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Filters and amplifiers for telephone modems

The filter consists of two parts - the line switch and the filter itself. The switch allows you to connect this device to any modem and excludes jingling of phones when dialing a modem number. Perhaps this is important if the set is conducted late at night. Filter - improves the modem's reconciliation with the telephone
Line at high frequencies and cuts noise above 4KHz. The C2-R3-HG2-D5 circuit is a rather complex circuit for dealing with impulse noise. Transistors Q1,2 perform the function of prohibiting the output to the line of the modem, if the line is busy - they are talking on the phone. This also increases the convenience of using the device.

Data and possible replacements

L1 - two windings with 0.2mm wire for 20-30 turns on the ring K10x6x3 (4.5), ferrite 2000HM1-17

L2 is a choke with two identical windings and a total inductance of 30-40mH.

Specific data is difficult to bring, because I applied the finished choke of imported production.

WARNING! This throttle MUST have a gap or be wound on a material with a "distributed" gap (MO-permoloy).

The procedure for choosing the size of the gap is as follows:

First, so many turns are wound around to get an inductance 10 times larger than necessary, and then such a gap is made to obtain the desired inductance. Relay - the operating voltage of 7-10V and the resistance is not less than 500 Ohm. I used a standard relay from imported telephone exchanges - they are very high-quality, and the extra bounce is clearly not needed. HG1,2 - any LEDs other than AL307 - these have too much leakage current.

This version of the filter is now used on the Motorola SM56 modem and gives a very noticeable effect - very often when a crack does not occur even a short retrain.

Condenser C2 for voltage not less than 160V, and C3 for 200V. These requirements are very well satisfied by capacitors of the type K73-16, K73-17.

Comments on the scheme

The L2 choke acts as a "passive amplifier" - it partially compensates for the capacity of the loop (telephone) line. A very simplified model is shown in the figure:

Here R2, R3, C1 ... C3 and R6, R7, C5 ... C7 are the equivalents of the "average" telephone line connected to the two modems - (L1, C4, R4) with compensation and (R8) without compensation. The following figure shows the results of calculating models on OrCAD'e 9.0 (PSPICE9):

Here the signal transmission (yellow) is clearly noticeable in the first variant in comparison with the second variant (blue color). And also - very good signal suppression above the modem signal (300Hz - 3.4KHz).

And still there is an important fact - the voltage on the transmitting side falls less in the region of 3KHz - this will facilitate echo cancellation in the modem.

In the process of calculations it was found out that the value of capacitor C4 can not be changed - it should be equal to the cable's cable capacity, and the resonance at 3.5-4KHz can be adjusted only by the size of the throttle L2.

As a further development of the circuit, I would try to increase the inductance of L2 - this would lead to an even higher rise at the upper frequency, and the limitation of the band of the spectrum can be compensated by a ban on the modem of extreme frequencies. All one, the modem does not feel good for them.

Author: Serj

Date of publication on 20.05.2004.

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