Pulse train generation - questions about PLL and jitter

[Eyal]

I need to generate a pulse train having a pulse width of 20ns and a period up to 100ms, selectable by software.

The jitter requirement of the period is 10ppm (1us over 100ms).

I thought to use PCI6221 to generate this pulse train but I could not find all answers in its documentation or anywhere else.

1. Can the PLL be used to get a clock frequency to the counters of 50MHz out of the internal clock of 80MHz (5/8 factor)?

2. Can the counters be set to output a pulse width of a single clock period in "Pulse Train" mode? (then the above 50MHz clock gives the 20ns pulse width).

3. The datasheet states the base clock accuracy is 50 ppm, in "Frequency Generator" mode. What is the clock period jitter in "Pulse Train" mode, including the PLL effect?

Is there any other counter/timer board that may be better for this job?

Thanks.

[hoffiz]

For this task first let’s take a look at the counter specification of the 6221.

3. The datasheet states the base clock accuracy is 50 ppm, in "Frequency Generator" mode. What is the clock period jitter in "Pulse Train" mode, including the PLL effect? NI 622x Specifications page 9 states as you said 50 ppm for the “Frequency Generator” for the pulse train mode it will be the same 50 ppm as it is stated I below the: “General-Purpose Counter/Timers” title.

I hope this knowledge base helps: How Do I Change The Frequency Output Of My Pulse Train On The Fly?

You will not be able to generate a 20 ns pulse, since you can only derive integer multiples of the 80 MHz time base in this case, 1/40MHz pulses will be the closest you can get to a 20 ns pulse.  Also another note is that the frequency generator does not have a selectable pulse width, it is a 50% duty cycle signal.

For more information look at the M Series User Manual  page 7-21.

Two other products that you might be looking at are High Speed Digital Inputs and Outputs and Function Generators; take a look at those and post on the right discussion forum if you have any questions.

Message Edited by Jaime F on 05-05-2008 05:38 PM