[time-nuts] Totally unrelated, but..

Bob Camp kb8tq at n1k.org
Wed Dec 7 11:43:08 EST 2016


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 


> On Dec 6, 2016, at 4:24 PM, Van Horn, David <david.vanhorn at 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 at backcountryaccess.com>
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