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Author Topic: Crystal too far away from XTAL pins?  (Read 1447 times)
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The last thing you did is where you should start looking.
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This maybe of interest:
https://solarbotics.com/product/kardw/
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The way you have it in your schematic isn't the same as how you have it wired up! That goes for me too.

Left Coast, CA (USA)
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Measurement changes behavior
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Actually, one (not uncommon) approach is to use a wire straddling the can into two through-holes.  One can trust the mechanical contact, or if it is necessary to solder the wire to the can in addition, this can be done using a soldering iron with much less heat/ time.


That's common in HAM radio equipment, however seen 4-8 crystals mounted vertically as in crystal filters used to set the IF bandwidth in high performance receivers.
http://ecee.colorado.edu/~mathys/ecen2420/psets/ps07/CrystalFilterP1.jpg
Lefty
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In a ""Staggered" array of crystals like the one in the picture the 'cans' are soldered together to prevent radiated signals from other places including adjacent crystals from affecting the signals being 'filtered'...
Ideally each crystal would be in it's own shielded enclosure, isolated from each other on both sides of the board. There is a lot of Military equipment that is designed that way.
In practice the cases are soldered together at an "Equidistant point" (for symmetry) and that point is grounded. You might have noticed the ground from the "Center" of the "X" shaped wires straight down to the PCB.  this is the equal point and any RF developed across the inductance of the ground wire affects all equally. Remember that these crystals are selected for their resonant frequencies so each will have a slightly different 'sensitivity' to radiation from external sources and the purpose of the ground is to make all equal in RF potential.
Finally the connections on the bottom of the PCB usually are shielded in separate compartments for the receiver filter but may well be unshielded in a transmit filter and just laid down close to the PCB copper.
Edit: I personally have soldered several thousand transceiver crystals to ground without a failure... from 6 to 50 MHz fundamental frequencies. Occasionally one will fail but it is my studied opinion that the failures were defective parts before being soldered to the case. This "Studied" opinion comes from the failure rates which according to some old notes I have dating to that period in my life that show about a 1% failure rate. Digital clock crystals are rarely used in RF applications (Poor relative stability) but when they are used for transceivers they should be grounded, For the reasons I stated and the reasons stated by others in this thread. I hadn't read all the thread before I added my $0.02's worth so a part of my addition is repetition of facts already stated.

Doc
« Last Edit: November 22, 2013, 05:41:41 pm by Docedison » Logged

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NSW Australia
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the 'cans' are soldered together to prevent radiated signals from other places including adjacent crystals from affecting the signals being 'filtered'...

Beat me to the punch!

I hadn't read all the thread before I added my $0.02's worth

Hardly surprising, there's quite an effort in keeping up with these discussions.   smiley-grin
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Good one, this...thread as it covers a lot of junk... the only thing not covered was the soldering of 32768 Hz crystals... Sometimes it works and sometimes the "Tuning Fork" is in contact with the case and the crystal should be replaced... It's commonly done to  keep the crystal isothermal to the chip oscillator mostly...  Although occasionally the oscillator has a bit too much gain and then the crystal either won't work or work right as the oscillator is subject to "Jumping Modes" internally.
Crystals are cheap, engineering time isn't.

Doc
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Quote
Crystals are cheap, engineering time isn't.

In the design phase while things are controlled by ET's, I would agree.  But out on the assembly line, over time, production runs may not be so well controlled - especially, when demand outstrips the first batch and reordering for run #n is occurs.  The cost for rework and later any warranty returns become significant product overhead.

Snippets from AN2606;
http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPC5121e&tab=Documentation_Tab&pspll=1&SelectedAsset=Documentation&ProdMetaId=PID/DC/MPC5121e&fromPSP=true&assetLockedForNavigation=true&componentId=2&leftNavCode=1&pageSize=25&Documentation=Documentation/00210KscRcb``Application%20Notes&fpsp=1&linkline=Application%20Notes
Quote
As a practical note, it should be firmly understood that 32768 Hz crystals are neither created equal, nor
close. Crystal parameters vary widely between different models produced by the same manufacturer, or
between manufacturers and styles of crystals. Designers must be aware that crystal oscillator circuits that
require “tweaking” or that must have very high tolerance components to make the oscillator work are
going to be marginal, at best. Most oscillators work reliably with components that vary over a reasonable
range of values. If a crystal is used that requires high-precision components, the circuit designer should
give very special consideration to using a different brand or style of crystal.
Quote
Low-frequency tuning elements are more sensitive to temperature than their high-frequency counterparts.
In general, low-frequency crystals can be damaged more easily than high-frequency elements in soldering
processes. Particular attention must be paid to keeping soldering operations within temperature ranges that
will not damage the crystal. While high-temperature damage can manifest itself in many ways, the most
common symptom is the failure of the crystal to operate. Another way for temperature damage to express
itself is for the crystal to display bizarre or unexplainable start-up characteristics. If problems with a crystal
suddenly appear in a product line that previously had no problems, assembly soldering temperatures are a
good place to investigate.
« Last Edit: November 23, 2013, 10:02:34 am by mrburnette » Logged

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