[time-nuts] Totally unrelated, but..

Scott Stobbe scott.j.stobbe at gmail.com
Wed Dec 7 18:58:31 EST 2016


You can buy opamps off the shelf with an input referred noise less than a
50 ohm resistor to build up a preamp (of course you can build up something
even lower with discretes, but then it's a time commitment over a basic
opamp preamp)

Even if your regulator is low noise if it's running with low phase margin
you can get noise peaking.

On Wed, Dec 7, 2016 at 3:06 PM Van Horn, David <
david.vanhorn at backcountryaccess.com> wrote:

>
>
> I replaced the original caps, and I added caps, I substituted good
> Jonhansen RF caps, and Tanceram caps.
>
> No help at all.
>
>
>
> The layout and routing is as good as I could do, and the only improvement
> I could see possible would be to move one cap closer to the reg.
>
> The difference would be less than the tolerance of part placement on the
> existing pads.
>
>
>
> Fortunately this isn't a critical piece of equipment, but I want it
> working RIGHT before I put it back in service.
>
> It's a custom receiver for 457 kHz.
>
>
>
>
>
> -----Original Message-----
>
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Graham /
> KE9H
>
> Sent: Wednesday, December 7, 2016 10:41 AM
>
> To: Discussion of precise time and frequency measurement
>
> Subject: Re: [time-nuts] Totally unrelated, but..
>
>
>
> Remember that the internal Voltage reference in the original three
> terminal regulator designs is a Zener.
>
> (Zeners are also useful as RF white noise sources.) The regulator is
> generally an amplifier with DC feedback.
>
> If you look at the application notes on the early regulators, they require
> capacitors to ground on both the input and outputs.
>
> If these capacitors are missing, or too low in value, or not good
> capacitors at RF frequencies, then the Zener noise is amplified by the
> regulator amplifier and pushed out the output port.
>
> I would experiment by putting a good ceramic 0.1uF cap to ground, right at
> the regulator output port.
>
>
>
>
>
> --- Graham
>
>
>
> ==
>
>
>
>
>
> On Wed, Dec 7, 2016 at 10:43 AM, Bob Camp <kb8tq at n1k.org> wrote:
>
>
>
> > Hi
>
> >
>
> > You probably have proven one of the most basic design truths: Parts
>
> > will
>
> > *always* oscillate just
>
> > outside the bandwidth of your test gear” :). A few other possible issues:
>
> >
>
> > 1) Something else is oscillating and it is simply interacting with the
>
> > regulator in an odd way.
>
> > 2) The oscillation / noise is at a very low level and it’s below your
>
> > test gear’s noise floor
>
> > 3) Testing stops the oscillation
>
> >
>
> > Bob
>
> >
>
> > > On Dec 6, 2016, at 4:24 PM, Van Horn, David <david.vanhorn@
>
> > backcountryaccess.com> wrote:
>
> > >
>
> > > Lots of discussion on here about low noise regulation so someone may
>
> > know what to look for.
>
> > >
>
> > > I have a receiver which is getting a lot of interference from
> somewhere.
>
> > > Antenna disconnected, interference still high.
>
> > > After much poking around, we found that replacing a voltage
>
> > > regulator
>
> > with a slightly different part cures the problem.
>
> > > Running that section on external battery is also fine, so it appears
>
> > > the
>
> > original regulator causes some problem.
>
> > > We tried various batteries over a range of voltages within the chip
>
> > spec, and couldn't make it have a problem.
>
> > >
>
> > > I looked at the reg input and output with scope and spectrum
>
> > > analyzer,
>
> > and I don't see anything that indicates excessive noise or oscillation.
>
> > > The PCB layout is as tight as you could ask for. Fat tracks, lots of
>
> > ground, I couldn't lay it out any better.
>
> > > Replacing the input and output caps didn't change anything.
>
> > > Replacing the input and output caps with parts that should be
>
> > > "better",
>
> > like Johanson Tancerams or tantalums has no effect.
>
> > >
>
> > > Just for laughs, we tried a number of different regulator chips, all
>
> > > new
>
> > from the reel.
>
> > > The parts with the quietest and with the most noisy specs caused
>
> > problems.
>
> > > One part, with a noise spec more or less in the middle of the spread
>
> > > is
>
> > the one that works.
>
> > >
>
> > > So what is it that a monolithic regulator (linear) can do which is
>
> > > not
>
> > observable on a scope or SA, which would cause a receiver to think
>
> > it's getting a signal or significant noise in band?
>
> > > Everything else in the system is shut down, I am sure the regulator
>
> > > chip
>
> > is the culprit, but so far I don't see how it's causing the problem.
>
> > > I could just use the quiet chip and move on, but experience tells me
>
> > that I'd just have problems again down the road.  That's voodoo, not
>
> > science.
>
> > >
>
> > >
>
> > > Ideas?
>
> > >
>
> > >
>
> > >
>
> > > --
>
> > > David VanHorn
>
> > > Lead Hardware Engineer
>
> > >
>
> > > Backcountry Access, Inc.
>
> > > 2820 Wilderness Pl, Unit H
>
> > > Boulder, CO  80301 USA
>
> > > phone: 303-417-1345 x110
>
> > > email: david.vanhorn at backcountryaccess.com<mailto:david.vanhorn@
>
> > backcountryaccess.com>
>
> > >
>
> > > _______________________________________________
>
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>
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>
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>
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>
> >
>
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>
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>
> >
>
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