Study On Radio Frequency Filter Computer Science Essay

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In 1957 the Soviet Union has send the first made man satellite in the space these satellite sends information to earth through radio frequency for many days. So with the passage of time the number of satellites is increased and now become the needs of modern communication. So repeater plays the main role in the satellite communication. The function of the repeater is to receive a signal on a given carrier frequency from a transmitting station and forward these signal to the receiving station. These specific filters were design for the space application, while on earth another carrier frequency was used, so there was loss of energy, as we know that there are limited resources of energy across the satellite. So to overcome this problem, a filter is used in the satellite repeater. Filter are used for the input Multiplexer (IMUX) and for the output (OMUX).and finally a low pass filter were used before the antenna, to eliminate unwanted high frequency harmonies generated by the power amplifier. So we can define the filter as "The electronic circuits which perform single processing functions, especially to remove the unwanted frequency components from a signal in order to enhance the wanted ones or both."

Filters are commonly known as pass filters are the circuits so arranged as to separate the signal.

Desire radio signal from undesired radio signal:-

In cellular, wimax and satellite networks the mobile station has the capability to extract and communicate on the selected frequencies only. They implement this feature by using high quality RF filters. E.g. in the case of cellular networks when a mobile is switched on it reads it SIM (Subscriber Identity Module) card and scans all the available frequencies. It measures their strengths, noise levels and then checks which frequencies are allowed by reading the carrier information from the SIM card. It then filters and scans only these frequencies. It then selects the most suitable cell to camp on. For better reception it periodically scans and filters frequencies of its neighboring cells and switch to the most appropriate cell (having lesser noise, good SNR, lower interference, Acceptable signal levels, etc.).

When a signal is modulated for transmission, none of the RF filters available are able to cut off the extra harmonics of the signal completely. Hence, sidebands are present in the transmitted signal. The sidebands which are higher in frequency than the original signal is called upper sideband (UF) and the ones lower in frequencies are called (LF). Sometimes both sidebands occur. We can filter out these sidebands using high and low pass filters at the transmitting and receiving end. By tuning the filter to filter higher and lower order frequencies than the original transmitted signal we can extract our original carrier signal.

In amplitude modulation the problem of sidebands' specially distorts the quality of the signal. DSB when occurs in AM is called DSM-AM. DSM in AM signals waste available bandwidth and hence reduces the efficiency of the communication system. Up to 66% of the bandwidths is wasted by these sidebands. To overcome this band pass filter is placed at the transmitter which filters out the higher frequency components and then the lower frequency components and original carrier signal are transmitted. This decreases the bandwidth by nearly half, giving more channels to AM system for communication

Suppose we have a series of long and short pulses in a signal and we have to separate them. Here RF filters can be used to filter them out. To separate the short pulses we can use Low pass filter and for higher high pass filter can be used. The following diagram illustrates this process.

Filters are used for separation unwanted objects from the mixture. Filters are of many types and perform different purposes. Paper filters are used in chemistry labs for filtering. Similarly, filters can be used to remove gravel from a mixture of sand and gravel.

Filtering out DC component from a mixture of AC and DC sources requires the use of filters. An inductive low pass filter is required to filter out the AC source present in the mixture.

Similarly, the AC component from a mixture of AC/DC source can be filtered using filters.

Filter circuits are normally designed by using inductors and capacitors inductors have low opposition to low frequency while in case of inductors it is otherwise. The action of a filter circuit depends upon the following principles:

The opposition offered by the inductance varies directly with the frequency. The inductance offers low opposition to D.C and low frequency A.C and high opposition to RF (XL= 2ПFL)

The opposition offered by a capacitor varies inversely to the frequency. A capacitor offers high opposition to D.C and low frequency A.C and low opposition to RF ( Xe = 1/2Пfc)

A series resonant circuit has got minimum impedance at resonance and offers low opposition to the currents of the frequencies that lie within the narrow band above and below resonance and higher opposition to other frequencies.

A parallel resonant circuit at resonance has maximum impedance to the flow of current and to frequencies that lie within the narrow band above and below resonance

Resistor doesn't provide any filtering action alone. It offers uniform resistance of A.C of any frequency or D.C when suitably arranged in a circuit containing L, C or both. It will help the filtering action. Normally when in the series will do limiting hence reducing the selectivity of the circuit

There are three main types of filters that is, High pass filter, Low pass filter, and band pass filter. Fourth type that is the band pass filter is also sometime used.

What is RF FILTER?



The RF filter is play an important role in communication system and due to the proliferation on radar, satellite and mobile wireless, there is a need for design, method that can satisfy the ever increasing demand for accuracy, reliability, and fast development time. While RF Filter is an electrical device that is utilize configuration design to have a specific characteristic with respect to transmission or attenuation of various frequencies. RF filters make possible the necessary frequencies to be passing through the circuit, otherwise reject this because those aren't needed. So more concisely we can define the RF filter Allow or stop a frequency or the ideal filter whether low pass, high pass, band pass or any other filter will show no loss in the pass band. That is frequency below the cut of frequency and also the filter will refuse all the stop band frequency. So it RF filter is used to eliminate unwanted signal. It means that RF Filter help to cut out RF interface that could occur if a Noisy device is activated. So we can say that RF filter is designed to authorize for the communication of a range of frequency in the mega Hz to gaga Hz that would be applied. The frequency range is the range used by the most broad band radio television, wireless communication and also majority of the microwaves devoice. Such filter is use as building blocks for diplexer and also having capability to combine or separate multiple frequency bands. Most RF filters are made of one or more resonator, so any technology that can use in resonator also having capability to make a filter. The main function of the RF filter is namely a high RF selectivity, with very low insertion lost, this feature is very beneficial for wireless communication system, because the wireless system is especially at risk to squalor from the strong unwanted interface. It place different system closed to each other in order to use more efficiently the limit of the spectrum in the frequency domain.



Pass band is the band of the frequency or wave length which is below the "cut off frequency"(Means point of the frequency, above or below the device fail to work or work efficiently).so the filter allow a signal through Pass band .so the cut of frequency of the filter can also called half or 3DB frequency. The filter is place in between the antenna input of a receiver and the antenna.The main function of the Filter is to remove the unwanted signal before it reach to the Receiver. The low pass filter and high pass filter depends on the frequencies which you want to eliminate. There are many advantage of to place the filter before the receiver.

Management of available Radio frequency is the key component in cellular networks. Spectrum is usually limited and is divided among many mobile operators in a country. So making the most of these frequencies is essential. Cellular networks have different technical departments dedicated to the efficient use of RF. The radio frequency departments can be divided into two main categories. RF Planning and RF Optimization These departments deal with RF Filters which are installed in the BSC (Base Station Controller) and BTS (Base Transceiver Stations). RF Planning is used where the mobile operator is laying out its network. Here it determines the area of the cells and their frequency distributions keeping in mind the precision of RF filters that would be used in vendor's equipment. RF optimization deals directly with RF filters. RF Engineer when performs Drive Tests he measures all the frequencies through RF filters of his mobile phone. The narrow allocation of frequencies allows the existence of many mobile operators simultaneously and they can have nearly the same frequencies but the filtering of RF filters makes it possible for them to operate without having interference between each others. Another key use is in manufacturing of Radios, Willkie talkie, Cellular towers and Mobile phones. Here again RF Filtering is the key component in communications. Example the radios select the channels by tuning their band pass filter to pass only the selected user channel. The TV's in our home also selects different channels by using a tunable RF filter.


There are many types of RF filter, which have a capability to accept or reject the unwanted signal in different way. These types of filter are below:-

1. Passive Filter 2. High pass Filter 3. Low pass filter

4. Band passes Filter 5. Band Reject filter 6. Active Filter

Note: There are four basic types of RF filters: High pass, Low pass, Band pass and Band Reject filter.


Passive filter is the combination of three elements includes Resister, indicator and capacitor, wit out any active elements such as vacuum tubes or transistor .passive filter do not produce any of the input signal. So we can say that the collection of the resister, indicator and capacitor is called passive filter. The selection of the signal depends on the size and the arrangement of these components actually passive filter doesn't contain on the active components like transistor. The filter is said to the simplest passive filter if it contain on a single reactive element Passive filter used the inductor which is an expensive and have high resistance at low frequency in passive filter the quality factor and the frequency cannot be set easily.

.passive filter there is no source of power

Indicator (has a capability to store energy in a magnetic field created by the electric current). Indicator is one of the basic reactive components used in an electronic and has a capability to block the high frequency signal and conduct the low frequency signal.

Capacitor is also one of the basic reactive component of the passive filter it. Capacitor is contains on pair of conductor separated by the insulator working opposite of the indicator. Block low frequency and conduct high frequency signal. When signal enter in the filter it pass through indicator where capacitor provide path, less attenuation to low frequency and high frequency signal.

Resister is the third component of the passive filter, resisters have no capability to select frequency but it is a part of the indicator. There are many types of the passive filter but some the basic are include Low pass filter, high pass filter and band pass filter.


It comprises a network of a series capacitors and shunt inductors. A high pass filter is the circuit which allows all current of frequencies above certain frequency to pass on to the desired circuit, and oppose or divert all the frequencies below that. The inductor only used in fig.b It must be remembered that if the internal resistance of source is zero it will not differentiate between frequencies. However if internal resistance of the source is Ri, it will drop most of the voltage at low frequency where XL is the extremely small voltage across XL and RL. If Ri is large it will share the A.C voltage with RL even at high frequency causing attenuation. Simple capacitor or inductor is seldom used as high pass filter, hence their combinations C and D is called T networks and of C П network. Figure 2 shows the behavior of the high pass filter to different frequencies. It is not possible to say exactly that frequencies above this are passed and below this are stopped, as the changes are in gradual. However for all the calculations to be made those frequencies for which output Amplitude is 70% or more as compared to maximum amplitude at any frequency. (In this case very high frequency) are considered to have been allowed to pass through the filter while other frequencies are considered to be stopped.


fig (a) fig (b)



As it is mention early that high pass filter will pass all the frequency from cut off to the customer specified upper limit and bloc al the frequency below the 60 DB , Also called miniature filter because it rejects all unwanted RF signal. High pass filter is use in many types of military application including RF communication and navigation system.

The main advantage of the high pass filter is its small size and better electrical performance, with an extended pass band range up to 100 times, the cut off frequency

The 2nd important advantage of the high pass filter is its sharp selectivity, low loss and high rejection and excellent frequency stability.


The 3ard advantage of the high pass filter is to receive excellence electrical performance, environmental stability, and mechanical.

A low pass filter is the filter which allows all the frequencies below certain frequency to pass through while others are stopped. Action of low pass filters is exactly like the high pass filters except that L and C are interchanged. Fig. and fig. are simple inductance and simple capacitance filters respectively. Fig c is sometimes called half T filters. Fig.4 shows the frequency response of high pass filter. In its simplest form the passive low pass filter is realized as a ladder network with a series inductors and shunt capacitors. At low frequencies the series reactance and shunt susceptance of the components are Insignificant and all the power from a signal is delivered to the load. As the frequencies increases the reactance and susceptance become more significant and the frequency response gradually rolls off. If the source and the load resistances are equal the filter built with ideal has zero transmission loss at D.C. When the transmission is not equal their impedance difference creates mismatch losses that apply to throughout frequency range. Such applications are mainly used in low frequency portion of a system. It is possible however to match unequal terminations through the specified pass band with a quasi low pass design that uses low pass topology yet perform as a band pass filter. Ideally we want a frequency response that has zero loss until a specified corner frequency and infinite loss from thereon. In filter design we approximate this response with various types of rational fractions to provide a gradual attenuation above the corner frequency. The selectivity of the frequency response just above the corner frequency depends on the order of filter and the type of filter response we chose. The required manual steps to prepare the low pass filter are:

Determine the filter order 2) Find the prototype component values

The inductor in series with the signal path is the main representation of the low pass filter. The LFC has the capability to attenuate all the signal frequency which is above from has cut off frequency and allow passing the entire signal which is below its cut off frequency. The Values of the capacitor and inductor can be determined by using the below equation:-

LµH=K1/MHz CpF=K1/MHz LµH=k1/FMHZ Since K1=15.88, FMHz=35

LµH=15.88/35=0.454µH CpF+K2/MHz CpF=3180/35=91pF





As it is mention that Low pass filter rejects all wanted RF signal above a specified pass band .these filter are used in many application like radar system, air born navigation system, electronic warfare and communication system.

The main advantage of the low pass filter is its small size and excellence RF performance with extended capability of stop band pass feature

The low pass filter is capable of excellence frequency flatness, low insertion loss and VSWR and sharp selectivity.

It receive excellence electrical performance ,mechanical reliability and sharp selectivity




A band pass filter is the one which allows a narrow band of frequencies to be passed on desired circuit, and opposes or divert all frequencies above and below the desired band of frequencies. A simplest form of band pass filter is series resonant circuit connected in series with a load as shown in the figure a or a parallel resonant circuit connected across the load as shown in the figure b. the series resonant circuit will pass all the frequencies in the vicinity of the resonant frequency. A parallel circuit connected across the load will not let the desired band of frequencies to be delivered away from output. But all the other frequencies fig and fig d are the other modified circuits.


Band pass filter is one of the form of RF filter, also include in a miniature device which has a capability to discriminate the frequency from unwanted frequency between the desired bands.

Band pass filter used in EW-system radar, aero space, navigation systems and also in communication and weather satellite system

The main advantage of the band pass filter is its small size and excellence RF performance with extended capability of stop band pass feature

Band pass filter has an excellent frequency, flatness low insertion loss ,VSWR and sharp selectivity

Band pass filter has ability to receive an excellent electrical performance mechanical reliability and environmental stability.

Band Reject filter/ BAND STOP FILTER


In this type of filter parallel resonant circuit and series resonant circuit are interchanged as compared to band pass filter. All frequencies are available o/p except a small band of frequencies in the vicinity of resonant frequency of the L.C circuits. This filter covers the frequency range of 10MHz to 20 MHz with narrow to wide customer. Specified frequency stop bounds .Typical notch attenuation is greater than 60 DB. Maintains loss pass bonds above and below the specified notch.


Custom application (300MHz-6GHz) 2. Custom design /connector

Rack amount or surface mount 4. High ceramic 5.Field application


Active filter is also type of the filter contains on one or two active components voltage, buffer amplifier .for example vacuum tube. The active filter has three main advantages over r the passive filter

In active filter the inductor is avoided, inductor has no capability to get a high quality factor. due to this quality the active filter become cheep and have a low internal resistance at low frequency

In active filter the quality factor and the frequency can be set easily regarding their both parameter


Active filter contains on one or more power source

Analog filter also one of the basic form of the filter .Analog filter is the type of filter which use the time varying element to perform various functions continuous time analog Signals. The analog signal is the signal which is not been digitize or and quantize in their strength as a function of time. So we can say that analog filter is a= type of filter that operate on any continuous time signal. It is described from the differential equation. The term Analog signal is a typically signal which have a single input and single output signal. So more concisely we can say that analog filter is form of digital filter as the sampling -rate is allowed to go to infinity.


Two additional groups are also present. One is used for equalization and the other is a multipoint circuit that passes different frequency bands to the different load terminations.


Circuits that pass all the frequencies with equal amplitude but lead to a predictable phase shift. The group delay of a component indicates how long it takes the signal to pass through the two-port. For accurate reproduction of the signals passing through the system all the components should have a flat group delay means that through the pass band the signal should take equal time to pass through the

component. Filters are designed to introduce sudden changes in the frequency response which causes non linear transmission phase. Since the group delay is a derivative of a phase a non linear phase response lead to the different group delays at the different frequencies. Components that are specially designed to compensate the group delay distortion of filters are called delay equalizers. These components do not affect the magnitude of signals of various frequencies but they compensate the group delay distortion introduced by the filter network. Some of the filter synthesis program also allows designing group delay equalizers with appropriate number of sections to offset the delay distortions of specified filters.


It is a type of single processing filter that is considered to have a frequency reply which is as level as possible mathematically in the pass band. It is also called maximally flat magnitude filter.


These are the filters which may be analogue or digital that has a steeper roll off and a more pass band ripple or stop band ripple than the Butterworth filters. It has a property to minimize the error between the ideal and the actual filter characteristics but with the ripples in the pass band over the range of filters. They show a smoother response in the pass band but a more irregular response in the stop band. This characteristic is desirable in some cases.


Elliptic function response

It is the response which is equal in both the pass band and the stop band

All pass response

Here the amplitude response at all the frequencies is unity so it is called all pass filters. A phase shift and group delay is produced by the all pass network.


Low-pass to High-pass transformations

Low pass filter prototype can be converted to high pass; band pass and band stop by simple mathematical operations and component modifications. Since even the basic synthesis programs provides different solution to bypass the transformations.

A high pass transmitter can be changed to low pass prototype before impedance and frequency scale. The low pass filter must be designed for mirror image symmetry around the pass band corner frequency of the high pass filter with the reciprocal stop band attenuation requirements.


Thanks to RF filters without which cellular networks would not exist as we know them today. We could not divide sharp frequencies to individual cells and hence the cellular networks would be very limited and expensive. RF filter is widely used in RF design and in all manner of RF and analog circuit. In general as the RF filter allow through a particular frequency, RF filter is an essential component for the RF design Engineer. Without today's RF Filters wireless digital communications would be impossible. RF Filters have many uses in satellite communications too.