Hi guys, A year ago I posted a question concerning the 1PPS output from my PRS10. I had thought it an anomaly, but other users indicated that the signal profile was common to other units. I have since added 2 PRS10s (actually TDS12s) to my bench and they also show the same profile. Jarl Risum was very helpful in my investigations, and proposed using inverters to get a better shaped signal. I tried this out, but am not satisfied with the induced delay. In the end I settled on an AND gate for the project. In the forwarded mail below (hope the pic passes the censors) you can see the object of my irritattion is that although I have a nicely conditioned output, there is too much delay between the initital 1PPS input rise and the gate response for even mildly afflicted timenut . I couldn’t figure out what the source of this was until I reviewed it the other day with an object of distributing 1PPS over the bench. From looking at 74HC device data sheets I think the issue is that the start of the 1PPS pulse input, (the bit I am interested in) does not reach the trigger level needed for CMOS gates.
Does that seem a reasonable assumption?, and if so is there a solution? I am wondering whether I could use a comparator? The trigger level of a LT1719 is better than 6mv but I fear that I might get false triggering as my scope indicates that the 0V level has <=2mv noise. I don’t have one to test so I would appreciate your input. I also can’t see how I might implement it if it is worth testing. Regards,Mike > Début du message réexpédié : > > De: mike cook <cook.mich...@orange.fr> > Objet: Rép : PRS10 question > Date: 1 mars 2014 16:50:12 UTC+1 > À: Jarl Risum <jarl.ri...@gmail.com> > > Hi Jarl, > Sorry about the late update but I was waiting for parts. > > I tried using a 74HC04 (I didn't have an HC14 but I only think the they > differ in performance) in your suggested config on a separately powered > breadboard but that doesn't give me what I want . > It just gives a slightly attenuated copy of the original, which is to say I > still have the slow rise to vcc after the initial 10ns of good rise. What I > was looking for is something that triggers on the first 10ns and then forgets > everything else after that, putting vcc on the output, until the input drops > to 0, when the output should also go to zero. That is a simple AND operation. > How about an AND gate? > So I tried a 74HC7001. Now I get 10ns rise to 2V and 20ns to Vcc. There is a > bit of overshoot but none of the ringing in the original plus a shorter > settling time that I can probably smooth out if I needed. There is as you > can see quite a bit of delay which I would need to characterize. However it > looks much more usable. > > Regards, > Mike > > > > > > > Le 14 févr. 2014 à 16:08, Jarl Risum a écrit : > >> Mike, >> >> Thanks for the feedback. OK - the coax cable explains your result so far. >> You may try an ordinary 10:1 probe directly on the BNC connector on the >> breakout board, but be careful to check - and if necessary correct - the >> probe response before use. The probe needs to have at least 100 MHz >> bandwidth. >> >> Your idea to use a buffer attached directly to the BNC connector on the >> breakout board is of course fine, but you should avoid loading pin 8 with >> anything except what it is intended for: as a supply to a lab. quality 10 >> kohm 10 turn potentiometer as an external means to correct the C-field in >> the Rb Physics Package. Pin 8 is a precision 5V reference also used by the >> internal 10 turn C-field potentiometer and you are almost sure to disturb >> the calibration if you load pin 8 with anything else. Please take a look at >> diagram RB_H1 (10MHz Ovenized Oscillator). The 5V reference circuit is at >> the top of the page, U100 and U102B. >> >> If you do not wish to add a 7805 5V regulator IC to your circuit yourself >> you might be able to get a few mA's from the 5V regulator on the breakout >> board. >> >> As a final remark I would like to draw your attention to the possibilty of >> using the 10 MHz output instead to feed a divider chain or a PIC divider >> which could output the 1 PPS where you need it. This solution would be >> somewhat more flexible with respect to interface problems since the 10 MHz >> could be distributed in coax cable over 10's of meters without any problem. >> >> Cheers >> Jarl >> >> >> >> >> 2014-02-14 14:57 GMT+01:00 mike cook <cook.mich...@orange.fr >> <mailto:cook.mich...@orange.fr>>: >> Jarl, >> Thanks for your tech comments. As far as electronics goes, I don't rise much >> above the level of a script-kiddie. I have in fact got a 2 meter 50Ohm coax >> into my scope. I will try to probe the output of the 74H140 to see what >> that looks like. Rather than use coax to connect in a buffer circuit, I was >> thinking of a breadboard dongle with BNC male and female connectors soldered >> to it, maybe slid into a metal tube once tested. I wouldn't be surprised to >> find it has already been invented. I might be able to steal some juice from >> the PRBB without braking anything. I should have 5V on pin 8 if I can get at >> it. I was only thinking of including the single output. If I can get a good >> square wave, I would use it via a distribution amp. I'll let you know how I >> get on. >> Mike >> >> Le 14 févr. 2014 à 12:17, Jarl Risum a écrit : >> >>> Mike, >>> >>> Thanks for the update. >>> >>> As you know already from the diagram of the Breakout Board RB_F7, the 1 PPS >>> output is a 74HC14-7 inverter with a series resistor of 1 kohm to protect >>> the gate from external "accidents". In order to speed up the risetime, SRS >>> has put a 100 pF capacitor in parallel to the 1 kohm resistor, but this >>> value cannot be increased while still protecting the 74HC17 output from a >>> short or similar abuse. >>> >>> What I would guess from the scope picture you attached is, that you use a 1 >>> m long piece of coaxial cable between the breakout board and your >>> oscilloscope (or a 1:1 probe with 100 pF input capacitance). That would >>> explain the waveform, since the fast risetime portion is about half of the >>> total height of the total step followed by the slow rise due to the 1 kohm >>> resistor charging the parallel 100 + 100 pF from the initial voltage step >>> and all the way up to Vcc. >>> >>> If you want to examine the waveform without capacitive loading effects you >>> would have to use a (frequency compensated) probe with a very low input >>> capacitance and perhaps even tap the waveform directly on pin 12 of the >>> 74HC14. >>> >>> If you want to make practical use of the 1 PPS output, I suggest that you >>> attach a buffer circuit to the breakout board output with the shortest >>> possible piece of 75 ohm screened cable. The buffer circuit could consist >>> of an inverter followed by 5 inverters in parallel. The output inverters >>> should each have a low value resistor in series (100 ohms ?) in order to >>> protect the outputs from unequal current sharing. With the eqvivalent of >>> 5x100 ohm series resistors in parallel you would have a much lower source >>> resistance available to drive an external load. You will need to decouple >>> the Vcc pin of the buffer circuit with a 0.1 uF ceramic capacitor to GND >>> with very short leads due to the large circulating currents. The buffer >>> circuit would add some delay, but that would be difficult to avoid in any >>> case. >>> >>> Hope you find a solution to your problem, Mike. Please let me know if I can >>> be of any help. >>> >>> Cheers from >>> Jarl in Denmark >>> >>> >>> 2014-02-14 10:54 GMT+01:00 mike cook <cook.mich...@orange.fr >>> <mailto:cook.mich...@orange.fr>>: >>> Hi Jarl, >>> I received my breakout board from SRS. Thought I'd give you an update. >>> I had a mechanical issue with one standoff being too long which I reported >>> back to them. They were kind enough to offer a replacement board at no >>> cost. However I am keeping it as the problem doesn't prevent it from >>> working. As you can see from the attached traces my problem with the slow >>> rise is only half solved (upper trace). The inverters on the board >>> certainly give me a quicker rise time for the start of the trace, better >>> than that of the GPS trace below , but it is only useful for about 650mv as >>> I get a little ringing, it then ramps up slowly as per the original. I'd >>> much rather have the shape of the GPS trace. I had an email chat with a guy >>> at SRS and he thought that the trace wasn't good, but couldn't tell me if >>> it was normal or not. He did suggest me terminating into 50Ohms, but that >>> was not the solution. Maybe I can recondition the input somehow, or on >>> output? The other parts of the interface, 1PPS in and 10MHz out are good. >>> Regards, >>> Have a good one as the Yanks say, >>> Mike >>> >>> >>> >>> <PastedGraphic-1.tiff> >>> >>> >>> Début du message réexpédié : >>> >>>> De : mike cook <michael.c...@sfr.fr <mailto:michael.c...@sfr.fr>> >>>> Date : 28 janvier 2014 21:48:29 HNEC >>>> À : Jarl Risum <jarl.ri...@gmail.com <mailto:jarl.ri...@gmail.com>> >>>> Objet : Rép : PRS10 question >>>> >>>> Jarl, >>>> I have ordered an interface board from SR. I hope it will lessen the >>>> amount of spaghetti I am wrestling with at the moment. >>>> I had figured that it would be easy enough to put a better shape on the >>>> pulse but didn't want to add to my open plan wiring scheme before I >>>> receive the breakout board. >>>> >>>> thanks for the thought . >>>> >>>> Mike >>>> >>>> >>>> Le 28 janv. 2014 à 19:40, Jarl Risum a écrit : >>>> >>>>> Mike, >>>>> >>>>> If you are still worried about the rise time of the 1 PPS output signal >>>>> from the PRS10 I suggest you to look into the description and diagram of >>>>> the RB_F7 Connector Interface Board for the PRS10, which you can find >>>>> somewhere on the SR homepage. On this Interface Board the 1 PPS from the >>>>> PRS 10 passes two 74HC14 Schmitt inverters before being passed on to the >>>>> BNC connector for external use. >>>>> >>>>> The 74HC14 has a rise time of around 7 ns, but of course adds some delay >>>>> to the pulse as well. >>>>> >>>>> Cheers from >>>>> Jarl >>>>> >>> >>> < snip the history> >>> >> >> > "Ceux qui sont prêts à abandonner une liberté essentielle pour obtenir une petite et provisoire sécurité, ne méritent ni liberté ni sécurité." Benjimin Franklin _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
