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 <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.
