[time-nuts] Assorted replies, and request for info

Dana Whitlow k8yumdoober at gmail.com
Tue Feb 27 05:50:15 EST 2018

In connection with ground loops, why not put "ferrite beads" on coaxial
cables in the system?

In this case where the offending frequencies would be in the 60 Hz regime
one would probably
need large toroids with multiple turns, but the approach ought to do some
real good, without
introducing the problems associated with leaving on end of a coax shield


On Mon, Feb 26, 2018 at 8:45 PM, John Miles <john at miles.io> wrote:

> Request for data from an off-list friend who is looking for info on an
> Ovenaire OCXO and/or the instrument it was used in:
> > Could you send a post on my behalf asking if anyone else has an
> > Ovenaire 42-15 or if not, a Spectracom 8131 Frequency Standard
> Oscillator.
> This is from Dennis Tillman, who can be reached directly at dennis (at)
> ridesoft.com.
> > Fun fact -- there's a wide spur at ~2 Hz on the 5065A phase noise plot.
> What
> > do you think that is? On a hunch I opened the front panel and reset the
> > blinking amber battery alarm lamp, and voila, that noise went away. Makes
> > sense when you think of the power variations associated with a blinking
> > incandescent lamp.
> I hadn't heard of that one, but some other examples include 1-pps
> crosstalk on the 10 MHz output of some of the HP GPSDOs (which I've run
> into myself), and the inadvertent ~5 MHz comb generator that drives the
> indicator LED on the HP 5370's reference clock interface PCB that Bruce
> Griffiths noticed several years ago.  Presumably neither of these faux pas
> were bad enough to be noticed by the original designers or their paying
> customers, but they probably would have been fixed if they had come to the
> attention of the people involved.  Goes to show how improved
> instrumentation can be a curse as well as a blessing.
> Poul-Henning's observation on the 5065A integrator cap highlights the risk
> of erring too far in the opposite direction:
> > When I experimented, I could hardly find *any* property that mattered
> > for that capacitor, not even the exact capacitance, because the
> > adjustment procedue handles that.
> I have a feeling this is true of most of the components in that circuit.
> The nice thing about an integrator is that it's also a low-pass filter.
> And the nice thing about a closed loop is that it's, well, closed.
> Before spending too much time arguing about whether the opamp should be
> replaced with an LT1012 or an AD797 or a cryocooled tunnel diode or
> whatever, I'd suggest replacing it with a 741 and seeing how much *worse*
> the performance gets.  It is easier to measure the effect of that kind of
> change.  If there is little or no harm in using the crappiest opamp you can
> find, that means that you can safely stop worrying about what the best one
> might be.
> > One of the TimePods that I had access to in the past was particularly
> good
> > at telling you it was sitting on top of a power transformer. It didn’t
> matter a
> > lot which instrument the power transformer was in. For some weird
> > reason it was a good magnetometer at line frequencies. I never bothered
> to
> > send it back for analysis. Simply moving it onto the bench top (rather
> than
> > stacked on top of this or that) would take care of the issue.
> >
> > As far as I could tell, it was just the one unit that had the issue.
> None of the
> > others in the fleet of TimePods seemed to behave this way. Given that
> they
> > normally are very good at rejecting all sorts of crud and ground loops,
> it was
> > somewhat odd to see.
> I've seen similar behavior here, not only with respect to units responding
> differently to 60/120 Hz magnetic interference, but also at higher offsets
> in the absence of an obvious coupling mechanism.  There was one case where
> a TimePod I was working with picked up an unstable low-level spur near 25
> kHz from an LED aquarium light fixture several meters away.  Other units
> swapped into the same position did not show the spur at all, and I was
> never able to narrow down the cause with any certainty.  I don't have a
> good explanation for any of the above, unfortunately.
> That being said, Phil Hobbs posted something on sci.electronics.design the
> other day that I thought was subtly insightful, even though he was just
> stating an obvious point.  Namely, ground loops are inherently very low
> impedance phenomena, often occurring in the milliohm range.  Especially
> when dealing with anodized aluminum hardware like the TimePod's enclosure,
> the difference between a test setup where all the coax shields act as a
> near-perfect shorted transformer turn versus one with significant loss
> might come down to small differences in fastener torque, or perhaps a
> missing star washer.  So it's possible to envision a scenario where
> tightening up all the proverbial loose screws actually makes a
> magnetically-coupled spur worse.
> Lifting a coax shield is usually not the best solution to ground loops,
> but Phil's offhand comment made me wonder about the effects of deliberately
> adding just a few ohms of series R.  It's on my list of things to look into
> when I have time.
> -- john, KE5FX
> Miles Design LLC
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