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

John Ackermann N8UR jra at febo.com
Mon Oct 23 16:43:29 EDT 2017


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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