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

Mattia Rizzi mattia.rizzi at gmail.com
Tue Oct 24 11:14:09 EDT 2017


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 <jelson at 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 at 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 <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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