INFICON PLO-10i Phase Lock Oscillator - Manual (Page 35)

PLO-10 PHASE LOCK OSCILLATOR
CRYSTALS, HOLDERS AND FLOW CELL 7-1
7 CRYSTALS, HOLDERS AND FLOW CELL
An essential part of the PLO-10 Oscillator is the sensing crystal. Careful handling of
both the crystal and the crystal holder must be observed to ensure proper and
reproducible measurements. Furthermore, the sensing crystal, the crystal holder and the
connecting cable must be orientated and connected correctly in order for the PLO-10 to
work properly. This is especially true if you design your own crystal or holder.
If you have purchased a INFICON crystal, holder and cable, the installation is simple.
Follow the instructions below.
If you plan to build your own crystal or holder or cable, see Section 7.3.2 HOLDER
CARE AND HANDLING.
7.1 1 INCH DIAMETER CRYSTALS
INFICON
pioneered the standard AT-cut, 5 MHz, 1-inch diameter crystals for use in liquid
applications. The AT-cut quartz is chosen for its superior mechanical and piezoelectric
properties, and the angle of cut can be adjusted to obtain a zero temperature coefficient at
a desired operating temperature. The 1 inch diameter was chosen to allow enough
distance between the active area of the crystal and the mounting o-ring. This improves
the overall stability of the crystal by reducing the frequency changes due to mounting
stress.
7.1.1 ELECTRODE CONFIGURATION
Figure 10 below shows INFICON’s 1” crystal electrode patterns. The left figure shows the
½ inch diameter front electrode (also called sensing electrode) with an extended electrode
that wraps around the edge of the crystal and extends into a semicircle shown in the top
half of the right figure. The lower half of the right figure shows the ¼ inch diameter rear
electrode (also called contact electrode).
This configuration enables both electrical contacts to be made on the backside of the
crystal allowing measurement in conductive liquids.
The oversized front electrode (½ inch in diameter as oppose to the ¼ inch diameter rear
electrode) was chosen to ensure a more consistence deposition across the active area of
the crystal. The exposed area of the front electrode is 0.212 in 2 (137 mm 2 ), but the active
oscillation region (displacement area) is limited to the overlapping area of the front and
rear electrodes (0.053 in 2 or 34.19 mm 2 ).