PBL3852 查看數據表（PDF） - Ericsson

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PBL3852

Universal Transmission Circuit

Ericsson  

PBL 3852

when the current and the voltage are not

enough for full signal swing in 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

mentioned before the output amplitude

can be limited by a resistor in series with

the pin 8. An other method is to connect a

series resistor with the earphone itself. In

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

ended resulting an undistorted signal. The

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

protection, two series diodes anti parallel

across its output to limit the signal to the

earphone and thus preventing an

acoustical shock. A resistor in series with

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

DC-load rather than using a capacitor but

it will give less swing with low line

currents, IL< 15mA.

Line length regulation

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)

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

(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.

To set the gain regulation:

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)

2). The values of R14, 15 and 16 are

dependent of telephone station feeding

system (2 x AΩ , bat. V), line type (cable

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

the network. (Ipin+C + eventual additional

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.

DC1 + +C

V GR =

R14 R16

1+1+1

R14 R15 R16

at 0 km :

≈ 2 + +C0km

1.237 =

R 14

1+

R 16

1+1

R14 R15 R16

≈ 2 + +CPkm

at P km :1.085 =

R 14

1+

R 16

1+1

R14 R15 R16

To calculate R14 and R16 for ex. choose R15 = 18k.

9

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