Triggering a CoaXPress camera: What about latency and jitter? - Part 1

Determining the latency and jitter in an imaging system is a difficult task. Each component in the system can contribute to these parameters. You have to take into account your frame grabber, the interface and the camera. Latency can occur at the start of the imaging process. In that case it is defined as the moment a trigger is created until the moment image integration starts. Latency can also occur at the end of the imaging process, then it is defined as the moment image integration is finished to the moment a decision can be made based on the image by a person or a computer. Jitter is defined as the variation on the latency. Where latency is a fixed number in a specific vision system, jitter is unpredictable.

In this article we will focus on the triggering part of the latency question. More specificly we will discuss the trigger options you have with a CoaXPress camera and the accompanying latency and jitter. The information that we provide should help you make a decision about what trigger option would be the best for you.

What trigger options do you have with a CoaXPress camera?

We have visualised the available options in Figure 1.

Frame grabber internal trigger
For most frame grabbers it will be possible to generate a trigger pulse on the frame grabber board. The frame grabber sends the trigger pulse to the camera over the CoaXPress interface.
Camera external trigger
For most cameras an external trigger port will be available using an I/O connector. This trigger will circumvent the CoaXPress interface.
Frame grabber external trigger
Frame grabbers often have an external trigger port as well. If you trigger by this port the frame grabber will convert the trigger signal to a CoaXPress trigger signal such that the trigger can reach the camera over the CoaXPress interface.

Figure 1. The three options for triggering with a CoaXPress camera.

What are the latency and jitter sources in the triggering scheme?

For the three options previously mentioned there are basically 2 main elements that can cause latency and jitter.

The CoaXPress interface
The external trigger port, i.e. the electronics that connect the external trigger signal with the internal electronics of the frame grabber or camera.

For the latter point it does not matter if the trigger port is located on the frame grabber or the camera. Both can have the same technical implementation to connect the external signal with the internal signal.

This means that the listed trigger options encounter the following delays:

Frame grabber internal trigger
This trigger signal only encounters the CoaXPress interface delay.
Camera external trigger
This trigger signal only encounters the delay caused by the electronic circuit of the trigger port.
Frame grabber external trigger
This trigger signal encounters both the delay caused by the electronic circuit and the CoaXPress interface.

Now that we know the sources of latency and jitter we can start to zoom in on the actual contributions of these sources. That however will be the topic of our next blog.