Ericsson PBL3852 Datasheet

April 1996

PBL 3852

Universal Transmission Circuit

Description

The PBL 3852 is an universal transmission circuit in bipolar technology that performs all the speech and line interface functions required to implement an electronic telephone set suitable for the majority of existing telephone network requirements.

Easy adaptation of the DC-mask to different line feed systems. A summing point for auxiliary signals to be transmitted like DTMF and hands-free audio signal. The PBL 3852 has a low current consumption that enables the circuit to work with reduced performance down to 2.1 volts (4.8 mA) across the circuit. The low current consumption for a speech circuit is essential in telephone line powered handsfree designs required to work at long line lengths. The PBL 3852 is especially suitable to be used with Ericsson handsfree circuits like PBL 3786, PBL 3786/2, PBL 3881 and PBL 3880 thanks to a specific interfacing arrangement.

The transmitting and receiving gains can be regulated in order to compensate for the attenuation of the signals due to increasing attenuation with increasing line length. It is also possible to limit high transmitting signal levels (soft clipping) thus preventing excessive distortion caused by signal clipping. The gain regulation is set with discrete external components.

The circuit is easily adapted to different markets by setting the application dependent parameters individually in certain order, this preventing the interaction between the same. PBL 3852 has up to four different power supplies to feed microphones, auxiliary circuits and functions.

All pin numbers in this paper refer to DIP package.

(+Line)

Power

down

Ref.

supply

18 9 8

DC1 DC2

supply

PBL 3852

Ref.

4 +

Telephone

line

Key Features

• Generates its own supply from the telephone line

• Adaptive to all types of telephone line feeding systems (i.e. 48V 2x200Ω, 60V 2x 600Ω, 48V 2x800Ω)

• Operates down to 2.1V (excl. polarity bridge)

• Adjustable DC-characteristic to the line

• Few inexpensive external components to function

• Easy adaptation for various market needs

• Dialler interface with DC-supply, mute, power -down and DTMF-input

• Confidence tone in the receiver at DTMF-dialling

• ”Soft clipping” that prevents distortion at high transmit signal levels

• Balanced microphone input for dynamic, and electret microphones

• Balanced receiver output for dynamic and magnetic receiver elements

• Transmitter and receiver gain regulation for automatic loop loss compensation (disabled in mute mode)

• Four separate DC supplies for different requirements

• High gain of the receiverfacilitates volume control function

• Microphone cut-off function possible by a switch

• All gain and frequency setting networks in Rx, Tx and side tone are referred to ground

• Excellent RFI performance

PBL3852

DTMF

A. Dynamic limiter B. Sidetone network C. Gain regulation with line length

Figure 1. Functional diagram.

(-Line)

PBL3852

18-pin plastic DIP 20-pin plastic SO

PBL 3852

Maximum Ratings

Parameter Symbol Min Max Unit

Line voltage, T Line current, continuous DIP I Line current, continuous SO package I Operating temperature range T Storage temperature range T Input level (all inputs) 0+C V

feed

E= 48.5V

= 2 s V

MUTE

R = 0-4KΩ

0 ohm when artificial line is used

5H+5H

= 400Ω+400Ω

C = 1µF when artificial line is used 470µF when no artificial line

600Ω

ARTIFICIAL

LINE

DC2

+ LINE

DC2

DC1

PBL 3852 with external components

See fig. 4

MIC

REC

- LINE

Amb

Stg

= 150Ω

Mic

= 150Ω

Rec

022 V 0 130 mA 0 100 mA

-40 +75 °C

-55 +125 °C

Figure 2. Test set up without rectifier bridge.

MUTE

E = 48.5V

= 400Ω+400Ω

feed

R = 0-4KΩ

5H+5H

1µF

Uz= 15-16V

600Ω

+ LINE

PBL 3854 with external components

See fig. 4

MIC

REC

DC2

DC1

= 150Ω

Mic

= 150Ω

Rec

- LINE

Figure 3. Test set up with rectifier bridge.

REC

R24

C10

(+Line)

(-Line)

R22

C12

MIC

C13

R23

Mute 6

Ref.

supply

18 9 8

DC1 DC2

supply

R5 R7

PBL 3852

R9 R11

R12

R13

R14

R15

Ref.

R18

R17

R16

4 +

R19

Figure 4. Reference figure with line length regulation. (Application for dynamic microphone)

R1 = - C1 = R2 = - C2 = 47µF R3 = 100Ω C3 = 47µF R4 = 7.5k C4 = 68nF R5 = 33k C5 = R6 = - C6 = 100nF R7 = 75Ω C7 = R8 = - C8 = 47nF R9 = 620Ω C9 = 47µF R10 = - C10 = 15nF R11 = 6.2k C11 = R12 = 130Ω C12 = 0.15µF R13 = 2.4k C13 = 0.15µF R14 = 27k R15 = 18k R16 = 120k R17 = 18k R18 = 62k R19 = 910Ω R20 = R21 = R22 = 10k R23 = 10k R24 = 150Ω

Electrical Characteristics

At T

= + 25° C. No cable and no line rectifier unless otherwise specified.

Amb

PBL 3852

Parameter fig. Conditions Min Typ Max Unit

Line voltage, V

note 1 2 IL = 15 mA 3.3 3.7 4.1 V

Transmitting gain, note 1 20 •

Transmitting range of 2 1 kHz, R regulation

note 1

Ref.

2R 2R 2R

= 100 mA 11 13 15 V

log (V2 / V3); 1 kHz

= 0 414345dB

= 400Ω 43.5 45.5 47.5 dB

= 900Ω - 2200Ω 46 48 50 dB

= 0 to 900 ohm 3 5 7 dB

Transmitting frequency 2 200 Hz to 3.4 kHz relative to 1 kHz -1 1 dB response Receiving gain, note 1 20 •

2R 2R 2R

Receiving range of regulation 2 1 kHz, R

log (V4 / V1); 1 kHz

= 0Ω -13 -11 -9 dB

= 400Ω -10.5 -8.5 -6.5 dB

= 900Ω - 2200Ω -8 -6 -4 dB

= 0 to 900Ω 357dB

Receiving frequency response 2 200 Hz to 3.4 kHz relative to 1kHz -1 1 dB Microphone input impedance 2 1 kHz, 1.7 kΩ pin 12 (14),13 (15) Transmitter input impedance 2 1 kHz 17 kΩ pin 3 Transmitter dynamic output 2 200 Hz - 3.4 kHz 1.5 V

≤ 2% distortion, IL = 20 - 100 mA

Transmitter max. output 2 200 Hz - 3.4 kHz 3 V

IL = 0 - 100 mA, V3 = 0 - 1 V

Receiver output impedance 2 1 kHz, R

= 0Ω, note 4 32(+150) Ω

Receiver dynamic output 2 200 Hz - 3.4 kHz 0.5 V

≤ 2% distortion, IL = 20 - 100 mA

Receiver max. output 3 Measured with line rectifier 0.9 V

200 Hz - 3.4 kHz, IL = 0 - 100 mA V

= 0 - 50 V Transmitter output noise 2 Psoph-weighting, Rel 1 V Receiver output noise 2 A-weighting, Rel 1V

rms

, RL = 0 -75 dB

rms

, with cable -80 dB 0 - 5 km, ø = 0.5 mm note 3 0 - 3 km, ø = 0.4 mm

Mute input current 2 20 µA DC1-supply voltage 2 I

DC2-supply voltaget (clamp) 2 I

= (20 - 100) mA note 2 1.75 2.0 2.25 V

= 1 mA

DC1

= 20-100 mA see text, I

= 1.9 mA note 2 3.4 3.7 4.0 V

DC2

at zero signal in the receiver amplifier

Psoph A

Notes

1. Adjustable to both higher and lower values with external components.

2. Lowest line current dependent of the set DC-characteristic. See page 14, fig 8.

3. Psofometric weighting will give (6-7) dB lower value. (-dB)

4. 150 ohm resistor in test set up.

PBL 3852

Pin Description

DIP SO Symbol Description

1 1 +L Positive line terminal 2 2 TO Slope setting for DC characteristic and sidetone balancing signal output 3 3 TI Transmitter amplifier input 4 4 +C Internal power supply 5 5 DCC Line voltage DC level adjustment input 6 6 MUTE Transmitter and receiver amplifier mute input 7 7 RCT Dynamic limiter ”soft clipping” input 8 8 DC2 DC supply 2 output, typically 3.7 V 9 9 DC1 DC supply 1 output, typically 2.1 V

10 NC Not connected

11 NC Not connected 10 12 GR The output of the rectifier to the dynamic limiter and gain regulation input 11 13 MO Microphone amplifier output 12 14 MI1 Microphone amplifier non-inverting input 13 15 MI2 Microphone amplifier inverting input 14 16 -L Negative line terminal 15 17 RI Receiver amplifier input 16 18 RO1 Receiver amplifier inverting output 17 19 RO2 Receiver amplifier non-inverting output 18 20 PD Power down input

DCC

Mute

RCT

DC2 DC1 GR

9 10

PD RO2 RO1 RI

-L MI 2 MI 1 MO

+C DCC Mute

RCT DC2

DC1

RO2

RO1

-L

MI 2

MI 1

DIP SO

Figure 5. Pin configuration.

Functional Description

Design procedure

1. Set the circuit impedance to the line, either 600Ω or complex. (R19 and C9). C9 should be big enough to give low impedance compared with R19 in the telephone speech frequency band. Too large C9 will make the start-up slow.

2. Set the DC-characteristic that is required in the PTT specification or in case of a system telephone in the PBX specification (R7). There are also internal circuit dependent requirements like supply voltages etc.

3. Set the attac point where the line length regulation is supposed to cut in (R14,R15 and R16). Note that in some countries the line length regulation is not allowed. In most cases the end result is better and more readily achieved by using the line length regulation (line loss compensation) than without.

4. Set the transmitter gain, regulation and frequency response. See text for the dynamic limiting feature.

5. Set the receiver gain and frequency response. See text how to limit the max. swing to the earphone.

6. Adjust the side tone balancing network.

7. Set the RFI suppression components in case necessary. In two piece telephones the often ”helically” wound cord acts as an aerial where especially the microphone input with its high gain and input impedance is the more sensitive.

+Line

Figure 7. System of DC-Characteristic.

R19

PBL 3852

Ref=1.16V

PBL 3852

C10

Figure 6. AC-impedance.

Impedance to the line

The AC- impedance to the line is set by C10, R19 and C9. Fig. 6. The circuits relatively high (≈ 20k with R7 = 75Ω) parallel impedance will influence it to some extent. At low frequencies the influence of the C9 can not be neglected. Series resistance of the C9 that is dependent on temperature and quality will cause that some of the line signal will enter pin 4 and generate a closed loop in the transmitter amplifier that will create an active impedance thus lowering the impedance to the line. The impedance at high frequencies is set by C10 that also acts as a RFI suppressor.

In many specifications the impedance towards the line is specified as a complex network. See fig. 6. In case a) the error

- I pin5

DCsupply

I pin5

R20

R21

DC1

PBL 3852

+Line

a) b) c)

R19

Rs ≈1Ω

signal entering pin 4 is set by the ratio ≈Rs/R19 (909Ω), where in case b) the ratio at high frequency will be Rs/220Ω because the 820Ω resistor is bypassed by a capacitor. To help up this situation the complex network capacitor is connected directly to ground, case c) making the ratio Rs/220Ω+820Ω and thus lessening the error signal. Conclusion: Use case c) when complex impedance is specified.

DC - characteristic

The DC - characteristic that a telephone set has to fulfill is mainly given by the network administrator.Following parameters are useful to know when the DC behaviour of the telephone is to be set:

• The voltage of the feeding system

• The line feeding resistance 2 x.... ohms

• The maximum current from the line at zero line length

• The min. current at which the telephone has to work (basic function)

• The lowest and highest voltage permissible across the telephone set.

• The highest voltage that the telephone may have at different line currents is normally set by the network owners specification. The lowest voltage for the telephone is normally set by the different voltages that are needed for the different parts of the telephone. For ex. for transmitter output amplifier, receiver output amplifier, dialler, speech switching and loudspeaker amplifier in a handsfree telephone etc.

Example:

The complex network 220Ω + 820Ω//115nF

-Line

PBL 3852

16 14

12 10

8 6

20 40 60 80 100 120

Figure 8. DC-Characteristics. (R7 = 75 Ω)

circ

. =

⋅

+k1⋅

Vref+R

⋅5.5⋅10

if function DC−control at pin5is used

()

⋅

V telephone line V line

V pin 4

V pin 2

V pin 8 V pin 9

line

⋅

ref

= 1.1

⋅

= 0.5⋅

4 ≈

The R7 will set the slope of the DC-char. and the rest of the level is set by some constants in the circuits as shown in the equation. The slope of the DC-char. will also influence the line length regulation (when used) and thus the gain of both transmitter and receiver. R7 acts also as current protection for the circuit, must be considered when low values are to be used. The level of DC-characteristic can be adjusted up at input pin 5 (some 100mV´s). The R21 adjusts a fix amount where R20 couples the adjusted value to line current. See fig. 32.

Microphone amplifier

The microphone amplifier in the PBL 3852 is divided into two stages. The first stage is a true differential amplifier providing high CMRR (-55 to -65 dB

⋅

line

typical) with voltage gain of 19 dB. This stage is followed by a gain regulated amplifier with a regulation range from 6.5 dB to 14.5 dB, see fig. 15. The input of the microphone amplifier can be used for electret, magnetic or dynamic transducers see fig. 9. The PBL 3852 has basically a higher gain regulation range (8 dB) than the more or less standard 6 dB´s for gain regulation with line length, this in order to be able to be used in applications where ”softclipping” is required. In case lower regulation range is necessary, it is possible with some additional components.

See reference figs. 4, 10c, 10f, 32 and

33. For an electret microphone the circuitry will be simple, see fig. 10f. A resistor is added from the microphone amplifier output, pin 11, to the positive termination of the microphone and further

via a capacitor to the - input at pin 13. The DC supply resistors for the microphone should be round 200Ω (in order not to overdrive the microphone amplifier) and the feedback resistor (17k) is of that magnitude that it either influences the CMRR balance at the input or destroys the send mute by bypassing signal round the microphone amplifier in mute state. For a dynamic microphone some more components are necessary, see fig. 10c. In order not to influence the send mute the feedback signal is taken from transmitter output at pin 2 and because this signal is in opposite phase with the signal at pin 11, it is taken to the other input at pin 12. Also in order not to influence the DC-balance of the microphone amplifier a capacitor has to be included in the feedback path and to maintain the CMRR of the

PBL 3852

microphone amplifier a similar RC combination ought to be connected from the other input, pin 13, to ground.

An electret microphone with a built in FET amplifier is to be seen from outside as a high impedance constant current generator and is normally specified with a load resistance of ≈ 2k. This is to be considered as max. value and by using it will render the max. gain from the microphone. This level of input signal that is unnecessary high will result in clipping in the microphone amplifier and in mute condition permeate through the input to

Strong cc gen.

DCload

ACload

the circuits reference and this way to all functions, resulting among other things in a bad mute. Hence it is better regarding noise perfomance and mute to rather use the gain of the microphone amplifier than the gain of the microphone itself (in case of electret) flat out. A more suitable level of gain from the microphone is achieved by using a load resistance of 200 - 470Ω. Gain setting to the line is done at the input of the transmitter.

It is possible to use the microphone amplifier as a limiter ( added to the limiter in the transmitter output stage ) of the

DC ( ref. ≈ 1.16V )

ref. minus a diode ≈ 0.5V

DC-load = R4+R5

DTMF

) //Z

AC-load = R4+R5// (R6+Z Z

DTMF

= DTMF generator impedance

transmitted signal (See fig. 9). The positive output swing is then limited by the peak output current of the microphone amplifier. The negative swing is limited by the saturation voltage of the output amplifier. The output of the amplifier is DC-vice at internal reference level (1.16V). The lowest negative level for the signal is reference minus one diode and sat. transistor drop (1.16-0.6-0.1 = 0.46V). The correct clipping level is found by determining the composite AC- and DCload that gives a maximum symmetrical unclipped signal at the output. This signal is then fed into the transmitter amplifier at a level that renders a symmetrical signal clipping on the line. (adjust with ratio R4, R5) The total transmitter gain when an electret microphone is used can then be adjusted with the load resistor of the electret microphones buffer amplifier.

Figure 9. Microphone amplifier output clipping.

(a)

PBL 3852

Dynamic microphone

PBL 3852

(h)

Balanced electret microphone. An additional RC filterlink is recommended if pin 4 is used as a supply.

(b)

Magnetic microphone

(g)

Mic. ampl. supplies the mic. current Im, set by Rx and Ry.

Figure 10. Microphone solutions.

PBL 3852

Unbalanced electret microphone

DC1

(c)

PBL 3852

For dynamic mic.with

reduced gain regulation

DC1

(f)

Balanced electret microphone with reduced gain regulation

PBL 3852

(d)

Unbalanced electret mic. with balanced signal, DC-supply from pin 4.

DC1

PBL 3852

(e)

PBL 3852

Balanced electret microphone

PBL 3852

(a),(c), (d)

(b),(e)

(f)

big C

small C

Transmitter amplifier

The transmitter amplifier in PBL 3852 consists of three stages. The first stage is an amplitude limiter for the input signal at TI, in order to prevent the transmitted signal to exceed a certain set level and cause distortion. The second stage amplifies further the signal from the first and adds it to a DC level from an internal DC-regulation loop in order to give the required DC characteristic to the telephone set. The output for this stage is TO. The third stage is a current generator that presents a high impedance towards the line and has its gain from TO to +L. The gain of this amplifier is ZL/R7 where ZL is the impedance across the telephone line. Hence, the absolute maxi-

11 3

(a)

(b)

mum signal amplitude that can be transmitted to the line undistorted is dependent of R7.(amplitude limiting) The figure 20 shows the range for the amplitude limiter dependent of the operating point on the DC characteristic.

The transmitter gain and frequency response are set by the RC-network between the pins MO and TI (See fig. 11). The capacitor for cutting the high frequency end is best to be placed directly at the microphone where it will also act as a RFI suppressor. The input signal source impedance to the transmitter amplifier input TI should be reasonably low in order to keep the gain spread down, saying that R4//R5//R6 (see fig. 32) must be at least a factor of 5 lower than the ZTin. Observe that the capacitor C9 should have a

(c)

reasonably good temperature behaviour in order to keep the impedance rather constant. The V+C´s influence on the transmitter DC-characteristic is shown in the fig. 8 therefore the transmitter gain would change if the transmitted signal gives reason to an ac-voltage leak signal across C9, this being a feedback point. If the transmitter has an unacceptable low sving to the line at low line currents <≈10mA the first should be to examine if the circuits DC- characteristic can be adjusted upwards and first secondly make use of the linear PD.

no attn.

(d)

attn.without dc.

no attn.

(e)

attenuation

(f)

attn.without dc.

Figure 11. Different possible types of networks between microphone amplifier and transmitter.

Receiver amplifier

The receiver amplifier consists of three stages, the first stage being an input buffer that renders the input a high impedance. The second stage is a gain

(a)

(b)

≈150Ω

150Ω

regulated differential amplifier and the third stage a balanced power amplifier. The power amplifier has a differential output that does not need a series capacitor with the load. The receiver

(c)

(d)

amplifier uses at max. swing (4-6) mA peak. This current is drawn from DC2 that can supply 2 mA continuous current, the C3 helping to supply the peaks, this applies for speech signals only. Continuous sinusoidal signals at this level will load the DC2 down. If a distortion appears in the earphone amplifier output at high signal levels, high line currents, low ohmic earphone load or at low frequencies, the most probable fault is that the filtering capacitor of the earphone amplifier supply C3 is too small. At low line

Figure 12. Receiver arrangements.

currents (normal case, IL < 10 mA)

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