WO1998038786A1

WO1998038786A1 - An adaptable impedance device for controlling direct current flow in a modem

Info

Publication number: WO1998038786A1
Application number: PCT/US1997/023013
Authority: WO
Inventors: Patrick J. Quirk; Trevor Hizzard; Ronald C. Parker
Original assignee: Motorola Inc.
Priority date: 1997-02-28
Filing date: 1997-12-12
Publication date: 1998-09-03

Abstract

An adaptable impedance device (120b) having an impedance that varies with the voltage applied across the device includes: a variable resistance element (208); and a nonlinear voltage divider (212) coupled to the variable resistance element and hence the impedance of the adaptable impedance device based on the voltage applied across the device.

Description

AN ADAPTABLE IMPEDANCE DEVICE FOR CONTROLLING DIRECT CURRENT FLOW IN A MODEM

FIELD OF THE INVENTION

This invention relates to an adaptable impedance device which controls the direct current flow in a modem, and more specifically to such a device which adapts its impedance to meet the DC loop requirements of a plurality of countries in which the modem may be placed in service.

BACKGROUND OF INVENTION

In a data communications system using modems, it is typically required that a called modem detect an incoming call and respond to the detected call in accordance with some predetermined procedure. The incoming call consists of a telephone signal, having a ring signal, generated at a central office ("CO") of a telecommunication service provider. The CO is physically connected to the called modem by a pair of telephone wires, hereinafter referred to as a telephone line. When the modem goes "off-hook" to answer the incoming call, a direct current source in the CO provides DC current to the modem answering the call, referred to as the loop current. The modem must provide a DC path for the loop current when the off-hook relay is closed, and maintain the DC path as long as the off-hook relay is closed. The amount of current flowing from the source in the CO depends on the characteristics of the source, and the impedance of the telephone line and the modem.

Different countries have different DC loop requirements which are stipulated by the relevant administrations of the countries in which the modems may be placed in service. Typically, each country will have a loop mask, which is a plot of modem voltage versus current and will indicate a region of permissible operation and one or more regions outside of the permissible region of operation. Thus, for a particular range of voltages, the mask indicates the acceptable modem currents. Since the current sources in the CO's vary from country to country and the impedances of the telephone lines differ from line to line, the impedance of the modem is the only thing that can be controlled in order to meet the specified loop requirements. That is, by providing the proper modem impedance, the modem will operate in the permissible voltage/current region of the loop mask.

The impedance of a modem to direct current flow is primarily controlled by the impedance of the circuit commonly referred to as the holding circuit. There are other components in the modem that do contribute to the impedance of the modem, such as the off-hook relay; however, in most cases the amount of impedance these other components contribute is significantly less than the holding circuit. Therefore, when analyzing the impedance of the modem to direct current flow to determine if the loop requirements of a particular country are met, one may analyze the DC impedance of the holding circuit.

One prior art method used, described below with regard to FIG. 3, is implemented in, e.g., 3400 series Motorola modems. This method adjusts the values of certain components in the holding circuit on a country by country basis in order to meet the different loop requirements of the various countries in which the modems are to be placed in service. Thus, a number of different holding circuits must be used for a particular modem to satisfy the different requirements. Some combinations of components of this holding circuit may satisfy more than one country's requirements; however, with this prior art approach, one circuit will not satisfy a plurality of different loop requirements. Overall this is not a very satisfactory approach from a manufacturing viewpoint or otherwise.

Thus, a need remains for a circuit which has an impedance that is capable of meeting the different loop requirements of the various countries in which a modem may be placed in service without requiring that the components be changed on a country by country basis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a modem coupled to a telephone central office;

FIG. 2 is a schematic block diagram illustrating a direct current model of the modem coupled to the central office of FIG.1 ; FIG. 3 is a schematic block diagram of a prior art holding circuit in a modem; FIG. 4 is a plot illustrating the voltage versus current characteristics of the prior art holding circuit of FIG. 3 and of the adaptable impedance device in accordance with the present invention;

FIG. 5 is a schematic block illustrating an embodiment of an adaptable impedance device in accordance with the present invention; and

FIG. 6 is a more detailed schematic diagram of the adaptable impedance device of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 there is shown a data terminal 100, such as a computer, connected to a modem 102. Modem 102 is coupled to one end of telephone line 104 and the other end of telephone line 104 is coupled to a telephone central office (CO) 106 within Public Switched Telephone Network (PSTN) 108. Modem 102 can receive incoming calls from and place calls to other modems (not shown) connected to PSTN 108 through their CO's. As described above, when a modem, such as modem 102, goes "off-hook" to answer the incoming call, a direct current source in CO 106 provides DC current over telephone line 104 to modem 102, referred to as the loop current. In FIG. 2, a direct current model 110 of modem 102 coupled to central office 106 is depicted. Within central office 106 is a current source 112 that provides the loop current, i_L, to modem 102 over telephone line 104 which has an impedance 114, Z_L. Current source 112 is actually comprised of a battery feed and a bulk resistance located within CO 106. The DC voltage across the modem is V_M. Within modem 102, the fixed DC impedances of the modem are depicted; namely, the DC impedance of the relay 116 , Z_c, and the total DC impedance 118, Z_D, of other devices within modem 102. These impedances are fixed for a particular modem regardless of the country in which the modem is placed in service. There is also impedance 120, Z_H, of the holding circuit which, in the prior art, varies according to the country in which the modem is placed in service. The voltage across impedance 120 is V_H and since the impedances Z_c and Z_D are significantly less than impedance Z_H, the voltage V_H closely approximates the voltage on the modem, V_M. As noted above, the values of Z_c and Z_D are fixed, therefore, in order to meet the variety of loop mask requirements of the telephone companies in various foreign countries, in prior art holding circuits, impedance 120, Z_H, is adjusted on a country by country basis. That is, as described above, with prior art modems, the modem is manufactured with an impedance 120 (holding circuit) designed specifically for the country in which the modem is to be installed, in order to meet the loop requirements of that country. The DC equivalent circuit of prior art holding circuit 120a is shown in

FIG. 3. This circuit includes a polarity diode bridge circuit 122, which regardless of the polarity of V_H at the input of holding circuit 120a, outputs a voltage V_H' of the same magnitude with a positive polarity at node 124 and a negative polarity at reference node 126. There is a voltage divider 128, comprised of resistors 130, 132 and 134, that biases the base of Darlington pair 136, comprised of transistors 138 and 140. There is also an over-voltage protection circuit 142 which includes resistor 144 and Zener diodes 146 and 148. Zener diode 148 also operates to limit the voltage at the emitter of transistor 140, which changes the resistance of Darlington pair 136 when the voltage across and current through holding circuit 120a reach certain predetermined levels, as described below. Capacitor 149 pervents transistor 138 from responding to AC signals.

As the DC voltage across the modem and hence the voltage, V_H (V_H'), across holding circuit 120a increases, the voltage from voltage divider 128 on the base of transistor 138 also increases and begins to turn on Darlington pair 136. The resistance of Darlington pair 136 is initially relatively high, yet it decreases as the voltage at the base of transistor 138 increases until Dariington pair 136 is fully turned on, at which time its resistance is fixed and linear. Resistor 134 limits the current i_D through Darlington pair 136 and sets the current point, i.e. the amount of current, i_D, that will produce a voltage across Zener diode 148 and cause it to clamp that voltage on the emitter of transistor 140. When the voltage on the emitter is clamped , voltage divider 128 divides the voltage across node 124 and the emitter of transistor 140 between resistors 130 and 132, instead of the voltage across nodes 124 and 126 among resistors 130, 132 and 134. This has the effect of producing a different fixed linear resistance by increasing the resistance of Darlington pair 136 and increasing the overall impedance of holding circuit 120a, as illustrated by portions 151 b and 155b of curves 152 and 154, respectively, FIG. 4, described below. As the voltage on the modem continues to increase, the voltage V_H(V_H') reaches a voltage level that turns on Zener diode 146 and it and Zener diode 148 limit the voltage on holding coil 120a to approximately that voltage level. This diverts some of the current that would be flowing through Darlington pair 136 and causes it to flow through resistor 144 and Zener diode 146 of over- voltage circuit 142, thereby preventing damage to the components of holding coil 120a. Examples of several voltage to current relationships achieved by different configurations of holding circuit 120a are depicted in FIG. 4. Curve 150 represents the VI characteristics of a holding circuit 120a configured to meet the loop mask requirements in the US. This holding circuit has an impedance that remains at one fixed linear level after Darlington pair 136 is turned on. This is because Zener diode 148 is not loaded and does not change the resistance of Darlington pair 136 after it is turned on, as described above.

Curve 152 represents the VI characteristics of a holding circuit 120a configured to meet the loop mask requirements in the UK. This holding circuit has different resistor values in voltage divider 128 and Zener diode 148 is loaded. Because of this, it has a greater initial fixed resistance as indicated by portion 151a of curve 152 and when the current through holding circuit 120a reaches the current set point at 153, holding circuit 120a has a different , increased fixed linear resistance as indicated by portion 151 b of curve 152. Curve 154 represents the VI characteristics of a holding circuit 120a configured to meet the loop mask requirements in France. This holding circuit has different resistor values in voltage divider 128 and Zener diode 148 is also loaded. Because of this, it has a greater initial fixed resistance as indicated by portion 155a of curve 154 and when the current through holding circuit 120a reaches the current set point at 156, holding circuit 120a has a different , increased fixed linear resistance as indicated by portion 155b of curve 154.

Accordingly, the resistor values and the loading of Zener diode 148 must be selected to produce a VI curve that meets the loop mask requirements of the country in which the modem is to be placed in service. Since the curves 150, 152 and 156 produced by holding circuit 120a are linear or piece-wise linear, once the Darlington pair 136 is turned on, a single curve produced by a set of resistor values will only comply with one or maybe two countries, if the countries' requirements are not stringent. A plurality of loop mask requirements cannot be met with prior art holding circuit 120a, because a linear or piece-wise linear impedance in this voltage range cannot satisfy a plurality of loop mask requirements. The adaptable impedance device according to the present invention has an impedance that varies nonlinearly in the voltage region between Darlington pair turn on and the voltage at which the over voltage protection circuitry is activated. This produces a more versatile VI characteristic which is capable of conforming to a plurality of countries' loop mask requirements without changing the components in the device.

An example of a VI characteristic produced is represented by curve 158, FIG. 4. The turn on voltage of this curve is at V volts which may vary depending on the application. From V volts up to the voltage level at point 160 the VI curve is substantially linear, indicating that the adaptable impedance device has a substantially linear impedance in this voltage range. However, for voltages above the voltage level at point 160, the VI curve 158 turns upwardly and becomes asymptotic in region 162, indicating an increasing non-linear impedance. The impedance increases until the voltage is limited or clamped at level 164 where the voltage remains constant, but the current increases, indicating a decreasing impedance.

When a plurality of loop masks are overlaid, a relatively narrow region of permissible operation exists. In region 162, the impedance of the modem must be non-linear and increasing in order for the modem to be in this narrow region of permissible operation. Since the prior art holding circuit has a linear impedance in region 162, it cannot satisfy a plurality of loop mask requirements.

Adaptable impedance device 120b, in accordance with this invention, shown in FIGS. 5 and 6, may be used in place of holding circuit 120a, FIG. 3. Adaptable impedance device 120b includes a polarity diode bridge circuit

200, which regardless of the polarity of V_H, at its input, outputs a voltage V_H' of the same magnitude with a positive polarity at node 202 and a negative polarity at reference node 204. Over-voltage protection circuit 206 limits the maximum voltage that can be applied across node 202 and reference node 204 to prevent damage to the circuitry within adaptable impedance device 120b should an excessive voltage be applied across adaptable impedance device 120b. There is a variable resistance circuit 208 connected between node 202 and node 204. Variable resistance circuit 208 accounts for the majority of the resistance of adaptable impedance device 120b and of the modem. A bias circuit 210 is connected to variable resistance circuit 208 and to reference node 204. Non-linear voltage divider 212, which includes linear voltage divider 214 and voltage sensitive switch 216, controls the resistance value of variable resistance circuit 208 and hence controls the impedance of adaptable impedance device 120b. Linear voltage divider 214 provides a voltage over line 218 to variable resistance circuit 208 to control the resistance value of that circuit. Linear voltage divider 214 is also connected to switch 216 over line 220 and switch 216 is connected to bias circuit 210 over line 222. There is a transient by-pass circuit 224 connected across node 202 and reference node 204 which diverts transients, that occur during on- hook to off-hook transitions, from the remainder of the circuitry in adaptable impedance device 120b. The operation of adaptable impedance device 120b is described generally as follows. When the voltage across nodes 202 and 204 is low, the voltage provided to variable resistance circuit 208 over line 218 is low, that is at levels which produce a voltage on line 218 below V volts, FIG. 4. This causes the resistance of variable resistance circuit 208 to be relatively high yet decreasing. As the voltage across nodes 202 and 204 increases to a level above V volts, the voltage on line 218 increases, causing the resistance of variable resistance circuit 208 to decrease and become substantially linear between V volts and the voltage at point 160, FIG. 4. As the voltage across nodes 202 and 204 increases, the current , i_D, that flows through variable resistance circuit 208 and bias circuit 210 increases. Above the voltage at point 160, current i_D reaches a predetermined level which produces a voltage on line 222 that causes switch 216 to close. The closing of switch 216 introduces a very low impedance into linear voltage divider 214 causing the voltage provided to variable resistance circuit 208 over line 218 from linear voltage divider 214 to decrease thereby increasing the resistance of variable resistance circuit 208 and the overall impedance of adaptable impedance device 120b. The effect of switch 216 on linear voltage divider 214, causing the combination to operate as nonlinear voltage divider 212, and on the impedance of adaptable impedance device 120b is illustrated by curve 158, FIG. 4, in region 162. Nonlinear voltage divider 212 causes variable resistance circuit 208 to have an increasing resistance in region 162 which limits the loop current i_L as the voltage on the modem increases.

Adaptable impedance device 120b is shown in more detail in FIG. 6. Polarity diode bridge circuit 200 includes polarity diode bridge 230, such as a full wave bridge rectifier. Over voltage protection circuit 206 includes a Zener diode 232 which clamps the voltage across nodes 202 and 204 at a predetermined maximum voltage when the voltage on the modem exceeds the predetermined voltage, which in this example is 39 volts. Variable resistance circuit 208 includes a Dariington pair 234 formed by transistor Q3 236 and transistor Q4 238. Transistor Q3 236 is a small signal switching transistor and transistor Q4 238 is a power transistor. Variable resistance circuit 208 also includes 1 Kohm resistor R5 240 connected to the collector of transistor Q3 236 and 56 ohm resistor R8 242 connected to the emitter of transistor Q4 238. Bias circuit 210, which is connected to the emitter of transistor Q4 238, includes 1.82Kohm resistor R6 244 and 2.74Kohm resistor R7 246. Linear voltage divider 214 includes 68.1 Kohm resistor R3 248 and 47.5Kohm resistor R4 250. Switch 216 includes transistor Q2 252, which is a small signal switching transistor, and a diode 254, connected to the emitter of transistor Q2 252. As noted above, the combination of linear voltage divider 214 and switch 216 forms nonlinear voltage divider 212.

Transient by-pass circuit 224 includes transistor Q1 256, which is a small signal switching transistor, with its collector tied to node 202. Transistor Q1 256 acts as a speed up transistor for Darlington pair 234 by setting a voltage on the base of transistor Q3 236 that causes Darlington pair 234 to have a low impedance during on-hook to off-hook transitions. There are two resistors R1 258 and R2 260 connected to the base of transistor Q1 256 . The other end of resistor R1 258 is connected to a .47 μF capacitor C1 262 which is also connected to node 202. Resistor R1 258, resistor R2 260 and capacitor C1 262 set the bias and response time of transistor Q1 256. The other end of resistor R2 260 is connected to the emitter of transistor Q1 256 and to 10 μF capacitor C2 264 which is also connected to node 204. Capacitor C2 264 prevents transistor Q3 236 from responding to AC signals. Adaptable impedance device 120b, as shown in FIGS. 5 and 6 has a non-linear impedance characteristic as illustrated by curve 158 of FIG. 4. However, the present invention can be more broadly utilized to attain different nonlinear impedance characteristics suitable for other applications by selecting different values for certain components in adaptable impedance device 120b. For example, different values for resistors R3 248 and R4 250 of voltage divider 214 may be used to change the voltage V_H which will fully turn on Darlington pair 234 of variable resistance circuit 208. Or, the values of resistor R6 244 and R7 246 may be changed to alter the point at which transistor Q2 252 of switch 216 turns on and forms nonlinear voltage divider 212 from the combination of linear voltage divider 214 and switch 216. This will cause the transition points V and point 160, respectively, of curve 158, FIG. 4, to vary.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the novel concept of the invention. It is to be understood that no limitation with respect to the specific methods and apparatus illustrated herein is intended or should be inferred. It is, or course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. The invention is further defined by the following claims.

What is claimed is:

Claims

1. An adaptable impedance device having an impedance that varies with the voltage applied across the device, comprising: a variable resistance element; and a nonlinear voltage divider coupled to the variable resistance element which varies the resistance of the variable resistance element and hence the impedance of the adaptable impedance device based on the voltage applied across the device.

2. The adaptable impedance device of claim 1 wherein nonlinear voltage divider has an output that is proportional to the voltage applied across the device and the output controls the resistance of the variable resistance element.

3. The adaptable impedance device of claim 2 wherein the variable resistance element includes first and second transistors; and wherein the first transistor has its base coupled to the output of the nonlinear voltage divider and its emitter coupled to the second transistor forming a Darlington pair, the resistance of which is controlled by the output of the nonlinear voltage divider.

4. The adaptable impedance device of claim 1 wherein the nonlinear voltage divider includes of a linear voltage divider coupled to the variable resistance element and the linear voltage divider is further coupled to a voltage sensitive switch having a low impedance.

5. The adaptable impedance device of claim 4 further including a bias circuit coupled to the voltage sensitive switch, wherein the bias circuit causes the voltage sensitive switch to close and introduce its low impedance to the linear voltage divider when a predetermined voltage is applied across the adaptable impedance device.

6. The adaptable impedance device of claim 5 wherein the bias circuit is further coupled to the variable resistance element which provides to the bias circuit a current indicative of the voltage applied across the adaptable impedance device.

7. The adaptable impedance device of claim 1 further including an over- voltage protection device which limits the voltage across the adaptable impedance device.

8. The adaptable impedance device of claim 1 further including a transient impedance element connected in parallel with the variable resistance element which diverts transients from the variable resistance element.

9. The adaptable impedance device of claim 1 wherein the device is contained within a modem to control the DC current flow in the modem.

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