This usually doesn't show up right away. It develops over weeks or months. Some fluxes, like "no-clean" can be pretty high impedance, especially right after soldering or reflow. Then, they absorb water, and the conductivity goes up. Many logic inverter-type oscillators need some amount of conductance between the two crystal terminals to put the inverter input near the transition. But, too much conductance and the oscillator may fail to start, or may hop in frequency. In some cases I've seen conductance between adjacent chip pins in the hundreds of Ohms, which would certainly kill the Q of an oscillator. Cleaning the boards carefully with a toothbrush and solvent will fix the problem. It can sometimes be hard to get solvent under SMT components, and that's where these deposits like to hide.
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.
True. In a past life, I designed marine radios. As one would expect, marine radios tend to get wet, usually from condensation.
Ionic contaminants on the PCB are NOT much of a problem, until the board gets wet. Then, the stuff really conducts. One board that I ran through a worst case test in our then modern wave soldering machine showed about 20K/square sheet resistivity. To high impedance circuits, long parallel traces, and voltage threshold activated circuits, that's almost like a short circuit.
The general solution is to design using low impedances wherever possible. However, that won't work for crystal oscillators, which are high impedance devices. So, you're stuck with keeping the board clean, or at least the area around the crystal clean. Once you get it clean, you might also need some conformal coating. (Not the entire board as that makes rework difficult. Just the areas that are deemed moisture sensitive).
When we switched from rosin flux to water soluable flux in the 1970's, we had nothing but problems. Initially, it was rather stupid problems, such as using an unfiltered water rinse with far too much calcium both in the water and sitting in the bottom of the water heater. Later, they became more difficult, such as uneven rinsing in the modified dish washer that was used for washing. Every board had several test traces which were used to estimate resistivity. When we knew they were baked dry, and all the rinse water was gone, they were conformal coated, usually with acrylic. In short, board cleaning after soldering with water soluable flux was not a trivial exercise.
I've never done anything with Zigbee, so I'm not really familiar with the frequency stability requirements. Googling... This app note goes into the requirements in detail: Reference Oscillator Crystal Requirements for the MC1320x, MC1321x, MC1322x, and MC1323x IEEE 802.15.4 Devices (Note that the chips have internal switched capacitors for both coarse and fine tuning.) Basically, they want +/-40ppm over the operating temperature range. That should be possible without any elaborate external TCXO style temperature compensation. Just buy a decent AT cut series resonant crystal, with a low series resistance, and keep the board clean.
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
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