Telephone line noise filter circuit

In the other method, loop series wiring, a single cable from the connector block connects to the nearest phone jack and continues to each of the system's phones in turn. Excepting the jack where the wires terminate, wires enter and leave each jack location. Fig 1 - Residential telephone systems usually depend on straight cable runs A or loop series wiring B for phone interconnection. Your system connects to the telephone company via the pole-to-house drop which may run underground in some installations ; a grounded, fused protector that minimizes lightning damage to your wiring and telephones, and the service entry - the terminal block at which the phone wires actually enter your house.

This drawing shows possible insertion sites X for the RF filters discussed in this article; not shown is the ground wire that goes from jack to jack in a properly installed system. Tip and ring are phone-company nomenclature for the talk-circuit wires. See text. Open each phone jack and check to see that the ground wire is connected to the jack. In loop circuits, a typical wiring error made by installers is to connect the pair and ground wires to the first jack, and to continue the pair, but not the ground, to the remaining jacks.

This deficiency may not be apparent until RF is present. If you find this condition, correct it by connecting the ground wire that leaves the jack to the jack ground terminal and the ground wire that enters. Do this with every jack that's wired incorrectly. While you're at it: Good telephone-wiring practice calls for grounding all unused conductors in active phone cables. This is easy to do at the service entry.

Simply locate all unterminated wires and attach them to the system ground. In a straight-cable-run system, entirely disconnect, rather than ground, all wires of cables to unused jacks. If the interference remains after you've ensured your phone system's physical integrity, RF currents - usually, common-mode currents Fig 2 - exist in the system. Your next job is to reduce or stop that RF flow with filters. Fig 2 - Audio, control and power signal energy appears in phone wiring as differential-mode signals - signals impressed "across" the Iwo wires of the pair.

At any instant, differential-mode currents in the wires flow in opposite directions, with the pair serving as source and return. The system ground wire - not shown here - is for protective purposes and does not serve as a return for talk-circuit signals. RF energy travels down the pair in common mode, in which both of the pair's wires act as a single wire. Common-mode RF signals may return to ground via multiple paths, or they may not need a ground return at all if the system wiring acts as a balanced antenna.

Ferrite-core chokes work well in reducing common-mode HF-RF current flow on telephone wiring.

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Fig 3 shows a bifilar choke with inductive characteristics appropriate for phone-line filtering in the 3- to MHz range. Both of the talk-circuit wires pass through the ferrite core together to block common-mode RF current on the telephone pair. Inserting such a choke at the telephone or telephone jack may solve your interference problem.

I'll refer to these chokes as filters hereafter because they filter out phone-line RF while allowing audio and dc to pass. Try this and see if the interference diminishes. If not, don't be discouraged. A tower-guy installation technique may help solve the problem. Schematically A , the choke consists of two 1. If necessary, you can use heavier- or lighter-gauge wire; heavier-gauge wire may necessitate the use of a larger-OD up to 1 inch core. The author reports that filters of this design have been effective in reducing or eliminating telephone RFI from transmitters operating between 3 and 30 MHz.

B shows how the two windings shown in A are wound together on a single ferrite core. To make construction easier, you can twist the wires together to a pitch of five to ten twists per inch and wind them as single wire. To keep track of which winding is which, use different-colored wires or if you use wire of one color for both windings use an ohmmeter to check for continuity through each winding.

C shows such a bifilar two-wire choke equipped with terminals and connecting wires, and mounted on a board for potting as a commercial filter. The terminals on this filter the contents of a K-COM RF-2 are available only in bulk quantities; you can use terminal strips Radio Shack instead when building your own filters.

K-COM also sells phone-line filters model RF-1 equipped with modular phone jacks; contact the author for details. If you've gone to the trouble of erecting a high antenna support for a directional antenna, you don't want resonance in and reradiation from the tower guys to distort that hard-won directivity. Breaking the tower guy wires into electrically short, nonresonant-in-ham-bands lengths, each of which is insulated from its neighbor s by strain insulators, can cure this. We can do the same thing in a telephone system. Rather than use insulators, we'll discourage RF-current flow in the system with filters.

Assuming that a filter at the telephone or wall jack did not reduce your interference to a sufficiently low level, let's move our attention to the telephone system's front end: Install a filter there and recheck the interference level. If your interference is gone, congratulations! If not, don't give up! Even if a service-entry filter doesn't reduce or cure the RFI, installing such a filter narrows your search by disallowing RF current flow into the house from the service drop.

In other words, now you know that in-house telephone wires are picking up the RF. Now, systematically breaking up your house telephone wiring with filters can greatly reduce the system's RF sensitivity. If your telephone system uses straight cable runs, you can take either of two approaches. The first, and arguably the faster, of the two approaches may involve installing more filters than the minimum necessary to cure the interference.


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The second and more methodical approach minimizes the number of filters installed but may take longer to complete. Study both methods before trying either:. Method 1: Insert a filter at the beginning of each cable run at the service-entry connector block and at its terminating phone jack. This approach usually solves all but the severest telephone-RFI cases. Curing stubborn interference may require you to physically break one or more long cable runs to insert additional filters.

Consider doing this only after you've filtered both ends of all runs. If you must cut a cable run to insert a filter, be sure that all cable wires, including ground, continue through the filter site. In other words, don't install new problems as you attempt to fix old ones!

In the U. However reliable it may be, the electricity is nonetheless susceptible to many outside sources of interference, including thunderstorms, nearby radio transmitters, power tools, household appliances and industrial equipment. These can add low-frequency buzzes, static, high-voltage spikes and other types of noise to the otherwise steady, regular flow of power.

Vintage electronic devices are particularly vulnerable to power problems. Modern equipment typically has some power noise filtering built in, but severe line noise can compromise the performance of nearly all electronics. Filters and surge suppressors are simple, relatively low-cost components that reduce AC line noise. Special electronic filters eliminate AC line noise where the unwanted signals have a higher or lower frequency than 60 Hz. For example, a low-pass filter greatly reduces high-frequency noise due to radio interference while allowing the 60 Hz AC to pass through.

A surge suppressor acts in a different way, absorbing the energy of sudden voltage spikes in the AC power that would otherwise fry sensitive electronic equipment. Although a severe spike might destroy a surge suppressor, the low cost to replace the component is worth it to save expensive electronics.

An uninterruptible power supply, or UPS, delivers clean, steady AC power even through line noise, brownouts and other problems. The transformer used in hybrid circuit must be a type which can handle at least 40 mA DC current without saturation. It is a good idea to use ohm trimmer in place of ohm resistor especially if you are trying to get the best performance with ordinary telephone.

In other way the circuits work in the same way as the hybrid circuits above. The possible uses for this circuits might be converting normal 2 wire modem for operating in 4 wire circuit for example for connecting to radio link or using normal telehone as microphone and speaker for computer soundcard when doing Internet telephony.

Clearing up Noisy Phone Line.

If you are planning to connect the circuit to your soundcard use the following wiring: Operational amplifier based hybrid circuits Modern modems use hybrid circuits built from operational amplifiers, resistors and one With operational amplifier circuit the circuit can be made cheaper and performing better. For receiving audio a differential amplifier must be used to separate the incoming signal form outgoing signal, but differential amplifier is very easy to implement using operational amplifiers. A better model provides better isolation between incoming and outgoing audio signals.

A quick note to mixing desk users: This makes it very easy to experiment with this type of circuit if you happen to own a good audio mixer. Here is the full operational amplifier based hybrid circuit diagram theoretical circuit: This circuit is an example of an "active" hybrid. Essentially it is a balanced network. Yet the receive signal from the "far-end" will appear on the receive line. In other words, two signals can use the same two-wire interface, yet are seperable.

Not theat the resistors which define the amplification of the opamps are not drawn here, so if you are plannign to build this circuit you will have to add them. Any imbalance in the balanced network creates sidetone - a small amount of the transmit signal will appear on the receive line. In typical situations the sidetone can be attenuated aroun dB with a well designed hybrid circuit. Another typical way to implement a hybrid circuit is to build an optoamplifier circuit which takes the signal over the transformer coil and subtracts the transmitted signal from it.

The following operational amplifier circuit does this: The circut below is partly redrawn optimized hybrid circuit from National Semicondictor application note "Optimum Hybrid Design" from that application note is no longer available. The transformer in this circuit is For best results you have to adapt the component values slightly to match the line impedance and the transformer you are using.

That upper amplifier the triangle with one input and output wire is just a buffer amplifier with amplification factor of one. Signal from transmitter is connected to the positive input of opamp. The negative input of that opamp is conected to the opamp output. More details implementing telephone line interface Telephone line interface has to provide two functions when it is off-hook: Provide DC path for current flowing in telephone line. Normally there flows about mA current in telephone line and telephone regulations typically sepcify that the DC resistance must be less than ohms.

Provide proper termination for telephone audio frequencies Hz. This is typically specified to be ohms. Wet type means that the transformer is designed to handle the DC current typically mA properly and does not saturate at this DC current. Typically "wet" transformers are more expensive, bigger and have worse specs than "dry" transformers which do not have to withstands any DC current. The proper termination in modems is provided by the electronic hybrid circuit connected to of the transformer. Another possibility is to use transformer which with center tap and build simple transformer and resistor hybrid circuit around it.

Here is a typical circuit for "wet transformer: The DC resistance of the circuit is determined by the resistance of the transformer primary coil typically in ohm range in The transformer is 1: Because the dry transformer can't stand DC then in telephone application where there is DC present the DC must be blocked by suitable capacitor usually Here is an example circuit: A large inductance coil can be used in this but it is not practical because you wanted to get rid of that bulky "wet" transformer using small "dry" transformer instead, so you don't want an expensive and bulky coil in your circuit.

Fortunately coils can be simulated electronically using gyrator circuit. With gyrator it is very easy to have a simulated coil which has low DC resistance and the circuit looks like high inductance coil few Henries simulated coil can be made easily. Another possibility is to use constant current sinking circuitry.

Constant current circuit provides path to DC current but has very high impedance before using constant current circuitry take a look if your telephone regulations allow constant current operation or you can make the circuit to work inside the specs in varying line conditions. When you add electronics to transformer primary side remember that those must work at both line polarities.

A bridge rectifier will help to make sure that the current going to DC path circuit is always at correct polarity. Another thing to consider is overvoltage protection because your circuit in the transformer primary side has to withstand the spikes which exist in telephone lines primary side. Make also the circuit so that it is not damaged by a little more current than normally present in telephone line sometimes there are overcurrent situations and you don't want your circuit to break down too easily. Secondary overvoltage protection In telephone circuits there are situations where there is some high voltage spikes on the line.

If such overvoltage get through the transformer it can destory the electronics connected to the transformer secondary unless there is some overvoltage protection. Even a normal telephone ring signal going through the transformer can cause harmful voltages on transformer secondary. Fortunatly the protection on the transformer secondary is usually quite easy, because the transformer itself redices the energy which can pass through it. Typically a pair of zener diodes voltage of few volts connected to the transformer secondary can do the protection nicely.

In audio output circuits where the signal levels are fraction of volt range you can use normal diodes like 1N in the following way to do the protection: Problems in linking telephone hybrid to audio system Every sound engineer has had to deal with telephone lines at one time or another. Linking the phone conversation to audio system like taking calls to radio studio can be more problematic than you first thought.

The main problem in making the audio connection is that the telephone line is full-duplex interface implemented using single twisted pair. You can hear this leakage in the earpiece of your telephone handset. Just listen to how much of your own voice comes back to you! Level matching on local and remote voice Typical commercial telephone hybrid allows the equalizing of levels of local and remote voices. Typically a hybrid needs adjustment for every new connection because of impedance changes. Today automatic digital hybrids are used for equalizing local and remote telephone conversations.

Trans-hybrid loss and announcer voice distortion Trans-hybrid loss is that portion of the announcer's voice that leaks through the hybrid to its audio output. The higher this spec, in db, the better isolation in the device. This leakage is distorted and phase shifted after its long journey. In the studio, the announcer audio is mixed at the console with the phone patch caller output to create the on-air mix. When you use a poor phone patch, its output includes a distorted, phase shifted version of the announcer signal. When this leakage is combined with the clean announcer audio, a "hollow" or "tinny" sound is produced as some frequencies are more affected by phase cancellation than others.

The greater the trans-hybrid loss, the less announcer audio that leaks into the hybrid output and the less the announcer voice distortion. Ideally, the output of the hybrid should consist of the caller audio only. Digital hybrids have signal processing electronics to get better trans-hybrid loss figures than which are available with simple analogue solutions.

You have to decide what's best for your application and your budget. There are different requirement depending the application broadcast, teleconference or remote training. It hase been suggestions that ISDN be used as it is full duplex is a good one, but it might be only practical if both sides of the telco path have ISDN. Echo problem in long distance calls Echo is caused because of the coupling between incoming and outgoing audio in the telephone circuit and the delay in the telephone line especially in long distance calls. Thus there is echo; ISDN or other digital telephone set on an all-digital connection would not cause echo because of conversion mismatch, but if normal handset or hadfree telephone is used the acoustic echo is still possible.

Echo doesn't become audible until the delay in the circuit exceeds a certain threshold value which depends on the losses in the circuit. Even milliseconds of terrestrial echo can be annoying, but typically the echo is not annoying if the delay stays below 25ms. Old Bell standards said that on calls of more than miles, an echo suppressor was used.

In general, you need echo cancellation when the delay exceeds some subjective value in the ms range. As it is practically impossible to prevent echo by perfectly matching the impedance in line circuits and by acoustically insulating all phones , it either has to be suppressed or cancelled when it does occur. For this reason, echo cancellers are deployed by telephone company on long-haul routes that, when used, bring the total circuit delay to above the echo threshold value determined by line loss.


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These echo cancellers are deployed on both sides of such long-haul routes and the echo canceller at the remote end of the call is responsible for ensuring that you don't hear any echo. The morale is therefore that if you hear echo, you can't do practically anything about it, as both the cause of the problem and the solution to it lie at the remote end of the connection typically at the telephone company equipments.

If the connection you're talking about is across a private network, make sure that the echo cancellers are correctly dimensioned because wrongly dimensioned echo canceller will be totally ineffective. Metallic sounding caller voice problem If your telephone connection is though a digital PBX or digital switch typical nowadays then you might encounter a problem that the voice which might sound OK on telephone but sound "metallic" when you connect it to the mixing desk through your high quality hybrid circuit.

The absence of the output filters causes that there are high frequency noise components added to the output audio signal. The audio sounds fine on normal telephone because it can only playback the normal telephone audio range. The problem is audible with your hybrid circuit of that circuit has wider bandwidth than normal telephone. The solution to make this signa sound normal telephone is to remove everything above 4 kHz by a sharp lowpass filter.

You can try if your mixing desk channel equalizers are effective enough to remove this problem. When you start equalizing the signal from telephone hybrid then you can also remove the bass frequencies also there is usable sound information below Hz on normal telephone line so you can also get rid of the possible low frequency noise mains 50 Hz or 60 Hz which is sometimes present on telephone line. Helpful tips for telephone hybrid circuit designers Transformer equivalent circuit Transformer equivalent circuit is very useful tool when you need to analyse the circuit operation using mathematical methods.

Transformers are usually modeled using "t" equivalent circuits. Here is the "t" equivalent circuit for a 1: Typically about ohms. Not necessarily equal. About 5 mH in "dry" tranformers. For the model above you can measure easily the primary and secondary copper resistance coil DC resistance. If you have the transformer datasheet you can usually find the values of those all parameters used in this model.

This model models the transformer behavior in all except two things: If your intuition needs to see the isolation in the circuit model you can thinks that you have an ideal 1: For core saturation in telecommunication applications you don't run the transformer core to saturation or near it so you don't have to model it. If more accurate equivalent model is needed for the transformer, then you can use the following model for the transformer: For Midcom tranformer the model has following parameters: Measuring return loss figures Return loss is a measure of match between the impedance of the line termination and the line itself.

If the impedance of the line is Zo and the termination or load is Zl then the return loss is given by the formula: The return loss must meet the regulations in the whole specified frequency range. The measurements can be quite easily made using a variable frequency signal sinewave generator and the reference impedance Zo can be built easily from resistors and capacitors.

The following circuit can be used to measure the return loss: Connect the reference impedance Zo and the measured telephone interface circuit Zl to this measurement circuit. Connect multimeter to the circuit to place marked with Vout to measure the Vout voltage. In ideally balanced circuit this voltage is always zero. Make sure that your multimeter can measure the AC voltages in the frequency range you are using accurately some multimeters have very large measurement error when frequencies go much higher than few hundred Hz.

Using the circuit is very simple. Just apply the input signal and measure the output voltage. Do as many measurements as necessary to cover the whole specified frequency range. When you have made the measurements you can calculate the return loss using following formula: You can separate the DC signal from the measurement circuit using capacitor 10 uF capacitor does not cause much error on telephone Hz frequency range, for lower frequencies use higher value.

The power to the measured telephone or other equipment must be fed from separate power supply and run through AC block circuit which prevents the power source for short circuiting the AC signals. This AC blocking circuit can be a large coil preferably more than 5 henries , gyrator circuit or constant current source. The only thing you muts consider is the possible measurement errors caused by the capacitor and AC blocking circuit. Distortion figures Distortion figures of the transformers have effect on voice quality on the telephone circuit.

Normal telephone voice communications are not very sensitive to distortion, but modem communications are very sensitive to it. The distortion of telephone line interfacing transformer can be caused by many factors, but is specially sensitive to the performance of the magnetic lamination within the transformer. If the transformer passes some DC current on the primary or secondary coils, the distortion figures will usually get worse when current increases and many transformer do not perform in any usable way if there is any DC current around.

If the transformer must handle DC current, you need a transformer designed to withstand some DC current "wet" to keep the distortion in some usable range in this kind of circuits. Typically low end modems use "wet" transformers and high end fast modems use "dry" transformers. The transmission speed of a modem is a function of many different design parameters. The performance of Modem Isolation Transformer MIT is one of the hardware design aspects which constrain the modem's transmission speed.

Specifically, the MIT's signal distortion is the main constraint on modem speed. The distortion of the MIT can be thought of as any change in the waveshape between the secondary output signal from the original primary input signal. Significant distortion can cause problems with the signal transmission from one telecom circuit to another. Connecting hybrids and telephone equipments Sometimes there is need to connect normal telephone equipments directly to the telephone hybrid circuit without any connection to public telephone network.

This kind of interfacing is needed for exampel for telephone equipment measurements using hybrids or for itnerfacing telephones to computers through a hybrid circuit. There are few different ways to do the interconnection of hybrid and telephone equipment. Simple interconnection with no power provided to the line Simplest inteconnection is just wiring the telephone equipment to the hybrid. This kind of simple interconnection works for cases where the telephone equipment does not need any telephone line loop current to operate normal telephone operated on loop current and can not be used in this way.

The current taken from it is limited by teh resistance in telephone itself and the DC resistance of the hybbrid. If you fear of excessivue current, you can put a ohm 1W resistor in seriws with the power supply. This will limit the current below 50 mA in all cases and does not cause too much impdeance mismatch to the circuit. General hybrid interface This is a general circuit suitable for interfacing "dry" hybrid circuits to practically any telephoen equipments works also for "wet" hybrids.

How to Clear Phone Line Noise | HowStuffWorks

The C1 shoudl have a voltage rating so high that it can withstand the voltages which might be present in the line. The value of C1 is not very critical, all values from 2 uF to 50 uF will work well. A "dry" capaictor type like polyrpopylene or duch is preferred capacitor type to be used. The current feed is an external circuit which is used to supply the current to the telephone equipment in use. For normal tephone equipments and ideal current source with nominal current in The source must be current source type.

Normal voltage sources like batteries or normal DC power supplied does not wotk for this because of their low internal impedance which would just short-circuit the audio. If you do not have a suitable ideal current source, you can use other methods for making "close enough" substitute for telephone applications. The closest thign to a traditional power supplied by telephone company would be a 48V power source fed through around 2 kohm resistor and 2H inductor.

If you use lower resistance values you can use lower voltages. The 2H coil is needed to keep the impedance on audio frequencies high so that the power supply does not "short circuit" the audio signal or cause serious impedance mistaches. If the actual impedance matches are not very important, them you can try methids like 12V power source fedh through the coil of small 12V relay or through ohm 1W resistor.

Both methods work in some cases, but can cause impedance mistaches which can cause poor operation of the hybrid the isolation between incoming and outgoing audio signals will not be very good. Components for telephone line interfacing If you are looking for components relays and transformers for making telephone interface, check the following companies: Clare Midcom Prem Magnetics Bourns. Using ready-made type approved interface can make designing small volume telecommunication product more easily, but unfortunately those ready made DAAs are usually more expensive than the discrete components.

The following companies make DAA products: The signal starts to attenuate in the frequencies below Hz because of the AC coupling of audio signals audio signal goes trough capacitors and transformers. The high frequency response is limited by the transformer and the available bandwidth in the telephone transmission system in digital telephone network the telephone audio is sampled at 8 kHz sample rate.

On the figure below you can see a typical frequency response of telephone line: In real life situations the high and lof frequencies can be more attenuated. The frequency curve information was taken from Dialup Line Quality in Houston web page.

How to Clear Phone Line Noise

The frequency response of the line depends on the line length. When line gets long high-tones drop-off much more quickly than the low tones with the obvious effect on speech. It's not all-that difficult to tell the distance an analog phone is from a central office assuming an analog line is the connector: Take also note that the telephone equipment has a huge effect on the speech quality. For example carbon and electret handset microphones have radically different frequency responses.

The frequency response and overall sound quality of carbon microphones used in old telephones are not very good. Many modern telephones with electret microphones give better sound quality. Telephone line details in different countries Normal telephone line is theoretically designed to be ohm resistive impedance.

This ohm is kept as international reference for designing telephone line equipment typically the signal powers are measured to ohm load. In practice the telephone line does lot look like pure ohm resistance. The cable and equipments used by the telephone companies have effect what the real impedance is. Telephone equipment which is designed to operate with ohm loads will operate with those real-life lines, but it's performance is worse than in ideal situation. Typically the modems are designed for ohm reference impedance because they can handle the sidetone, but for best performance the telephones are designed to the exact line impedance.


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  • When best performance is needed the circuit should be exactly matched to the impedance of the real telephone lines. Matching the hybrid circuit to the real line impedance instead of ohm will improve the feedback typically by dB. Different countries have different characteristics on the telephone line parameters. Here are some impedance models for typical lines in different countries: Some telephone lines can have higher impedance typically ohms in lines with loading coils or telephone air cables.

    All power specs and return loss measurements are taken so that the reference impedance is ohm resistive. This measurement applies to telephones, modems and other terminal equipments. NET4 technical specs are European specs and they are used in many European countries NET4 is actually a collection of different specs in use in different countries. Here is a picture of that complex reference impedance: Because the telephones designed to meet the needed characteristics measured against ohm reference impedance does not always work satisfactory when connected to telephone switches.

    A better results can be obtained if the phone meets all other NET4 regulations, except the return loss is matched to TPL06 regulations. The reference model used in TLP06 for telephone line: Loading coils Loading coils are lumped inductance added in series with the telephone line to compensate for the mutual capacitance of the cable pair s. They are placed at specific intervals on loops of 18, feet or greater to improve voice grade transmission.

    Placing load coils at other than the specified intervals actually degrades voice grade transmission. It is generally accepted that the upper cutoff frequency of these devices is Hz. Therefore loaded loops do not lend themselves well to high frequency or high data rate transmission. Loading coils introduce phase delays which are fine for voice but unacceptable for high speed data and are best confined to the past, or to very long local voice loops where they can't be done without.