My circuit's ringing

George Herold · 2010-11-24T20:06:06+00:00

Testing a counter today. The input comes from a PMT, the pulsetriggers a comparator that then goes to an inverter (74HC14) and outinto the digital world. (Goes into the counter of a PIC.)Here=92s a =91scope shot with just the comparator and inverter.My problem is that the output has a rather lengthy trace to drive ~15cm. Andwhen that=92s attached the signal looks like this,Channel 1 is from the pmt and channel 2 the output of the inverter.I don=92t seem to be getting any double counting when I set the level onthe comparator lower so that it would overlap with the ringing, but itreally looks ugly and I=92d like to clean it up. Any suggestions?Would it help to terminate the long trace with maybe a 1k ohmresistor? Should the trace width be adjusted so that it=92s close to 50ohms?There are separate grounds under the input section and the rest of thedigital section, was this a mistake? (I didn=92t layout the board, justtrying to make it look nice.)ThanksGeorge H.

Nov 24, 2010 20 Replies

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Thanks whit3rd, The PMT is a side veiw (R212 from Hamamatsu.) the socket and high voltage (one of these Cockroft-Walton sources) is built into a single unit. So I don't have much opportunity to play with the HV resistors. We want the students to set the threshold level of the comparator, a stand alone comparator seemed easier. And yes I could add more resistance from the PMT anode to ground. (currently set at 300 ohms) but as you most likely know this comes at the expense of the pulse width (~RC where C is the capacitance of the anode plus whatever my circuit adds.) Actually it's not clear that increasing the resistance will give me any more signal amplitude..... Doesn't it just make the impulse response longer? (I better check this.)

George H.