Hi,

On 17 Mar 99 at 12:54, John Esposito wrote:
> But what are the differences between a serial and parallel but crystal?

This is from an FAQ on crystals from Fox (They make crystals).  You
can find the whole FAQ and some more data on their web site at:
http://www.foxonline.com/

What is the difference between series resonant and parallel resonant
crystals?

All quartz crystal resonators have a series resonant frequency (f s ,
frequency of lowest impedance). At this frequency, the crystal
appears resistive in the circuit. Crystals can be "pulled" from this
series frequency by adding reactance (capacitance) in series with the
crystal. When operated in combination with an external load
capacitance (C L ) the crystal oscillates in a frequency range
slightly above its series resonant frequency. This is the parallel
(load resonant) frequency. When ordering a parallel crystal, always
specify the nominal parallel resonant frequency and the amount of
load capacitance in picofarads (pF). Alternatively, a standard value
of C L (such as 20pF) can be ordered; the capacitor values are then
calculated to match the crystal C L . Note that a series type will
oscillate in a parallel oscillator, and vice versa. The observable
difference is a frequency offset of less than + 1000 PPM (0.1%) from
the nominal frequency.

<snip> During the final plating the crystal is dynamically tuned. The
crystal is oscillated in a test circuit and the frequency is read via
a counter. While oscillating, a fine stream of silver is applied via
depositioning process in a vacuum chamber. As the target frequency is
approached the stream ( plating thickness) is turned off. As you can
see from the process above the only difference between series
crystals and parallel crystals is that tuning is offset to a lower
frequency in the parallel crystal to compensate for the system
frequency shift cause by the parallel load of C1 & C2.

Hope this helps,
Guy - mlsirton@inter.net.il