Re: [time-nuts] Isolated +20V for lamp supply
I will guess to minimize radiated noise. Regards Paul On Fri, Aug 14, 2020 at 4:34 AM wrote: > >Corby, > >so why did HP use a coaxial cable for the lamp power? it seems nonsense. > >Luciano > >Luciano P. S. Paramithiotti >tim...@timeok.it >www.timeok.it > >Da "time-nuts" time-nuts-boun...@lists.febo.com >A time-nuts@lists.febo.com >Cc >Data Thu, 13 Aug 2020 10:46:01 -0700 >Oggetto [time-nuts] Isolated +20V for lamp supply >Luciano, > >You should "Ohm out" the lamp connector shield to the chassis with >nothing connected to the connector. >Depending on how your optical unit is mounted there is a good chance the >lamp assy is grounded to the chassis through the threaded stud! >Especially so if the optical is the newer unpainted one. >If so isolating it as you detailed does not change anything! > >Cheers, >Corby > > >___ >time-nuts mailing list -- time-nuts@lists.febo.com >To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >and follow the instructions there. > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Isolated +20V for lamp supply
Hi, On 2020-08-14 11:31, Poul-Henning Kamp wrote: > > tim...@timeok.it writes: >>Corby, >> >>so why did HP use a coaxial cable for the lamp power? it seems nonsense. > Because it generates a LOT of noise, being in essense a VHF/UHF generator. > Recall it is about 100 MHz and 5 W or thereabouts. So, you have the DC current and you have the 100 MHz AC current. Both can cause interference. The total isolation approach really only work for DC, at 100 MHz you jump gaps through capacitive or magnetic coupling, which exists because of voltage difference or current flow respectively. The best way to handle both of these is to provide a low impedance path on the preferred way, as that will dominated the current flow. For DC, this means adding a low resistive path for the current. For AC it is all about keeping a low-inductive path and keep the loop area low, which is also about low-inductance. The chassi path for ground should be a very low-resistive and low-inductive path, but the +20 V does not match that. The AC-return path can be made shorter by providing a capacitive path back, with inductance on the 20 V feed. Care should be made to ensure inductance can handle the current through it, as you do not want the core to be saturated. If one wants to separate coupling, one can do "star-ground" feed, but for the 5065 it's mostly the +20V and not the ground that needs it because of significant difference in conductance area between wires and chassi, so reasonable to assume wired to provide higher resistance and thus higher problem with common path. So sure, pulling a separate wire for the +20V from the PSU to the lamp would help. Less so for the ground, but you can do that too if you want to, but that is on the diminishing returns effort list. In the end, adding more wires in parallel to existing conductions paths will lower the resistance, and thus lower the cross-coupling. You have a number of different ways around it. However, for RF-signals you always want an AC return-path for that current very close to the signal, and if that happens to bridge between two different DC points you will lower the voltage difference between these, if that current is high, add a parallel thick wire. This is also true for AC-power at 50-60 Hz. Many attempts to break the connection for DC or 50/60 Hz tends to cause severe isolation problems for the signal, so instead shunting with a thick wire to connect things tighter together ends up making sense. So, isolation can maybe be a local strategy, but it doesn't work as one builds larger things in a box or even a system of boxes. As soon as you start interconnect you get into issues. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Isolated +20V for lamp supply
tim...@timeok.it writes: > >Corby, > >so why did HP use a coaxial cable for the lamp power? it seems nonsense. Because it generates a LOT of noise, being in essense a VHF/UHF generator. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Isolated +20V for lamp supply
Corby, so why did HP use a coaxial cable for the lamp power? it seems nonsense. Luciano Luciano P. S. Paramithiotti tim...@timeok.it www.timeok.it Da "time-nuts" time-nuts-boun...@lists.febo.com A time-nuts@lists.febo.com Cc Data Thu, 13 Aug 2020 10:46:01 -0700 Oggetto [time-nuts] Isolated +20V for lamp supply Luciano, You should "Ohm out" the lamp connector shield to the chassis with nothing connected to the connector. Depending on how your optical unit is mounted there is a good chance the lamp assy is grounded to the chassis through the threaded stud! Especially so if the optical is the newer unpainted one. If so isolating it as you detailed does not change anything! Cheers, Corby ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
[time-nuts] Isolated +20V for lamp supply
Luciano, You should "Ohm out" the lamp connector shield to the chassis with nothing connected to the connector. Depending on how your optical unit is mounted there is a good chance the lamp assy is grounded to the chassis through the threaded stud! Especially so if the optical is the newer unpainted one. If so isolating it as you detailed does not change anything! Cheers, Corby ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
