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

Jeremy Elson jelson at gmail.com
Mon Oct 23 20:55:18 UTC 2017

Is there an easy way to measure the per-device load, or do you typically
just go by whatever the datasheet says?

I suppose I could just hook everything up and then check the amplitude of
the signal at the far end.

On Mon, Oct 23, 2017 at 1:43 PM, John Ackermann N8UR <jra at 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 <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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