meauring the period of a differential TTL signal
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From: shyammenon on 18 Jul 2008 12:40 Hi all, My system has a pci-6052e series card connected to an scxi-1000 chassis and i am using an scxi-1302 terminal block to access the counter pins from the card. I have a differential square wave (ttl compatible according to the manufacturer) signal that i want to measure the period (and frequency) of. The signal is a low-mid range frequency signal (upto 300 hz). When i use the measure period.vi to measure the period of the signal (which i connect to the gate pin of the counter), i keep getting incorrect measurements (i am checking using an oscilloscope). But the same vi works perfectly fine if i use a pulse train generated by another counter(even at very low frequencies). So i am wondering if it is because this is a differential ttl signal and if thats so, how do i fix this problem. Where do i connect the reference end of the signal to..? Would be glad to get any help i can on this. Thanks in advance, Shyam.
From: ShotSimon on 18 Jul 2008 13:10 Sounds to me like a signal level problem. Can you get an o-scope screen shot of the working signal and the non-working signal? Which of the two signals are you driving your logic with? If for example you are using the low end, you may want to try using the high. Sometime differential signals use a different reference than ground say 2.5V and one signals swings to 5V while the other swings down to 0. The level of the signal you are using to drive your logic may not be adequate. Regards, -SS
From: shyammenon on 18 Jul 2008 14:10 Hi, I am attaching a picture of the scope outputs from the two signals. Is there a grounding pin on the scxi-1302 where i can connect the reference (ground) of the signal..guess i should be checking the manual!!.Thanks, Shyam. oscilloscope pic.JPG: http://forums.ni.com/attachments/ni/170/342267/1/oscilloscope pic.JPG
From: AnalogKid2DigitalMan on 18 Jul 2008 14:10 I can't get to the manuals now (slow web) but could it be a slow rise time issue on your sensor signal? You may want to look in the manual for any min/max risetimespecs on the counter inputs. Signal looks TTL compatible, you could use a schmitt trigger circuit to get faster edges.
From: ShotSimon on 18 Jul 2008 14:40
Can you include the excel file so we can see the rise time between data points and transistions. Sorry but your plot is formatted so it's hard to make out details other than that the amplitudes look the same. -SS |