Hi The only other thing to consider is switching stuff on and off. A distribution amplifier or a tap system gives you some isolation between devices. One can go short circuit or back feed a noise burst and the others will not be impacted.
Does this matter? In a lot of cases the answer is no. If you routinely run very long data runs, the answer might be yes. Another datapoint on the 8140: ADEV through the typical tap comes in around 2x10^-11 at tau = 1 second. That’s not terrific if you happen to have a Maser as your reference :) It’s “as good as” most small Rb standards. It’s also not going to hurt most GPSDO’s. They are a cute system if you “suddenly” need 100 KHz to run something odd. They show up on eBay from time to time. A complete system consists of a head end box, the taps (up to some limit I forget at the moment) and the more or less unobtanium DC blocked 50 ohm load for the end of the coax (hint: build one yourself ….). Lots of fun !! Bob > On Oct 23, 2017, at 4:43 PM, John Ackermann N8UR <j...@febo.com> wrote: > > To some extent, it depends on the load presented by each device. The "EXT > REF" input on many pieces of test equipment is fairly high impedance (maybe > 10k?) and you can drive several of those with a single output, putting a 50 > ohm load at the end of the line to provide a reasonable termination.* I seem > to recall that three HP boxes worked nicely off one line, but when I added a > fourth things got flaky. > > But if any of the EXT REF are low-z inputs, that won't work so well. > > FWIW, Spectracom had a distribution system (8140 series) that had amplified > "tap" boxes that were daisy-chained together on a single coax run. The > driver put 12 volts DC on the cable along with 10 MHz, and that powered the > taps. You could put several taps on a single line. I once measured the > phase noise of the system and while it wasn't up to a really good > distribution amplifier, it was perfectly adequate for normal RF testing. > > John > > * Mismatch causes reflections, which can screw up square wave edges or sine > wave zero-crossings, increasing jitter. SWR is usually a bigger issue for RF > distribution than amplitude loss. > ---- > On 10/23/2017 01:49 PM, Jeremy Elson wrote: >> I was about to ask a related question of the list: when do you need a >> distribution amplifier, and when is it sufficient to just have a single >> (linear) run of coax? >> I have a GPSDO (Nick Sayer's device) that I want to use to feed a few other >> pieces of equipment in my lab (an HP5335A, John Ackermann's beautiful TICC, >> and a Rigol signal generator). Is it safe to have RG174 coming out of the >> GPSDO, tapping into it with a BNC T-junction that plugs into the back of >> each device that needs the 10mhz input, and then terminating the strand >> with a 50 ohm terminator? (In other words, the way thinnet Ethernet was >> wired back in the day.) As long as the signal goes in a straight line, not >> a "Y" (i.e. no cables attached to the t-junction taps, just a direct input >> into a high-z input) it seems like it should work. Do I need a distribution >> amplifier? Or is that, say, if the signal needs to split off in multiple >> directions and you don't want to fill your lab with a space-filling curve >> of coax? >> -Jeremy >> On Mon, Oct 23, 2017 at 10:26 AM, Bob kb8tq <kb...@n1k.org> wrote: >>> Hi >>> >>> The correct answer to any real question like this is “that depends”. >>> >>> For anything that I normally run as test gear, noise outside a very narrow >>> bandwidth really >>> does not matter much. The test gear *assumes* (by design) that the >>> reference signal going >>> into the “ref in” jack is not very clean. It does various tricks with >>> filters and PLL’s to “scrub” >>> the input. >>> >>> If we are talking about the reference into one side of a phase noise test >>> set, then >>> the situation is a bit different. The test set is simply going to tell me >>> what the combined >>> noise is on the two inputs. If one is significantly more noisy than the >>> other, that’s pretty >>> much all I will see. In this case, my answer is “don’t use a distributed >>> signal”. Use a >>> stand alone source as your reference and isolate it from the rest of the >>> world. >>> >>> In any case, making a super duper distribution gizmo and feeding it with a >>> noisy signal >>> is not going to make the signal any better. Most GPSDO’s have relatively >>> noisy outputs. >>> Some are better than others. None that I have seen on the surplus market >>> are what >>> I would call quiet at the output jack of the GPSDO. They either have an >>> ocean of spurs >>> or a lot of phase noise. Some have both …. >>> >>> Any time you boost a bunch of signals up to high levels, you create “crud” >>> running around your >>> lab / shack. One of the most basic questions should always be “do I really >>> need this signal?”. Next >>> should be “how can I have a shorter run?”. I have many pieces of gear >>> that are rarely used. >>> They use odd references. When I need to use them I rig a reference. That >>> gets shut down >>> once the gear goes back to storage. …. no more birdies every 100 KHz …. No >>> need for >>> tripple shielded coax …. >>> >>> Simple answer: >>> >>> Square up the 10 MHz (or whatever) by matching it into a 5.5 V powered >>> high speed CMOS >>> gate. The NC7SZ series is one of many families you can use. A NC7SZ125 is >>> not a bad gate >>> to pick. Distribute the square wave to however many output amps as you >>> need. Each one >>> is another of the same gates with the output matched via a 50 ohm to 50 >>> ohm lowpass Tee network >>> with a low Q ( < 2). Likely pad down the output a bit to keep it at a >>> rational level. Build up however >>> many you need for however many frequencies you require. Very normal linear >>> regulator chips >>> are fine for the power. Careful bypassing and solid ground planes are >>> always a good idea. >>> Parts cost wise, postage is likely to cost you more than the components. >>> There are …. errr… >>> many thousands …. of multi output amps of this basic design out there …. >>> they seem to >>> work pretty well. >>> >>> Yes, there are *lots* of possible twists and turns to this. I’m only >>> guessing about the gear you >>> are trying to run and what you are trying to do with it. >>> >>> Bob >>> >>> >>> >>> >>> >>>> On Oct 23, 2017, at 12:45 PM, Tom Van Baak <t...@leapsecond.com> wrote: >>>> >>>> List -- Don is having email trouble, but here's his posting: >>>> >>>> ---------- >>>> >>>> From: donaldbcol...@gmail.com >>>> Date: Tue, Oct 24, 2017 at 1:38 AM >>>> Subject: Distribution divider/amplifier for 10MHz GPSDO >>>> >>>> Hello group. I`m intending to distribute, via 50 Ohm coax, frequency >>>> reference signals to my test equipment in my test bay [no relation to >>> eBay, >>>> except that most of the equipment came from there]. I`ll be using RG58/U >>>> coax, and 50 ohm terminations, with the highest reasonable signal level >>>> reticulated. Given that the name of the game seems to be to avoid any >>>> severe reduction in SNR of the 10MHz signal comming out of the GPSDO, by >>>> the logic dividers, and impedance lowering buffer amplifiers, what >>>> considerations should be made regarding the choice of logic families, and >>>> transistors to be used? The frequencies required by the test equipment >>> vary >>>> from 500kHz to 10MHz, and amplitudes from 100mV P-P sinewave, to 5V peak >>>> squarewave. How good must the PSU be to stop the rot getting worse, and >>> is >>>> 1/f noise in the active devices important? Your thoughts will be >>>> appreciated. >>>> >>>> P.S.: How accurate is the Trimble Thunderbolt for this >>>> application?.............................................. >>> Thankyou,............Don >>>> Collie. >>>> >>>> ---------- >>>> >>>> _______________________________________________ >>>> 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. > _______________________________________________ > 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.