Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Hi Said, Late reply... (been busy) The purpose of the straight drive is to maximize the signal amplitude on the cable. The test intends to demonstrate that it is perfectly adequate if only the far end is matched, and yet you get twice the amplitude you would get with a 50 ohm drive signal. On the other hand, if the near end is driven like that and the far end is open, you get very significant ringing even with a modest bandwidth oscilloscope (100MHz in my case). The longer the cable, the lower the ringing frequency and potentially the worse the uncertainty on the rising edge timing. Tom asked how much cable reflections could bother a 1 PPS signal and the answer is that it can bother the leading edge a whole lot, no matter what the frequency is. Of course, if you receive the 1PPS with a serial port connected to a busy CPU with multiple uS of latency, it won't make much difference unless the cable is so long as to be impractical. Didier KO4BB On Thu, Sep 18, 2014 at 10:00 AM, Said Jackson via time-nuts time-nuts@febo.com wrote: Hi Didier, You showed the effect of standard end-termination and no end-termination both while driving the cable with a mis-matched low signal source impedance. These are the same type of plots that Tom posted a link to last week. The interesting plots will be when you set R1 to match the cable impedance and then vary R2. Bye, Said Sent From iPhone On Sep 18, 2014, at 3:41, Didier shali...@gmail.com wrote: Hi Tom, This page will show you the kind of problems to expect: http://ko4bb.com/Test_Equipment/CoaxCableMatching.php Didier KO4BB On September 15, 2014 8:43:21 AM CDT, Tom Van Baak t...@leapsecond.com wrote: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. -- Sent from my Nexus 7 tablet. ___ 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. ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
It's the rise-time that kills you. You can under certain circumstances avoid termination, and gain better amplitude, but you then need to make sure that the reflections is not going to disturb your measurements. The general recommendation is to do proper termination. If you know the signal and reflections, you can play it differently. Remember, the sharper rise-time, the more high-resolution ringings you wake up. Sometimes you get better signal integrity simply by reducing the rise-time, as there is less energy to confuse you. In the end, this is a topic one can go deeply into. There are many quirks. Cheers, Magnus On 09/15/2014 03:43 PM, Tom Van Baak wrote: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Hi Tom, This page will show you the kind of problems to expect: http://ko4bb.com/Test_Equipment/CoaxCableMatching.php Didier KO4BB On September 15, 2014 8:43:21 AM CDT, Tom Van Baak t...@leapsecond.com wrote: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. -- Sent from my Nexus 7 tablet. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Hi Didier, You showed the effect of standard end-termination and no end-termination both while driving the cable with a mis-matched low signal source impedance. These are the same type of plots that Tom posted a link to last week. The interesting plots will be when you set R1 to match the cable impedance and then vary R2. Bye, Said Sent From iPhone On Sep 18, 2014, at 3:41, Didier shali...@gmail.com wrote: Hi Tom, This page will show you the kind of problems to expect: http://ko4bb.com/Test_Equipment/CoaxCableMatching.php Didier KO4BB On September 15, 2014 8:43:21 AM CDT, Tom Van Baak t...@leapsecond.com wrote: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. -- Sent from my Nexus 7 tablet. ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Hi Hal, I think its a slow driver, and a complex output impedance, not necessarily a weak driver. I have a Tbolt hidden somewhere and would need to dig it out. My 30 foot cable disappeared though, so I need to buy a new one :( On the small hump, i don't think that is a reflection. The signal risetime is just too slow to create such fast spurts. I think it may be a capacitor bridging a larger series resistor. Yuck. Oh, on the z3801 I thought it had a bnc 1pps output as well.. Bye, Said Sent From iPhone On Sep 16, 2014, at 13:07, Hal Murray hmur...@megapathdsl.net wrote: saidj...@aol.com said: here are some plots from two GPSDOs, one series terminated (CSAC GPSDO), and one load-terminated (Agilent 58503A) product. Nice pictures. Thanks. My reading of your pictures is that the 58503A has a weak driver. Do you have a TBolt? ... and there is a little hump right at the beginning. That could be a reflection from an impedance change. I wonder what's inside the box. I don't know what the output circuit of the 58503A looks like, but assuming that it probably is the same as on the Z3801 units many folks here have it is quite relevant to this discussion, and comparing these two approaches really shows the quality difference between these two products' signals. The Z3801 doesn't have a simple BNC connector for the PPS. Their are 2 pairs of differential PECL signals on the DB-25. I haven't looked at them. If all you want to do with the PPS is feed it to NTP, you don't need ns level accuracy. There is a standard recipe for converting the RS-422 levels to RS-232 and borrowing one of the drivers for the PPS signal. RS-232 drivers are deliberately slow enough to avoid most of these problems. The 10 MHz comes out on a BNC. That's what most hams use for multiplying up to GHz. -- These are my opinions. I hate spam. ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
saidj...@aol.com said: here are some plots from two GPSDOs, one series terminated (CSAC GPSDO), and one load-terminated (Agilent 58503A) product. Nice pictures. Thanks. My reading of your pictures is that the 58503A has a weak driver. Do you have a TBolt? ... and there is a little hump right at the beginning. That could be a reflection from an impedance change. I wonder what's inside the box. I don't know what the output circuit of the 58503A looks like, but assuming that it probably is the same as on the Z3801 units many folks here have it is quite relevant to this discussion, and comparing these two approaches really shows the quality difference between these two products' signals. The Z3801 doesn't have a simple BNC connector for the PPS. Their are 2 pairs of differential PECL signals on the DB-25. I haven't looked at them. If all you want to do with the PPS is feed it to NTP, you don't need ns level accuracy. There is a standard recipe for converting the RS-422 levels to RS-232 and borrowing one of the drivers for the PPS signal. RS-232 drivers are deliberately slow enough to avoid most of these problems. The 10 MHz comes out on a BNC. That's what most hams use for multiplying up to GHz. -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Fast risetime pulses _are_ RF and need to be treated as such. Tom - Original Message - From: Tom Van Baak t...@leapsecond.com To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Monday, September 15, 2014 9:43 AM Subject: Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver. How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Some good refs on coax driving showing scope traces for ringing etc: http://www.ti.com/lit/an/snla043/snla043.pdf http://www.ti.com/lit/an/sboa075/sboa075.pdf On Mon, Sep 15, 2014 at 9:43 AM, Tom Van Baak t...@leapsecond.com wrote: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
On 9/15/2014 10:01 AM, Tom Miller wrote: Fast risetime pulses _are_ RF and need to be treated as such. You say that as if simply saying it provides an explanation, or even a reason. Exactly what ill effect on a triggered measurement is there if one does not terminate a PPS signal properly? Does/can termination increase the slew rate or make the speed of propagation more consistent, which might make the measurement more accurate? Like Tom said, what comes after the leading edge of a PPS signal (which is the measurement trigger) seems irrelevant. A simple though experiment. If I take a high impedance measurement at a tap 1M from the source, and the cable ends another 100M away, how can the termination or lack thereof at that end effect my measurement of a single event? It's over 700 ns round trip away? If I repeat that event 1/sec, is it any different? I can see where there would be a difference when I get close to a 700 ns cycle time, and likely before because of ringing. But for a 1 second cycle? Someone will have to provide more than a dismissive just because to convince me. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Hi guys, Tried to bring my point across, but I guess I failed to do so properly. What happens after the edge is very important because what happens after the edge settles is up to 100mA DC current is flowing through all the coaxes AND your building ground. Pumping ~5V into 50 Ohms (Thunderbolt) results in up to 100mA DC current flowing. This current flows out into the center conductor then through the 50 Ohms termination resistor at the sink and then back through ALL your grounds due to the finite resistance of your coax. This includes the instruments' AC power cord, as well as any 10MHz coax you have connected! This DC ground current now does many bad things: 1) it can corrode the connectors over time in humid environments (eg shipboard) 2) it causes measurable and significant (~0.5W!) heating in the termination resistor ( I have IR video that shows the termination resistor blink like a christmas tree once a second) 3) it causes significant dips in the source power supply and heating of the driver ICs in the source 4) it causes a high voltage drop across all coax connections which results in a corresponding shift in the ground potential of the 10MHz signal and thus results in amplitude modulation of the 10MHz signal (CMOS). RG-142 shield has 0.0075 ohms per meter, so the AM modulation of the 10MHz signal over several meters could be in the millivolts - not conducive for measuring stability in ppt 5) if the termination fails or you leave the coax end-termination unconnected then your driver (a number of standard AC gates in parallel in case of the Thunderbolt) will get the full brunt of the reflected pulse which will be up to 10V for a significant amount of time so you are over-stressing that gate. If the termination fails or is disabled, your counter input or scope input may also be overstressed by the double amplitude. On the falling edge it gets even worse: the reflections generate negative voltages far below ground level and can also cause driver over-stress. In summary: End-termination is designed for maximum power transfer for RF signals. It should not be used for transmitting DC signals such as 1PPS signals (the 1PPS pulse is a very high frequency AC signal until the reflections settle in some 10's of nanoseconds, then it is a DC signal) Series termination such as used for reflected wave switching (ie PCI) is the way to go for 1PPS signals and has essentially no drawbacks for fast rising edges other than that a resistor must be inserted at the output of the driver. Hope I made the advantages of series rather than end termination clear. I understand that we all were taught in school that a coax needs to be terminated, and series termination is just that - but at the other end of the cable which is somewhat counter intuitive. The above except item 1) is easy to verify and a lot if fun to do. All you have to do is insert that single series resistor after the driving gates and remove the end-termination and your system will be updated to 21st century standards. Btw I have extensive scope plots comparing series- to end-termination over 10+ feet of coax if anyone is interested. Bye, Said Sent From iPhone On Sep 15, 2014, at 6:43, Tom Van Baak t...@leapsecond.com wrote: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Looks like this email did not make it: Hi guys, Tried to bring my point across, but I guess I failed to do so properly. What happens after the edge is very important because what happens after the edge settles is up to 100mA DC current is flowing through all the coaxes AND your building ground. Pumping ~5V into 50 Ohms (Thunderbolt) results in up to 100mA DC current flowing. This current flows out into the center conductor then through the 50 Ohms termination resistor at the sink and then back through ALL your grounds due to the finite resistance of your coax. This includes the instruments' AC power cord, as well as any 10MHz coax you have connected! This DC ground current now does many bad things: 1) it can corrode the connectors over time in humid environments (eg shipboard) 2) it causes measurable and significant (~0.5W!) heating in the termination resistor ( I have IR video that shows the termination resistor blink like a christmas tree once a second) 3) it causes significant dips in the source power supply and heating of the driver ICs in the source 4) it causes a high voltage drop across all coax connections which results in a corresponding shift in the ground potential of the 10MHz signal and thus results in amplitude modulation of the 10MHz signal (CMOS). RG-142 shield has 0.0075 ohms per meter, so the AM modulation of the 10MHz signal over several meters could be in the millivolts - not conducive for measuring stability in ppt 5) if the termination fails or you leave the coax end-termination unconnected then your driver (a number of standard AC gates in parallel in case of the Thunderbolt) will get the full brunt of the reflected pulse which will be up to 10V for a significant amount of time so you are over-stressing that gate. If the termination fails or is disabled, your counter input or scope input may also be overstressed by the double amplitude. On the falling edge it gets even worse: the reflections generate negative voltages far below ground level and can also cause driver over-stress. In summary: End-termination is designed for maximum power transfer for RF signals. It should not be used for transmitting DC signals such as 1PPS signals (the 1PPS pulse is a very high frequency AC signal until the reflections settle in some 10's of nanoseconds, then it is a DC signal) Series termination such as used for reflected wave switching (ie PCI) is the way to go for 1PPS signals and has essentially no drawbacks for fast rising edges other than that a resistor must be inserted at the output of the driver. Hope I made the advantages of series rather than end termination clear. I understand that we all were taught in school that a coax needs to be terminated, and series termination is just that - but at the other end of the cable which is somewhat counter intuitive. The above except item 1) is easy to verify and a lot if fun to do. All you have to do is insert that single series resistor after the driving gates and remove the end-termination and your system will be updated to 21st century standards. Btw I have extensive scope plots comparing series- to end-termination over 10+ feet of coax if anyone is interested. Bye, Said In a message dated 9/15/2014 06:44:21 Pacific Daylight Time, t...@leapsecond.com writes: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
I found the schematics of the Mini-T output circuit. It actually is a single 10 Ohms series resistor (R32) driven by six parallel 74AC04 gates. Thus only a single resistor change on the Mini-T would fix the issue. Since the gates probably have about 2 Ohms equivalent impedance, simply changing R32 to a 47 Ohms resistor would fix the problem. Also, another issue with the end termination happens when driving very long coax cables: RG-142 for example has about 60 Ohms center conductor resistance and 7.5 Ohms shield resistance at 1km length. So the cable DC resistance is almost 70 Ohms at 1km, which would make the voltage across the end termination drop to around 2V from the desired 5V. In this scenario the series terminated system would generate a nice 5V at the end of the cable after the reflections have calmed down (within some us). bye, Said In a message dated 9/15/2014 07:24:27 Pacific Daylight Time, tsho...@gmail.com writes: Some good refs on coax driving showing scope traces for ringing etc: http://www.ti.com/lit/an/snla043/snla043.pdf http://www.ti.com/lit/an/sboa075/sboa075.pdf On Mon, Sep 15, 2014 at 9:43 AM, Tom Van Baak t...@leapsecond.com wrote: How important are all these cable / termination / impedance issues for 1PPS signals? I know ringing and reflections are undesirable in many applications. But for 1PPS? I often use pick whatever cable, termination, and trigger level gives the cleanest edge, the best risetime. What happens to the signal tens or hundreds of nanoseconds after the edge seems irrelevant to me. Could one of you RF experts comment? /tvb ___ 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. ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
I don't claim to be able to do the math, but one could probably easily calculate it from the Fourier frequencies by looking at the attenuation by frequency over a 1km lengh: Loss per 100m over frequency: 10M100M400M1300M2300M 7dB14dB28dB 49dB72dB So looking at these numbers I am guesstimating a risetime in the us alongside a good handful of microseconds of propagation delay. bye, Said In a message dated 9/15/2014 11:31:28 Pacific Daylight Time, hmur...@megapathdsl.net writes: Also, another issue with the end termination happens when driving very long coax cables: RG-142 for example has about 60 Ohms center conductor resistance and 7.5 Ohms shield resistance at 1km length. RG-142 is far from low-loss. Does anybody use it at that length? What's the rise time at the end of 1 km with a 1 ns rise time at the input? -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Also, another issue with the end termination happens when driving very long coax cables: RG-142 for example has about 60 Ohms center conductor resistance and 7.5 Ohms shield resistance at 1km length. RG-142 is far from low-loss. Does anybody use it at that length? What's the rise time at the end of 1 km with a 1 ns rise time at the input? -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
So does adding ~80 pF per meter or 8 nF for 100 meters (RG58) to your output have any effect on the risetime? Because that is what it will see with an open cable. I am sure you can make a case for some condition(s) where an unterminated cable will still work. But it is not something we have been shown necessary to make precise measurements (something this group strives for). YMMV, Tom - Original Message - From: Mike S mi...@flatsurface.com To: time-nuts@febo.com Sent: Monday, September 15, 2014 10:29 AM Subject: Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver. On 9/15/2014 10:01 AM, Tom Miller wrote: Fast risetime pulses _are_ RF and need to be treated as such. You say that as if simply saying it provides an explanation, or even a reason. Exactly what ill effect on a triggered measurement is there if one does not terminate a PPS signal properly? Does/can termination increase the slew rate or make the speed of propagation more consistent, which might make the measurement more accurate? Like Tom said, what comes after the leading edge of a PPS signal (which is the measurement trigger) seems irrelevant. A simple though experiment. If I take a high impedance measurement at a tap 1M from the source, and the cable ends another 100M away, how can the termination or lack thereof at that end effect my measurement of a single event? It's over 700 ns round trip away? If I repeat that event 1/sec, is it any different? I can see where there would be a difference when I get close to a 700 ns cycle time, and likely before because of ringing. But for a 1 second cycle? Someone will have to provide more than a dismissive just because to convince me. ___ 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. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
On 9/15/2014 3:04 PM, Tom Miller wrote: So does adding ~80 pF per meter or 8 nF for 100 meters (RG58) to your output have any effect on the risetime? Because that is what it will see with an open cable. It's not nearly that simple. 8 nF distributed along 100 M is not the same as an 8 nF cap at the source. The example I gave was measuring 1 M from the source, then 100 M of cable beyond that. If the signal has a rise time of, say, 20 ns (Thunderbolt max spec), then anything added cable of more than 3 M (~ 10 ns one way) simply doesn't matter - there's no time for anything which happens to the signal beyond that distance to return in less than the rise time. The signal during the rise time can't know whether there's 3 M or 1000 M of additional cable, or whether the far end is terminated or not. I don't pretend to fully understand transmission lines, but I do know the basics. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
tmiller11...@verizon.net said: So does adding ~80 pF per meter or 8 nF for 100 meters (RG58) to your output have any effect on the risetime? Because that is what it will see with an open cable. That way of thinking only works if the risetime is long relative to the cable length. In this context, long enough is ballpark of 4 to 10 times the prop time of the cable. For long cables, the cable initially looks like the characteristic impedance of the cable. You can see that on a scope with the classic reflection pictures. In vacuum, the speed of light is very close to 1 ft/ns. Coax varies from 60-90% of that. So if you have 10 ft of cable, it's unlikely that your driver is slow enough for the lumped capacitor approximation to be valid. With shorter cables and old gear using HC chips, you might get there. -- These are my opinions. I hate spam. ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
They say that the system clock is 12.504 MHz and that they use both the rising and falling edges. That is about 40 nS between quantization time slots. The PPS can only appear on a 40 nS edge. I should be seeing 40 nS jumps in the waveforms. I do see ~40 nS jumps but they are less common. The PPS can only appear on a 40 ns edge -- of the imprecise, unstable, uncompensated, unshielded, crystal oscillator on the Resolution-T board. It may be 12.504 MHz, but it certainly isn't (and not intended to be) 12.50400 MHz. There's also instability in your 'scope or counter. Thus you will see significant jitter, drift, and wander in the observed 40 ns edges. All this is normal and expected any time you beat two oscillators against each other. The net result is that the jumps are very evenly (and not Gaussian) distributed anywhere from 0 ns to 40 ns. Again, look at the raw data, plots, and histograms that I provided. Especially the zebra plots which show just how varied the sawtooth error is over the span of minutes, hours, and days: http://leapsecond.com/pages/MG1613S/tic-72-hour.gif (3600x1800 pixels!) The waveform timing is clearly quantized but I am seeing ~2 nS jumps. My scope is a Rigol DS2202 which samples at 1 GSP in 2 channel mode. Could this 2 nS quantization be a result of the scope? Perhaps I should get my 400 MHz analog scope out (Tek 2465B) and repeat the measurements? Off-list you mentioned you don't have a ns or sub-ns time interval counter, so yes, I guess you should try the analog 'scope. I'm wondering now if your GPS receiver more stable than your scope's timebase. /tvb ___ 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.
Re: [time-nuts] Correcting jitter on the 1 PPSsignalfromaGPSreceiver.
Hi Actually your “best case” is where the clock in the Res-T is *not* 12.504000 MHz or a frequency that is +/- (N * 40 ppb) of that frequency. If it is, you get a “hanging bridge” in the data. At +/- (N * 20 ppb) you don’t get the classic hanging bridge, but you still get a bias. All of that assumes for simplicity that the sawtooth is at 40 ns rather than just a bit off from there. Bob On Sep 14, 2014, at 2:06 PM, Tom Van Baak t...@leapsecond.com wrote: They say that the system clock is 12.504 MHz and that they use both the rising and falling edges. That is about 40 nS between quantization time slots. The PPS can only appear on a 40 nS edge. I should be seeing 40 nS jumps in the waveforms. I do see ~40 nS jumps but they are less common. The PPS can only appear on a 40 ns edge -- of the imprecise, unstable, uncompensated, unshielded, crystal oscillator on the Resolution-T board. It may be 12.504 MHz, but it certainly isn't (and not intended to be) 12.50400 MHz. There's also instability in your 'scope or counter. Thus you will see significant jitter, drift, and wander in the observed 40 ns edges. All this is normal and expected any time you beat two oscillators against each other. The net result is that the jumps are very evenly (and not Gaussian) distributed anywhere from 0 ns to 40 ns. Again, look at the raw data, plots, and histograms that I provided. Especially the zebra plots which show just how varied the sawtooth error is over the span of minutes, hours, and days: http://leapsecond.com/pages/MG1613S/tic-72-hour.gif (3600x1800 pixels!) The waveform timing is clearly quantized but I am seeing ~2 nS jumps. My scope is a Rigol DS2202 which samples at 1 GSP in 2 channel mode. Could this 2 nS quantization be a result of the scope? Perhaps I should get my 400 MHz analog scope out (Tek 2465B) and repeat the measurements? Off-list you mentioned you don't have a ns or sub-ns time interval counter, so yes, I guess you should try the analog 'scope. I'm wondering now if your GPS receiver more stable than your scope's timebase. /tvb ___ 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. ___ 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.
