Sunday, February 6, 2011

Crystal Oscillator Design: Parallel and Series

On the Rockmite yahoo group today, Chuck Carpenter, W5USJ posted a very nice explanation of series vs. paralel resonant crystals along with a link to a FAQ on crystal oscillator design at:

http://www.foxonline.com/techfaqs_cry.htm#a4

Chuck' post follows:

RMers,

Here's some info sent to me by Rich at ESS.



Chuck,

The group that requested 7114 requested it in Series Resonant crystal. Most
of the other crystals that I sell are Parallel Resonant. I have included an
explanation from Fox Crystal Co. web site. They are not my supplier but the
information applies to all crystals and their manufacturers. This should
get you where you want to be.

The information that everyone needs to know is #6. 4 and 5 will help people
make a circuit oscillate where they want it.

This is from: http://www.foxonline.com/techfaqs_cry.htm#a4

More information can be found at:
http://www.oscilent.com/spec_pages/PNDescrpt/Load_Cap.htm

4. The load capacitance (CL) of my parallel resonant crystal is rated at
20pF. How do I calculate the value of the load capacitors used in my
parallel resonant oscillator circuit?

Use this formula to approximate the value of capacitors needed:

CL=((C1 x C2) / (C1 + C2)) + Cstray

Cstray is the stray capacitance in the circuit, typically 2-5pF. If the
oscillation frequency is high, the capacitor values should be increased to
lower the frequency. If the frequency is low, the capacitor values should be
decreased, thus raising the oscillation frequency. When CL =20pF, C1 and C2
will be approximately 27-33pF each, depending on the amount of stray
capacitance.

5. What is load capacitance?

Load capacitance is defined as being the total capacitance present in an
oscillator circuit as measured or calculated across the pins of the crystal
socket. Load capacitance has the effect of increasing the frequency of a
crystal unit. Please see the tutorial section for a complete explanation.

6. Will a series crystal work in a parallel circuit and vice versa?

Yes to both, but a crystal unit manufactured as a series resonant crystal
will operate at a frequency higher than expected if used in a parallel
resonant circuit. A crystal unit manufactured as a parallel resonant crystal
will operate at a frequency lower than expected if used in a series resonant
circuit.

If you need any other information then feel free to contact me,

Rich Kennedy - N4ESS
Expanded Spectrum Systems
6645 Founders St.
Zephyrhills, FL. 33542-1828
USA

813-620-0062 Office

www.expandedspectrumsystems.com

n4ess@tampabay.rr.com



Chuck Carpenter, W5USJ, Point, -|- Rains Co., TX -|- EM22cv -|- 72/73
56 years -|- 19 as K2OFN and 37as W5USJ -|- Most fun = QRP since 1984.
Website = http://www.w5usj.com

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