Electronic circuit

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

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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 the characteristic impedance. This 
means that the terminating impedance is mainly formed by the 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 
brined 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 the 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 that 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 the 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 other aspects of the invention will be apparent from and 
elucidated with reference to the embodiments described hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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 the 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 the invention. The supply 
voltages are denoted Vcc and Vee, respectively. The signal coming in over 
the transmission line is referenced signin. This signal is applied to the 
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 the transmission line 
termination and the reference potential there is a series combination of 
an active impedance 6 and a passive impedance 7 to control the signal 
behavior. 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 the output impedance of a common collector stage. The common 
collector stage consists of setting resistors R1, R2 and R3 and parallel 
transistors Q1, Q2, Q3, Q4. The terminating impedance is formed by the sum 
of the resistance Rmatch and the output impedance of the common collector 
stage. The output impedance of the common collector stage has a small 
value as is widely known. For low frequencies this output impedance may be 
approached by the inverted value of the sum of the transconductance gm of 
the employed transistors Q1, Q2, Q3, Q4. 
A frequently used characteristic impedance of transmission lines is 50 
Ohms. This implies that the terminating impedance is to be 50 Ohms too to 
have a proper termination of such a transmission line. Since the output 
impedance of the common collector stage decreases at high frequencies, it 
is necessary to select the resistance Rmatch of the same order of 
magnitude as the characteristic impedance of the transmission line. The 
greater part of the terminating impedance is then formed by the resistance 
Rmatch. The remaining part is formed by the output impedance of the 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 the output impedance of the common collector stage. This capacitive 
part is caused by the junction capacitances and diffusion capacitances of 
the transistors Q1, Q2, Q3, Q4. The value this capacitive part exactly has 
depends on the employed types of transistors and the configuration used. 
At low frequencies the terminating impedance is equal to the sum of the 
value of the resistance Rmatch and the 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 the 
resistive part and the capacitive part of the active impedance plotted 
against frequency. It is observed that the impedance ratio drops to about 
1 at high frequencies. 
The resistive part of the active impedance Rout is approximately equal at 
low frequencies to the inverted value of the sum of the transconductances 
of the transistors Q1, Q2, Q3, Q4 and can thus be matched by varying the 
collector current flowing through these transistors. This property may be 
employed for readjusting the terminating impedance. This may be necessary, 
because the value of the resistance Rmatch, if manufactured on-chip, is 
not exactly defined. 
The DC voltage at the transmission line termination and thus also the DC 
voltage at other locations in the circuit may be set as desired. They 
depend on the ratio of setting resistance R1 to R2. 
The circuit according to the invention can also be used in a balanced 
version for even better performance. This is shown in FIG. 2b. The 
components R3', R4', Rmatch', Q1', Q2', Q3', Q4', Q5' shown in the left 
portion have the same value as the corresponding components shown in the 
right portion R3, R4, Rmatch, Q1, Q2, Q3, Q4, Q5. The signal invsignin 
applied to the left portion of the circuit shows a 180.degree. phase 
difference with the signal signin applied to the right portion. 
FIG. 4 shows a second embodiment of the circuit according to the 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 RS, 
R6 and Rshunt. When the resistances R5 and R6 and Rshunt are properly 
selected, the input impedance of the shunt stage is low, as is widely 
known. By selecting the resistance Rmatch of the same order of magnitude 
as the characteristic impedance of the transmission line, there is 
achieved that the terminating impedance is mainly formed by this 
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 the invention. 
In this embodiment the active impedance 6 is the input impedance of a 
shunt stage. The shunt stage is formed by transistor Q7 and resistors R7, 
R8 and Rshunt. If the setting impedances R7, R8 and Rshunt are properly 
selected, the output impedance of the shunt stage will be low. By 
selecting the resistance Rmatch of the same order of magnitude as the 
characteristic impedance of the transmission line, there is achieved that 
the terminating impedance is mainly formed by this resistor Rmatch. 
Different configurations from those shown in FIGS. 2a, 4 and 5 are 
possible. In general there may be observed that the configurations in 
which a small active impedance is combined in series with a passive 
impedance which lies in the neighbourhood of the characteristic impedance, 
lead to a favourable behaviour at high frequencies. 
FIG. 6a shows simulations of the signal reflection coefficient S11 of the 
circuit shown in FIG. 2a and the circuit as used in Japanese Patent 
Application 60-102011 plotted against frequency. The dashed curve denotes 
the curve of the circuit shown in FIG. 2a, the solid line denotes the 
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 both circuits is set to 50 Ohms. It 
is noticeable that the signal reflection in the circuit according to the 
invention remains below -15 dB up to about 14 GHz, whereas the reflection 
in the circuit shown in Japanese Patent Application 60-102011 already 
starts exceeding this value at approximately 2.5 GHz. 
FIG. 6b shows simulations of the signal reflection coefficient S11 of the 
circuit shown in FIG. 2a and the 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 the curve of the circuit shown 
in FIG. 2a, the dashed curve denotes the curve of the 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 that the 
signal reflection in the circuit according to the invention continues to 
be -15 dB up to about 14 GHz, whereas the reflection in the circuit 
according to Hauenschild and Rein exceeds this value for the first time 
already at about 8 GHz.