Once upon the time, there were some "three legged" voltage regulators,
which were very sensitive to the ESR value of the capacitor which was
connected to their output. With to low ESR values they oscillated, [the
oscillation was "inside" of the regulator, which became warm, or
sometimes hot, because of the high AC output current feeding into the
output capacitor] that is actually logical since the dominant pole of
the control loop is determined with that output capacitor. For noise
critical application use one resistor -- 0,5 to 2,00 ohms -- serial with
the output capacitor, and use an inductor -- 200uH to 1mH -- with very
low -- 20mili-ohm to 100mili-ohm depend how much voltage drop could
you tolerate with the actual current load -- ohmic resistance serial
with the output, and a large capacitor--- it could be even10uF -- with
very low ESR value -- as low as available, ceramic multi-layer
capacitors are the best -- after the inductor, at the load site -- that
way the second capacitor is isolated from the output of the regulator,
and will be not part of the feedback loop. If you have the luxury, to
design your own regulator, separate the DC feedback and the AC feedback,
that way you could keep the output voltage more stabile without oscillation.
73
KJ6UHN
Alex
On 12/7/2016 8:43 AM, Bob Camp 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
<[email protected]> 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:
[email protected]<mailto:[email protected]>
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