Active networks and signalling equipment

An active network incorporating a gyrator, having a first and a second operational amplifier each said operational amplifier having an inverting input and a non-inverting input, the inverting inputs of said first and said second operational amplifiers being directly connected, and wherein the outputs of said first and said second operational amplifiers are resistively connected to the inverting inputs of said operational amplifiers, an input terminal is connected to said non-inverting input of said first operational amplifier which said input terminal is connected resistively to said output of said second operational amplifier, said output of said first operational amplifier is capacitively coupled to said non-inverting input of said second operational amplifier and said non-inverting input of said second operational amplifier is resistively connected to earth.

This invention relates to active networks and signalling equipment. The 
invention is particularly concerned with the use of simulated inductance 
in tone signal receivers in which the tone signal is used for switching 
purposes. 
According to the present invention there is provided an active network 
incorporating a gyrator, having a first and a second operational amplifier 
each said operational amplifier having an inverting input and a 
non-inverting input, the inverting inputs of said first and said second 
operational amplifiers being connected, and wherein the outputs of said 
first and said second operational amplifiers are resistively connected to 
the inverting inputs of said operational amplifiers, an input terminal is 
connected to said non-inverting input of said first operational amplifier 
and said input terminal is connected resistively to said output of said 
second operational amplifier, said output of said first operational 
amplifier is capacitively coupled to said non-inverting input of said 
second operational amplifier and said non-inverting input of said second 
operational amplifier is resistively connected to earth. 
An output terminal may be connected to the output of either said first or 
said second operational amplifier.

Referring now to FIG. 1 the active network gyrator circuit includes a pair 
of operational amplifiers 1 and 2 each of which has two inputs and an 
output. The non-inverting input of the amplifier 1 is connected directly 
to an input terminal 3 and by way of a resistor 4 to the output of the 
amplifier 2. The non-inverting input of the amplifier 2 is connected by 
way of a resistor 5 to earth and by way of a capacitor 6 to the output of 
the amplifier 1. 
Resistors 7 and 8 are connected between the outputs of the amplifiers 1 and 
2 and their inverting inputs respectively. The inverting inputs to the 
amplifiers 1 and 2 are also directly interconnected. 
In operation the circuit simulates an inductance between the terminal 3 and 
earth. Terminal 3 is directly connected to the amplifier 1, and for 
suitable values of the components the voltage of the output of the 
amplifier 1 is related to the voltage at point 3 in a useful manner, so 
that an output signal may be taken from the output of the amplifier 1 to 
an output terminal 9 (or for similar reasons from the output of amplifier 
2). 
By taking an output from the amplifier 1 it eliminates the need for a 
buffer amplifier to be connected to the terminal 3. This circuit is used 
in signalling equipment to discriminate between a tone signal of 2,280 Hz 
and speech signal. 
Part of the signalling equipment containing the frequency discrimination 
circuit in shown in FIG. 2. Referring now to FIG. 2, the circuit includes 
a tuned circuit formed by a capacitor and simulated inductance in series. 
The simulated inductance comprises a gyrator circuit similar to that 
described with reference to FIG. 1 and the integers of this gyrator 
circuit are given the same reference numerals as similar integers shown in 
FIG. 1. The resistor 5 of FIG. 2 comprises a fixed resistor 5' and a 
variable resistor 5". The gyrator- capacitor circuit of FIG. 2 is coupled 
to an input amplifier circuit 10 by way of a capacitor 11. The input 
amplifier 10 receives input signals from a line 12 by way of a transformer 
13 having a secondary winding 14 directly connected to the base of a 
transistor 15. The collector of the transistor 15 is connected to a 
positive supply line 16 by way of a resistor 17 and the emitter of the 
transistor 15 is connected to earth by way of a series pair of resistors 
18 and 19 the junction between which is coupled to earth by way of a 
decoupling capacitor 20. The base bias for the transistor 15 is obtained 
by way of the winding 14 from the junction point between resistors 21 and 
22 which are connected in series between the supply line 16 and earth. The 
output from the input amlifier circuit 10 which is coupled to the 
capacitor 11, and thence to the gyrator circuit, is derived by way of a 
capacitor 23 and a resistor 24. The output from the gyrator at terminal 9 
is connected by way of a resistor 25 and a capacitor 26 in series to a 
rectifying network formed by diodes 27 and 28. The rectified output from 
the diode 28 is integrated by means of a capacitor 29 and resistors 30 and 
31 and is then applied to an amplifier 32. 
The amplifier circuit 10 has three functions; these are: 
i. to provide isolation between the receiver and the speech path, 
ii. to provide the correct driving resistance, and 
iii. to have gain such that a specified range of signal levels at the input 
from the line 12, (i.e. 3dBm to -18dBm,) is transformed to the optimum 
range of voltage levels for the amplifier-rectifier circuits. 
The driving resistance for the tuned circuit is the parallel combination of 
resistors 17 and 24. The coupling capacitor 23 appears in series with the 
capacitor 11 of the tuned circuit and has the effect of making the 
frequency of the signal having a minimum voltage at the junction of 
capacitor 11 and resistor 24, which signal can be used as a "guard" or 
"inhibit" signal in some circumstances, slightly lower than the frequency 
of the signal having a maximum voltage across the simulated inductance. 
This difference of frequency has little affect on the operation of the 
equipments since, in practical embodiments, the effective Q of the tuned 
circuit plus driving resistance may be very low (about 4). The gain of the 
amplifier circuit 10 may be set so that, for the range of input levels 
+3dBm to -18dBm, the output voltage of the amplifier 1 falls within the 
range defined by overload of the amplifier 2 at the high level and by 
forward voltage of the rectifiers 27 and 28 at the lower level. The 
amplifier circuit 10 is coupled to the line 12 via a 2 : 1 ratio winding 
on the input transformer 13 but the change in input impedance has a 
negligible effect on the input return loss of the signalling equipment. If 
desired, a buffer amplifier without an input transformer may be used. In 
such circumstances the single transistor amplifier circuit 10 could be 
replaced by a balanced differential input stage using two transistors 
connected directly across the line 12. 
The inductance in the series tuned circuit is simulated by the gyrator 
circuit. It can be shown that if the resistors 4, 5, 7 and 8 have the 
resistance values R4, R5, R7 and R8 respectively, and if the capacitance 
of the capacitor 6 is C then the impedance of the circuit is j.omega.C. 
R4. R5. R7/R8 which is equivalent to an inductance with one terminal 
earthed. 
If the value of the resistor R8 is made equal to the value of any of the 
resistors R4, R5 or R7 then the value of the simulated inductance L is 
equal to: C.R4.R5 (assuming that R7 = R8). If the simulated inductance L 
is connected in a series tuned circuit with a capacitor having a value 
C.sub.S, then the resonant frequency of the tuned circuit is given by: 
1/2.tau..sqroot.C.sub.S CR4 R5. 
It is convenient to make all the resistor values nominally equal, say to R; 
the inductance value is then CR.sup.2 and if this is turned by a series 
capacitor which has the value C.sub.S, the resonant frequency is 1/2.tau. 
R.sqroot.C.sub.S.C. 
Therefore it is possible to minimize the effects of temperature changes 
causing variations of the resonant frequency by using resistors and 
capacitors with equal and opposite temperature coefficients. The 
capacitors 6 and 11 should have low losses; polystyrene-dielectric 
capacitors are suitable. To obtain the "operate" signal to be fed to the 
following circuit, it is necessary to derive a voltage proportional to the 
voltage across the simulated inductance (the voltage at the terminal 3) 
without loading it and reducing the Q of the tuned circuit. Such a voltage 
is available from the output terminal 9, or from the output of the 
operational amplifier, as the outputs from both amplifiers are low 
impedance they can be loaded without significantly affecting the tuned 
circuit. It can be shown that the output voltage of the amlifier 1 is 
(V-V/j.omega.CR) where V is the voltage across the simulated inductance 
(at the terminal 3). 
Gyrator circuits of the type shown in FIG. 1 have low sensitivity to small 
changes in the values of the components. The Q of the simulated inductance 
is generally high, and can be positive or negative depending upon the 
distribution and magnitude of the stray capacitance. 
Care must be taken in the layout of the circuit to keep the capacitive 
coupling between the components of the circuit low, especially from the 
junction of the resistors 7 and 8 to earth or to either of the amplifier 
output terminals. The effect of stray capacitance is worse when the values 
of the resistances are high; therefore the resistance values should be low 
consistent with a reasonably low value for the capacitor 6 and an 
acceptable effective load resistance for the amplifier. In a practical 
circuit, using 33 kilohm resistors for each of the resistors R4, R5, R7 
and R8 and 0.005 pF capacitors for the capacitor C6 the Q is high (about 
600) and is not too sensitive to strays. Suitable amplifiers for the 
differential input amplifiers 1 and 2 are sold by several manufacturers 
under the general type and one specific example is made by Motorola under 
the type number MC 1741CG. 
It will be appreciated that the gyrator is also applicable to other 
arrangements requiring simulated inductance. In some tone signal receivers 
it is necessary to attenuate an unwanted tone, in order that the operation 
of the detection equipment shall not be impaired. In particular, in 
telephone signalling systems, dial tone may be present. A tone signal 
receiver may have to operate in the presence of dial tone, but normal dial 
tone has harmonic components extending into the signalling band used for 
multi-frequency telephone tone signalling. The dial tone will have been 
filtered at source to attenuate harmonics above 650 Hz to a level which 
will not affect the multi-frequency signalling. 
There is shown in FIG. 3 a dial tone filter in which the components of dial 
tone up to 650 Hz can be removed by a high pass filter which incorporates 
the gyrator circuit shown in FIG. 1. The criteria used in the design of 
this dial tone filter are that the attenuation in the pass band should not 
vary by more than 0.5 dB from 680 Hz to 1670 Hz and all the components of 
dial tone should be attenuated to below -30 dBm. The circuit arrangement 
is based on an equally terminated LC filter with the inductance replaced 
by the gyrator circuit of FIG. 1. In FIG. 3 the integers of the gyrator 
circuit are given the same reference numerals as similar integers shown in 
FIG. 1. 
The signals, including dial tone, are applied to an input port between an 
input terminal 42 and earth. The signals at the terminal 42 are coupled by 
way of resistor 44 to the filter composed of capacitors 41, 45 and 40 and 
the gyrator circuit of FIG. 1. The filter output from the capacitor 45 is 
applied to an emitter-follower buffer amplifier composed of a resistor 47 
and a transistor 48. A resistor 46 provides the correct terminating 
impedance for the filter circuit and bias for the base of the transistor 
48. 
In practical realisation of this circuit it is advantageous if the 
resistances of the resistors have preferred values. 
Component values used in a circuit for filtering standard United Kingdom 
dial tone were: 
resistor 44 = 75 kilohms and 110 kilohms in parallel; 
resistor 46 = 110 kilohms and 160 kilohms in parallel; 
resistor 47 = 51 kilohms; 
resistor 4 = 36 kilohms; 
resistor 8 = 39 kilohms; 
resistor 7 = 36 kilohms; 
resistor 5 = 36 kilohms; 
capacitor 41 = 1.5nF; 
capacitor 45 = 3nF; and 
capacitor 40 = 13nF. 
the frequency response of the dial tone filter of FIG. 3 is shown 
graphically in FIG. 4, where line 50 shows the response against the normal 
scale on the left-handside of the diagram and 51 shows the response on an 
expanded scale on the right-hand side of the diagram. A further 
application for the active network circuit in FIG. 1 is in a tuned circuit 
and detector in a tone signalling receiver. 
Referring now to FIG. 5 there is shown a frequency-discrimination circit 
incorporating the gyrator circuit of FIG. 1 which can detect a given 
frequency. A tuned circuit 80 includes a capacitor 55 and an inductance 
simulated by the gyrator circuit of FIG. 1. In FIG. 5 similar integers are 
given the same reference numerals as in FIG. 1. A resistance 56 is 
provided between the input terminal 57 and the capacitor 55 in order to 
provide the correct input impedance. A tuned circuit 80 is coupled via its 
output terminal 9 to a detector circuit. A differential input operational 
amplifier 60 is used as the detector because it has good discrimination 
and variations of the input offset voltage are small. The inverting input 
of the operational amplifier 60 is connected to the output of operational 
amplifier 2 and the non-inverting input is connected to a reference 
voltage V.sub.ref, which is about 7V. The resonant response at the 
terminal 9 is shown graphically in FIG. 6. The output of the amplifier 60 
is coupled by way of a diode 58 to a junction 59 between a resistor 61 and 
a capacitor 69 connected in series between a +15 volt supply line 66 and 
earth. The junction 59 is connected via a resistor 67 to the input of an 
output trigger circuit 81 having transistors 64 and 65 as the active 
components. The transistors 64 and 65 are NPN transistors and have their 
commoned emitters connected to a -15 volt supply by way of a resistor 68. 
The collectors of the transistors 64 and 65 are coupled to the +15 volt 
supply line 66 by way of resistors 62 and 63 respectively. The output of 
the trigger circuit 81 is coupled to an output circuit 82 via resistor 70 
to the base of a transistor 71 which provides, from its collector 
electrode, the output signal of the tone signalling receiver. The output 
circuit 82 is coupled to the output line by a matching resistor 72 between 
earth and the collector of the transistor 71 which is connected via a 
resistor 73 to the line 66. 
In operation, while the output voltage from the operational amplifier 2 is 
less than V.sub.ref, the output of operational amplifier 60 remains at 
about +13V, which reverse biases diode 58 and allows capacitor 69 to be 
charged to about +0.5V from the +15V supply via resistor 61. 
When the capacitor 69 is charged to +0.5V, transistor 64 is conducting and 
transistor 65 is non-conducting and the common emitter voltage is 2.9V. 
Thus, the output transistor 71 is cut off and, due to the proportioning of 
the resistors 72 and 73, is passed to logic circuit (not shown) from the 
output terminal 74. The voltage at terminal 9 is approximately 2 times the 
voltage at terminal 3. When the output voltage from the operational 
amplifier 2 becomes greater than V.sub.ref, as it does at positive peaks 
of the frequency to which the tuned circuit is sensitive, the output of 
operational amplifier 60 becomes negative and capacitor 69 is rapidly 
charged to about -13V. This causes transistor 64 to be cut off and 
transistor 65 to be saturated. This causes the common emitter voltage to 
rise to +2.6V, causing output transistor 71 to be saturated and thus pass 
a voltage close to zero (earth) to the terminal 74 and hence the logic 
circuit. Between the positive peaks of the signal output from the 
operational amplifier 2, the output of operational amplifier 60 becomes 
positive and capacitor 69 charges towards the potential of the +15V 
supply line 66 +15V resistor 61. The charging time constant given by the 
product of capacitance of capacitor 69 and the resistance of resistor 61 
is arranged to give a 3.5 to 4.5ms delay before the trigger circuit 81 
changes state. This delay bridges the gap between positive peaks of the 
output voltage from operational amplifier 2 and enables the receiver to 
ignore short breaks of up to about 3ms in the tone signal input. 
In one practical circuit the following component values were used: 
resistor 56 = 2 kilohms; 
resistors 
4, 7 and 8 = 36 kilohms; 
resistor 61 = 510 kilohms; 
resistor 62 = 15 kilohms; 
resistor 63 = 12 kilohms; 
resistor 73 = 15 kilohms; 
resistor 72 = 7.5 kilohms; 
resistor 67 = 100 kilohms; 
resistor 68 = 10 kilohms; 
resistor 70 = 47 kilohms; and 
capacitor 69 = 0.01 F. 
the capacitance of capacitors 55 and 6 and the resistance of the resistor 5 
are determined by the resonant frequency to which it is desired to tune 
the circuit. 
Although the component configuration is critical, it will be appreciated 
that there are many further applications for the gyrator circuit of FIG. 
1, and those described in this specification are intended as examples of 
the use of a simulated inductance inductance in telephone signalling 
equipment.