EP0694246A1 - Procede et couplage pour la reduction de la distorsion harmonique d'un transducteur capacitif - Google Patents

Procede et couplage pour la reduction de la distorsion harmonique d'un transducteur capacitif

Info

Publication number
EP0694246A1
EP0694246A1 EP94913038A EP94913038A EP0694246A1 EP 0694246 A1 EP0694246 A1 EP 0694246A1 EP 94913038 A EP94913038 A EP 94913038A EP 94913038 A EP94913038 A EP 94913038A EP 0694246 A1 EP0694246 A1 EP 0694246A1
Authority
EP
European Patent Office
Prior art keywords
capacity
preamplifier
transducer
negative
coupling
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
EP94913038A
Other languages
German (de)
English (en)
Inventor
Erling Frederiksen
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Hottinger Bruel and Kjaer AS
Original Assignee
Bruel and Kjaer AS
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Bruel and Kjaer AS filed Critical Bruel and Kjaer AS
Publication of EP0694246A1 publication Critical patent/EP0694246A1/fr
Ceased legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R19/00—Electrostatic transducers
    • H04R19/04—Microphones
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R3/00—Circuits for transducers, loudspeakers or microphones
    • H04R3/04—Circuits for transducers, loudspeakers or microphones for correcting frequency response
    • H04R3/06—Circuits for transducers, loudspeakers or microphones for correcting frequency response of electrostatic transducers

Definitions

  • the invention relates to a method of reducing the harmonic distortion of a capacitive transducer, such as a capacitor microphone, the capacity of which is altered in response to a sound pressure on the electrode (the membrane) of the capacitor microphone, said distortion originating from undesired capacities in the transducer, by means of a negative capacity connected to said transducer.
  • the invention relates furthermore to a coupling for carrying out the method.
  • Capacitor microphones present a very high fidelity and are therefore used in almost all professional systems. They are also used in consumer devices and in personal equipment, such as tape recorders and hearing aids.
  • the high fidelity is of particular importance in measuring systems and other professional systems, where large dynamic ranges free of noise and distortion are the object.
  • the dynamic range is limited at low sound levels by the noise of the microphone and by the noise of an amplifier placed after the micro ⁇ phone.
  • the limit is in practise set by a non-linear distortion uniformly increasing with the signal level and being caused by the micro ⁇ phone; or by an abrupt cutting of the microphone signal caused by the signal level in the succeeding amplifier. It is known that an electric capacity in parallel to the set noise of the microphone increases the distortion of the microphone, cf. for instance Br ⁇ el & Kjaer Technical Review No. 4, 1979, page 18.
  • the amplifier provides the necessary impedance matching and has typically a voltage gain of slightly below 1.
  • the amplifier is optimized to drive long cables and presents such a high input impedance that the micro- phone is only insignificantly loaded.
  • the input impedance is typically between 10 and 50 times 10 9 ⁇ , and the input capacity is typically between 0.3 and 1 pF. This low input capacity can for instance be provided by means of a preamplifier in form of a field effect transistor coupled as source follower.
  • a screen around the input terminal of the preamplifier can be connected to the output of the preamplifier, the signal voltage therein then being in phase with the input voltage and only being slightly lower than said voltage.
  • the resulting input capacity is reduced in the preamplifier because a stronger current in the capacity between the input terminal and the frame has then been replaced by a weaker current in the capacity between the input terminal and said screen.
  • Measuring microphones are cylindrical and often characterised by their external diameter. The most conventional sizes are 1 ", 1/2", 1/3", and 1/8" with capacities of 60 pF, 20 pF, 6.5 pF, and 23.5 pF, respectively. Some of the capacity is formed by the active signal-generating capacity between the membrane and the back electrode, whereas another portion of the capacity is passive and originates from the mounting of said back electrode in the microphone housing and from the output terminals of the microphone capsule.
  • the passive portion C s of the capacity, cf. Fig. 2, is almost the same for all microphone sizes, viz. 2 to 3 pF.
  • the ratio of the passive to the active capacity is typically 4% for the largest types of microphone and 200% for the smallest types of microphones.
  • the object of the invention is to provide a method of reducing the har ⁇ monic distortion of a capacity transducer by optimizing the load thereof.
  • a method of the above type is according to the invention characterised by the negative capacity being coupled in parallel to the transducer and being dimensioned such that it corresponds substantially to the sum of the undesired capacities.
  • the resulting optimizing is performed by neu ⁇ tralizing the effect of the dissipation capacity of the microphone and thereby minimizing the harmonic distortion.
  • the invention relates furthermore to a coupling for carrying out the method according to the invention for reducing the harmonic distortion of a capacity transducer, such as a capacitor microphone, the capacity of which is altered in response to a sound pressure on the electrode (the membrane) of the capacitor microphone, said distortion originating from undesired capacities in the transducer, by means of a negative capacity connected to said transducer.
  • a capacity transducer such as a capacitor microphone
  • the coupling is according to the invention characterised by the negative capacity being coupled in parallel to the transducer and substantially corresponding to the sum of the undesired capacities.
  • the resulting capacitive transducer presents a lower distor ⁇ tion than previously known.
  • the nega- tive capacity may according to the invention be provided by the preamplifier with a positive capacitive feedback implying that the preamplifier has a negative input capacity. In this manner the available preamplifier is utilized, and substantially no more than a single additional component is required for establishing the negative capacity.
  • the nega- tive input capacity of the preamplifier is preferably 2 to 3 pF.
  • the negative input capacity of the preamplifier may be variable, whereby the input capacity can be set until the harmonic distortion has a minimum value.
  • Fig. 1 illustrates a capacitive transducer with a coupling according to the invention
  • Fig. 2 illustrates the imperfections of the capacitor microphone.
  • the capacitive transducer of Fig. 1 can for instance be formed by a microphone 1 , such as a capacitor microphone, charged from for instance a DC source V charge through a resistance R.
  • a microphone 1 such as a capacitor microphone
  • V charge a DC source V charge through a resistance R.
  • R a resistance
  • the voltage increases proportional to the distance between the plates when said plates are removed from one another, and when said distance is reduced the voltage drops proportional thereto.
  • This is known and applies to a capacitor without rim effects and without a load.
  • This proportionality is the ideal for a capacitor microphone.
  • the ideal image is, however, disturbed both by unavoidable stray capacities and by the membrane not being moved to an equal extent across the entire surface, cf. Fig. 2.
  • V Q _ 2 - k
  • Q is the charge on the capacitor
  • C k is the capacity of the capacitor
  • d is the distance between the capacitor plates
  • k is a constant.
  • the above problem has been solved by a negative capacity being coupled in parallel to the microphone.
  • This negative capacity serves to eliminate the effect of undesired capacities and corresponds substantially to the sum thereof.
  • the negative capacity can be provided by means of a succeeding preamplifier 2 providing an impedance matching to a connecting cable.
  • the preamplifier 2 is coupled such that it has a negative input capacity C jnd .
  • the negative input capa- city C jnd is provided by means of a positive capacitive feedback, such as by a capacity C
  • the capacity C 1 can be variable in such a manner that an optimum value can be obtained in response to the microphone on which the coupling is used.
  • the sum of the passive parallel capacity and the input capacity must not become negative because the circuit may otherwise oscillate.
  • the positive capacitive feedback must be so strong that the preamplifier corresponds to a negative input capacity C jnd of a few pF, preferably 2 to 3 pF.
  • C can have a value of 25 pF if the ratio of R ⁇ to R 2 is 10.
  • R 2 can have a value of 5 K ⁇
  • C 1 is for instance set by a pure sinusoidal sound signal of for instance 1000 Hz being fed to the microphone 1. Then the harmonic distortion or the distortion is measured and C 1 is set until said harmonic distortion or distortion assumes a minimum. In connection with a typical capacitor microphone, the harmonic distortion should be reduced by approximately 10 to 20 dB. The improvement depends, however, on the size of the capacitor microphone. In order to determine the DC level on the input of the preamplifier and in order to isolate said level from a possible polarisation voltage V charge , a complex comprising a capacity C 2 and a resistance R 3 is in front of the preamplifier 2.
  • the capacity C 2 is situated in the current path from the microphone 1 to the preamplifier 2, whereas the resistance R 3 is coupled between the connection of the capacity to the preamplifier 2 and the frame. In this manner the DC level is determined on the input to frame, C 2 is for instance 100 times greater than C k .
  • the resistance R 3 must be very high as it together with C k it determines the lower limit frequency.
  • the principle can be used in connection with pressure and pressure gradient microphones. It can, however, also be used in connection with electrete microphones.

Landscapes

  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Acoustics & Sound (AREA)
  • Signal Processing (AREA)
  • Circuit For Audible Band Transducer (AREA)
  • Electrostatic, Electromagnetic, Magneto- Strictive, And Variable-Resistance Transducers (AREA)

Abstract

L'invention concerne un procédé et un couplage pour la réduction de la distorsion harmonique d'un transducteur capacitif (1) tel qu'un microphone du type à condensateur dont la capacité (Ck) est modifiée en réponse à une pression sonore sur l'électrode du condensateur (membrane) au moyen d'une capacité négative. Selon ledit procédé, la capacité négative est couplée en parallèle au transducteur acoustique (1) et est dimensionnée de manière à éliminer l'effet des capacités indésirables (CS) à l'origine de la distorsion. Ainsi, la distorsion harmonique du transducteur de capacité (1) eat réduite au minimum.
EP94913038A 1993-04-07 1994-04-06 Procede et couplage pour la reduction de la distorsion harmonique d'un transducteur capacitif Ceased EP0694246A1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
DK415/93 1993-04-07
DK041593A DK170196B1 (da) 1993-04-07 1993-04-07 Fremgangsmåde og kobling til at reducere den harmoniske forvrængning af en kapacitiv transducer
PCT/DK1994/000142 WO1994023547A1 (fr) 1993-04-07 1994-04-06 Procede et couplage pour la reduction de la distorsion harmonique d'un transducteur capacitif

Publications (1)

Publication Number Publication Date
EP0694246A1 true EP0694246A1 (fr) 1996-01-31

Family

ID=8093260

Family Applications (1)

Application Number Title Priority Date Filing Date
EP94913038A Ceased EP0694246A1 (fr) 1993-04-07 1994-04-06 Procede et couplage pour la reduction de la distorsion harmonique d'un transducteur capacitif

Country Status (5)

Country Link
EP (1) EP0694246A1 (fr)
JP (1) JPH08509327A (fr)
AU (1) AU6534694A (fr)
DK (1) DK170196B1 (fr)
WO (1) WO1994023547A1 (fr)

Families Citing this family (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH08233581A (ja) * 1994-12-28 1996-09-13 Yoshiro Tomikawa 静電型変換手段の駆動装置
JPH08307199A (ja) * 1995-01-11 1996-11-22 Yoshiro Tomikawa 静電型変換手段の容量成分低減回路および静電型変換手段の駆動装置ならびに検出装置
JP4057212B2 (ja) * 2000-02-15 2008-03-05 三菱電機株式会社 マイクロフォン装置
WO2002073792A2 (fr) 2001-03-09 2002-09-19 Techtronic A/S Preamplificateur de microphone a condensateur electret, insensible aux courants de fuite a l'entree
WO2003023418A2 (fr) 2001-09-06 2003-03-20 Sumitomo Metal Industries, Ltd., Daspositif et procede de fixation de potential
TWI221196B (en) 2001-09-06 2004-09-21 Tokyo Electron Ltd Impedance measuring circuit, its method, and electrostatic capacitance measuring circuit
EP2317645B1 (fr) 2009-10-16 2013-04-10 Nxp B.V. Capteur capacitif
US10153740B2 (en) 2016-07-11 2018-12-11 Knowles Electronics, Llc Split signal differential MEMS microphone
EP3379204B1 (fr) 2017-03-22 2021-02-17 Knowles Electronics, LLC Agencement pour étalonner une interface de capteur capacitif

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3116366A (en) * 1959-08-18 1963-12-31 Arnold L Seligson Capacitive source signal generators
GB2003364B (en) * 1977-08-24 1982-03-03 Post Office Electroacoustic transducer for a microphone
IT1112691B (it) * 1978-07-12 1986-01-20 Sits Soc It Telecom Siemens Microfono a condensatore

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO9423547A1 *

Also Published As

Publication number Publication date
DK41593D0 (da) 1993-04-07
WO1994023547A1 (fr) 1994-10-13
JPH08509327A (ja) 1996-10-01
AU6534694A (en) 1994-10-24
DK41593A (da) 1994-10-08
DK170196B1 (da) 1995-06-06

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