Hi, > 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?
Besides the usual signal integrity issues, remember to provide a solid earth connection between instruments. Depending on the scenario, failure to do so can result in worse performance than mismatched termination. cheers, Mattia ---------- Forwarded message ---------- From: Jeremy Elson <jel...@gmail.com> Date: 2017-10-23 19:49 GMT+02:00 Subject: Re: [time-nuts] Distribution divider/amplifier for 10MHz GPSDO To: Discussion of precise time and frequency measurement <time-nuts@febo.com > 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.
