Ceramic RF & IF Filter

- notes, information and overview of the ceramic filter used for IF and RF filter applications, particularly in radio receiver applications.

Ceramic filters are widely used in IF and RF bandpass filter applications for radio receivers and transmitters.

These RF & IF ceramic filters are cheap, easy to use and in many ways ideal for a host of applications where the performance of more costly filters such as crystal filters are needed.

RF & IF ceramic filter basics

As the name implies, RF & IF ceramic bandpass filters are manufactured from ceramics that exhibit the peizo-electric effect. These filters were initially available for lower frequencies, with centre frequencies typically being within the range of a few kilohertz up to frequencies of 10.7 MHz which is a standard IF for many FM broadcast receivers. However significant investment in the development of these filters enabled much higher frequencies and specification levels to be achieved. Frequencies up to UHF and beyond are now available.

Having a lower Q level than quartz, these ceramic RF & IF filters have bandwidths that are typically measured between 0.05 and 20% of the operating frequency. Often the Q levels range between around 500 up to 10 000 or possibly more as the technology improves.

Ceramic resonator outline

The elemental format for the ceramic filter is a simple ceramic resonator that has electrodes plated onto either side. This behaves in much the same way as a single quartz crystal, although with a lower level of performance. Also the separation between the parallel and series resonant frequencies is greater.

Ceramic resonator
Ceramic resonator used as the basis for the ceramic bandpass filter

The ceramic bandpass filter has a very similar equivalent circuit diagram to that of a quartz crystal.

Ceramic resonator equivalent circuit
Ceramic resonator equivalent circuit

From this it can be seen that there are two ways in which the circuit can become resonance - one being a parallel resonance and the other a series resonant point.

Ceramic resonator response curve
Ceramic resonator response curve

Ceramic bandpass filter overview

It is possible to make a complete bandpass filter using a ceramic resonator by splitting and separating one of the electrodes. This gives a common connection - the single electrode - and input and output connections.

Ceramic passband filter equivalent circuit
Ceramic passband filter equivalent circuit

Ceramic RF & IF bandpass filter circuit symbol

The circuit symbol for the ceramic filter gives a representation for the ceramic resonator together with the common electrode at the bottom, and the input and output electrodes separated in distance across the ceramic resonator element itself.

Ceramic bandpass filter circuit symbol
Ceramic bandpass filter circuit symbol

It must be remembered that unlike a transformer style LC filter used for inter-stage coupling and tuning, there is no DC path within the ceramic filter - it is an insulator. Therefore any circuit using ceramic filters must take account of this providing all the bias and source current from outside the filter. A transistor circuit if the format shown below would work, although many IF amplifier or complete radio ICs have ports specifically for use ceramic filters and will incorporate the correct current and bias paths outside the ceramic filter.

Ceramic bandpass filter circuit using transistors and showing DC arrangements
Ceramic bandpass filter circuit using transistors
and showing DC arrangements

Ceramic filters are widely used as cheap RF and IF bandpass filters for a variety of applications. Initially ceramic filters were primarily used to provide IF selectivity in low cost broadcast receivers. Now the technology has improved significantly and they are widely used at much higher frequencies and to meet more exacting requirements. Accordingly they are sued in many forms of radio systems from cellular to wireless applications as well as in broadcast radios, televisions and other more traditional applications.

By Ian Poole

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