PARTNER FOCUS
A simple Guide to RF Filter
n this modern world of wireless communication and Internet of Things (IOT), many devices are at risk of being swamped by a barrage of radio signals. For systems to function correctly it is necessary to filter out undesirable signals and only permit those that are required to pass through – this is the essential function of an RF filter.
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There are many types of RF filter available on the market which for buyers can be bewildering, but the type of filter used is dependent on several factors such as the centre frequency, attenuation and power rating.
In all filter applications the matching circuitry is critical for optimum filter performance and careful design and testing is important to obtain the best results.
The five most frequently used types are: crystal filters, surface acoustic wave (SAW) filters, helical filters, cavity filters, and ceramic dielectric filters.
Table 1 below sets out the basic parameters needed to specify a typical RF filter.
Centre Frequency 1dB bandwidth 3dB bandwidth
Stopband frequency Insertion Loss Ripple
Voltage Standing Wave Ratio (VSWR) Attenuation
Stopband Loss Input Power
Input/Output Impedance Termination
MHz kHz kHz kHz dB dB
Typically 3 to 1 dB dB dB
Ohms Pin, BNC or SMA
The following is a brief description of each filter type and some of the applications that use them.
Quartz crystal filters – 1.4 to 200MHz There are two basic types of crystal filter: multi-element and monolithic.
The multi-element filter as its name suggests comprises several pairs of matched crystals connected with balanced capacitors and inductors to produce an RF filter.
32 JULY/AUGUST 2021 | ELECTRONICS TODAY
These exhibit an extremely high Q factor, narrow bandwidth and operate from 1.4 to 200MHz housed in a connectorised metal case. A band-pass filter – one that allows a particular band of frequencies to pass through – may be characterised by its Q-factor, defined as the reciprocal of the fractional bandwidth. Thus, a high-Q filter will have a narrow passband while a low-Q filter will have a wide passband. This means that if a narrow signal acceptance is required then the filter must have a high Q factor to restrict unwanted signals.
Common applications for crystal filters include radio receivers and maritime communication systems.
In contrast, monolithic crystal filters (MCFs) are a single element of quartz with two sets of electrodes plated onto the crystal surface and coupled together to provide a highly selective filter. These are typically produced in limited frequencies such as 21.4MHz, 21.7MHz, 38.85MHz and 45MHz but other frequencies are available. MCFs are generally available in two-pole formats, but four-pole versions are available depending on the required filter response. Common applications for monolithic crystal filters include IF channel filter in radio base station receivers, two-way communication sets and pager systems
SAW filters – 50MHz to 3GHz
Surface Acoustic Wave filters are usually made from Lithium Niobate or Lithium Tantalate material. These filters work by converting electrical impulses into an acoustic waves by means of interdigital transducers deposited onto a piezoelectric substrate such as Lithium Niobate.
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