What Is a Crystal Filter?
Crystal filters are filters that use quartz crystals. Crystal filters are mainly used as band-pass filters to extract the required frequency components.
Crystal filters are effectively used to selectively extract frequencies because quartz crystals have high Q and steep passband characteristics.
Similar to crystal filters, SAW filters are sometimes used because they can extract the required frequency components.
Applications of Crystal Filters
Crystal filters can select only specific frequency components from a wide range of frequency components and attenuate unwanted components. For this reason, they are often used in equipment that requires highly accurate frequency selection. The most common application is in wireless communication equipment.
Wireless equipment uses a signal called a carrier wave. Carrier waves are the basic waves, such as radio waves, used to transmit and receive information. When creating a wireless signal, the information to be sent is superimposed on the carrier wave to create a communication signal. The carrier wave is then separated when it is received. In wireless equipment, it is necessary to separate the necessary frequencies from the unnecessary frequencies, and filters such as crystal filters are often used.
Principle of Crystal Filters
Crystal filters are filters that use the high Q of crystal units, where Q stands for Quality Factor and is called selectivity.
The larger the Q value, the narrower and sharper the bandwidth, and the smaller the Q value, the wider and gentler the bandwidth and frequency response.
The Q value can be calculated as follows and is expressed in terms of how much bandwidth characteristic it has in relation to the center frequency.
Q = ωo / (ω2 – ω1)
where ωo: center frequency, ω2, ω1: frequency of 1/√2 intensity relative to ωo.
A band-pass filter using LC, which is a common passive component, has a Q value of about 10^2. Crystal Filters have a high Q value of about 10^3 to 10^6, and Crystal Filters can be configured to have a high Q value. When a high Q filter is used as a band pass filter and the signal is passed through the filter, there is almost no output at frequencies other than the center frequency, making it possible to create a circuit that selectively obtains a specific frequency.