TERMINATING TRANSMISSION LINE IMPEDANCE-MATCHING CIRCUIT

Abstract: In order to avoid signal reflections, a transmission line (3) connected to an electronic circuit (4) is terminated by a terminating impedance (5). The value of the terminating impedance (5) is to match in the best possible way in the frequency zone of the circuit (4) with the value of a characteristic impedance of the transmission line (3) to be terminated. The invention provides an electronic circuit (4) in which the terminating impedance (5) is a series combination of an active impedance (6) and a passive impedance (7). The passive impedance (7) is of the same order of magnitude as the characteristic impedance. The terminating impedance (5) matches well with the characteristic impedance of the transmission line (3) even at high frequencies. By modifying the settings of the active impedance (6), both the value of the terminating impedance (5) and the DC voltage on a transmission line termination (p) can be set.

Description: TERMINATING TRANSMISSION LINE IMPEDANCE-MATCHING CIRCUIT The invention relates to an electronic circuit comprising a terminating impedance for terminating a transmission line connectable to the circuit, in which in a frequency range of the circuit the terminating impedance is matched with a characteristic impedance of the transmission line. The matching of the terminating impedance with the characteristic impedance is necessary to avoid signal reflections on the transmission line. The electronic circuit is preferably realised in the form of an integrated circuit. An electronic circuit as defined in the opening paragraph is known from Japanese Patent Application 60-102011. In the circuit shown in that document the terminating impedance consists of an active impedance. The active impedance is formed by the output impedance of a common collector stage. The circuit shown does not properly function for high frequencies, because then the output impedance of the common collector stage and thus the terminating impedance is not accurately determined as a result of parasitic influences. Consequently, the terminating impedance is no longer well-matched with the characteristic impedance and strong signal reflections occur. It is an object of the invention to realise an electronic circuit as defined in the opening paragraph, which causes less strong signal reflections than the known circuit when a transmission line is connected to the circuit at high frequencies. The electronic circuit according to the opening paragraph is thereto characterized, in that the terminating impedance is a series combination of an active impedance and a passive impedance while the passive impedance is of the same order of magnitude as die characteristic impedance. This means that the terminating impedance is mainly formed by me passive impedance and to a smaller extent by the active impedance. At high frequencies it is especially the active impedance that is not accurately determined as a result of capacitances present, whereas the passive impedance retains a fairly constant value especially so if the latter consists of a single on-chip resistor. As a result, the terminating impedance in the electronic circuit according to the invention is better matched with the characteristic impedance of the transmission line at high frequencies than in the electronic circuit discussed in cited Japanese Patent Application. Therefore, the signal reflections occurring when the circuit according to the invention is used at high frequencies is less strong than when the prior-art circuit is used. An electronic circuit in which the terminating impedance as a whole is formed by a passive impedance which consists of a single resistor is known from "Influence of Transmission-Line Interconnections Between Gigabit-per-Second IC's on Time Jitter and Instabilities" by J. Hauenschild and H.M. Rein, IEEE Journal of Solid-State Circuits, Vol. 25, No. 3, June 1990, pp. 763-766. The resistor is inserted between the collector and the base of a transistor which forms part of a buffer stage located at the input of the electronic circuit. The drawback of this circuit is that the DC voltage at the point where the transmission line is connected to the electronic circuit, also termed d e transmission line termination, cannot be set at will but depends on the D.C. voltage on a reference potential, usually the positive supply voltage, to which the resistor is connected. This means mat the DC voltage at other locations in the circuit cannot be set at will either, but depends on the DC voltage on the reference potential. In the circuit according to the invention the DC voltage at the transmission line termination can be set indeed by a selection of the settings of the active impedance. This results in greater flexibility for setting the DC voltage at points in the remaining part of the circuit. Furthermore, the terminating impedance in the known circuit, if an on- chip resistor is used, is not very accurate, which may result in enhanced reflection coefficients. In the circuit according to the invention the terminating impedance may be accurately readjusted, as required, by readjusting the active impedance. This is possible by modifying the setting of these impedances. Finally, the known circuit operates less well for high frequencies than the circuit according to die invention. Since the DC voltage between the collector and the base of the buffer transistor is small, the transistor junction capacitance present between the collector and base has a relatively large value, so that this makes the value of the terminating impedance decrease at high frequencies. These and od er apsects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows a printed circuit board comprising two integrated electronic circuits connected by a transmissin line, Fig. 2a shows an electronic circuit according to die invention, Fig. 2b shows the electronic circuit according to die invention wid balanced operation, Fig. 3a shows what elements mainly determine the terminating impedance. Fig. 3b shows me impedance ratio of me resistive pan and capacitive part of die active impedance plotted against frequency, Fig. 4 shows a second embodiment for me circuit according to e invention, Fig. 5 shows a third embodiment for the circuit according to die invention and Fig. 6a shows a simulation of me signal reflection coefficient of the circuit, shown in Fig. 2a and of the circuit shown in cited Japanese Patent Application, plotted against frequency, and Fig. 6b shows a simulation of the signal reflection coefficient of the circuit shown in Fig. 2a and of me circuit according to "Influence of Transmission-Line Interconnections Between Gigabit-per-Second IC's on Time Jitter and Instabilities" by J. Hauenschild and H.M. Rein, IEEE Journal of Solid-State Circuits, Vol. 25, No. 3, June 1990, pp. 763-766. Fig. 1 shows a printed circuit board 1 comprising two integrated electronic circuits 2, 4 connected by a transmission line 3. The transmission line is formed by a microstrip line. To avoid signal reflections, a terminating impedance 5 is inserted at d e end of the transmission line between a transmission line termination p and a reference potential ref. The terminating impedance is to match substantially with the characteristic impedance of the transmission line in a frequency range of the circuit. Fig. 2a shows an electronic circuit according to die invention. The supply voltages are denoted Vcc and Vee, respectively. The signal coming in over me transmission line is referenced signin. This signal is applied to die further part of the circuit (not shown) via a buffer transistor Q5 which is set by means of a resistor R4. This further part may consist of logic gates, amplifier stages and so on. Between d e transmission line termination and me reference potential there is a series combination of an active impedance 6 and a passive impedance 7 to control the signal behaviour. The passive impedance 7 is preferably formed by a single resistor Rmatch as is shown in the drawing Figure. The active impedance is formed by die output impedance of a common collector stage. The common collector stage consists of setting resistors Rl, R2 and R3 and parallel transistors Ql, Q2, Q3, Q4. The terminating impedance is formed by me sum of the resistance Rmatch and die output impedance of the common collector stage. The output impedance of me common collector stage has a small value as is widely known. For low frequencies this output impedance may be approached by d e inverted value of me sum of the transconductance gm of the employed transistors Ql, Q2, Q3, Q4. A frequently used characteristic impedance of transmission lines is 50 Ohms. This implies tiiat the terminating impedance is to be 50 Ohms too to have a proper termination of such a transmission line. Since the output impedance of die common collector stage decreases at high frequencies, it is necessary to select the resistance Rmatch of the same order of magnitude as me characteristic impedance of die transmission line. The greater part of the terminating impedance is then formed by die resistance Rmatch. The remaining pan is formed by me output impedance of me common collector stage. Fig. 3a shows what elements mainly determine the terminating impedance. There is a frequency -dependent capacitor part C(f) combined in parallel with the frequency-dependent resistive part Rout(f) of die output impedance of die common collector stage. This capacitive part is caused by d e junction capacitances and diffusion capacitances of the transistors Ql, Q2, Q3, Q4. The value this capacitive part exactly has depends on die employed types of transistors and die configuration used. At low frequencies die terminating impedance is equal to the sum of the value of the resistance Rmatch and die resistive part of the output impedance Rout. The capacitive part causes the terminating impedance to drop at high frequencies. Fig. 3b shows the impedance ratio of me resistive part and me capacitive pan of the active impedance plotted against frequency. It is observed that die impedance ratio drops to about 1 at high frequencies. The resistive pan of the active impedance Rout is approximately equal at low frequencies to the inverted value of die sum of the transconductances of d e transistors Ql, Q2, Q3, Q4 and can dius be matched by varying the collector current flowing through these transistors. This property may be employed for readjusting die terminating impedance. This may be necessary, because die value of die resistance Rmatch, if manufactured on-chip, is not exactly defined. The DC voltage at the transmission line termination and ius also the DC voltage at other locations in the circuit may be set as desired. They depend on me ratio of setting resistance Rl to R2. The circuit according to die invention can also be used in a balanced version for even better performance. This is shown in Fig. 2b. The components R3\ R4\ Rmatch', Ql', Q2\ Q3', Q4', Q5' shown in the left portion have the same value as die corresponding components shown in the right portion R3, R4, Rmatch, Ql, Q2, Q3, Q4, Q5. The signal invsignin applied to die left portion of the circuit shows a 180° phase difference widi die signal signin applied to die right portion. Fig. 4 shows a second embodiment of the circuit according to die invention. In this embodiment the active impedance 6 is the active impedance of a shunt stage. The shunt stage is formed by transistor Q6 and resistors R5, R6 and Rshunt. When die resistances R5 and R6 and Rshunt are properly selected, d e input impedance of d e shunt stage is low, as is widely known. By selecting the resistance Rmatch of the same order of magnimde as d e characteristic impedance of the transmission line, there is achieved that the terminating impedance is mainly formed by mis resistance Rmatch. This results in the fact that, in analogy with the circuit shown in Fig. 2a, also the circuit shown in this Figure has a terminating impedance which remains reasonably constant at high frequencies. Fig. 5 shows a third embodiment for the circuit according to die invention. In d is embodiment die active impedance 6 is me input impedance of a shunt stage. The shunt stage is formed by transistor Q6 and resistors R5, R6 and Rshunt. If die setting impedances R7, R8 and Rshunt are properly selected, die output impedance of the shunt stage will be low. By selecting the resistance Rmatch of the same order of magnitude as die characteristic impedance of die transmission line, there is achieved diat the terminating impedance is mainly formed by mis resistor Rmatch. Different configurations from those shown in Figs. 2a, 4 and 5 are possible. In general diere may be observed diat die configurations in which a small active impedance is combined in series widi a passive impedance which lies in die neighbourhood of die characteristic impedance, lead to a favourable behaviour at high frequencies. Fig. 6a shows simulations of me signal reflection coefficient Sl l of the circuit shown in Fig. 2a and die circuit as used in Japanese Patent Application 60-102011 plotted against frequency. The dashed curve denotes die curve of die circuit shown in Fig. 2a, d e solid line denotes die curve of the circuit from the Japanese Patent Application. A transmission line having a characteristic impedance of 50 Ohms is assumed. The low- frequency terminating impedance in bo circuits is set to 50 Ohms. It is noticeable that die signal reflection in me circuit according to die invention remains below -15 dB up to about 14 GHz , whereas die reflection in the circuit shown in Japanese Patent Application 60- 102011 already starts exceeding diis value at approximately 2.5 GHz. Fig. 6b shows simulations of me signal reflection coefficient Sl l of the circuit shown in Fig. 2a and die circuit as used in "Influence of Transmission-Line Interconnections Between Gigabit-per-Second IC's on Time Jitter and Instabilities" by J. Hauenschild and H.M. Rein, IEEE Journal of Solid-State Circuits, Vol. 25, No. 3, June 1990, pp. 763-766, plotted against frequency. The solid curve denotes d e curve of die circuit shown in Fig. 2a, the dashed curve denotes the curve of die circuit according to Hauenschild and Rein. A transmission line having a characteristic impedance of 50 Ohms is assumed. The low-frequency terminating impedance is set to 50 Ohms in the two circuits. There can be observed diat the signal reflection in the circuit according to die invention continues to be -15 dB up to about 14 GHz, whereas die reflection in the circuit according to Hauenschild and Rein exceeds diis value for die first time already at about 8 GHz.

Claims: CLAIM: 1. Electronic circuit (4) comprising a terminating impedance (5) for terminating a transmission line (3) connectable to the circuit (4), inwhich in a frequency range of die circuit (5) the terminating impedance (4) is matched widi a characteristic impedance of me transmission line (3), characterized in that the terminating impedance (4) is a series combination of an active impedance (6) and a passive impedance (7) while die passive impedance (7) is of die same order of magnitude as die characteristic impedance.

Inventors: HAGERAATS JOHANNES JACOBUS EMI, HAGERAATS, JOHANNES, JACOBUS, EMILE, MARIA
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H03H11
IPC codes: H03H7, H03H11, H03K19, H04B3, H04L25