[time-nuts] Totally unrelated, but..

Van Horn, David david.vanhorn at backcountryaccess.com
Tue Dec 6 16:24:31 EST 2016

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.


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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