PBL 3852
when the current and the voltage are not
enough for full signal swing in the
mentioned before the output amplitude
can be limited by a resistor in series with
protection, two series diodes anti parallel
across its output to limit the signal to the
receiver amplifier, a sort of ”soft clipping”
is activated and lowers the gain so that
no distortion will appear. A capacitor is
needed at the output with low ohmic DC
loads (some of the earphones have
extremely low DC resistance) because
even a small DC offset at the output will
cause a great current drain from DC2
continuously. This capacitor is also
needed if DC2 is used as a back up
supply for some memories and the
isolation is not done with a diode. The
gain and frequency response is set at
the input RI with a RC-network. The
receiver gain can be regulated. The
range of regulation from the input to the
output is 5.8 dB (23.7 to 29.5dB). As
the pin 8. An other method is to connect a earphone and thus preventing an
series resistor with the earphone itself. In acoustical shock. A resistor in series with
case of no signal at the input of the
receiver, very little current is drawn from
DC2. The same is valid at mute
condition, understood that no DC current
is drawn to somewhere else, as for
example to a low ohmic DC load at
earphone amplifier output. The receiver,
contrary to most of our previous speech
circuit families, can be loaded single
the output can very well be used to
increase the protection level. Note, that
the noise in the receiver is allways
transmitter noise that has been more or
less well balanced out in the side tone
network.
Figure 12 b) shows a 150Ω resistor in
series with a 150Ω earphone load. This is
to minimize distortion and to decrease the
ended resulting an undistorted signal. The DC-load rather than using a capacitor but
load should be 10x the standard (150Ω)
load of the amplifier with a capacitor in
series, without a capacitor somewhat
higher, depending on the required signal
swing. The receiver has, as a principal
it will give less swing with low line
currents, I < 15mA.
L
Line length regulation
Ω/km) and DC - characteristic of the
telephone set (see fig. 14). Therefore
calculate or measure the voltage at pin
+C at 0 and P km. (the DC - characteristic
ought to have been set at this stage) The
voltage drop a) in the graph is across the
discrete components like the polarity
guard bridge, protection components and
series transistor for LD - dialling. The
voltage drop b) is across R19 or in case
of comlex line impedance the drop across
(dB/km). The approximate centre point of
the gain regulated line length portion is
P/2. The line length above point P is not
regulated in any sense and therefore
followes the attenuation due to the
increase in impedance at increasing line
length.
Line length regulation is used to
compensate the gain loss in both trans-
mitter and receiver due to increasing
attenuation at increasing line length.
Setting the parameters for line
length regulation (See fig.13)
To set the gain regulation:
The dotted line from dB axis to km axis
indicates the attenuation versus line
length that originates from the impedance
in the increasing length of the specific
cable used.It is generally desired that
there is a gain regulation that
compensates for this attenuation. The
regulation should operate across a line
length that comprises most of the
subscribers. This will give the value for
the line length P and is in most cases
given by the network owner in their
specification about the telephones
acoustical behaviour. The amount of
regulation is given by the portion of
attenuation q. The slope of attenuation
change within this area is given by q/P
1). Determine from the acoustics spec.
diagram, that is given by the network
owner, where the line length P is and
what q value has to be used (a value dB/
km = q/P) and adjust the microphone
amplifier gain regulation accordingly with
a feed back resistor between pins 11 and
13. The q value is a gain, the micro-
phone amplifier regulation has to be set
to.The receiver gain is fixed. (normally the
transmitter and receiver regulation gains
are set to the same value, it is only in the
case of ”soft clipping” the transmitter
regulation is bigger)
the network. (I
+ eventual additional
pin+C
current taken from pin +C ) x R19.
Condition: The network with R14, 15, and
16 should not be too low ohmic because
it would load the +C unnecessary,
increasing the DC mask. (<100µA) The
network should not be too high ohmic
either thus influencing the precision of
the current into the GR input. (≈20µA)
The GR input current<1µA.
3). Set the gains for transmitter and
receiver.
2). The values of R14, 15 and 16 are
dependent of telephone station feeding
system (2 x AΩ , bat. V), line type (cable
≈ 2 +C0km
≈ 2 +CPkm
DC1 +C
+
+
+
R
1
14
R
16
R
1
14
R
16
at 0 km : 1.237 =
at P km :1.085 =
R
14
R
16
V
GR =
1
1
1
1
1
1
1
+
+
+
+
+
+
R
14
R
15
R
16
R
14
R
15
R
16
R
14
R
15
R
16
To calculate R14 and R16 for ex. choose R15 = 18k.
9
ERICSSON [ ERICSSON ]
