Re: [time-nuts] Isolated 1PPS-input for distribution-amp?
Anders wrote: I'm looking for a solution for galvanic isolation of a 1PPS signal-input to a distribution amplifier. * * * Does anyone have experience with a circuit with good rise-time and low jitter? Sometimes the simplest solutions can be best. In this case, capacitive coupling of the source into the load may be appropriate (see attached diagram). If the two chassis can be connected safely, all you need is to cap-couple the signal line (upper diagram). While there may still be a ground loop between the two chassis, it will not affect the PPS signal because the small capacitor used to couple the signal line (probably around 100pF, depending on the load impedance and the PPS pulse width) will not carry any significant current at line and line harmonic frequencies. If the two chassis cannot be connected safely, then cap-couple the common line as well (lower diagram). The common cap must be rated for the maximum possible voltage between the two chassis. The two caps should have approximately the same value. The capacitive isolation can be at either end (source or load). I prefer putting the AC coupling at the source end, so the interconnect cable is shielded at all frequencies to the load. This scheme will only work if the PPS output is robust enough to drive the load impedance without further buffering. Also, there is no DC reference at the load end. This may not be a problem if the PPS pulse is short (tens of uS). Just make sure there is a DC path to ground on the signal line (input termination resistor). Otherwise, you would need to use a clamp to DC-restore the PPS signal. Best regards, Charles ___ 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] Isolated 1PPS-input for distribution-amp?
Hoi Anders, On Thu, 31 Aug 2017 15:12:00 +0300 Anders Wallinwrote: > I'm looking for a solution for galvanic isolation of a 1PPS signal-input to > a distribution amplifier. What is the goal of the isolation? Breaking ground loops? Or safety? How much isolation voltage do you need? > We have an old box using HCPL-2411 at the input followed by 74LS04, but the > degraded rise-time (from below 3ns out of a clock, to >12 ns out of the > box) and increased jitter (from below TIC-resolution of 20ps out of the > clock to 700ps std.dev at 100s out of the box) are now limiting our > measurements. Yeah.. These optocouplers are not ment for stuff like this. > Does anyone have experience with a circuit with good rise-time and low > jitter? > Should I look for better optoisolators, or pulse-transformers, or something > else? My first pick would have been an ADuM, like the ADuM1280 or so. They offer decent rise/fall times and I guess the delay variations should not be too much. But they have a temperature coefficient for the delay, so beware of that! Another idea would be to use one of the LVDS isolators like the ADN4650, which has a jitter spec of 4.8ps RMS max (random). You will need a TTL->LVDS and LVDS->TTL conversion, of course. But the jitter added there should be small compared to the one added by the isolator. > PS. I started on publishing my distribution-amplifier (non isolated) on the > ohwr.org site, see https://www.ohwr.org/projects/pda-8ch-fda-8ch/wiki/wiki Cool! Thanks a lot! Attila Kinali -- You know, the very powerful and the very stupid have one thing in common. They don't alters their views to fit the facts, they alter the facts to fit the views, which can be uncomfortable if you happen to be one of the facts that needs altering. -- The Doctor ___ 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] Isolated 1PPS-input for distribution-amp?
HCPL7101 and similar 50MBaud optocouplers claim 50ps rms jitter. Some non optical isolated couplers specify ~30ps jitter. Bruce > > On 01 September 2017 at 00:12 Anders Wallin> wrote: > > Hi all, > I'm looking for a solution for galvanic isolation of a 1PPS signal-input > to > a distribution amplifier. > > We have an old box using HCPL-2411 at the input followed by 74LS04, but > the > degraded rise-time (from below 3ns out of a clock, to >12 ns out of the > box) and increased jitter (from below TIC-resolution of 20ps out of the > clock to 700ps std.dev at 100s out of the box) are now limiting our > measurements. > > Does anyone have experience with a circuit with good rise-time and low > jitter? > Should I look for better optoisolators, or pulse-transformers, or > something > else? > > thanks for your input! > Anders > > PS. I started on publishing my distribution-amplifier (non isolated) on > the > ohwr.org site, see https://www.ohwr.org/projects/pda-8ch-fda-8ch/wiki/wiki > > ___ > 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] Isolated 1PPS-input for distribution-amp?
Hi all, I'm looking for a solution for galvanic isolation of a 1PPS signal-input to a distribution amplifier. We have an old box using HCPL-2411 at the input followed by 74LS04, but the degraded rise-time (from below 3ns out of a clock, to >12 ns out of the box) and increased jitter (from below TIC-resolution of 20ps out of the clock to 700ps std.dev at 100s out of the box) are now limiting our measurements. Does anyone have experience with a circuit with good rise-time and low jitter? Should I look for better optoisolators, or pulse-transformers, or something else? thanks for your input! Anders PS. I started on publishing my distribution-amplifier (non isolated) on the ohwr.org site, see https://www.ohwr.org/projects/pda-8ch-fda-8ch/wiki/wiki ___ 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.