Automatic gain control amplifier for compensating cable loss

Abstract: An AGC amplifier for compensating a signal loss in a cable through which a signal having a frequency is passed, the signal loss being defined as a function of a square-root of the frequency, having a wide compensation ability and adaptable to the forming of a LSI circuit construction. The AGC amplifier includes a first amplifier (10) amplifying an input signal (IN) to be compensated at a first gain (A₀), a differential circuit (40) having a constant differential coefficient defined by the frequency, and differentiating the input signal at the differential coefficient, a second amplifier (20) amplifying the differentiated signal at a second gain (A₁), and an adding circuit (30) adding the amplified signals therefrom to output an output signal (OUT) in which the signal loss is compensated.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control (AGC) amplifier for compensating a cable loss, more particularly, to an AGC amplifier for compensating a cable loss, having a wide compensation ability and adaptable to the forming of a large scale integration (LSI) circuit construction. 2. Description of the Related Art An equalizing amplifier system, which carries out a wave shaping of a pulse signal having an amplitude which is attenuated by a cable, for example, a coaxial cable or a paired-cable, to a pulse signal having a predetermined equalization characteristic, usually includes a build-out network (BON), a preamplifier, an AGC amplifier, a wave equalizer, a post-amplifier, a peak detector connected to the post-amplifier to detect a peak of an output from the post-amplifier, and a DC amplifier connected to the peak detector and supplying an amplified peak signal to the AGC amplifier. The BON is an adjustment circuit enabling a level compensation at the AGC amplifier even when a cable relaying distance is short and thus an input level is too high. The preamplifier is a low noise amplifier for suppressing noise. The AGC amplifier compensates cable loss to restore an amplitude so that a gain thereof is controlled by the peak detector and the DC amplifier to restore a peak of a wave equalized at the equalizer. The equalizer equalizes the AGC amplified signal to form a signal having a predetermined wave characteristic, in accordance with a predetermined equalization function. The post-amplifier supplies a high level signal to a timing circuit and a discrimination and recovery circuit. The present invention pertains to the AGC amplifier. A Bode type variable equalizer and modifications thereof are used as an AGC amplifier in a frequency division multiplexing (FDM) system, but a variable equalization range thereof is narrow, and accordingly, this type is not used for a pulse coded modulation (PCM) system. The present invention particularly relates to an AGC amplifier for compensating a cable loss in the PCM system. From US-A-3,173,110 a transmission system is known including a fixed-gain amplifier and a variable-gain amplifier, the gain of the latter being controlled by temperature sensitive means. Both amplifiers receive an input signal from a matched dividing network. The output signal of the fixed-gain amplifier and the variable-gain amplifier is fed to a delay network and a slope equalizer, respectively. A combiner receives the output signals of both, the delay network and the slope equalizer and generates an output signal. From US-A-2,895,111 means for destortion correcting devices for the frequency response curve of a four-terminal network are known. An equalization is carried out by several equalizing stages, comprising stages of a first and a second type. The first type stage comprises a circuit which is differentiating the input signal twice and outputting it to an amplitude control member. The input signal and the second derivative are combined in an adder to generate the output signal of the stage. Prior art AGC amplifiers, some of which will be described later in greater detail, suffer from a narrow variable equalization range and a relatively bulky circuit size, and cannot be adapted to form an LSI circuit construction. An object of the present invention is to provide an AGC amplifier for compensating a cable loss in a PCM system, and having a wide variable equalization range. Another object of the present invention is to provide an AGC amplifier for compensating a cable loss in a PCM system, and able to be adapted for forming a LSI circuit construction. Yet another object of the present invention is to provide an AGC amplifier for compensating a cable loss in a PCM system, by which the cable loss is easily compensated and having a simple circuit construction. According to the present invention there is provided an automatic gain control amplifier for compensating signal loss in a cable through which a PCM signal is passed, the signal loss being defined as a function of the square-root of a frequency of said PCM signal, having a total gain being proprotional to the square-root of the frequency of said PCM signal, comprising: - a first amplifier having a first fixed gain, receiving an input signal to be compensated and amplifying said input signal at said first fixed gain; - a differential circuit having a constant differential coefficient defined by said frequency, receiving said input signal and differentiating said input signal at said differential coefficient; - a second amplifier having a second fixed gain, receiving said differentiated input signal from said differential circuit and amplifying said differentiated input signal at said second fixed gain, the gain of said second amplifier thereby being controlled by said differential circuit (such that it changes linearly in a narrow frequency range in which compensation is required; and - an adding circuit connected to said first and second amplifiers, adding said amplified signals and generating an output signal in which said cable loss is compensated. The first amplifier may comprise a first gain change circuit for changing the first gain in response to a first gain control signal; a differential-operation type amplifying circuit including transistors; and a first operation point compensation circuit for rejecting an affect of a variation of base-emitter voltages of the transistors in the first differential-operation type amplifying circuit. The second amplifier may comprise a second gain change circuit for changing the second gain in response to a second gain control signal; a differential-operation type amplifying circuit including transistors; and a second operation point compensation circuit for rejecting an affect of a variation of base-emitter voltages of the transistors in the second differential-operation type amplifying circuit. The first and second amplifiers can be formed by a large scale integrated circuit. The differential circuit may comprise a capacitor and a resistor designed to give the differential coefficient. According to the present invention, there is also provided an AGC amplifier circuit including a plurality of the above AGC amplifiers connected in series, to give a continuous-compensation characteristic. Other objects and features of the present invention will be described in more detail with reference to the accompanying drawings, in which: Figs. 1a to 1c are graphs representing general characteristics of a gain equalization; Fig. 2 is a circuit diagram of a basic prior art AGC amplifier for compensating a cable loss; Fig. 3 is a circuit diagram of a prior art AGC amplification circuit as a combination of the circuits shown in Fig. 2; Fig. 4 is a graph representing a total gain equalization characteristic obtained by the circuit shown in Fig. 3; Fig. 5 is a graph representing a change of a gain equalization of the prior art; Fig. 6 is a block diagram of an AGC amplifier for compensating a cable loss in accordance with the present invention; Figs. 7 and 8 are graphs representing the basic operation of the AGC amplifier shown in Fig. 6; Fig. 9 is a circuit diagram of an embodiment of the AGC amplifier of Fig. 6; Fig. 10 is a graph representing a change of a total gain of the AGC amplifier shown in Fig. 9; Fig. 11 is a circuit diagram of another embodiment of the AGC amplifier of Fig. 6; Figs. 12 and 13 are circuit diagrams of AGC amplifiers of the present invention; and, Figs. 14 and 15 are graphs of actual cable loss curves and actual cable loss compensation curves. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiments of the present invention, the principle of cable loss compensation will be described with reference to Figs. 1a to 1c. Figure 1a shows a relationship between a frequency f (Megabits per second: Mb/s) on an abscissa of a logarithm scale and a cable loss LOSS (dB) on an ordinate. The cable loss LOSS depends upon a square root of the frequency f of a signal passing through a cable, for example, a coaxial cable, and a length ℓ of the cable, and is expressed by the following formula (Re: Equation (6) in JP-A-59-4335): LOSS = exp (A f + Bf)ℓ = exp (A f )ℓ where, A and B denote constants. In Fig. 1a, curve LOSS L represents a cable loss when the length of the cable is long, curve LOSS S represents a loss when the length is short, and curve LOSS M represents a loss when the length is medium. Figure 1b is a graph showing a characteristic of a gain of an AGC amplifier for compensating the cable loss, for example, the cable loss shown by curve LOSS M in Fig. 1a. In Fig. 1b, an abscissa indicates the frequency (Mb/s) of a logarithm scale, and an ordinate indicates a gain (dB) of the AGC amplifier for compensating the cable loss. The AGC amplifier should compensate the cable loss within a predetermined equalization range. Figure 1c is a graph representing a total gain compensated by the AGC amplifier. In general, the AGC amplifier should compensate the cable loss in response to the frequency f and the length ℓ , and within the predetermined equalization range. Before describing the preferred embodiments of the present invention, prior art AGC amplifiers will be described. Figure 2 is a circuit diagram of a basic prior art AGC amplifier shown in Fig. 4.77 on page 218 of a publication titled "HIGH SPEED PCM", by Shigei et al., published on Feb. 10, 1975, CORONA Co., Ltd. The AGC amplifier consists of a DC component cutting capacitor CO, a transistor TR, resistors R0 to R3 having resistances R0 to R3, respectively, and a variable capacitor CV having a capacitance C which is varied in response to a control voltage V C . A frequency characteristic A(f) of the AGC amplifier can be approximately expressed by the following formula: A(f) = R1 R2 · 1+jωC(R2+R3) 1+jωC·R3 where, ω = 2πf The frequency characteristic is the same in a hyperbolic first-order form wherein, in a bridge T type circuit, an impedance Z a thereof is sL or 1/sC(s=jω). Accordingly, by combining the AGC amplifiers, more specifically, by a cascade-connection of a plurality of the AGC amplifiers of Fig. 2 and a suitable selection of parameters, an AGC amplifier for compensating a cable loss, which is expressed by a function of a square root of the frequency, can be realized. Figure 3 is a circuit diagram of such a combined AGC amplifier (Re: Fig. 4.78 in the above-mentioned publication). The AGC amplifier circuit shown in Fig. 3 consists of four tandem-connected AGC amplifiers each corresponding to the AGC amplifier in Fig. 2. Figure 4 is a graph representing a gain of the AGC amplifier circuit shown in Fig. 3, and corresponds to Fig. 1a. Assuming that the cable loss is varied from a standard loss (a √f) by (Δa √f), a gain G(f) necessary for the AGC amplifier circuit is expressed by the following formula: G(f) = G O + a f + Δa f = G O + (1+ Δa a )²·f This is equivalent to a parallel shift by (1+Δa/a)² of the frequency characteristic along the frequency axis. The parallel shift of the frequency characteristic can be realized by simultaneously shifting all poles and all zero points, and thus can be achieved by changing the capacitances of the variable capacitors CV11 and CV12 to CV4 shown in Fig. 3 to which a control voltage V C is supplied. Figure 5 is a graph showing the shift operation. Curves GAIN1 to GAIN3 show gains (dB) on an abscissa of the AGC amplifier circuit of Fig. 3 when the capacitances of the variable capacitors are changed at a same ratio. The AGC amplifier circuit shown in Fig. 3 is an example used in a PCM repeater having a frequency of 400 (Mb/s). The capacitance of the variable capacitors can be varied between 5 to 30 pF, and have a variable gain range of 15 (dB). The above AGC amplifier circuit has the following defects. First, since the capacitance variable range is based on a multiple of 6, the equalization range is narrow and cannot be commonly used for a communication system in which cables having a variety of lengths are used. Second, since the variable capacitors having large capacitances cannot be easily formed in LSI chip, the AGC amplifier circuit shown in Fig. 3 can not be adapted to form the LSI chip. In addition, the AGC amplifier circuit requires many resistors, and thus can not be adapted to form the LSI chip. Third, the AGC amplifier circuit is relatively complex. JP-A-59-4335 discloses a variable gain amplification circuit including a circuit having a characteristic of a square root of a frequency. The variable gain amplification circuit realizes a high compensation and has a wide equalization range, but uses two variable capacitors in each AGC amplifier, and thus still cannot be used when forming the LSI chip. The present invention will now be described. Figure 6 is a block diagram of an AGC amplifier circuit for compensating a cable loss in accordance with the present invention. In Fig. 6, the AGC amplifier circuit includes a first amplifier 10, a differential circuit (or a low frequency range cut-off circuit) 40, a second amplifier 20, and an adder 30. The AGC amplifier circuit can be provided in an PCM repeater, and an input signal IN, which is attenuated by a cable, is supplied to the amplifier 10 and the differential circuit 40. The operation of the AGC amplifier cirucit shown in Fig. 6 will be described. Referring back to formula (2), this formula can be rewritten as follows: T(s) = R 1 R 2 1+ sC ( R 2+R3) 1+ sCR 3 = A₀ 1+ s τ₂ 1+ s τ₁ where, A₀ = R1/R2 s = j ω τ₁ = C R₃ τ₂ = C(R₂+R₃) The equation (4.1) is modified as follows: T(s) = A₀ 1+ s τ₁+ s (τ₂-τ₁) 1+ s τ₁ = A₀ [1 + s τ₁(τ₂-τ₁)/τ₁ 1+ s τ₁ ] T(s) = A₀ + A₁ s τ₁ 1+ s τ₁ where A₁ = A₀ (τ₂ - τ₁) / τ₁. Figure 7 is a graph showing the characteristic of the equation (4.3). In Fig. 7, an abscissa indicates a frequency f or s of a logarithm scale, and an ordinate indicates a gain (dB). A transition zone is repressed by the term sτ₁/(1+sτ₁) in the formula (4.3). Obtaining an absolute value of the second term in the formula (4.3). Considering a frequency range of 0 < sτ₁ < 1, a development of a Malaurin's series to the formula (5.1) is carried out: A₁ |sτ₁| 1 - 1 2 (sτ₁)² + ...... Furthermore, a first-order approximation is applied to the formula (5.2), and the following formula is obtained: A₁ |sτ₁| = A₁(2πfτ₁) The formula (6) shows that the second term in the formula (4.3) is briefly expressed by a dotted linear line as shown in Fig. 8 in a narrow frequency range which actually required the compensation. Accordingly, the formula (4.3) is expressed as follows: T(s) = A₀ + A₁ sτ₁ Referring back to Fig. 6, the amplifier 10 has a gain A₀ , the differential circuit 40 has a differential coefficient (a time constant) τ₁, and the amplifier 20 has a gain A₁. The adder 30 adds the first term component and the second term component in the formula (7). In Fig. 6, the amplifiers 10 and 20 and the adder 30 can be easily formed by an LSI technology. The differential circuit 40 can be formed by a conventional differential circuit consisting of a capacitor C1 and a resistor R1, as shown in Fig. 9. Note, that the differential coefficient τ₁ is constant, and accordingly, a variation of the capacitance of the capacitor C1 is not required. The gain A₁ of the amplifier 20 should be varied in response to a length of the cable, and can be easily carried out in a conventional manner. The capacitance of the capacitor C1 in Fig. 9 may be approximately several tens pico Farad (pF). In Fig. 9, the amplifiers 10 and 20 and the adder 30 can be easily formed in a single LSI chip, but the differential circuit 40 is provided outside of the LSI chip. Figure 10 shows a change of the total gain of the AGC amplifier shown in Fig. 9 by changing the gain A₁ of the amplifier 20. Note, that the gain A₁ is changed in Fig. 10, but the capacitance of the capacitor C1 in the differential circuit 40 is not changed. By connecting the AGC amplifier shown in Fig. 9 in series, as for the circuits shown in Figs. 2 and 3, a desired gain characteristic can be obtained as shown in Fig. 4. Figure 11 shows a circuit diagram of another embodiment of the AGC amplifier of Fig. 6, and corresponds to Fig. 9. In Fig. 11, the differential circuit 40 is formed by a well known constant resistance circuit. The constant resistance circuit comprises a capacitor C1 having a capacitance C1, resistors R1 to R3 each having a resistance R, and an inductor L1 having an inductance L1. In Fig. 11, L₁/C₁ = R², and a total impedance Z = R. Figure 12 is a circuit diagram of a specific AGC amplifier of the present invention. In Fig. 12, the amplifiers 10 and 20 shown in Fig. 9 or 11 are formed by differential-operation type amplifiers, and the adder 30 shown in Fig. 9 or 11 has a current adding points N1 and N2. For example, the amplifier 10 includes a pair of transistors Q11 and Q12, a constant current source CS10, and load resistors R11 and R12 connected to a power supply V CC . The AGC amplifier shown in Fig. 12 does not have a gain changing circuit for changing the gain A₀ and/or A₁ , but the change of the gain, for example, A₁ , can be easily realized by adding a gain changing circuit 21 consisting of control transistors and supplied with a gain control signal GC, as shown by dotted lines. In Fig. 12, the differential coefficient τ₁ of the differential circuit 40 is determined by the frequency f and expressed by the following relationship: 2πτ₁f = 1 When the frequency f is 32 MHz or Mb/s, the time constant τ₁ = 0.05 µS. To obtain the total gain characteristics as shown in Fig. 4, a predetermined number of the AGC amplifiers shown in Fig. 12 should be combined. Figure 13 is a circuit diagram of another specific amplifier used for the amplifiers 10 and 20. In Fig. 13, the amplifier includes gain control transistors Q1 to Q4, a pair of differential-operation transistors Q5 and Q6, a transistor Q8 for a constant current source, and load resistors RL1 and RL2. An input signal Vin is supplied to the bases of the transistors Q5 and Q6; a reference voltage Vref is supplied to the bases of the transistors Q1 and Q4; and a gain control signal Vagc is supplied to the bases of the transistors Q2 and Q3. In Fig. 13, an operational point compensation circuit consisting of a resistor Rm and a transistor Q7 is further provided. The operation of the operational point compensation circuit will be described. An output voltage Vo is expressed as follows: Vo = Vb - I1·Px·R1 where, Px denotes a gain change parameter, and R1 denotes a resistance of the load resistor RL1 or RL2. Vb = Va - Vbe where, Vbe denotes a base-emitter voltage of the transistor. Va = Vcc - (I1·(1-Px) + I2·(1-Px))Rm Substituting the relationship: I1 = I2 = I0/2 in the formula (8.3), an output operational point Vo1(DC) of an output Vo1 is expressed as follows: Vo1(DC) = Vcc - Vbe - I0·RI/2 As seen from the formula (9), the output operational point Vo1(DC) does not depend upon the gain parameter Px, but only upon a variation of the base-emitter voltage, and thus the amplifier shown in Fig. 13 is very stable. A gain of the amplifier shown in Fig. 13 is expressed by the following formula: G = I0·R1/(2·Vt(1+exp(Vid/Vt))) where, Vt = kT/q Vid = Vagc - Vref Figure 14 shows actual attenuation (cable loss) characteristics, and Fig. 15 shows actual compensation characteristics obtained by the AGC amplifier shown in Fig. 13. In Fig. 15, dotted lines LN1 and LN2 indicate an approximate compensation obtained by the time constant circuit. Curves GAIN-A. GAIN-B, and GAIN-C show total gains when the cable length is 500 m, 250 m, and 0 m, respectively.

Claims: An automatic gain control amplifier for compensating signal loss in a cable through which a PCM signal is passed, the signal loss being defined as a function of the square-root of a frequency of said PCM signal, having a total gain being proprotional to the square-root of the frequency of said PCM signal, comprising: a first amplifier (10) having a first fixed gain (A₀), receiving an input signal (IN) to be compensated and amplifying said input signal at said first fixed gain (A₀); a differential circuit (40) having a constant differential coefficient defined by said frequency, receiving said input signal (IN) and differentiating said input signal (IN) at said differential coefficient; a second amplifier (20) having a second fixed gain (A₁), receiving said differentiated input signal from said differential circuit (40) and amplifying said differentiated input signal at said second fixed gain (A₁), the gain of said second amplifier (20) thereby being controlled by said differential circuit (40) such that it changes linearly in a narrow frequency range (TRANSZONE) in which compensation is required; and an adding circuit (30) connected to said first and second amplifiers (10, 20), adding said amplified signals and generating an output signal (OUT) in which said cable loss is compensated. An automatic gain control amplifier according to claim 1, wherein said first amplifier (10) comprises a first gain change circuit for changing said first gain in response to a first gain control signal. An automatic gain control amplifier according to claim 1 or 2, wherein said first amplifier (10) comprises a differential-operation type amplifying circuit including transistors. An automatic gain control amplifier according to claim 3, wherein said first amplifier (10) comprises a first operation point compensation circuit for overcoming an affect of a variation of base-emitter voltages of said transistors in said first differential-operation type amplifying circuit. An automatic gain control amplifier according to claim 4, wherein said second amplifier (20) comprises a second gain change circuit for changing said second gain in response to a second gain control signal. An automatic gain control amplifier according to claim 3 or 5, wherein said second amplifier (20) comprises a differential-operation type amplifying circuit including transistors. An automatic gain control amplifier according to claim 6, wherein said second amplifier (20) comprises a second operation point compensation circuit for overcoming an affect of a variation of base-emitter voltages of said transistors in said second differential-operation type amplifying circuit. An automatic gain control amplifier according to claim 7, wherein said first and second amplifiers (10, 20) are formed by a large scale integrated circuit. An automatic gain control amplifier according to claim 8, wherein said differential circuit comprises a capacitor and a resistor, said capacitor and said resistor giving said differential coefficient. An automatic gain control amplifier circuit according to any one of the preceeding claims, comprising a plurality of said automatic gain control amplifiers connected in series to give a continuous-compensation characteristic.

Inventors: IBUKURO SADAO, IBUKURO, SADAO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: H04B3
IPC codes: H04B3, H03H11