Re: From an audio forum... FR4 question
- From: John Larkin <jjlarkinSNIP@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 12 Nov 2005 15:29:57 -0800
On Sat, 12 Nov 2005 22:26:22 GMT, mw <mw@xxxxxxxxxxxxx> wrote:
>John Larkin wrote:
>> I had a pll running at 155.52 MHz with a narrowband (2 khz) loop, on a
>> VME module. Whenever one pulled an SMB test cable out of its
>> connector, the loop briefly lost lock. After some tapping and bending,
>> we found it was the expensive Vectron crystal oscillator, not the
>> caps. We designed some tiny springs to isolate the oscillator can,
>> made it wobble like one of those gooney-head dolls you see in the
>> backs of cars.
>
>I agree that crystals are prone to vibration problems.
>
>I am surprised that springs solved your problem. Many times a spring
>will merely change the frequency of the mechanical resonance. It's
>better to eliminate the resonance entirely by more firm mounting, or if
>that is impractical, use a vibration dampening material. Sometimes just
>moving the component to a place on the pcb that is less disturbed by
>vibration (close to a mounting screw) is good enough. Then you'd run
>sweeps on an electrodymaic shaker to prove the fix.
>
The oscillator was sensitive to high-frequency vibration, and the
resonant frequency of the spring-mass system was way below the
sensitive range, so the sharp shocks from connector mating couldn't
penetrate to the crystal. The crystal *was* firmly mounted to the
board, which was exactly the problem, and relocating it would have
trashed the entire design. The springs work great.
>> "Safe to say?" I can't imagine any significant pcb microphonics in an
>> audio system in any real-world sense. More audio nonsense.
>
>Vibration induced piezolectric noise from capacitors is a well-known
>problem. Ref: Linear Tech AN83, page 14:
>
>"A piezoelectric device generates voltage
>across its terminals due to mechanical stress, similar to
>the way a piezoelectric accelerometer or microphone
>works. For a ceramic capacitor the stress can be induced
>by vibrations in the system or thermal transients. The
>resulting voltages produced can cause appreciable amounts
>of noise, especially when a ceramic capacitor is used for
>noise bypassing. A ceramic capacitor produced FigureÊ B4?s
>trace in response to light tapping from a pencil. Similar
>vibration induced behavior can masquerade as increased
>output voltage noise."
I submit that if you open up your amp and start tapping parts with a
pencil, the loudest noise you'll hear is the sound of parts being
tapped with a pencil. To solve this particular problem, quit hitting
parts with pencils.
>I think it all depends on how "quiet" is "quiet" for your application.
My NMR gradient drivers have a couple of PPM noise, dc to 50 KHz, and
use regular surface-mount parts on FR-4, with lots of noisy fans.
Aside from pure semiconductor and resistor noise, the next biggest
hazard is magnetic loop pickup from fields leaking out of transformers
and fans.
>Some people can accept hissy, buzzy audio, and others know that things
>can be made quieter.
How will a microphonic cap make hissy, buzzy audio?
John
.
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