Surface Acoustic Wave (SAW) Filters

A surface acoustic wave (SAW) filter is a solid-state filter that can replace a conventional passive inductive-capacitive LC filter. It offers excellent amplitude and phase response over wide bandwidths and frequency ranges. SAW filters are made from piezoelectric materials such as lithium niobate (LiNbO3) and quartz. A filter made from quartz offers excellent temperature stability over wide temperature ranges, and a lithium-niobate filter simplifies electromagnetic-to-acoustic coupling. These filters have relatively high insertion losses, so they typically require an amplifier in series with the SAW to recover lost signal strength.



Figure 1 Crystal in holder.

Crystal Frequency Standards

Crystals used as frequency standards are made from piezoelectric materials that resonate at high frequencies when subjected to an alternating current. Selectively cut quartz crystals generate more stable frequencies than coil-and-capacitor tank circuits. Crystals for generating frequencies for timing or other purposes are packaged in radial-leaded metal cases, as shown in Fig. 1.

Quartz wafers are ground to precise thicknesses, and metal-film electrodes are deposited on both sides. The electrodes are connected to the leads that extend through the base. When powered by AC, the quartz wafer vibrates at a frequency determined by its thickness. Thin crystals resonate at higher frequencies than thick crystals. The highest fundamental frequency of a quartz crystal wafer is 15 to 20 MHz. Harmonics or multiples of this frequency provide higher radio frequencies. Quartz crystals in holders serve as oscillator tank circuits. Crystals can also serve as selective filters because of their high Q factors.