scan clock gating for very fast signals?

[Specter]

Hello,

I am using a modulator to pulse a laser.  The laser's rep rate is ~50MHz...pulse width is ~5nsec (so roughly a 25% duty cycle -- it's "ON" for 5nsec and then "OFF" for 15nsec).  The modulator requires a DC bias voltage in order to maintain its extinction ratio, and this bias voltage tends to drift over time.  I need to design a feedback circuit which will tune the DC bias voltage such that the modulator maintains its maximum extinction ratio, as all sorts of nasty things happen to the laser when the extinction ratio isn't good.

What I am hoping is possible is to use scan clock gating to eliminate the "ON" part of the signal and look only at the "OFF" part.  I've attached a scribble of roughly what I want to do.  My analog acquisition would be performed on what is labeled the "Resulting signal".

The drift takes place on a time scale of seconds, so I don't have to acquire data and perform adjustments at 50MHz.  I can acquire data at a lower repetition rate (i.e. something a multi-function DAQ board could conceivably handle).  What I do have to do, however, is gate at 50MHz.  I can't have any of the laser pulse corrupting the "OFF" signal.

Is it possible for an M-series multifunction DAQ board to gate at 50MHz, then acquire the gated signal at some lower rate?  For instance, I could gate as described above at 50MHz and then collect data every 1000th counter gate (which would be an acquisition frequency of 50kHz) and that would be plenty good enough for me to tune the bias voltage since the drift is slow.

Is this do-able? 

Thanks,

d

[Speedy_Badger]

Hello Specter,

I have looked over the diagram you included and the description of your application. I am assuming that the signal used to gate your input would be external to the M-series device. While scan clock gating (referred to as pause triggering in the context of DAQmx) is theoretically feasible to control your application, the frequencies you require are not within the capabilities of the M-series device. The largest external timebase that an M-series device can use is 20MHz.

Additionally, the bandwidth for analog input on an M-series device is much smaller than that of the signal you are attempting to acquire. The PCI-6251 for instance has a small signal bandwidth of 1.7 MHz. Any information above 1.7 MHz will be filtered out and spread across the remaining signal.

The above consideration could in fact be beneficial to your feedback application. If your modulation pulses are constant and consistent throughout the signal, they will be evenly spread across your signal due to the filtering. This means that you will be left with your bias signal plus a constant offset. Overall, ignoring this constant offset, you should still be able to see the change in bias voltage over time.

I hope this gives you some insight towards developing your application.

Regards,