Low Noise Electronic Circuit, Transducer Using the Same, and Method

The present invention relates to low noise electronic circuitry and more particularly to such circuitry for use with transducers.

All electronic circuit components internally modulate electrical signals producing internally generated noise due to some finite value of random internal electronic motion. This characteristic is more pronounced in active circuit components and places a minimum noise level on signals emanating from circuits using semi-conductor elements. Additional difficulty has been experienced in the past with circuitry provided in electro-mechanical transducers, such as the circuitry disclosed in U.S. Pat. No. 3,109,145. Such a circuit requires feedback means for acceptable operation because the active circuit components have widely varying gain characteristics caused by changes in ambient temperature and physical variation within a component group. The active components have limiting signal levels to which they may be exposed and beyond which they suffer permanent structural damage. The resulting functional variations can only be controlled within the narrow operating limitations by providing feedback within input circuit to closely control the amount of drive at the inpu to maintain normal operating levels within the circuit.

Many circuits operating in conjunction with electro-mechanical transducers provide a signal proportional to the transduced quantity which is detected and amplified by a high gain amplifier. A minute amount of noise modulation by the active circuit components is amplified by a large factor; for example, 1,000. Amplifier output as a consequence provides an erroneous indication of the quantity being monitored.

Whenever requirements arise calling for the conversion of the deflection of a moving system into an electrical output with an accompanying wide band width of flat response, the physical displacement must be by design small, so that the natural frequency of the mechanical system will be high. In such cases, the noise level of the transducer and its electronics becomes a limiting factor in system performance. Obviously a need exists for a low noise electronic circuit independent of such internal noise modulation for use with transducers of the above type.

One embodiment of the low noise electronic circuit and method for use with transducers has an AC source, the output of which is used to drive variable Q pickoff coils located each in two separate signal channels. The pickoffs are series tuned to resonance to provide for a maximum signal to noise ratio. A moving member is disposed within a magnetic field between the pickoffs and is displaced due to the effect of a quantity being measured. Output from the two pickoffs is converted to DC signals of opposite polarity which are smoothed in filters and presented to a summing network wherein AC source internally generated noise transmitted through each of the two signal channels as common mode signals cancels due to the opposite polarity and identical amplitude. Noise levels lower than those produced internally in the AC source are presented at the output of the summing network. Pickoff output induced by the effect of the transduced quantity causing the moving member to be displaced within the magnetic field between the pickoffs appears in the two signal channels as differential mode signals but of like polarity due to the inversion of one of the DC signals and is summed and presented noise free at the output of the summing network.

In general it is an object of the present invention to provide an electronic circuit and method for use with a transducer which is independent of the noise produced internally in the AC source.

Another object of the invention is to provide an electronic circuit and method for use with a transducer of the above character which provides low output noise without a feedback path to control the AC source amplitude.

Another object of the invention is to provide an electronic circuit and method for use with a transducer of the above character which combines noise signal cancellation with AC source amplitude control feedback to obtain the ultimate in low output signal noise.

Another object of the invention is to provide an electronic circuit and method for use with a transducer of the above character which provides high signal to noise ratio by using high pickoff drive signals limited only by pickoff coil power dissipation capability.

Another object of the invention is to provide an electronic circuit and method for use with a transducer of the above character which limits the noise in the two signal channels to coherent or synchronous noise only so that it may be cancelled directly in the summing means.

Additional objects and features of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.

FIG. 1 is a block diagram of the low noise electronic circuit for use with transducers without AC source control feedback.

FIG. 2 is a block diagram of the low noise electronic circuit with identically connected detectors and AC source control feedback.

FIG. 3 is a schematic diagram of the low noise electronic circuit with identically connected detectors and AC source control feedback.

FIG. 4 is a block diagram of the low noise electronic circuit with reverse connected detectors and AC source control feedback.

FIG. 5 is a block diagram of the low noise electronic circuit without AC source control feedback and shown in association with a closed servo loop.

FIG. 6 is a schematic diagram of the low noise electronic circuit without AC source control feedback and shown in association with a closed servo loop.

In describing the preferred embodiments certain terminology will be used which has gained wide acceptance with persons skilled in this field of art. Reference to "active devices" as used herein includes any device where an equivalent circuit of the device would include an energy source. Such devices would include electrical energy generators, semiconductor devices, and vacuum tubes. An input noise characteristic for an active device is defined as the input noise that would appear at the output for a gain through the device of unity using a conventional noise test circuit. Common mode signals are synchronous signals arising from a common source. Differential mode signals are synchronous signals which display a differential change in amplitude at their respective sources. The term "elements" when used herein in reference to active components such as semiconductor devices or vacuum tubes, refers to the number of separate terminals carried by the active device in the manner of the accepted vacuum tube terminology. The term "pickoff means" refers to a general transducer element. The term "pickoff coils" refers to wire wound transducer coils which are the generating means for electrical signals proportional to a quantity being measured.

As seen in FIG. 1 the most general configuration of the low noise electronic circuit described herein consists of an AC excitation source 10 and a pair of spaced pickoff means 11 and 12 excited by the AC source 10 and providing output to two separate signal channels. A moving element 13 is disposed between the pickoffs 11 and 12 and is capable of modifying the magnetic field existing between them producing differential output signals from the pickoffs. Detector means 14 and 15 convert the AC output signals into DC signals in the signal channels and filter circuits 17 and 19 smooth the detected DC levels. The two rectified and smoothed signals from the pickoffs are presented from each of the two signal channels to a summing network 22 which combines the signals, cancelling noise appearing as common mode signals and summing the differential mode signals representing the summation as an output proportional to the displacement of the moving element.

Referring to FIG. 2 there is a block diagram of one embodiment which illustrates the cooperation between an AC source, a transducer movable element, the low noise circuit, and feedback to control AC source amplitude. In this embodiment the noise signals appearing as common mode or synchronous signals are maintained as signals of like polarity through the choice of configuration for the detectors. This requires a summing of the common mode signals from the two signal channels for the feedback to control AC source amplitude and a differencing of the signals from the two channels to cancel common mode signal at the circuit output which then exists as the sum of the differential mode signals alone. The diagram of FIG. 2 shows an AC source 10 which excites both pickoffs 11 and 12 in first and second signal channels with a moving element 13 disposed between them. The pickoffs 11 and 12 are preferably, but not necessarily, the resonant type disclosed in U.S. Pat. No. 3,321,753. One signal channel exists in which the pickoff 11 is connected to a detector 16 which in turn provides an input to filter 17. A second channel includes pickoff 12 connected to a detector 18 which in turn supplies an input to filter 19. Detectors 16 and 18 are configured to provide like polarity output signals. Matched filters 17 and 19 provide smoothed detected inputs to a summing network 23 and a differential amplifier 24. Summing network 23 is, in this embodiment, a preamplifier which sums the common mode current from the two signal channels and directs it to a feedback amplifier 25 for controlling the output level of AC source 10. Amplifier 25 is provided with a control reference level 27 which defines the amplitude at which the AC source 10 is regulated. The differential amplifier 24, sensitive only to differential mode signals from the two channels, provides the circuit output signal, free of coherent or synchronous noise due to its common mode rejection characteristic.

The circuit configuration represented in the block diagram of FIG. 2 may be seen in FIG. 3. The AC souce 10 is comprised of a tank circuit containing a capacitor C1 and an inductance L1. Inductance L1 is the primary winding on a transformer T1. The tank circuit is connected between a positive voltage source and the collector of a transistor Q1. The base of transistor Q1 is driven by a winding L2 on transformer T1. The tank circuit, transistor Q1, and winding L2 comprise an oscillator which excites separate secondary windings 28 and 29 on transformer T1. A pickoff coil 30 is in turn excited by secondary winding 28. In this embodiment the output from secondary winding 29 is scaled by resistor R4 to provide an output equivalent to the output from coil 30 when the quantity to be measured is in its zero state. The signal channels are generally balanced for zero input state, in some fashion such as this. A dummy pickoff coil may be utilized in lieu of resistor R4.

Pickoff coil 30 is series tuned to resonance by a capacitor C4 thereby amplifying the excitation provided by secondary winding 28. The amount of drive delivered to pickoff coil 30 by the excitation from secondary winding 28 and the resonance obtained through series tuning with capacitor C4 is limited only by the power dissipation capabilities of the coil 30 itself.

The AC signal from coil 30 is coupled to the detector 16 through a capacitor C5. The scaled AC signal from secondary winding 29 is coupled to the detector 18 through a capacitor C6. Detector 16 and 18 are connected in identical fashion to provide signals of like polarity which are smoothed in the two matched filters represented by the capacitor-resistor combinations C7-R5 and C8-R6.

The summing network 23 consists of transistors Q5 and Q6. Transistor Q5 receives the reference channel output from detector 18 at its base connection. Transistor Q6 receives the signal channel output from detector 16 at its base connection. Low noise components are required for transistors Q5 and Q6 since they amplify the desired output signal and the component input noise characteristic as well.

The equivalent noise level at the input of the differential amplifier 24 is the square root of the sum of the squares of the two major noise components. The gain factor and input noise characteristic of preamplifier 23 produce a product which is one noise component. The input noise characteristic of the differential amplifier 24 is the other. The input noise of the summing network 23 is amplified, along with the desired signal, by the gain of Q5 and Q6. Assuming the input noise characteristics of the summing network components times the selected gain of summing network 23 is greater in amplitude than the input noise characteristic of the differential amplifier 24, the latter's input noise becomes effectively determined by the predominating amplified input noise level of the summing network 23.

Common mode currents arriving through the signal channel and the reference channel are synchronous in nature and of like polarity due to the connection of detectors 16 and 18 as mentioned above. These currents are summed as the emitter currents from Q5 and Q6 and are utilized to drive a feedback amplifier 25, represented by transistor Q7, which thereby provides negative feedback to the base of transistor Q1 for controlling the amplitude of oscillator 10. A zener diode D5 provides a control reference level for transistor Q7 which determines the constant amplitude at which the oscillator 10 operates. Any tendency for the oscillator 10 to change amplitude causing common mode level variation in the signal channels is reduced by the gain of the feedback loop containing transistor Q7. The output from the differential amplifier 24 is shown driving a torque motor 31 which servos the movable element 13. An output signal may be taken across a precision resistor R13 in series with the winding of the torque motor 31.

Turning now to FIG. 4 a block diagram is shown which differs from FIG. 2 in that the detectors are connected to provide outputs of opposite polarity. The AC source 10, the pickoff coils 11 and 12, and the movable element 13 are the same as those shown in FIG. 2. A detector 33 in the upper channel and a detector 34 in the lower channel are configured to provide outputs of opposite polarity. The ripple is removed from the detected signals in filters 17 and 19 as before. Since the signal channels now produce signals of opposite polarity, it is necessary to utilize a differential amplifier 35 to sum the common mode signals now appearing in differential form from the two channels. The summed common mode signal is passed to feedback amplifier 25 associated with reference voltage 27 in the feedback path for controlling the output level of AC source 10. Differential mode signals appearing as signals of like polarity are summed in a resistive network 36 wherein common mode noise signals appearing as signals of opposite polarity are cancelled providing a noise free output.

Referring to FIG. 5 a block diagram is presented which depicts another embodiment of the invention. This embodiment of the low noise circuit does not incorporate feedback to control the AC source amplitude. Noise suppression obtained through coherent or synchronous noise signal cancellation in the summing network is sufficient for the purpose in this case, yielding, for example, an effective transducer input noise of 1 nanoinch RMS for a 2OOHz band width, the transducer having a 6 milli-inch linear range. The circuit is shown in conjunction with a servo loop to clearly illustrate the cooperation of the circuit with a transducer in a typical application. The AC source 10 operates into a first channel including pickoff coil 11 detector 33, and filter 17 and into a second channel including pickoff 12, detector 34, and filter 19. Moving element 13 is disposed between the pickoffs 11 and 12 as described above. Resistive summing network 36 receives the outputs from the filters 17 and 19 in the first and second channels providing an output which is delivered to an amplification and response circuit 37 which governs system response characteristics. The amplification and response section 37 is optional, depending entirely on desired output characteristics. The output from summing network 36 is connected to a torque motor 31 which servos the moving element 13. A precision resistance R13 connected to ground in series with the torque motor 31 provides an output voltage by virtue of the current from the torque motor 31 passing through it.

The embodiment displayed in block form in FIG. 5 is shown in the schematic diagram of FIG. 6 for purposes of explaining the operational characteristics. An AC source 10 similar to the type shown in FIG. 3 provides the drive for transformer T1. Secondary winding 40 provides the drive for both signal channels in this embodiment. Similar pickoff coils 11 and 12 are placed in each of the signal channels and are series tuned to resonance by selection of capacitors C6 and C8 respectively. The drive to the pickoffs 11 and 12 from the combination of excitation from winding 40 and series tuning is limited only by the power dissipation capabilities of the pickoff coils 11 and 12. The production of relatively high AC voltages on pins 2 and 4 through tuning of the pickoffs 11 and 12 presents the advantage of high scale factor without using active components with internal noise modulation characteristics for amplification in the signal channels.

The AC voltages appearing at pins 2 and 4 are coupled to detectors 33 and 34 through capacitors C5 and C10. Detectors 33 and 34 are configured as described before to provide output signals of opposite polarity which are in turn smoothed by the filter capacitors C7 and C9. The smoothed DC output signals are then summed in the resistive network 36 providing the basic output from the system. Resistor R2 is in series with the parallel combination of resistors R3 and R4. Resistor R5 is in series with the foregoing combination. The R2, R3, and R4 combination is provided to afford adjustment in the null signal to offset electronic inbalance in the circuit components and/or minor mechanical offset.

Common mode signals from internally or externally generated noise at the AC source 10 or nominal variations in amplitude thereof will appear in the signal channels as coherent or synchronous signals. The detecting configuration performs an inversion in one channel rendering the common mode signals of opposite polarity. These signals of substantially identical magnitude, but of opposite polarity, cancel in the resistive summing network 36. On the other hand variations in voltage levels at pins 2 and 4 caused by mechanical displacement of the moving element 13 appear as differential mode signals rendered of like polarity due to the aforementioned inversion in one of the signal channels. A summing of the differential mode signals in the network 36 provides a DC output voltage proportional to the actual displacement of the movable element 13 from null position.

The signal proportional to the displacement of movable element 13 causes a current to flow through torque motor 31 which also flows through a sampling resistor represented by components R9, R15, R18 and a bias and scale factor temperature compensation network 43. Torque motor 31 acting on moving element 13 serves to close the servo loop to provide a well known force balance configuration. An output voltage is sensed across the sampling resistor to provide a signal proportional to the transduced quantity. Bias and scale factor temperature compensation circuit 43 is of the type disclosed in copending patent application Ser. No. 476,063 filed June 3, 1974.

The optional circuit 37 for providing desired system response characteristics may be inserted between the output of the summing circuit 36 and the torque motor 31. Circuit 37 contains an amplifier A1 with associated gain and frequency response determinative feedback paths as seen in FIG. 6. In this embodiment the feedback path contains resistor R6 in parallel with the series combination of resistor R14 and capacitor C12. Such a network provides for gain proportional to R6 at DC, rolling off from the first corner frequency. Flat response is provided above the second corner frequency proportional to the parallel combination of R6 and R14 up to the response limit of the amplifier A1.

The system as shown in FIG. 6 does not incorporate a common mode feedback path because the circuit is almost entirely passive. The significance of this is that the oscillator 10, if it is self regulating within 10% to 20%, containing the only active component other than the diodes D1 through D4, introduces only non-coherent or asynchronous noise into the signal channels. The feature allowing the signal channels to be driven arbitrarily hard, limited only by the power dissipation capabilities of the pickoffs 11 and 12, removes the factor compelling the use of feedback to keep the oscillator 10 at a finely regulated constant operating point. Such nominal variations of oscillator 10 cancel in the summing circuit 36 as common mode signals.

The embodiments described herein which utilize diode connections in the detectors providing Dc voltages of opposite polarity provide common mode cancellation and differential mode summing directly in the passive summing network 36. The advantage obtained in this configuration is that the output of one pickoff is reversed and added to the other, eliminating the need for an active inverting component which would represent a potential asynchronous noise source when contained independently in one signal channel only. When the ultimate in low noise is required it becomes immaterial whether the detectors are configured to provide outputs of opposite or identical polarity from the first and second signal channels. In such a situation where the output of the oscillator 18 is caused to vary by internal or external causes, a feedback loop containing an additional amplifier 25 is directed to the oscillator 10. An improvement on the order of 2 to 1 in signal to noise ratio is obtained when AC source feedback is implemented due to the reduction in residual output noise by virtue of source amplitude control.

When the detector diodes are connected in identical fashion the common mode current from the two channels may be combined in a summing network similar to that represented by the differential pair Q5 and Q6 in FIG. 3 and fed back to the Ac source 10 to control the common mode oscillation. When the detectors are connected in reversing form, the detector outputs from the two signal channels must be combined in a differential amplifier and fed back to the Ac source 10 to control the common mode oscillations in the signal channels. In the former case it is necessary to use a differential amplifier 24 to obtain differential mode output from the circuits since the desired system output signal is in differential form. In the latter case summing in a passive resistive network 36 is sufficient to obtain the desired system output since the differential output signals have been rendered of the same polarity by the detectors 33 and 34.

The low noise circuit of FIG. 6 has the possibility of good low-noise performance, despite the fact that the signal is attenuated by 6 db, since all components are passive in form between the AC source 10 and the output of the summing network 36. The only non resistive noise generator is the diode network containing junctions D1 through D4 which raises the circuit noise only slightly over the Johnson noise level which represents the lowest achievable noise in the input network. Such noise generator signals are no longer in balanced form in the two channels since independent rectifiers are used. In one embodiment the output at the summing circuit is 0.2 volts or 200,000 microvolts per milli-inch (mil) of travel of element 13. One milli-inch being a typical displacement of element 13 for full design input level for this transducer, and equivalent noise levels below one microvolt being attainable, it is seen that noise equivalent to something less than 5 .times. 10.sup..sup.-9 inch mechanical displacement results. As a matter of comparison one Angstrom unit is 4 .times. 10.sup..sup.-9 inches.

When the output of summing circuit 36 is amplified the input noise characteristic of the operational amplifier is approximately one microvolt. This is equivalent to five parts in one million as a noise level for the same scale factor. Prior art scale factors and noise levels display typical noise levels of 250 parts in one million. The circuit disclosed herein has provided an improvement of approximately 50 to 1 reduced noise level in the worst case where output amplification is required.

The circuit of FIG. 3 is capable of up to 3 decibels better signal-to-noise ratio because of its active, differential amplifier summing of signals, instead of the passive summing described above. In addition, the AC source feedback removes oscillator noise as a potential noise source.

As stated earlier, the pickoff means are not necessarily wire wound inductive pickoff coils. They may be the variable capacitance elements operated in a resonant circuit and driven arbitrarily hard, limited only by the power dissipation capability of the passive circuit elements. Similarly, the detector means are not necessarily diode rectifier circuits. A phase sensitive detector may be utilized which would either synchronously detect the relative amplitudes in the first and second signal channels to provide an analog of the transduced quantity or provide an output proportional to the relative phases of the signals in the first and second signal channels to provide an analog of the transduced quantity. Pickoff means and detector type combinations which are known to those skilled in the art are included in the disclosure set forth herein.

A low noise electronics circuit for use with transducers is provided which produces a noise level which is independent of noise produced internally in the AC excitation source 10 and wherein large excitation amplitude variation may occur without affecting output noise. The noise in the two signal channels is limited to coherent or synchronous noise signals so that it may be cancelled in a summing network. A low noise signal is available without feedback control of the amplitude of oscillator 10. Should the ultimate in low noise be desired, an improvement may be obtained through the use of feedback for AC source amplitude control. Since residual noise in the circuit is substantially constant, driving the signal generator coils hard provides an improved signal to noise ratio. Only the coil power dissipation limits the coil drive where inductive coil type pickoffs are used.