Re: [SI-LIST] : Microphonics problem.

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From: Ray Anderson (Raymond.Anderson@eng.sun.com)
Date: Mon Feb 05 2001 - 17:21:12 PST


Microphics in a synthesizer can come from a lot of sources.

Physical vibration can cause inductors and traces to move. This
will modulate the synthesizer if the vibration takes place at a
frequency above the loop bandwidth of the synthesizer.

Vibration can cause the PCB to flex and cause microphonics in
some cases. In some cases it can cause the capacitance between
a ground plane and a signal trace or plane to vary very slightly
if the flexure is in the correct orientation.

Physical vibration of ceramic capacitors can cause a piezoelectric
voltage to be injected into the loop which can cause microphonics.

Vibration of the reference oscillator (xtal or SAW) can modulate the
reference oscillator which will cause the output of the synthesizer
to vary as well.

As you can see, there are multiple mechanisms, but the end result is the same,
something causes noise to be injected into the circuit or L or C to vary
which in turn causes rapid deviations in frequency.

-Ray Anderson
Sun Microsystems.

>From: "Liang, Hongbo" <hliang@harris.com>
>To: si-list@silab.eng.sun.com
>Subject: [SI-LIST] : Microphonics problem.
>Date: Mon, 5 Feb 2001 16:42:17 -0500
>MIME-Version: 1.0
>
>Hi gurus:
>
>We met microphonics problem with our frequency source design. I think there
>might be experts
>out there.
>
>Here's our situation: we have four different sources on one FR4 board. Named TX
>PLL, RX PLL,
>TX SYNTHESIZER, RX SYNTHESIZER. The TX and RX used different references.
>
>We could isolate the microphonics problem with TX PLL, which is 2.6GHz. The RX
>PLL uses almost the
>same design as TX PLL and has no problem. Only different is the RX is 1.7GHz.
>Both of them use VCO.
>
>My question as: what's the physics behind microphonics? What're the roots of
it?
>
>
>Wish someone out there could enlightened me.
>
>Best Regards,
>Hongbo Liang
>
>
>
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