[time-nuts] Totally unrelated, but..

Wed Dec 7 19:02:51 EST 2016

Hi

Just for reference, KTB in 1 Hz at 50 ohms is roughly 0.9 nV at room 
temperature. The previously mentioned 2 nV is equivalent to about 250 ohms.

Bob

> On Dec 7, 2016, at 6:58 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
> 
> 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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