Dial-FILTERS FOR TELEPHONE LINES
AV Marchenko, Kyiv
Those who use dial-up svyayu (Fidonet users and others, as
well as the Internet) quadrupeds problems caused by poor
quality of local telephone lines. If the user has a strong
desire improve the quality of the signal and the modem,
offer simple solutions to this
problem.
Often the signal is overloading the ADC home ATS modem that is
harmful affects, primarily on identifying signal "BUSY"
To eliminate these troubles should be included in the gap of
each wire telephone line to resistor 60-100 Ohms (1). Resistor
s must be identical
Vym.
You can try beneath
take the resistance results
transistors. Top before
ate for them to 400 ... 600 Ohm
when the modem is already clearly
dials the number, the lower limit of 20 ... 30 ohms
when their inclusion has no appreciable
influence. These resistors also protect your
modem from the possible surge in the line.
The other most important problem yavlyayutsa telephone line
noise, increasing the level which the signal becomes illegible
and lost. Can significantly reduce the noise by installing a
telephone line filter.
Figure 2 shows a simple filter scheme pokazazana
pa lower frequencies, which
th "cuts" all the frequencies
above it raschetnoy.Sobran
on the RC-elementah.Nastroy
ka by hit
rum elements C1, R1, R2.
The capacitor C1
for working voltage up to
200 1-1,5 uF. Resisted
Op R1 used to
coarse adjust treble.
Its resistance can be chosen around 10
ohms. Resistor R2 (about 2kOm) is used
for fine tuning. In the open circuit
between C1 and the line can set the switch to turn off if
necessary filter.
Another version of the filter is shown in Figure 3.
This is also a low pass filter on the RC-elements, only here
for a high
RFI filtering is applied
Transformer T1.Eto allows successful penetration protistoyat
interference from appliances and such unpleasant phenomena as
interference from radio transmission network.
Elements R1, C1, R2, C2 are designed to
frequency correction of the input signal.
Transformer T2 acts as a passive amplifier signal.
Transformer T1 has two independent
winding to 7 turns each, wound on
ferrite ring in one direction. Wire -
- Painted with a diameter 0,4 - 0,5 mm.
Resistance of resistors R1, R2 (10 - 50 ohms)
determined empirically from zavisemosti
prodyazhennosti line from the PBX to the modem. Capacitance C1,
C2 0,1 uF. Transformer T2 has two independent windings 30 - 60
turns each, wound on ferrite ring toward each other varnished
wire diameter 0,4-0,5 mm. Try to place the transformer T1
and T2 away from each other.
The circuit shown in Figure 4, is also a RC-low pass filter,
but when -
meniem diodes VD1
and VD2, which
work as a rectifier
yamiteli signal
freeing it from
Surge drew
polyarnosti.Ispita
of the filter pokozali very good
results. The resistance of resistors R1 and R2, 360 ohms
capacitor C1 0.15 uF; diodes VD1, VD2 type D226.
Try to put other napremer COP
each PBX its strangeness.
If there is no dial tone, change the polarity of the diodes.
