[time-nuts] Totally unrelated, but..

[David G. McGaw]

One has to be very careful with LDOs.  As we say in applications, 
RTFDS.  :-)  The output capacitor often needs a finite ESR (effective 
series resistance).  A too-large ceramic cap WILL make them oscillate.  
I have noticed that they are used in some GPSDOs.

David


On 12/8/16 11:39 AM, Van Horn, David wrote:
> Yes, I hit one of those. I forget the frequency other than around 1 MHz, but it was pretty warm when it shouldn't have been, and it was several volts amplitude.
>
> This thing I'm chasing is much more subtle.
>
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Chuck Harris
> Sent: Wednesday, December 7, 2016 1:57 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Totally unrelated, but..
>
> I don't think so.  I first ran into a batch of LM340-5's that were excellent oscillators back in the 1970's... long before counterfeiting was even remotely possible.
>
> The symptom is the regulator puts out only 4.5 out of 5V.
>
> LM309's were, however, totally immune.
>
> Usually, I had to be really bad to make it happen, things like using clip leads between the power supply and load with the LM340-5 dangling in between.
>
> The answer is as simple as a couple of 0.1uf ceramic caps soldered right at the input and ground, and the output and ground pins.
>
> LDO (low dropout) regulators are very susceptible to oscillation.  They need to have a couple of hundred uf of good quality capacitance right on the input and output leads.  Where people usually get in trouble, is in not knowing that electrolytic capacitors lose most of their capacitance as the temperature starts hovering around 0C.
>
> The circuit works great on the bench, but fails when out hanging on a light pole...
>
> -Chuck Harris
>
> Joe Leikhim wrote:
>> Could the low noise parts actually be counterfeit, relabeled as such?
>>
>> Is the circuit the regulator feeds sensitive to a narrow band of
>> voltage that the "good regulator" is outside of?
>>
>> Try replacing the regulator with a battery supply and resistor divider
>> to attain the working voltage. Move the voltage around. A good
>> potentiometer and stiff filter capacitors are recommended so as not to introduce "pot noise".
>>
>> Is something corrupting your test procedure?  I had a circuit that
>> misbehaved due to floating logic pins reacting to static electricity
>> on the work bench. Another time a diode was photosensitive.
>>
>>
>>
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