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Filters for telephone modems

It's no secret that the 220V network has interference and it is unlikely that they have a positive impact on the quality of communication. To understand the process of causing interference, consider the structure:

Before you do think, do you need it? For my line, this turned out to be very useful, if only because the influence of the refrigerator was virtually eliminated.

Explanations of the scheme:

Capacitors C1 and C2 eliminate RF interference ... but only if the computer is grounded, but this, in practice, never happens, and if desired, this is extremely difficult.

Capacitors C4 and C5 eliminate RF interference from its own power supply to the modem. Here they do not need to be touched - they are clearly in place.

Interference from the network goes along the path A (B) - C1 (C2) - case - C3 (C4) - capacitive communication on TV2 - and in the modem. Thus. С1 and С2 bring great harm and it must be somehow eliminated.

True, there is a second path of interference propagation: A (B) - D1-5 - TV1 (power transformer) - rectifiers - the power supply of the modem. As you can see, this does less damage, because Affects not the signal circuit, but the modem's power, but it would not be correct to ignore it.

The method of struggle is presented in the figure below:

Here, C1 and C2 are enlarged by two orders of magnitude and are not connected to the body, but to the pseudo-ground of the power unit. This option nicely removes and another bad feature of impulse power supplies - the generation of jamming complex shapes.

I will explain - just as the diodes open only 30% of the time from the switching period, the interference in the 220V network (and hence at the modem's input) has the appearance of packs of 3ms duration and a period of 100Hz. This is perfectly detected by the parasitic non-linear circuits of the input modem circuit, which generates interference with a frequency of 100Hz and a very rich spectrum. It seems to me that it will not be better for communication from this. So - this modification of the circuit somewhat reduces the amplitude of the interference (because the capacitors are much larger than the original ones) and, most importantly, excludes the effect of packets. This leads to the appearance of a constant component at the input of the modem, which is easily eliminated during processing.

Capacitors С1, С2 - for voltage not less than 400V and good quality (К73-16, К73-17).

Please take into account that the same procedure will have to be done with the power supply of the monitor - otherwise it is not worth starting.

In some schemes it is recommended to put not two, but four capacitors - parallel to each diode. This is a very bad decision, because Interference with the "+" wires C3 fall into the network, which is not good.

Author: Serj

Date of publication on 20.05.2004.

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