GB2103041A

GB2103041A - Waveform generator

Info

Publication number: GB2103041A
Application number: GB08220514A
Authority: GB
Inventors: Bruce Francis Orr
Original assignee: International Standard Electric Corp
Current assignee: International Standard Electric Corp
Priority date: 1981-07-24
Filing date: 1982-07-15
Publication date: 1983-02-09
Also published as: GB2103041B

Abstract

In many telephone systems it is desired to send metering pulses over the speech path in such a way as to be inaudible to the answering subscriber. To do this it is desirable to produce triangular or trapezoidal pulses. A wave shaper to produce such pulses in response to the application of a ramp voltage (Control Voltage) to the base of a transistor (TR5) is used. This wave shaper includes two differential amplifiers (TR1-TR2 and TR3-TR4) with their collectors cross-connected in complementary manner (TR1-TR4 and TR2-TR3). The first control transistor (TR5) is connected between the bases of two transistors (TR1 and TR3) in the two amplifiers, and it also controls a second transistor (TR6) between the bases of the other transistors (TR2 and TR4) in the two amplifiers. <IMAGE>

Description

SPECIFICATION Waveform generator This specification relates to a waveform generator and uses the characteristics of differential transistor amplifier pairs to provide desired waveforms.

Fig. 1 shows the current changeover characteristic of a differential amplifier (long tailed pair) when driven by a ramp input. As can be seen the circuit provides a smooth transition.

This invention relates to an electronic wave shaper including a pair of series-coupled complementary differential amplifiers, wherein the first differential amplifier has first and second transistors whose emitters are connected to a first internal load impedance, wherein the second differential amplifier has third and fourth transistors whose emitters are connected to a second internal load impedance, wherein the collector of the fourth transistor is connected to the collector of the first transistor and the collector of the third transistor is connected to the collector of the second transistor, and wherein control circuit means is connected between the bases of the first and third transistors and between the second and fourth transistors. The load impedance is connected between the collector pairs.

In a preferred embodiment the control circuit means includes a fifth transistor and a potential divider connected to control the base currents of the first and second transistors and a sixth transistor controlled by the fifth transistor, the sixth transistor controlling the base currents of the second and fourth transistors.

Specific output characteristics of the circuit may be controlled by the waveform fed to the control input. In the case of "silent" metering pulses, a ramp input has proved to be suitable. Triangular or trapezoidal inputs provide suitable "silent" output pulses. Thus a particular application of the circuit is in telephone metering where it is customary to transmit pulses at known rates representing the time cost of the call over the trunk line. In many telephone systems such pulses must be of a specified shape as they are required to be inaudible to the call parties. A square wave pulse is unsuitable because of the transients at each transition of the pulse. To overcome that it is known to use very large inductances to damp the transients. These inductors are physically large and expensive.

The present circuit as specified above provides an electronic circuit which can achieve the desired smooth transitions without the need for large inductance, and an embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 shows the transfer characteristic of a differential amplifier; Fig. 2 shows a wave generator embodying the invention; Fig. 3 shows an outgoing signalling driver embodying the invention, suitable for use with a two wire loop signalling system; and Fig. 4 shows the current distribution paths during the changeover.

In Fig. 2 the first differential amplifier comprises PNP transistors TR1 and TR2 whose emitters are connected to R2. Zener diode Z1 is connected to the bases of TR1 and TR2 to provide a steady state current lo. NPN Transistors TR3 and TR4, with their emitters connected to R9 form the second differential amplifier and their collectors are connected to the collectors of TR2 and TR1 respectively.

Z2 provides the steady state control. The load connection points are shown at A and B on the common collectors.

Fig. 4 shows the operation of the circuit when a ramp signal is applied. The initial conditions (tO) are assumed to be:

IC1 =Collector current of TR1 =25mA IC2=Collector current of TR2= 0 IC3=Collector current of TR3= 0 fizz -Figure 4A IC4=Collector current of TR4=2 5mA IL =Load current =25mA At time tl, Figure 4B, IC4=lCi=i8mA; IC3=lC2=7mA; and lL=lCi-1C2=i imA.

At t2 (Figure 4C) IC1 =IC2=lC3=lC4=1 2.5mA; and IL=O.

At t3 (Figure 4D) IC1 =IC4=7mA; IC2=lC3=1 8mA; and IL=-1 1 mA.

At t4 (Figure 4E) IC 1 =lC4=0; IC2=lC3=25mA; and IL=-25mA.

Fig. 3 shows a PCM multiplexer outgoing signalling driver embodying the invention. The components of the generator are as shown in Fig. 2.

The NORM/REV signal from a local controller such as a PCM rack drives a ramp generator comprising TR7, TR8 and TR9 and capacitor C1.

In addition, loop detection is incorporated in the circuit by monitoring the voltage across R2. With 25mA loop current flowing R2 drops about 5V. This is reduced to about 2V when the loop is broken.

This is used in the reception of dialling pulses.

A further feature of the circuit is the ON/OFF control used to indicate forced release of the line.

Response to the ON/OFF signal switches and bases of TR1 and TR2 to the +5V line and holds these transistors off.

Claims

1. An electronic wave shaper, including a pair of series-coupled complementary differential amplifiers, wherein the first differential amplifier has first and second transistors whose emitters are connected to a first internal load resistance, wherein the second differential amplifier has third and fourth transistors whose emitters are connected to a second internal load resistance, wherein the collector of the fourth transistor is connected to the collector of the first transistor and the collector of the third transistor is connected to the collector of the second transistor, and wherein control circuit means is connected between the bases of the first and third transistors and between the bases of the second and the fourth transistors.

2. An electronic wave shaper as claimed in claim 1, wherein the load impedance is connected between the collector pairs.

3. An electronic wave shaper as claimed in claim 1 or 2, wherein the control circuit means includes a fifth transistor and a potential divider connected to control the base currents of the first and second transistors and a sixth transistor controlled by the fifth transistor, the fifth transistor controlling the base currents of the second and fourth transistors.

4. An electronic pulse generator which includes a wave shaper as claimed in claim 3, and an input to the base of the fifth transistor via which a ramp waveform is applied to control the shaper.

5. An electronic pulse generator substantially as described with reference to the accompanying Figs. 1, 2 and 4 or Fig. 3.