GB1603530A - Filtering circuit - Google Patents

Description

(54) FILTERING CIRCUIT (71)We, WESTINGHOUSE AIR BRAKE COMPANY, a Corporation organised and existing under the Laws of Pennsylvania, of Pittsburgh, Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a filtering circuit and in particular concerns a fail-safe electronic filtering circuit.

In a typical speed control system for either a railroad or a mass and/or rapid transit operation, a band-pass filter would generally form a part of the vehicle-carried or car-borne apparatus. For example, in a standard cab-signalling speed control system the cab signals are received from the rails and are fed to the cab-signalling car-carried receiver for processing. By comparing the desired-speed command signal with the actual vehicle speed signal produced by an axle driven generator, it is possible to determine whether a vehicle is proceeding at the appropriate authorised speed for any given section of track. In such operations, it is mandatory that any overspeed condition be immediately detected and that the necessary measures, such as braking, be instituted to correct the situation.A further requirement of such operation entails that under no circumstance should a critical circuit or component failure simulate a true condition. Thus, each and every vital portion including filtering circuits of the vehicle-carried apparatus must operate in a fail-safe manner.

Further, since a separate filtering circuit is employed for detecting each of the different frequencies of the various speed command signals, it is required that each filter only respond to a signal having its particular frequency. That is, the filter should not be capable of passing signals having frequencies other than the signal of the pre-selected or particular frequency. Such operation is necessary in order to insure that it is impossible to produce an erroneous output which may simulate a less restrictive speed command than the actual speed command signal being received from the rails.

In prior art filtering circuits. passive types of tuned L-C networks were extensively employed as band-pass filters for selecting the particular frequency of an incoming signal which represented the authorised speed command of a moving vehicle in a railroad or mass and/or rapid transit operation. These previous passive types of tuned circuits are generally acceptable and operate satisfactorily when employed in high frequency signal applications.

However, when the frequencies of the speed command signals approach the lower end of the electro-magnetic spectrum. the L-C networks become expensive in cost, bulky in size, and heavy in weight. Thus, each of these adverse factors obviously detracts from the continued usage of passive L-C networks in vehicular speed control systems utilising low frequency command signals. While a number of attempts have been made in an effort to solve this low frequency problem, none of these previous endeavours have been completely successful in every aspect which is demanded by vital types of speed control systems.For example, while prior types or electronic filters were relatively inexpensive, small and light-weight and were generally adequate for non-vital applications, these previous active filters are usually considered completely unacceptable for use in mass and/or rapid transit systems in that a component or circuit failure could result in an unsafe and hazardous condition. For example, previous types of electronic filter circuits generally employed a negative feedback which could result in an unsafe failure if and when degeneration is lost due to opening of the feedback loop since the active element becomes a high gain stage for all signal frequencies. In speed control systems, it is of utmost importance to exercise extreme care in safely designing and constructing filtering circuits in order to preclude injury to persons and to prevent damage to the equipment.That is, in order to insure the highest degree of safety to individuals as well as to apparatus, it is important that under no circumstances should a failure cause or be capable of simulating a true or valid indication.

Accordingly, it is self evident that the filtering circuits, like every other portion or network of the speed control system, should operate in a fail-safe manner so that any conceivable failure will result in a condition at least as restrictive and preferably more restrictive than that preceding the failure. For example, when a circuit malfunction or component failure occurs in the filter, it is important that no output be produced during the presence of a false input or presence of an input of another frequency, and it is also important that no output be produced during the absence of an input. Thus, it will be appreciated that an acceptable filtering circuit should operate in a fail-safe manner so that the integrity and security of the speed control system is maintained at all times, and then and only then can all eminently hazardous and dangerous conditions be avoided.Further, it will be recognised that the electrons filtering circuit should not be adversely affected by variations of fluctuations that occur in the d.c. supply source. That is, the vital filter should exhibit a high immunity to inherent noise and ripple present in the supply voltage. It is highly advantageous from an economical as well as an operational standpoint to encompass the immunity circuitry as an integral part of the filter rather than provide external ancillary circuitry to accomplish the desired result.

For example, it has been found that the filtering circuit of U.S.A. Patent No. 3725802 could be adversely affected by fluctuations or ripple voltages which are normally present in the d.c. supply source so that, for a vehicle-speed control system, it would be necessary to employ ancillary apparatus or circuits to remove the ripple voltage which could result in irregular operation. In a vehicular speed control system, the low code rate or frequency of the speed command signals produces noise in the power supply which can result in fluctuation of the d.c. bias point. Thus, it is desirable to provide a circuit in order to stabilise and improve the operation of the electronic filter.

According to the invention there is provided an electronic filtering circuit comprising an amplifier circuit having a gain greater than unity, a feedback circuit including an attenuating twin-T network connected to the amplifier circuit to form a feedback amplifier loop wherein the twin-T network is nulled to provide regenerative feedback at a preselected frequency and degenerative feedback at all other frequencies, a filter circuit input connected to the feedback amplifier loop so as to introduce thereinto an excitation voltage such that the filtering circuit produces an output signal when and only when a signal at the input has the preselected frequency, a constant current source connected between a d.c.

supply voltage and the amplifier circuit and, a biasing network for the constant current source including voltage breakdown means for providing immunity to fluctuations in the d.c. supply voltage.

By way of non-limiting example, one embodiment of this invention will now be described with reference to the accompanying drawing which is a schematic circuit diagram of a fail-safe electronic filtering circuit arrangement embodying the present invention.

The illustrated circuit provides a fail-safe electronic low frequency band-pass filtering circuit including a common-emitter transistor amplifier having a gain greater than unity and including a feedback circuit extending from the collector electrode to the base electrodes of the amplifying transistor. The feedback circuit includes a twin-T network and a pair of Darlington connected transistors. The twin-T network is an unbalanced symmetrical circuit made up of a plurality of resistors and capacitors which provide a 180 degree phase shift at a preselected signal frequency. The transistors of the Darlington circuit are connected in an emitter-follower configuration so that no signal inversion occurs and less than unit gain is exhibited.The attenuation of the twin-T network along with the less than unity gain and the inherent losses occurring in the emitter-follower Darlington circuit offset the greater than unity gain of the transistor amplifier so that unwanted spurious oscillations cannot be produced during the absence of a signal having the preselected frequency. The unbalancing effect causes the twin-T network to be imperfectly nulled at the preselected frequency and ensures that all other signal frequencies are not phase inverted and therefore degeneration occurs at the other signal frequencies. The twin-T network also operates as the collector load impedance for the common-emitter transistor amplifier. A constant current source including a PNP transistor and a voltage regulating device is connected to the common-emitter transistor amplifier. The voltage regulating device, namely, a Zener diode, regulates the d.c. supply potentials applied to the NPN transistor which causes a constant current to be fed to the collector electrode of the transistor amplifier. Thus, the biasing and operating potentials are independent of the d.c. supply voltage so that a high immunity to ripple voltage is exhibited by the filter. A relatively high input resistor and its particular circuit connection insures that substantial signal losses will occur at any input signal during a critical circuit or component failure. Thus, the below-described and illustrated fail-safe filter will only pass an input signal having the preselected frequency and will only produce an output signal during the presence of an input signal having the preselected frequency and in the absence of a critical circuit or component failure.

In describing the circuit in detail, let us assume for the purpose of convenience that the circuit parameters and characteristics of the electronic band-pass filter 1 have been chosen or selected such that the circuit will only pass a signal having a frequency of two (2) hertz.

Accordingly. an output signal should appear across output terminals 2a and 2b when and only when a two (2) hertz signal is present on input terminals 3a and 3b and a critical circuit or component failure is not present in the circuit. As shown, a high impedance resistor R1 interconnects the input terminal 3a to junction J1 of a parallel or twin-T resistancecapacitance network TT. The twin-T circuit TT is preferably an unbalanced symmetrical network consisting of resistors R2, R3, and R4 and capacitors C1, C2, and C3. As shown, resistor R2 and capacitors C2 and C3 form one-Tee of the network T,T, while capacitor C1 and resistors R3 and R4 form the second-Tee of the network TT.In viewing the drawing, it will be noted that the upper end of resistor R2, namely, junction J1, is directly connected to the junction J2, the common point between the right and left plates of capacitors C2 and C3, respectively, while the lower terminal of resistor R2 is connected to the common lead L1 extending between terminals 2b and 3b. Likewise, the upper plate of capacitor C1 is connected to the junction J3 which is the common point between the right and left ends of resistors R3 and R4, respectively, while the lower plate of capacitor C1 is connected to the common lead L1.The remote or left plate of capacitor C2 and the remote or left end of resistor R3 are connected together at junction J4, and likewise the remote or right plate of capacitor C3 and the remote or right end of resistor R4 are connected together at junction J5.

As mentioned above, the twin-T network TT is symmetrical in that parameters of capacitor C2 and capacitor C3 are equal, and the resistors R3 and R4 have identical values.

Further, in the present case, the parallel-T network TT is unbalanced from the standpoint that resistive value of resistor R2 is not equal to or a whole multiple of resistances R3 or R4 and that capacitive value of capacitors C2 or C3 is not a factor of capacitance C1. It has been found that a twin-T network having these parameters or values has a unique characteristic that at the center frequency, namely, two (2) hertz, the signal undergoes a 180 degree phase shift in passing from the common junction J5 to the common junction J4 While the twin-T network T,T itself will pass signals of other frequencies, the network will provide a phase shift which is less than 180 degrees and in fact less than 90 degrees.At zero and infinity frequencies, a zero phase shift takes place while at all other frequencies the phase angle follows a rising a decaying curve exponential toward and from + 90 degrees as the frequencies approach and recede from the center frequency. Thus, the unique phase inversion of a nulled twin-T network permits its usage in a positive feedback type of band-pass amplifier circuit, as will be described presently.

As will be more readily apparent hereinafter, the band-pass filter 1 includes a single voltage gain stage TA comprising an active amplifying element, namely, NPN transistor Q1. As shown, the NPN transistor Q1 includes an emitter electrode el, a collector electrode c, and a base electrode bl. The collector electrode cl of transistor Q1 is directly connected to the common junction J5 of capacitor C3 and resistor R4 while its emitter el is connected to the common lead L1 through resistor R5. The base electrode bl of transistor Q1 is connected to the junction point J formed between the common ends of resistors R6 and R7.

The remote or lower end of resistor R6 is connected to the common lead L1 while the remote or upper end of resistor R7 is connected to the output of a two stage Darlington circuit configuration DC. The Darlington configuration includes a pair of cascaded NPN transistors Q2 and Q3, each having an emitter, a collector and a base electrode. As shown, the output emitter electrode e2 of transistor Q2 is connected to the upper end of resistor R7 remote from junction point J while the collector electrode c2 of transistor Q2 is directly connected to the positive terminal +V of a suitable source of d.c. supply voltage, such as, a rectified power supply (not shown). The base electrode b2 of transistor Q2 is directly connected to the emitter e3 of transistor Q3.As shown, the collector electrode c3 of transistor Q3 is also directly connected to the positive terminal +V of the power supply voltage. The base electrode b3 of transistor Q3 is electrically connected via resistor R8 to the common junction J4 of capacitor C2 and resistor R3. Thus, a feedback loop or path is provided from the output collector electrode cl to the input base electrode bl of the transistor amplifying stage TA. In effect, the feedback circuit path comprises the twin-T network TT, the resistor R8, the base and emitter electrodes of transistors Q2 and Q3 of the Darlington circuit DC and the resistor R7.

As shown, a voltage regulated current source CS including transistor Q4 is arranged to provide a constant current to the collector electrode cl of transistor Q1 as well as a constant biasing and operating voltage which is independent of normal power supply fluctuations. It will be seen that the collector electrode c4 of transistor Q4 is directly connected to collector electrode cl of transistor Q1. The emitter electrode e4 of transistor Q4 is connected to the positive voltage terminal +V via resistor R9. The base electrode b4 of transistor Q4 is connected to a voltage dividing network including a voltage regulating device, e.g. Zener diode Z1, and a resistor R11.As shown, the base electrode b4 of transistor Q4 is connected to the junction point J6 to provide the necessary biasing and supply voltages for transistor Q4. It will be seen that the use of the Zener diode Z1 in the voltage divider results not only in a constant current but also causes the output bias voltage to have a fixed value independent of normal power supply fluctuations.

Let us assume that the gain of the transistor Q4 is relatively large, namely, B > 100, so that the following assumptions can be made: If, i2 > > iln then it follows that, i2=I.

If we then neglect the base-emitter junction voltage drop, then, I ~ VZ/R9, where VZ is the Zener breakdown voltage, and, R6 1 i2 = Vo ( ) x (-), R7 + R6 R5 where Vo is the output voltage at junction J5 and by substituting we get, R6 1 VZ Vo ( ) x (S R7 + R6 R5 R9 and therefore, VZ (R7 + R6) R5 Vo ~ - R6 R9 thereby demonstrating that Vo is essentially independent of the variations which are present on the power supply voltage terminal +V.

As shown in the drawing, the output terminal 2a is directly connected to the collector electrode cl of transistor Q1 and a smoothing or filtering capacitor C4 is connected across both of the output terminals 2a and 2b.

In describing the operation, it will be initially assumed that the input signal appearing across terminals 3a and 3b is at center frequency, namely, two (2) hertz, and that the band-pass filtering circuit 1 is intact and operating in a proper manner. It will be appreciated that in addition to increasing the amplitude of the 2 hertz input signal the common-emitter gain transistor Q1 inverts the incoming signal so that the amplified output signal appearing on collector electrode cl is 180 degrees out of phase with the signal applied to base electrode bl. Thus, the 180 degree phase shift of the common-emitter configuration along with the 180 degree phase shift produced by the twin-T network TT insure that positive feedback occurs at the preselected center frequency, in this case, two (2) hertz.

Accordingly the center frequency signal appearing across terminals 3a and 3b is reinforced by regenerative feedback and the gain of the amplifier transistor Q1 insures that a sufficient level of output voltage is produced across terminals 2a and 2b.

Further, it will be appreciated that the gain of the feedback loop should be less than unity in order to preclude unwanted spurious oscillations to be produced during the absence of a two (2) hertz signal on input terminals 3a and 3b. That is, if sufficient degradation or attenuation does not occur in the feedback loop, the circuit could assume a condition of oscillations in which an erroneous output signal having a center frequency of two (2) hertz could appear across the output terminals 2a and 2b even during the absence of a two (2) hertz signal on terminals 3a and 3b. In order to preclude such unwanted oscillations, the gain of the feedback loop is designed to be less than unity. It will be noted that the loop gain is the sum of the gain of the twin-T network TT times the gain of the common-emitter amplifier times any other gain in the feedback loop. In practice, the gain of the amplifier Q1 is approximately 25 while the gain of the twin-T network TT is 1/15. The Darlington transistor circuit is arranged in an emitter-follower configuration so that the voltage gain is less than unity. The feedback signal is also reduced or attenuated by a factor of R6/R6 + R7 so that the overall gain of the loop is less than unity. It will be noted that the parallel twin-T network T,T is effectively the collector load impedance of transistor Q1 so that the gain of the common-emitter amplifier Q1 is the ratio of the input impedance of the twin-T network TT over the impedance of the emitter resistor R5.

Let us now assume that the two (2) hertz input signal is no longer applied to input terminals 3a and 3b. Under this condition, the off-center frequency signal undergoes a 180 degree inversion due to the common-emitter amplifier transistor Q1. However, the twin-T network TT fails to invert the off-center frequency signal so that the feedback signal effectively opposes the signal appearing on terminals 3a and 3b. Thus, degeneration or negative feedback occurs at all frequencies other than the center frequency, namely, two (2) hertz. Accordingly, an erroneous output is incapable of being produced by the presence of signals other than the two (2) hertz signal on input terminals 3a and 3b.

As previously mentioned, the presently described active electronic band-pass filter circuit operates in a fail-safe manner in that no critical circuit or component failure is capable of producing a false output across terminals 2a and 2b. It will be appreciated that the Zener diode Z1 is unable to cause an unsafe condition when it fails since opening or shorting of the Zener diode Z1 results in the removal of the necessary biasing and operating voltage from the transistor Q4 which in turn causes the filter to stop operating which is a safe condition.

Further, the input signals are assured of being attenuated a given amount so that subsequent amplification is necessary in order to produce any appreciable amount of output on terminals 2a and 2b. Further, an open-circuited or short-circuited circuit of any other element will either destroy the necessary circuit amplifying characteristics or derange the d.c. biasing conditions of the amplifier. If the gain amplifier transistor Q1 becomes either open-circuited or short-circuited, the amplifying characteristics of the stage are destroyed so that the circuit losses cannot be overcome. An open condition of transistor Q4 interrupts the current flow to the collector of transistor Q1. The opening of transistors Q2 and Q3 interrupts the feedback loop so that no feedback signal is available for the amplifying transistor Q1.The opening of resistor R1 completely removes any input signal to the band-pass filter circuit 1. The opening of other elements either interrupts the feedback loop or removes the input signal and the necessary d.c. biasing to the fail-safe bandpass filter circuit 1. Further, the use of positive feedback ensures that the opening of the feedback loop causes degeneration rather than regeneration which would be the case if negative feedback was employed. That is, negative feedback would allow regeneration at all signal frequencies and thus would result in an unsafe condition during an open circuit failure.As an additional safeguard, the resistors employed in the circuit are preferably constructed of a carbon composition which will insure that these elements are incapable of becoming short-circuited, and the circuit is meticulously designed and laid out to ensure that leads in proximity of each other are incapable of touching each other to create a short circuit. With these safeguards and other precautionary measures being taken, it will be noted that no circuit or component failure is capable of producing an erroneous output across terminals 2a and 2b.

It will be apparent from the foregoing that the illustrated circuit provides an active band-pass filter including a transistorised constant current source having a Zener diode included in the biasing network for providing a high immunity against fluctuations in the d.c. supply voltage and including a low frequency selective transistorised amplifier circuit having a feedback loop which is nulled to a particular frequency so that an output signal is produced when and only when an input signal having the particular frequency is present and no critical component or circuit failure exists.

It will also be apparent from the foregoing that the above-described exemplary embodiment of this invention provides a fail-safe electronic filtering circuit in the form of a vital type,of low frequency band-pass filter employing an amplifier circuit having a feedback loop which includes a parallel-T network and employing a constant current source having a voltage breakdown device for providing immunity to fluctuations in the d.c. supply voltage.

The illustrated embodiment operates as a fail-safe self-regulating solid-state filter for passing a signal having a certain frequency and for blocking signals having all other frequencies; and employs a twin-T network which is nulled to provide positive feedback at a preselected frequency, and a constant current circuit which is insensitive to variations in the source of d.c. supply voltage. The illustrated embodiment equally provides a semiconductive filter including a regenerative feedback amplifier employing a frequency selective network and a voltage regulating device to produce an output signal only during the presence of an input signal having a given frequency and in the absence of a critical component or circuit failure; and provide an electronic band-pass filtering circuit which operates in a fail-safe manner.The illustrated embodiment can take the form of a vital type of active filtering circuit or a transistor filter which is light in weight and small in size, and/or which is economical in cost, simple in construction, reliable in operation, durable in use and efficient in service.

While the illustrated embodiment of this invention has been described with reference to a vehicle speed control system for mass and/or rapid transit systems, it should be understood that the fail-safe electronic band-pass filter circuit may be used in other applications which require the vitality herein described. That is, it is readily evident that this invention is not limited to vehicle speed control systems but may be employed in other various systems and apparatus, such as logic circuitry which require the security and safety inherently present in this invention. In addition, it is considered that the presently described filter circuit may be used in any railroad, industrial, commercial as well as other environmental places where similar needs and conditions exist.

In addition, it is readily understood that the complementary type of the transistors may be employed in place of those shown and described by simply reversing the polarity of the d.c. supply voltage, as is well known. Further, it will be appreciated that the input signal may be applied to various other points in the circuit rather than being applied to the junction point J1. However, in changing the input point, it is necessary to insure that a readily accessible low impedance circuit path is not capable of being established between the input and output during certain types of failures. It is further understood that other values of the resistors and capacitor can be combined to make up the parallel or twin-T network TT which may be selected in accordance with the characteristics that are desired to be obtained.That is, nulling at a frequency of two (2) hertz may also be obtained by using other resistance and capacitance values, and nulling is also obtainable at other signal frequencies with other resistors and capacitors.

WHAT WE CLAIM IS: 1. An electronic filtering circuit comprising an amplifier circuit having a gain greater than unity, a feedback circuit including an attenuating twin-T network connected to the amplifier circuit to form a feedback amplifier loop wherein the twin-T network is nulled to provide regenerative feedback at a preselected frequency and degenerative feedback at all other frequencies a filter circuit input connected to the feedback amplifier loop so as to introduce thereinto an excitation voltage such that the filtering circuit produces an output signal when and only when a signal at the input has the preselected frequency, a constant current source connected between a d.c. supply voltage and the amplifier circuit and, a biasing network for the constant current source including voltage breakdown means for providing immunity to fluctuations in the d.c. supply voltage.

2. An electronic filtering circuit according to Claim 1, wherein said voltage breakdown means forms part of a voltage divider for biasing said amplifier.

3. An electronic filtering circuit as defined in Claim 1, wherein the constant current source includes a transistor and said voltage breakdown means biases the base-emitter junction of said transistor.

4. An electronic filtering circuit according to any preceding claim, wherein said voltage breakdown means comprises a semiconductive avalanche device.

5. An electronic filtering circuit according to any preceding claim, wherein said voltage breakdown means comprises a Zener diode.

6. An electronic filtering circuit according to Claim 5, wherein the Zener diode has its cathode electrode connected to the positive terminal of the d.c. supply voltage.

7. An electronic filtering circuit according to either one of claims 5 and 6 when dependent from Claim 3, wherein said Zener diode has its anode electrode connected to the base electrode of said constant current source transistor.

8. An electronic filtering circuit substantially as herein described with reference to and/or as shown in the accompanying drawing.

9. A speed control system for a vehicle, the system incorporating (preferably on the vehicle) at least one fail-safe electronic filtering circuit according to any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. of active filtering circuit or a transistor filter which is light in weight and small in size, and/or which is economical in cost, simple in construction, reliable in operation, durable in use and efficient in service. While the illustrated embodiment of this invention has been described with reference to a vehicle speed control system for mass and/or rapid transit systems, it should be understood that the fail-safe electronic band-pass filter circuit may be used in other applications which require the vitality herein described. That is, it is readily evident that this invention is not limited to vehicle speed control systems but may be employed in other various systems and apparatus, such as logic circuitry which require the security and safety inherently present in this invention. In addition, it is considered that the presently described filter circuit may be used in any railroad, industrial, commercial as well as other environmental places where similar needs and conditions exist. In addition, it is readily understood that the complementary type of the transistors may be employed in place of those shown and described by simply reversing the polarity of the d.c. supply voltage, as is well known. Further, it will be appreciated that the input signal may be applied to various other points in the circuit rather than being applied to the junction point J1. However, in changing the input point, it is necessary to insure that a readily accessible low impedance circuit path is not capable of being established between the input and output during certain types of failures. It is further understood that other values of the resistors and capacitor can be combined to make up the parallel or twin-T network TT which may be selected in accordance with the characteristics that are desired to be obtained.That is, nulling at a frequency of two (2) hertz may also be obtained by using other resistance and capacitance values, and nulling is also obtainable at other signal frequencies with other resistors and capacitors. WHAT WE CLAIM IS:

1. An electronic filtering circuit comprising an amplifier circuit having a gain greater than unity, a feedback circuit including an attenuating twin-T network connected to the amplifier circuit to form a feedback amplifier loop wherein the twin-T network is nulled to provide regenerative feedback at a preselected frequency and degenerative feedback at all other frequencies a filter circuit input connected to the feedback amplifier loop so as to introduce thereinto an excitation voltage such that the filtering circuit produces an output signal when and only when a signal at the input has the preselected frequency, a constant current source connected between a d.c. supply voltage and the amplifier circuit and, a biasing network for the constant current source including voltage breakdown means for providing immunity to fluctuations in the d.c. supply voltage.

2. An electronic filtering circuit according to Claim 1, wherein said voltage breakdown means forms part of a voltage divider for biasing said amplifier.

3. An electronic filtering circuit as defined in Claim 1, wherein the constant current source includes a transistor and said voltage breakdown means biases the base-emitter junction of said transistor.

4. An electronic filtering circuit according to any preceding claim, wherein said voltage breakdown means comprises a semiconductive avalanche device.

5. An electronic filtering circuit according to any preceding claim, wherein said voltage breakdown means comprises a Zener diode.

6. An electronic filtering circuit according to Claim 5, wherein the Zener diode has its cathode electrode connected to the positive terminal of the d.c. supply voltage.

7. An electronic filtering circuit according to either one of claims 5 and 6 when dependent from Claim 3, wherein said Zener diode has its anode electrode connected to the base electrode of said constant current source transistor.

8. An electronic filtering circuit substantially as herein described with reference to and/or as shown in the accompanying drawing.

9. A speed control system for a vehicle, the system incorporating (preferably on the vehicle) at least one fail-safe electronic filtering circuit according to any preceding claim.