PXI 5152 High Speed Alternating Reference Trigger Source

[tkmn]

Hi,

I am trying to measure the time difference between triggers on two different channels with very high time resolution.  Currently, I believe I can use one of my sources as an advance trigger and the other as a reference trigger and use the relativeInitialX for each record as the time between these two triggers.  However, as I understand it, the time of the advance trigger (absoluteInitialX) is measured only with the time resolution of the maximum real-time sampling period of the digitizer (1 ns in our case) while the time between the advance trigger and the reference trigger (relativeInitialX) can be measured with higher time resolution (50 ps?).  Thus, I believe this method only gets me high time resolution on one end of the measurement and doesn't achieve the maximum possible time resolution on the calculated time difference.

I am now wondering whether it is possible to configure the digitizer to look for reference triggers from alternating sources before I start an acquisition.  That is, would it be possible for me to configure the reference trigger so that it first looks on Channel 0 and then when it is triggered on this channel, look on Channel 1 and then back on Channel 0 and so on?

Thank you for your time.

[S_Hong]

Hi tkmn,

Unfortunately there is no way to directly access the TDC on your PXI-5152 to make finer time measurements between your two triggers and therefore, the best time resolution you can achieve with this device is 1 ns. Is there any particular reason why such a time resolution would be required in your application?

[tkmn]

Hi S Hong,
We want to accurately time single photon events which may occur very quickly after an excitation signal in order to generate a time histogram of when these events have occured.  While I understand that I do not have direct access to the TDC, I believe I can get precise timing of a reference trigger by using the absolute and relative initialX values.  Since, our excitation and response signals are on two different channels, I wonder if there was a way of forcing the reference trigger configuration to switch rapidly between one channel and another.  I believe we can work with a rearm time of 10+microseconds because we can run the timing in reverse (find the time between a photon event and the next excitation event).  Any ideas?

[S_Hong]

Hi tkmn,

Unfortunately, according to the NI-SCOPE Help file, the relative initialX value only gives you the time between the first point in your waveform (determined by your reference clock) and the trigger. Therefore, you will not be able to use this to get the time difference between two triggers. However, I think that you may be able achieve your task by using two PXI digitizers and synchronizing their clocks. You can use the first digitizer for the first trigger and the second, for the second trigger. And since the relative initialX value should be based off the same reference clock, you may be able to take the difference of the two to find your final time difference.

I hope this helps,

[tkmn]

Hi S Hong,
Perhaps I have misunderstood the usefulness of relative and absolute initialX.  I was thinking of using the absoluteInitialX, which is the time of the first point in the waveform and which I understand to be comparable between records and acquisitions, and subtracting the relativeInitialX value, which is the time between the first sample and the reference trigger, in order to get the "absolute" time of the reference trigger.  Then I believe I can subtract two reference trigger times derived in this manner in order to get a precise time difference.  Do you think this is a viable solution?

Thanks again for you time,
tkmn

[S_Hong]

Hi tkmn,

You are correct in saying that you can achieve sub-nanosecond resolution for your reference trigger by using the following equation:

absolute trigger time = absolute initial x – relative initial x

where "absolute initial x" is the time of the first point in the waveform in units of seconds (this has a resolution of the maximum real-time sampling period of the digitizer) and "relative initial x" is the time between the first point in your waveform and the reference trigger (this has the time resolution of the TDC).

Although this will allow you to get sub-nanosecond resolution for one reference trigger, since you can you only trigger off of one source in any given session, you will be unable to get the absolute trigger time from a second trigger source. However, if the time between triggers is long enough, you may be able to trigger your data acquisition to sample one record containing a single data point and reinitiate a new data acquisition to receive your second trigger. In the event that this is not possible you can use two scopes on the same chassis and synchronize their clocks and each have a different trigger source to obtain their respective absolute trigger times and find the time difference.

I would also like to point that depending on the type of trigger ou are receiving, there may be a way of using interpolation to give you sub-clock period resolution.