the scope could certainly be a factor here in case there has been an update. I hope the problem has been sorted by now, but good eye!

Cheers,

Andy C.

Applications Engineer

National Instruments