[time-nuts] Distribution divider/amplifier for 10MHz GPSDO

Bob Bownes bownes at gmail.com
Mon Oct 23 16:49:10 EDT 2017


Will it work? Probably. Up to a point.

Is is best practice? Not even close.

Each device will (should?) generally present a 50Ω termination. In the case
of instruments that provide their own ref output which is then looped back
in, they may be different, but I don't recall seeing any that said anything
other than nothing or 50Ω.

That being said, I've daisy chained a few together to get quick results
without trouble.

Bob


On Mon, Oct 23, 2017 at 1:49 PM, Jeremy Elson <jelson at gmail.com> 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 <kb8tq at 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 <tvb at LeapSecond.com> wrote:
> > >
> > > List -- Don is having email trouble, but here's his posting:
> > >
> > > ----------
> > >
> > > From: donaldbcollie at 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.
> > >
> > > ----------
> > >
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