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From: upsided...@downunder.com
Newsgroups: sci.electronics.design,comp.arch.embedded
Subject: Re: 802.15.4 RF issues
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On Mon, 29 Oct 2012 12:38:52 +0100, Oliver Betz <ob...@despammed.com>
wrote:

>"Martin Riddle" wrote:
>
>[...]
>
>>Make sure the board is cleaned properly. The No clean flux can cause 
>>problems for crystals, especially under the crystal.
>
>could you explain this further, what kind of influence are you talking
>about?
>
>Oliver

I have not been followed this discussion with great attention, but
there are several issues related to PCB material humidity and
temperature dependencies in oscillators.

When building free running HF oscillators (e.g. VFOs) never use two
(or multi) layer constructions near the LC resonant circuit. The stray
capacitance between the resonant LC components and the PCB ground
plane affects the frequency. Unfortunately, this stray capacitance
varies with the air humidity and temperature, which affects the
dielectric constant of the PCB material and hence affect the
frequency.

For simple oscillators in the UHF/microwave range, a free running
oscillator made of 1/4 wavelength PCB traces are extremely sensitive
to these issues. 

Anyway, one should remember that a frequency drift of 1 Hz at 25 MHz
is 100 Hz at 2.45 GHz. I was once debugging a GHz signal source based
on some VHF overtone crystals and wondered, why the frequency was
shifting every few seconds. I finally discovered that I was breathing
on the crystal, that cased the frequency drift :-).

When using HF fundamental or VHF overtone crystals as a reference to
frequency multipliers or PLLs, you really need to pay attention to the
PCB material and layout around the crystal oscillator.