Ericsson DIALOG - Page 23

22
Two component's resistor r3 and varistor Vl are connected in
parallel with the 2 - 5 - 3 branch of the IC winding. The varistor
Vl is a non-linear element whose impedance is high with low
voltages across it but decreases (becomes low-ohmic) with in-
creasing voltage.
The transmission circuit is designed to provide a constant trans-
mission level from O Ohm up to about 1 000 Ohm line resistance.
In order to achieve this the shunting circuit (Vl-r3) is dimensioned
to give an attenuation of about 4 dB at 0 Ohm line resistance with a
continuous fall - off to 0 dB at approx. 1 000 Ohm line reisistance.
The 2 - 5 - 3 branch is in the primary winding and 4 - 5 in the
secondary of a transformer circuit. Considering the above, when
the primary winding is almost short-circuited at OOhm line resis-
tance the transformer efficiency is reduced by about 4 dB and
consequently the receiver is fed with a lower voltage.
The instrument' s balancing impedance (c-r2) is designed to give
as good as possible sidetone attenuation over lines of greatest
possible length. In fig. 33 the side tone reference equivalent is
shown as a function of various cable lengths.
Fig. 33
Instrument impedance
Cables: 0,4 mm, r = 280n. /km, c%40 n F/k m « = 1,62 dB/km
0,Smm,r • 1 BO n /km.c •40 n{ /km~• 1,22 dB/km
0,6mm,r •125si./km,c•40nf-'/km°'"' Q9SdB/km
dB Sidetone Reference Equivalent rel. to NOSFER
0,5mm
20
O.L.mm
SIRE
5 km
The instrument impedance must be variable so that. seen from the
line, it coincides with the line impedance and causes minimum
reflection losses.
In fig. 34 an RX diagram shows the instrument impedance and
phase angle's variation at a number of different line resistances.
Another function is a requirement which has become stronger
during the last few years, namely the instrument to line adaptation.
Attempts have always been made to achieve the best results in
this respect, but it is only with the coming of long-distance interna-
tional connections that a real need has arisen. If we consider the
case of a call from one point of the earth to another point. no
appreciable transmission losses take place since amplification is
applied over the entire length of the connection. However, noise.
added and amplified at every point of amplification, does enter. as
do reflections at every poor matching point. One point where
reflections can arise occurs where the instrument is connected to
the line. In this way a poor connection is established right from
TCI Library www.telephonecollectors.info