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DG645 Tech Support

If you are experiencing trouble with your DG645 you can either send your instrument back to SRS for service, or you can try and fix the problem yourself. If you wish to send the instrument back now click here.

If you wish to try and fix the problem yourself, the following troubleshooting guide may help you.

DG645 Troubleshooting Guide

Symptom 1 — The trigger rate error light is on

DG645 Digital Delay Generator is a precision 8-channel delay generator that can output arbitrary delays from 0 to 2000 s with 5 ps resolution. The DG645 includes an internal rate generator that provides triggers at rates of up to 10 MHz. However, under default settings the DG645 will generate rate errors for trigger rates above about 8.7 MHz. Furthermore, even if every delay is set to 0 ns, the DG645 will generate rate errors for trigger rates above about 9.5 MHz. This sets an upper limit for the effective maximum trigger rate for the DG645.

The reasons for this are discussed below.

The maximum trigger rate that the DG645 can handle is governed by three parameters: the insertion delay, the longest programmed delay, and the reset time. The timing diagram presented summarizes the relationships between these three parameters.

The insertion delay is the fixed delay from when a trigger is applied to the external input to when T0 fires. For the DG645 this insertion delay is about 85 ns. The DG645 provides 8 channels of programmed delay, labeled A through H. On the front panel, adjacent delays are combined to provide the rising and falling edges of a pulse. These outputs are identified in the diagram as AB, CD, EF, and GH. For computing the

maximum acceptable trigger rate, only the longest programmed delay is relevant. In the diagram, channel H has the longest programmed delay. The final component to consider is the reset timing. This is the time it takes the DG645 to reset its circuitry and arm it for the next trigger. For the DG645 this is about 25 ns. The minimum period between triggers is given by:

Tcycle = Tinsertion + Longest Delay + Treset = 85 ns + Longest Delay + 25 ns = 110 ns + Longest Delay

If we set all delays to 0 ns, the longest delay will be 0 ns. In this case, the minimum Tcycle = 110 ns, which corresponds to a maximum rate of about 9 MHz. Under default settings the longest programmed delay is 10 ns. In this case, the minimum Tcycle = 85 ns + 10 ns + 25 ns = 120 ns, which corresponds to a maximum rate of 8.3 MHz. A typical DG645 will actually trigger up to about 9.5 MHz with 0 ns delays and 8.7 . This is indicative of the fact that the actually insertion delays and reset timing are a bit shorter than specified. Using the formula above, the user can predict the maximum acceptable trigger rate for any given set of programmed delays.

There is one special circumstance where the DG645 can accept triggers at a rate exceeding 10 MHz. This is when bursts are used. The insertion delay between triggers of a burst is shorter than the insertion delay of the first trigger. This makes it possible to process N burst triggers at a rate exceeding 10 MHz. However, successive triggers of the burst will have to obey the normal timing presented above.

If you are still having trouble your DG645, it will need to come back to SRS for service. Click here to get an RMA number.

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