GB2417393A - Central Office Line Circuit - Google Patents

Abstract

The present invention is directed towards a low cost and high performance CO line interface circuit. The line side and application side circuits are isolated with a transformer T1, and the line side circuit includes a high voltage transistor Q1 that modulates the line current for transmitting the D-A (digital-to-analog) voice signal. This eliminates the use of a bulky high voltage DC blocking capacitor. Moreover, the transformer is actively driven by an amplifier with a low output impedance on both primary and secondary side. This allows for the use of low inductance transformer. Removal of the DC blocking capacitor and low output impedance of the amplifiers driving the transformer, improves the low-end frequency response.

Description

CENTRAL OFFICE LINE INTERFACE CIRCUIT

BACKGROUND OF THE INVENTION

1. Field of the Invention

10001] The present invention relates generally to telephone systems, and more particularly to a low cost and high performance central office line interface circuit with improved low-frequency response.

2. Description of the Related Art

10002] Central office (CO) line interface circuits (also referred to as LS/GS (loop start/ground start) trunk circuits or Direct Access Arrangements (DAAs)) are used to interface the CO telephone line (Tip and Ring pair), commonly referred to as line-side circuitry, with communications systems, commonly referred to as application-side circuitry, such as PBXs, telephones, modems, codecs and fax machines. A CO line interface circuit or trunk circuit is required to perform several functions in accordance with the specifications of each country.

3] Transformers have traditionally been used on CO line interface circuits to provide isolation between the CO line-side circuitry and the local application-side circuitry (e.g. a PBX). Although CO line circuits with isolation transformers provide numerous advantages, as discussed in greater detail below, there are certain drawbacks. During off hook operation (i.e. voice/data transmission), considerable DC voltage is present on the Tip and Ring pair. Hence, large audio transformers with high saturation current are required to handle the high DC voltages (e.g. typical transformer inductance values are between 3 and 5 H (primary and secondary)).

4] In order to reduce the size of the transformers, AC coupling capacitors have been used, such as described in US Patent 4,776,007 (Styrna). However, the capacitors used to block direct current from the transformer must be high enough in value (e.g. typically between 1uF and 2.2 uF) so as not to effect AC signal transmission. The AC coupling capacitors must also be able to handle ringing voltages with high amplitude for the duration of time the CO takes to acknowledge a remote off-hook and then remove the ringing signal.

5] In order to meet the above two conditions, it is known in the art to use a high value (typically 1-2.2 uF) capacitor with high voltage ratings (typically 100- V). However, this results in large sized capacitors that contribute to circuit board real estate.

[00061 Another problem with prior art isolation transformers is that the transformers have finite inductance, thereby affecting the signals passing therethrough. Normally, a lower inductance results in a higher impact on the phase and magnitude of the signal passing through the transformer.

Consequently, in or to simplify design and lower the component count, it is highly desirable that a transformer and DC blocking capacitor have minimal impact on the magnitude and particularly the phase of the signals passing through these components. Also, since the size of a transformer increases with an increase in inductance, it is beneficial for circuit real estate to lower the primary and secondary inductance.

10007] One solution to the above problems is to eliminate the transformer used for isolation. To that end, optocouplers are known in the prior art for providing isolation. Optocouplers are devices for providing isolation while yielding a flat response down to very low frequencies. Optocoupled devices contain an LED that transmits light as a function of current traveling through the diode. This light is sensed by a light detecting diode (LDD). The current flowing through the detector diode is modulated based on light transmitted by the LED. Optocouplers are normally low height components. However, one drawback of optocouplers is that the coupling between the LED and LDD is not constant over temperature.

For this reason, each LED in an optocoupler must be provided with two matched LDDs where one of the LDDs provides feedback to compensate for coupling variation due to temperature variations. This is possible since the ratio of coupling factor associated with first LDD and second LDD remains fairly constant over temperature. This reduces the gain variations through the optocouplers.

Nonetheless, the use of optocouplers still results in considerable gain variations due to variation in coupling from component to component and over temperature.

[00081 US patent number 5,381,606 (Andriew) and US patent number 5,946,393 (Holcombe) each discuss the foregoing drawbacks of high gain variation due to the limitations of the optocouplers, and the fact that the coupling factor varies with temperature and from device to device.

9] Capacitors have also been used in the prior art as an alternative to transformers, for achieving isolation of the local circuit from the line. In operation, a signal on the Tip and Ring is digitized by a line side circuit, modulated by a high frequency and passed through the capacitive isolation barrier. On the system side of the communication path the signal is demodulated to recover the voice signal. However, circuits incorporating capacitive isolation are of complicated design and, for practical purposes, must be implemented in an IC (integrated circuit) .

SUMMARY OF THE INVENTION

10010] According to the present invention, a circuit is provided that eliminates the DC blocking capacitor and requires only a low inductance transformer thereby resulting in a reduction in circuit board real estate. The topology of the present invention also improves low-end frequency response as compared to prior art solutions, resulting in design simplification and improved input impedance and line impedance matching.

10011] The circuit of the present invention is divided into two parts one powered by the line from the CO (Tip and Ring), or line side circuit, and the other powered by the local communication system (e.g. PBX), or application side circuit. These two parts are isolated by a transformer, as is known from the prior art. However, the line side circuit includes a high voltage transistor that modulates the line current for transmitting the D-A (digital-to-analog) voice signal.

This eliminates the use of a bulky high voltage DC blocking capacitor. Removal of the DC blocking capacitor improves the low-end frequency response as well. The received A-D (analog-to-digital) signal received from the CO is AC coupled via a low voltage and low-valued (i.e. 220 nF) capacitance to a receive op-amp having high input impedance.

2] The primary side of the transformer is driven by the receive opamp through a low output impedance. The secondary side of the transformer is driven by a transmit op-amp also with a low output impedance. The lower values of the output and input impedances enables the use of a transformer characterized by low primary and secondary coil inductance and improved low end frequency response of the signals passing through the transformer. Moreover, the transformers with lower primary and secondary coil inductance are smaller in size and consume less real estate.

BRIEF DESCRIPTION OF THE DRAWINGS

3] Figure 1 is a block diagram of a conventional CO line circuit; [0014] Figure 2 shows a prior art isolation transformer for use in the CO line circuit if Figure 1; [0015] Figure 3 is a block diagram of a transformer coupled CO line circuit according to the preferred embodiment of the invention; and [0016] Figure 4 is a simplified high side model of the transformer coupled CO line circuit of Figure 3.

7] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 10018] Figure 1 shows a block diagram of a typical LS trunk circuit. The trunk circuit contains protection circuitry 1 to prevent the interface circuitry from high voltages on the line. These voltages are present on the line either from signaling or environmental conditions such as lightning, power cross or ESD (electrostatic discharge).

9] A polarity bridge is 3 used to define the polarity of the line. A ring detector 5 is used to detect the incoming ringing signal from the CO. A line current sense or battery detector 7 is used to detect if the line is available for use.

An isolated hook-switch is used to control the on/off hook status of the device.

0] Gyrator 9 is used to draw a specified amount of DC current (typically 20mA-120 mA) while presenting a high AC impedance. The function of hybrid 11 is to convert the bi-directional line signal to a unilateral transmit and a unilateral receive signal, also known in the art as 2-to-4 wire conversion.

lO021] As discussed above, the line side circuitry must be isolated from the application side circuitry. The trunk circuit must present a specified input impedance (impedance looking into the Tip and Ring (T/R) interface of the trunk) to the line. This impedance is 600 ohms in North America. When a signal is transmitted to the line, part of it also leaks back into the receive path as a received signal. Hence, if the load impedance of the line is known the leaked signal can be subtracted from the received signal. The ratio of this received signal from the transmitted signal is known as the transhybrid loss. A trunk circuit is required to yield a satisfactory transhybrid loss against a specified impedance (i.e. 600 ohm).

lO022] Conventionally, a transformer (Xformer) is used to provide isolation from T/R and a capacitor (C) is used to block DC voltages, as shown in Figure 2.

However, large values of transformer inductance (i.e. > 3 H) and a large capacitor (i.e. > 2,uF) are required for good low frequency response. The capacitor (C), in some configurations, must also be capable of handling high AC line voltage (caused by ringing signal) further justifying larger capacitor size. As discussed above, large values of inductance and capacitance result in increased size of these components and hence add to overall cost. However, as discussed above, use of such large discrete components is not suitable in applications where PCB (printed circuit board) real estate and component height are constraints. It is desirable to minimize the component count and use small sized component.

lO023] Therefore, it is an object of an aspect of the present invention to provide a trunk design with lower real estate requirements while yielding good frequency response at low frequencies.

4] According to the present invention, a transformer is used for isolation, as is known in the art, but with low primary and secondary inductance, for example < 500 uH, such that the transformer is of smaller size than is known in the prior art. Also, contrary to the prior art, the traditionally used high voltage DC blocking capacitor is eliminated. Instead, only a low valued, low voltage and hence smaller size capacitor is required to couple the AC signal on T/R. As discussed in greater detail below, the CO line circuit of the preferred embodiment also yields enhanced frequency response at low frequencies.

5] Figure 3 is a block diagram of the CO line circuit of the preferred embodiment, wherein the transformer (T1) is driven by low resistances, R3 and R4. (e.g. 50 ohms as compared to 600 ohms traditionally used). The low values of R3 and R4 cause the transformer transfer function to have a smaller impact on the low-end frequency response than prior art designs. Moreover, the low values for R3 and R4 allows for the use of low inductance, and hence a smaller size transformer. As discussed in greater detail below, low-values of R3 and R4 are driven from a low impedance point (i.e. an op amp).

6] A high voltage transistor, A, is used to modulate the line current thereby eliminating the need for a high-value DC blocking capacitor. AC voltage at Q1 emitter is proportional to VRX (the signal to be transmitted by the trunk circuit towards the CO). Resistor R' conducts AC current through Tip and Ring proportional to VRX. This current in conjunction with the loop impedance results in an AC voltage, proportional to VRX, across Tip and Ring. The gain of the Q1 stage is dependent on R1 and Zline. R1 also controls the amount of DC current flow through Q1. Hence, the value of R1 must be chosen carefully to meet the specifications of the trunk circuit. For 600 ohm trunk circuit, the value of R1 is preferably about 100 ohm, (see detailed analysis below).

O027] As discussed above, eliminating the prior art requirement of a high value DC blocking capacitor results in saving real estate and decreases the phase shift at low-end frequencies that would otherwise be introduced by the DC blocking capacitor.

10028] Instead, according to the present invention, a low-value (e.g. less than 680 nF) and low voltage rating (e.g. less than 50V) capacitor C1 is used to replace the prior art high-value DC blocking capacitor. Since C1 is in series with a very high, typically > 200kQ resister R2, it does not need to be either of high value or high voltage.

lO029l Differential amplifiers U1A and U1B form part of the line side circuitry and are powered by the line, while differential amplifiers U2A and U2B form part of the application side circuitry and are powered by the system (e.g. PBX, telephone, etc.) The input impedance is actively generated by the loop involving U1B and U1A.

An incoming AC signal, A-D, on the tip and ring is sensed and a proportional signal with the same phase as the sensed signal is transmitted to the tip and ring by Q'. This results in the required input impedance. The outgoing signal, D-A, is transmitted by Q' that modulates the line current proportional to the transmit voltage. The incoming signal, A-D, is received by U 1 B and U2A and transmitted to the application side circuitry. The balance impedance for transhybrid loss is matched through the loop involving Ret. A person of ordinary skill in the art will appreciate that balance impedance is different from input impedance, and that an LS trunk circuit must match both the input impedance and balance impedance for acceptable transmission performance. Hence R', is used in the transhybrid network to match the balance impedance. Better balance impedance matching results in better transhybrid loss.

10030] Calculation of the transfer functions, input impedances and signal gains for the preferred embodiment of Figure 3, will be understood by a person of ordinary skill in the art with reference to the circuit model of Figure 4. R3 is the source output impedance. Rm is a resistance to model the core loss due to hystersis. Lp is the mutual inductance. Req is a copper winding equivalent resistance. Leakage is the leakage inductance and accounts for flux leakage. R4 is the load impedance on the secondary.

1] From Figure 4, it will be appreciated that: [0032] Vab = (S*Lp II Rm)/(s*Lp II Rm + R3)* V,n [0033] VCNab = R4/ (R4 + Req + s*L'eakage) [0034] VouNn = VcuNab * VabN,n [0035] Assuming that a transformer with low leakage inductance is used, the calculations are further simplified: [0036] Vout/Vin= R4/ (R4 + Req) * (s*Lp 1I Rm)/(s*Lp II Rm + R3) [0037] From the expression above if R3 is small VouNin R4/ (R4 + Req) [00381 Therefore, the transfer function (VouN'n) dependency on frequency is insignificant if R3 is chosen to be small. Since, in context of the present invention, the path through the transformer is bi-directional, both R3 and R4 are chosen to be small.

9] Transfer function equations for Figure 4 are: [0040] Let U1B gain -R7/R2 = g1 [0041] Let U1A gain -R5/R6 = g2 [0042] Let U2B gain -R12/R13 = g3 [0043] Let U2A gain -Rg/R8 = g4 [0044] Let gs = [R10 / (R10 + R11)]*[(1+ RslRs)l [0045] Moreover for simplicity assume R3 = R4 and R1 = k*Z'ne [0046] From the forgoing, the input impedance may be calculated, as follows: [0047] If VRX = 0 [0048] Vab = g1.g2/2 (Z'ne/(k.Z'ne) Vab + Vin [1] [0049] For an input impedance of ZLine [00501 Vab = V'n /2 [2] [0051] From [1] and [2] [0052] g'*g2 = 2*k [0053] The transmit (A-D) gain may be calculated, as follows: 10054] V'n 91 94 /4 = VTX [3] 10055] Vab = V'n/2 [4] 100561 From [3] and [4] 0057] VTxNab = 91 94/2 [5] O058] The receive (D-A) gain may be calculated as follows: 10059] Assume V,n = 0 loges] Vab = [93 VRX 92/2] Zune/(k ZLine) + [dab 91 92/2] Z'ne/(k Zone) [6] O061] The transhybrid (D-D) gain may be calculated as follows: lO062] Assume V,n = 0 10063] VTX = Transmit gain * Receive gain * VRX + 93*95*VRX O064] VTXNRx = Transmit gain * Receive gain + g3*g5 lO065] For ideal D-D gain i.e. VTxNRx = 0 10066] So transmit gain * receive gain + g3*g5 = 0 [8] lO067] From [5] [7] and [8]: 10068] gs = [g,*g4* g2/(4*k+2*g,*g2)] [0069] In summary, the present invention, as exemplified by the preferred embodiment in Figure 3, provides an enhanced trunk circuit topology based on transformer coupling, but which eliminates the bulky DC blocking capacitor and allows the use of transformers with low inductance and hence reduces the real estate. Another key advantage of the circuit according to the present invention is the minimal impact on low-end frequency response. This is in contrast with the prior art wherein the high drive impedance of the transformer and DC blocking capacitor(s) introduce high phase shift particularly at the low frequencies, resulting in a poor match to the line impedance and input impedance at the lower frequencies and/or added design complications.

0] The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the sphere and scope of the invention. For example, the transmission specifications differ from country to country and sometimes in areas within a country. The present invention can easily be altered to adapt to various transmissions requirements of different countries. Also, a person skilled in the art will appreciate that the present invention can be used in any product containing a CO line interface or a 2 to 4 wire hybrid, including phones, modems, fax machines, call answering machines, bank transaction/lottery units, point of sales etc. Since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

Claims (8)

1. What is claimed is: 1. A central office line circuit for interfacing central office line side circuitry to application side circuitry, comprising: a small low inductance isolation transformer for bi-directional coupling of signals between said central office line side circuitry and said application side circuitry; a receive amplifier having a high input impedance and a low output impedance; a low value and low voltage capacitor for AC coupling signals received from said central office line side circuitry to said receive amplifier which in response applies said signals to a primary coil of said transformer for transmission to said application side circuitry; a transmit amplifier having a high input impedance and a low output impedance for applying signals received from said application side circuitry to a secondary coil of said transformer; and a high voltage transistor connected in a circuit to said primary coil for applying said signals received from said application side circuitry to said central office line side circuitry and applying an in-phase portion of said signals received from said central office line side circuitry back to said central office line side circuitry resulting in a specified input impedance, whereby said low output impedance of the receive and transmit amplifiers allows the use of said small low inductance transformer resulting in low circuit board real estate and enhanced low end transmit and receive frequency response.
2. 2. The central office line circuit of claim 1, wherein said low inductance is approximately 500 uH.
3. 3. The central office line circuit of claim 1, wherein said low output impedance is approximately 50 ohms.
4. 4. The central office line circuit of claim 1, wherein said capacitor is approximately 680 nF.
5. 5. The central office line circuit of claim 1, wherein said high impedance of said receive amplifier is provided by a high value input resistor.
6. 6. The central office line circuit if claim 5, wherein said high value input resistor is approximately 200 Kohm.
7. 7. The central office line circuit of claim 1, further including a resistor connected to an output of said high voltage transistor, for controlling gain of and DC current flow through said high voltage transistor.
8. 8. The central office line circuit of claim 7, wherein said resistor is approximately 100 ohm.