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Crystal parameters [1 Attachment]

Steven Dick
 

Jack, see http://pages.suddenlink.net/wa5bdu/crystal_slide_show.pdf pages 33-34. Generally when the crystal filter bandwidth is narrower than expected, it is because you didn’t take crystal’s lead to case capacitances into consideration. There’s a C from each crystal’s plate to the case. If you tie the cap’s two leads together and measure this capacitance to case, when you measure lead-to-lead, you measure Co (capacitance across the xrystal’s leads) plus Cc1/2. To find Cc1, connect the leads together and measure to the case. That gives 2*Cc1.
If in your filter you have a coupling capacitor Cjk between crystals j and k to ground, you should reduce its design value by Cc1 * 2, assuming your cases are grounded.
 
“Digital Steve”, K1RF
Sent: Friday, March 18, 2016 1:35 PM
Subject: Re: [PHSNA] Crystal parameters [1 Attachment]
 
 

Well,

 
Works much better -- I redid the filter with a 1.65KHz Dishal design.  You can see the results in the attached pic.  Not perfect but what I expected -- BW is a little low 1.4.  Thanks again for the help.
 
73,
Jack (W0FNQ)

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