Discussion:
Bodnar "Precision Frequency Reference (GPS Clock)" AND LeoNTP Networked Time NTP Server Questions
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jimlux
2018-05-18 13:59:03 UTC
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Interested to know how much noise would be from USB signalling and how much
is " machine noise" from the PC as my understanding of USB signalling is
that it's differential so such should be low noise?
Every device I've worked with that had a USB interface has radiated
significant power around 12 or 24 MHz. (I've not worked much with
devices that support 480Mbps USB3..)

Indeed, the data wires are twisted pair (although the "twist rate" in
most cables is pretty sketchy), but the signaling is voltage mode, not
switched balanced current like LVDS. So I think you get significant
current spikes on the power wires as you charge and discharge the
capacitance/inductance of the (unterminated) data lines.


The shielding is, in a lot of consumer devices, common to the power
supply negative lead. So the shield carries part of the power supply
current, and then radiates.

In theory, USB devices sold in the United States should meet FCC Part 15
(and the similar requirements in the EU and elsewhere) - but I'll bet
there's a LOT of stuff out there that isn't tested, nor would it pass.


High Speed USB might be better for EMI/EMC - you're just not going to
push 480Mbps through an unterminated system with crummy noise properties.

TI has an application note
https://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf

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Denny Page
2018-05-18 16:11:19 UTC
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Have you considered asking Leo? In my experience, he is very helpful.

Denny
*"GPSDO"* - Once configured, unit can run on an external DC source (5-15VDC). I am NOT using it to power the mast mounted timing antenna. I don't anticipate leaving unit connected to a computer when not configuring UNLESS I can figure out how to grab data from the unit via the USB port for reporting/analysis.
1) What are the specs of the DC plug on the back? If I use an external supply, I want to eliminate the addition of yet another SMPS/wall wart to the clutter.
2) IF you were choosing a voltage to run JUST this unit on big/short/twisted/ferited leads, what voltage would you choose (regulating down from 13.8VDC or so). Anything above the required minimum will be converted to heat in either the unit or at the regulator. Considering this uses a TXCO (I think) and not an OXCO or DOXCO, is running it hotter to try and achieve a more consistent internal temp worth the tradeoff in potentially shortened life of the unit. (Bottom Line: What voltage will make it run most accurately/consistently?)
*LeoNTP Networked Time NTP Server* - Can be powered via USB or PoE according to website. Detailed specs are seriously lacking. Trying to wade through how to power it. USB is obviously 5vdc or a bit less.... But the PoE could be much higher with an internal regulator(s)
- I haven't bought my new switch yet, so I can buy one with PoE capability or otherwise inject PoE on the Ethernet cable from the NTP Server to the Gbit switch.
- All Ethernet cables in my network are CAT-7 STP (shielded twisted pair) or better. (Yeah, yeah, I know, wired Ethernet can be noisy, but you do the best you can. IF wired Ethernet becomes an issue, I can back up to the wireless network.)
- Not sure how much I plan to have the unit connected via USB. Beyond curiosity data gathering for a while, I'm thinking at some point I want it to be disco'd and forgotten. One less potential USB cable radiating or needing ferrites.
There's more... but this is a good start. Just want to try and get parts on the way. Have to build a separate outboard regulator for the timing GPS antenna, too.
Thanks!
--
*Clay Autery
(318) 518-1389
MONTAC Enterprises*
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Mark Goldberg
2018-05-18 19:26:06 UTC
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Leo and I are already discussing it off list and he is trying to duplicate
my measurements.

Regards,

Mark
Post by Denny Page
Have you considered asking Leo? In my experience, he is very helpful.
Denny
Anyone who is using one (or both) of these, and/or folks who have a
*"GPSDO"* - Once configured, unit can run on an external DC source
(5-15VDC). I am NOT using it to power the mast mounted timing antenna. I
don't anticipate leaving unit connected to a computer when not configuring
UNLESS I can figure out how to grab data from the unit via the USB port for
reporting/analysis.
1) What are the specs of the DC plug on the back? If I use an external
supply, I want to eliminate the addition of yet another SMPS/wall wart to
the clutter.
2) IF you were choosing a voltage to run JUST this unit on
big/short/twisted/ferited leads, what voltage would you choose (regulating
down from 13.8VDC or so). Anything above the required minimum will be
converted to heat in either the unit or at the regulator. Considering this
uses a TXCO (I think) and not an OXCO or DOXCO, is running it hotter to try
and achieve a more consistent internal temp worth the tradeoff in
potentially shortened life of the unit. (Bottom Line: What voltage will
make it run most accurately/consistently?)
*LeoNTP Networked Time NTP Server* - Can be powered via USB or PoE
according to website. Detailed specs are seriously lacking. Trying to
wade through how to power it. USB is obviously 5vdc or a bit less.... But
the PoE could be much higher with an internal regulator(s)
- I haven't bought my new switch yet, so I can buy one with PoE
capability or otherwise inject PoE on the Ethernet cable from the NTP
Server to the Gbit switch.
- All Ethernet cables in my network are CAT-7 STP (shielded twisted
pair) or better. (Yeah, yeah, I know, wired Ethernet can be noisy, but you
do the best you can. IF wired Ethernet becomes an issue, I can back up to
the wireless network.)
- Not sure how much I plan to have the unit connected via USB. Beyond
curiosity data gathering for a while, I'm thinking at some point I want it
to be disco'd and forgotten. One less potential USB cable radiating or
needing ferrites.
There's more... but this is a good start. Just want to try and get
parts on the way. Have to build a separate outboard regulator for the
timing GPS antenna, too.
Thanks!
--
*Clay Autery
(318) 518-1389
MONTAC Enterprises*
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Didier Juges
2018-05-18 13:15:47 UTC
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A shield will definitely help with differential mode noise (the USB
communication signal) but has little effect on common mode noise (the
digital stuff coming from other parts of the circuit), a choke is the fix
for common mode (if you can't get rid of the noise at the source, or
otherwise return common mode noise directly back to earth instead of
through the cable).

Didier KO4BB
Interested to know how much noise would be from USB signalling and how much
is " machine noise" from the PC as my understanding of USB signalling is
that it's differential so such should be low noise?
I'm also not sure I've ever seen a non screened usb cable?
Caution, folks, about USB cable radiation. While the intended signals
flowing through the
cable presumably contribute a bit to the overall picture, common-mode
currents on the cable
are the most likely cause of severe radiation problems. These currents
arise not merely from
intended USB signals, but from *all* the digital activity within the
device, and will be present
whether or not USB communications are going on.
1) Shielding on the USB cable.
2) Proper design inside the USB device, rarely done beyond the point of
(barely) meeting
government regulations, which are far too forgiving IMHO.
3) Use of a ferrite choke on the cable to reduce those residual currents
that get by anyway.
Note that only item 3 might be under the control of the user to any
useful
extent.
So please don't just assume that using a USB cable for charging only
solves
anything.
Note that the above comments are also generally applicable to *all
*cables,
including power
cables, that are plugged into *any* digital device.
Dana K8YUM
On Fri, May 18, 2018 at 5:15 AM, David J Taylor via time-nuts <
Anyone who is using one (or both) of these, and/or folks who have a
[]
There's more... but this is a good start. Just want to try and get
parts on the way. Have to build a separate outboard regulator for the
timing GPS antenna, too.
Thanks!
Clay Autery
=============================
Clay,
I have all three of these units, all powered off USB. I don't see any
reason why you shouldn't use a USB cable with just the DC part
connected,
so that there's no issue about radiation. A standard USB port would
only
supply 0.5A so that's an upper limit on the power consumption. If
it's a
very long USB cable (you mention mast-head) check the resistance.
As a crude guide...
- The (larger box) NTP server feels cold to the touch.
- The smaller, single output frequency source feels very slightly warm
to
the touch (feeding an Icom IC-R8600)
- The larger, dual-output frequency source feels slightly warm to the
touch.
Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
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Mark Goldberg
2018-05-18 06:54:19 UTC
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Anyone who is using one (or both) of these, and/or folks who have a
- Not sure how much I plan to have the unit connected via USB. Beyond
curiosity data gathering for a while, I'm thinking at some point I want it
to be disco'd and forgotten. One less potential USB cable radiating or
needing ferrites.
I have the Bodnar Mini which only has a USB connection. You can connect it
to a USB power only source. I have found a large variation (20 dB) in close
in phase noise, depending on how clean the power source is. I am using a
power only wall wart with an added series ferrite core / shunt cap to get
acceptable results ( -103 dBc/Hz at 10 Hz offset). I don't know if there is
more internal filtering on the separate power input of the 2 output
version. So far peak ADEV at Tau = 20s is about 2e-10. It would be nice to
improve that.

You can also get widely varying spurs, phase noise and ADEV depending on
the register values chosen to provide a specific output frequency. Many
different sets of register values may be used to provide the same output
frequency. I have not determined a pattern. I just calculated different
values and tried them out.

Regards,

Mark
W7MLG
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Magnus Danielson
2018-05-18 12:23:21 UTC
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Hi,
Caution, folks, about USB cable radiation. While the intended signals
flowing through the
cable presumably contribute a bit to the overall picture, common-mode
currents on the cable
are the most likely cause of severe radiation problems. These currents
arise not merely from
intended USB signals, but from *all* the digital activity within the
device, and will be present
whether or not USB communications are going on.
1) Shielding on the USB cable.
2) Proper design inside the USB device, rarely done beyond the point of
(barely) meeting
government regulations, which are far too forgiving IMHO.
3) Use of a ferrite choke on the cable to reduce those residual currents
that get by anyway.
Note that only item 3 might be under the control of the user to any useful
extent.
So please don't just assume that using a USB cable for charging only solves
anything.
Note that the above comments are also generally applicable to *all *cables,
including power
cables, that are plugged into *any* digital device.
I could not agree more.

Shielded cables helps a lot. We used to have issues at the radio club
with Ethernet cables, but as we shifted to shielded cables with reduced
the noise level considerably.

Ferrite helps to suppress common mode currents, but the milage may vary
depending ont he ferrite material.

There is USB isolators, which breaks "ground loops" (sigh), and that do
helps on the low frequency side of things, but not necessarilly at HF,
where the isolation is just a capacitive gap which couples fairly well.

One thing that does help is not to have unnecessary rise/fall times of
signals.

Cheers,
Magnus
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