[time-nuts] Totally unrelated, but..
Van Horn, David
david.vanhorn at backcountryaccess.com
Wed Dec 7 13:32:09 EST 2016
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.
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.
On Wed, Dec 7, 2016 at 10:43 AM, Bob Camp <kb8tq at n1k.org> wrote:
> You probably have proven one of the most basic design truths: Parts
> *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@
> 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
> > 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
> > 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@
> > _______________________________________________
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