Patent Publication Number: US-6222928-B1

Title: Universal impedance matcher for a microphone-to-radio connection

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of official duties by employees of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
     1. Field of the Invention 
     The invention relates generally to microphone-to-radio connections, and more particularly to an impedance matcher that allows a microphone (e.g., a powered piezoelectric microphone) to automatically be impedance matched to almost any two-way radio. 
     2. Background of the Invention 
     Military and civilian two-way radios are typically designed for use with a specific microphone. In each case, the microphone&#39;s impedance value is matched to an input impedance of its corresponding radio. Typical input impedances for HF, VHF, or SATCOM radios fall between the broad range of 5-2000 ohms. Thus, interchanging microphones (necessitated by failure, breakage, lost, etc.) between radios often results in an impedance mismatch between microphone and radio. The impedance mismatch can cause a significant reduction in the microphone&#39;s apparent performance. 
     As mentioned above, microphones are subject to failure and breakage. This is especially true in rugged environments. Accordingly, the U.S. Navy has developed a waterproof, powered, piezoelectric microphone that operates extremely well in rugged and high-noise environments. However, as also mentioned above, the number of different types of radios used by the military means that impedance matching of the microphone to the different radios is a time consuming and costly endeavor. Eliminating risk of a mismatch during operations and minimizing inventory requirements are two major benefits. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an impedance matcher that would allow a microphone to be automatically impedance-matched to a variety of radios. 
     Another object of the present intention is to provide an impedance matcher for use with powered microphones. 
     Still another object of the present invention is to provide an impedance matcher that is small and economical to build. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, an impedance matching circuit couples a microphone&#39;s output to a radio&#39;s microphone input. The input impedance for the microphone input is within a prescribed range. A tap circuit is coupled to the microphone&#39;s output for extracting an audio signal therefrom. The audio signal has an AC component and a DC component. An amplifier circuit having DC blocking circuitry is coupled to the tap circuit for passing the AC component and blocking the DC component. An audio transformer is also provided with its primary and secondary windings defining a turns ratio of approximately 1:1. Further, the primary winding has a characteristic DC resistance that is less than approximately 100 ohms and the secondary winding has an impedance that is within the prescribed range of the input impedance for the microphone input. A driver circuit is coupled between the amplifier circuit and audio transformer for continuously driving the audio transformer with the AC component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a functional block diagram of the impedance matching circuit according to the present invention; 
     FIG. 2 is a schematic diagram of one circuit implementation of the present invention; and 
     FIG. 3 is a schematic diagram of an alternative gain control adjustment that can be used in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, an impedance matching circuit according to the present invention is illustrated and referenced generally by numeral  10 . Impedance matching circuit  10  is used to couple a microphone  100  to a two-way radio  200 . More specifically, impedance matching circuit  10  matches the output impedance of microphone  100  to the input impedance of a microphone input  202  of radio  200 . For purposes of the present invention, it will be assumed that input  202  has an input impedance that falls within an expected prescribed range such as 5-2000 ohms as is generally the case with HF, VHF and SATCOM radios. Microphone  100  is representative of both powered and non-powered microphones. Microphone  100  is connected to an audio signal tap circuit  12  that extracts the audio signal from microphone  100 . If microphone  100  is a powered microphone, tap circuit  12  includes a power source and resistance element as will be described further below by way of a specific example. The audio signal output from tap circuit  12  includes an AC and DC component. 
     An amplifier  14  with DC blocking circuitry is connected to tap circuit  12 . Amplifier  14  blocks the DC component of the audio signal while amplifying the AC component. The DC blocking capability provided by amplifier  14  insures that audio transformer  18  does not get “saturated” which could desensitize audio transformer  18  to the AC component of the audio signal. The gain of amplifier  14  can be pre-set to a value that provides acceptable performance for a few types of radio  200 . The gain of amplifier  14  could also be adjustable by means of a gain control for greatest versatility as will be described further below. 
     The amplified AC component of the audio signal is then passed to the series combination of a transformer drive circuit  16  and an audio transformer  18 . Audio transformer  18  should have a primary-to-secondary winding turns ratio of approximately 1:1. Further, the secondary winding of audio transformer  18  should have an impedance that falls within the prescribed range (e.g., 5-2000 ohms) of radios that might be used with the impedance matching circuit  10 . The secondary winding of audio transformer  18  should also have a low characteristic DC resistance on the order of approximately 100 ohms or less. Since audio transformer  18  represents a low impedance load, transformer drive circuit  16  is coupled between amplifier  14  and audio transformer  18 . One such drive circuit that is used to drive low impedance loads is an emitter follower circuit. 
     As illustrated in FIG. 2, the present invention will now be explained by way of example for use with a powered microphone  102  designed by the U.S. Navy and disclosed in an allowed U.S. patent application Ser. No. 08/136,856, filed Oct. 18, 1993, entitled “Surface Laminated Piezoelectric Film Sound Transducer”, the contents of which are hereby incorporated by reference. The output of microphone  102  is connected to tap circuit  12 . More specifically, the output of microphone  102  is coupled across a combination of a power source  120  (e.g., a battery) and a resistor R 1  serving as a pull-up resistor that allows microphone  102  to receive power. Note that if the microphone is a non-powered microphone, resistor R 1  is not needed. A smoothing capacitor C 1  can be coupled to power source  120  if necessary. For microphone  102  described in the above-noted patent application, power source  120  should be able to supply 5-30 volts DC. 
     The AC and DC components of the audio signal extracted from tap circuit  12  are amplified and blocked, respectively, at amplifier  14  where capacitors C 2 , C 3  and C 4  block the DC component. The gain of amplifier  14  can be adjusted over a range of gains by a continuously variable gain control adjustment  140 , e.g., a potentiometer. Alternatively, as illustrated in FIG. 3, a gain control  142  can be used in place of gain control  140  to provide a number of predetermined discrete resistances R 3 - 1 , R 3 - 2 , . . . , R 3 -N, any one of which can be selected by a user. While gain control  140  provides for fine tuning of impedance matching circuit  10  to achieve the optimum performance at radio  200 , gain control  142  is not subject to drift. The presence of either gain control  140  or  142  allows the present invention to adapt to high-noise environments. 
     The AC audio signal is passed to transformer drive circuit  16 , which continuously drives audio transformer  18  with the AC audio signal. In the illustrated example, drive circuit  16  is an NPN transistor Q 1  connected in an emitter follower configuration. More specifically, the base of transistor Q 1  is coupled to amplifier  14  and the emitter of transistor Q 1  is coupled to primary winding  180  of audio transformer  18 . In order to insure that transistor Q 1  is always in the ON state so that audio transformer  18  is always being driven with the AC signal, a biasing circuit  162  is coupled to the base of transistor Q 1 . 
     The output of drive circuit  16  is coupled to primary winding  180  of audio transformer  18 . Because biasing circuit  162  introduces DC voltage into drive circuit  16 , a capacitor C 5  can be coupled between drive circuit  16  and audio transformer  18  to block any DC voltage passed thereto. As mentioned above, the turns ratio of primary winding  180  to secondary winding  182  should be approximately 1:1. The impedance of secondary winding  182  should fall within the input impedance of microphone input  202  and the characteristic DC resistance of secondary winding  182  should be less than approximately 100 ohms. In terms of microphone  102 , a good economical choice for audio transformer  18  was the SP 67  from MagnaTek, Goodland, Ind. 
     The advantages of the present invention are numerous. Impedance matching circuit  10  allows a microphone to be used with a variety of types of radios without significant loss of performance. The present invention was field-tested for the above-referenced U.S. Navy-designed microphone in a variety of noisy environments and found to provide excellent performance when connected to radios having microphone input impedance of 5, 1000 and 2000 ohms. Since the circuit can be built with off-the-shelf discrete electronic components, its design is both economical and small. The present invention will be useful in both military and civilian applications. 
     Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.