[time-nuts] Totally unrelated, but..

Bob Camp kb8tq at n1k.org
Wed Dec 7 13:44:52 EST 2016


Hi

Usually what happens is you have a broadband oscillation at 3.5 to 4 GHz (based on your 3 GHz 
upper limit). The “munge” mixes with this and that creating interference at RF.

Bob

> On Dec 7, 2016, at 1:28 PM, Van Horn, David <david.vanhorn at backcountryaccess.com> wrote:
> 
> Well so far at least #3 is not true.
> 
> It may be something happening below the noise floor or outside the bandwidth, but I was looking from 0-5MHz.
> I have 3Ghz+ available, but I wouldn't expect these parts to be that fast.
> 
> It's a mystery, but I love solving mysteries.
> 
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Bob Camp
> Sent: Wednesday, December 7, 2016 9:43 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Totally unrelated, but..
> 
> 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 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 backcountryac
>> cess.com>
>> 
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