US 3564416 A
Description (OCR text may contain errors)
we .16, 1,97 gap'm a 3,564,416
CORDLESS, SELF-CONTAINED MICROPHONE TRANSMITTER Fil ed March 29, 1968 I OSCILLATOR- ANTENNA, -AM I MODULATOR- .3 v ,EDWARD apmce INVENTOR.
ATTORNEY United States Patent 3,564,416 CORDLESS, SELF-CONTAINED MICROPHONE TRANSMITTER Edward G. Price, 1471 East 6050 South, Salt Lake City, Utah 84121 Filed Mar. 29, 1968, Ser. No. 717,224 Int. Cl. H04b 7/ 20 US. Cl. 325--111 1 Claim ABSTRACT OF THE DISCLOSURE A cordless, self-contained, frequency modulated microphone transmitter including a housing adapted to be hand-held; a microphone pickup element; and a circuit comprising a battery, an input impedance matching and equalization network, a triplex Colpitts radio frequency oscillator-modulator, an antenna that includes components of the microphone pickup element; and an isolating buffer amplifier to couple frequency modulated energy to the antenna.
BRIEF DESCRIPTION There has long been a need for a cordless microphone transmitter that can be effectively used by radio and television announcers, performers, public speakers and others, as they move about, or that can be moved from place to place for use by a number of different persons, without the audio cable that is usually associated with microphones, being dragged behind. Cordless microphone transmitters have been used in the past, but the ones with which I am familiar have not been entirely satisfactory. Most have been unsatisfactory because of their antenna construction and the manner in which the antennas perform their functions. These known cordless microphone transmitters have incorporated antennas that are wrapped around the users body, inserted into the users garments or otherwise conspicuously and obtrusively placed where they distract the user and/or persons viewing him, or that form long, difficult to handle, extensions of the microphone transmitter. Generally, the users body is required to serve as a ground plane for the antenna, and all too often the body acts as a shield, blocking effective transmission from these elaborate and bulky antennas.
Principal objects of the present invention are to provide an unusually compact and eflicient, cordless, microphone transmitter that can have the appearance and size of a conventional cord-type microphone, but that does not utilize an audio interconnecting cable or an exterior antenna.
Other objects are to provide such a microphone transmitter that can-be hand held, that can be easily suspended from an inexpensive cord, placed around the users neck, or that can be rested on a support or suspended in many other ways, without the suspension apparatus or support apparatus becoming an integral part of the transmitting circuit.
Still another object is to provide a microphone transmitter having an antenna with elliptical polarization so that the resulting radio frequency field is not as subject ICC to disruption as that developed by the linear polarization of the wire antennas heretofore used.
Principal features of the invention include a self-conrained microphone case, which has a power supply stage; a circuit including an amplifying, input impedance matching, and equalization stage, an oscillator-modulator stage having a radio frequency oscillator, and an isolating bulfer amplifier stage to couple frequency modulated energy to an antenna; a microphone cartridge; and an antenna that is integrally connected to the shield structure of the microphone cartridge, such that the cartridge forms a part thereof and the overall antenna structure launches an elliptically polarized Wave.
Additional objects and features will become apparent from the following detailed description and drawing, disclosing what is presently contemplated as being the best mode of the invention.
THE DRAWING FIG. 1 is a perspective view of the invention; FIG. 2, a side elevation view, with the housing cut away to show the interior thereof; and
FIG. 3, a circuit diagram, schematically showing the circuit of the invention, including the antenna construction.
DETAILED DESCRIPTION Referring now to the drawing:
In the illustrated preferred embodiment, the invention includes a tubular metal case 10 that has a conventional microphone head 11, at one end. The microphone head contains a pickup cartridge 12, FIG. 2. Although other microphone heads can be used, the R4M and other models manufactured by Shure Brothers, Inc., have proven suitable. Screws 10a inserted through holes provided therefor in case 10 and threaded into the microphone head 11 secure the head in place.
The other end of case 10 is closed by a cap 13 having a spring 14 therein that projects into the case to engage a battery 21. Cap 13 telescopes over case 10 and is secured in place, in conventional fashion, by a stub post 15 that projects inwardly from the wall of the cap and a matching angled slot 16 in the case. The stub post slides into the slot and when the cap is twisted, it is securely held.
Small openings 17 and 18 through the side wall of case 10 allow a screwdriver blade (not shown) to be inserted to tune and adjust the power of the circuit for maximum performance. A switch 19 protrudes through another opening through the sidewall of case 10 to connect or disconnect a circuit, shown generally at 22, from the battery 21.
A split ring 20 fits in a groove provided therefore, inside case 10 and divides a power supply compartment containing the battery 21 from a circuit compartment containing the encapsulated circuit 22.
The encapsulated circuit 22 and the antenna connected thereto are best shown, schematically in FIG. 3.
As illustrated, the pickup cartridge 12 of the microphone head has lead wires 23 and 24, leading from an induction coil 25 to plugs 26 and 27 respectively, and another lead wire 28, leading from the metal portions 29 of the microphone cartridge, and a metallic plate 30 normally used as an electrostatic shield rolled inside the microphone head, is connected to another plug 31.
Plug 26, and therefore wire 23, are connected through a plug receptacle 32 and a line 33 to the base of an NPN amplifier transistor 34. A capacitor 35 in line 33 acts to block DC current into the microphone coil winding. Another line 36 serving as a circuit ground, is connected to plug 27 and, therefore, wire 24, through a plug receptacle 37, and to a post 38 adapted to be contacted to one terminal of battery 21.
The other terminal of the battery is adapted to be connected through spring 14, cap 13 and the metal case 10, line 39 and a line 40 having a resistor 41 therein, to the line 33. Another line 42, having a resistor 43 therein interconnects lines 40 and 36 and lines 44 and 45, respectively interconnecting the emitter and collector of the transistor 34 with lines 39 and 36. Line 44 has a resistor 46 therein and line 45 has a pair of resistors 47 and 48 connected in series and with a line 49 connected to line 45 between resistors 47 and 48 and to line 36, such that the capacitor 50 therein is in parallel with resistor 48.
The lines 40, 42, 44, 45, and 49, resistors 41, 43, 46, 47, and 48 and capacitor 50 form with the transistor 34, a pre-amplifier stage of the circuit that receives an audio signal from the pickup cartridge, through line 33, and that transmits the signal through a line 51 having a blocking capacitor 52 thereon to the oscillator-modulator stage of the overall circuit.
The amplifier stage, through proper component selection, is designed to match a particular microphone head and also equalizes audio amplification with frequency to provide a suitable preemphasis to the subsequent oscillater-modulator stage. The preemphasis is obtained by choosing the resistor 48 and the capacitor 50 to have a time constant such that the audio gain of transistor 34 is increased with frequency. It will be apparent that some amount of preemphasis will also occur in the oscillator-modulator stage.
The oscillator-modulator unit of the circuit is of triplex type. It includes a common base, Colpitts type oscillator, with a silicon NPN transistor 55, the base of which is connected to line 51 to receive the audio signal from the amplifier circuit. Transistor 55 is selected so that there is just suflicient magnitude and degree of phase shift in a feedback circuit that includes line 56 and capacitors 57 and 58 to maintain stable oscillation at the desired frequency, i.e. 100 megahertz.
At radio frequency the capacitors 61 and 62 offer very low impedance and the circuit functions as a Colpitts oscillator whose frequency of oscillation is governed by the particular values of the capacitor 58 and variable inductor 63. At audio frequencies the tank circuit, comprising capacitor 58 and a primary winding of a variable inductor 63 is not significant and the transistor 55 functions as an audio common emitter amplifier.
The audio input voltage at the base of transistor 55 causes a coincident voltage change in the collector current. This, in turn, causes a change in the collector voltage, which linearly changes the output capacitance of the transistor. The change in output capacitance appears across the capacitor 58 in the Colpitts connection, and frequency, therefore, deviates the oscillator, under the direct control of the input audio voltage.
As is common in circuitry of this type, lines 64 and 65, respectively containing resistors 66 and 6-7, are provided to maintain the base voltage of transistor 55 constant and thus set a stabilized bias. Line 68 and resistors 69 and 70 therein and line 71 and capacitor 72 maintain the emitter voltage constant while providing some further preemphasis.
The secondary winding of coil 63 couples the oscillatormodulator stage to an output buffer amplifier stage between the oscillator and the antenna. This buffer stage serves to provide power gain at radio frequencies and load isolation between the antenna and the oscillating tank circuit including capacitor 58 and coil 63. The load isolation is obtained since there is only an extremely low feedback capacitance and leakage of the NPN silicon transistor 75 in the common base amplifier configuration. The transistor 75 operates as a constant current source to the antenna load.
The output tank circuit, connected between the collector of the transistor 75 and the antenna member 30 by plug 31, consists of the inductor 77 connected in the line 76, the choke 79 in the line 78 that interconnects the lines 39 and 36 and a variable capacitor 80 that serves to match the highly reactive and changeable load impedance of the antenna, which includes member 30 and other components of the microphone head 11, to the output impedance of the transistor 75.
The transistor 75 in turn is impedance matched to the oscillator secondary tank coil by capacitor 81 in line 82. A capacitor 83 in line 84 sets the base of transistor 75 at radio frequency ground potential in order for it to operate as class A common base amplifier while providing a DC block that will allow resistors 85 and 86 in line 87 and resistor 88 in line 89 to establish a proper DC bias voltage.
The circuit is tuned by adjusting tuning coil, i.e. inductor 63 with a screwdriver blade or other suitable tool inserted through opening 17 in the case 10 and the circuit is adjusted for optimum power transfer to the antenna by inserting a tool through opening 18 to adjust variable capacitor 80.
The on-olf switch 19 is provided in line 89, between case 10 and the other circuit elements, and it extends through the case such that it can be easily manipulated by a user.
With the circuit and antenna construction herein described, it has been found that a strong signal is continuously transmitted and that the signal does not fade out with the repositioning of the body of the user or even when the users hand comes in contact with the microphone head. The antenna is slightly elliptically polarized and is biased along the longitudinal axis of the microphone-transmitter.
By encapsulating the elements of the circuit, except of course for the tuning coil, variable capacitor and, switch and lead wires, in an epoxy, it is possible to protect the circuit components against damage due to rough treatment of the microphone-transmitter.
Although the microphone-transmitter of the invention can be suspended by a lavalier hung around the neck of a user or can be otherwise supported, the lavalier and/or support structure do not comprise any part of the microphone-transmitter circuit or antenna.
Although a preferred form of my invention has been herein disclosed, it is to be understood that the present disclosure is made by way of example and that variations are possible without departing from the hereinafter claimed subject matter I regard as my invention.
1. A cordless, self-contained microphone-transmitter comprising a case;
a power supply positioned within said case;
a microphone head including a non-metallic housing,
a metallic mesh extending across one end of the housing, a metallic shield rolled inside said housing and positioned between the shield and the mesh, and an induction coil within the housing and adjacent to said microphone cartridge, the other end of said housing being attached to one end of said case;
a solid-state circuit including an amplifying input impedance matching and equalization stage electrically connected to the power supply and to the microphone head, an oscillator-modulator stage including a tuning coil electrically connected to the amplifying input impedance matching and equalization stage, and an oscillating buffer amplifier stage including a tank circuit with a transistor, a coil and a variable capacitor; and
means electrically connecting the oscillating buffer stage to the microphone cartridge and the shield as an antenna, whereby the oscillating buffer stage matches the load impedance of the antenna to a common base network including a transistor that thereby acts as a constant current source to the antenna.
References Cited UNITED STATES PATENTS 2,840,694 6/1958 Morgan 325111 6 OTHER REFERENCES General Electric Transistor Manual, 7th edition, p. 386.
2,810,068 10/1957 iWeisz et a1. 325-411 5 RICHARDMURRAY Primary Examiner A. H. HANDAL, Assistant Examiner