Small, battery operated RF transmitter for portable audio devices for use with headphones with RF receiver

• US 5,771,441 A
• Filed: 04/10/1996
• Issued: 06/23/1998
• Est. Priority Date: 04/10/1996
• Status: Expired due to Term

• Current Assignee: AJZN, Inc.
• Original Assignee: National Group, Inc.
• Inventors: Altstatt, John E.
• Primary Examiner(s): Urban, Edward F.
• Assistant Examiner(s): SOBUTKA, PHILIP
• Application Number:
  US08/630,502
• Time in Patent Office: 804 Days
• Field of Search: 455/129, 455/66, 455/575, 455/91, 455/95, 455/100, 455/107, 455/269, 455/270, 455/280, 455/282, 455/286, 455/83, 455/525, 343/718, 343/720, 381/25, 381/74, 381/86, 381/88, 381/14, 331/158
• US Class Current: 455/66.1
• CPC Class Codes: H01Q 1/24 with receiving set
• CPC Class Codes: H04B 1/034 Portable transmitters
• CPC Class Codes: H04B 1/086 Portable receivers
• CPC Class Codes: H04M 1/6041 Portable telephones adapted for handsfree use

Abstract

A portable RF transmitter having an audio plug extending therefrom which mates with the earphone or output jack of an audio source such as a portable battery operated CD or tape player and having no external antenna. The portable RF transmitter modulates audio signals from the audio source onto an FM carrier and transmits them to an FM receiver mounted on a headset worn by a user. The RF transmitter uses its own ground circuit and the ground circuit of the audio source as two elements of a short dipole. The two ground circuits are electrically isolated at RF by an RF choke but connected together at audio frequencies by the low impedance of the choke at audio. The choke'"'"'s leakage inductance also reduces the capacitive reactance of the dipole antenna for better power dissipation. A transformer coupling the RF output to the dipole transforms the impedance of the dipole to a value closer to the output impedance of the RF transmitter.

Claims

1. An apparatus, comprising:

any audio source having an audio output jack and having a signal ground conductor coupled to a first terminal of said audio output jack, said audio source having right and left stereo audio channels which output left and right channel signals and which are coupled to second and third terminals of said audio output jack;

an RF stereo transmitter positioned within a housing made of any material and having left and right audio inputs and a ground conductor all of which are electrically coupled to an audio plug sized to mate with said audio output jack of said audio source and having first, second and third terminals which electrically connect to said first, second and third terminals, respectively, of said output jack when said audio plug is inserted into said audio output jack, said RF transmitter having a signal ground conductor coupled via a first coil of a three coil common mode radio frequency choke to said signal ground conductor of said audio source through said first terminals of said audio output jack and said audio plug when said audio output jack and audio plug are mated, said first coil of said three coil common mode radio frequency choke having a high impedance at the frequency of transmission of said RF stereo transmitter but small enough impedance at audio frequencies to render said signal ground conductors of said RF transmitter and said audio source essentially one conductor at audio frequencies, and wherein the three coil common mode choke includes second and third coils which are wound around the same core of magnetically permeable material as said first coil and have exactly the same impedance as said first coil, and wherein said left and right channel signals from said audio source are coupled to left and right channel audio inputs of said RF stereo transmitter through said second and third coils of said common mode RF choke, said RF stereo transmitter having an RF output comprising an RF ground output terminal coupled to said signal ground conductor of said RF stereo transmitter and an RF output terminal coupled to said first terminal of said audio plug so as to be connected to said signal ground conductor of said audio source when said output jack and audio plug are mated but so as to be electrically isolated at the frequency of said RF stereo transmitter from the signal ground conductor of said RF stereo transmitter by said first coil of said three coil common mode radio frequency choke, whereby said signal ground conductor of said RF transmitter and said signal ground conductor of said audio source act as dipole antenna elements of a radiating antenna.

2. The apparatus of claim 1 wherein said audio source is any portable audio source such as a radio, cassette player, CD player or other device, and further comprising an impedance matching network comprised of a fixed capacitor and fixed inductor and connected so as to couple said RF transmitter to a dipole antenna comprised of the ground conductors of said audio source and said RF stereo transmitter, each of said fixed capacitor and fixed inductor having values selected to provide the best impedance match between the output impedance of said RF transmitter and the average input impedance of said dipole antenna.

3. The apparatus of claim 1 wherein said RF transmitter has its transmit frequency tuned so as to be in the FM radio band at a position to not interfere with local stations, and further comprising a portable headphone FM receiver for receiving signals from said RF transmitter, said receiver having an IF filter comprised of two piezoelectric resonators substituted for the normal IF filter of said receiver and coupled together so as to at least double the steepness of the skirts of the passband of the IF filter of a normal FM receiver with only a single piezoelectric resonator in the IF filter so as to improve the selectivity of said receiver, and further comprising an amplifier coupled to said piezoelectric resonators to amplify signals passing therethrough to add gain sufficient to make up part or all of the loss imposed by said second piezoelectric resonator, said two piezoelectric resonators selected to have 3 dB bandwidths that are wider than the 3 dB bandwidth of the IF resonator that was replaced by them such that the 3 dB point bandwidth of the composite transfer function of the two piezoelectric IF resonators satisfies Carson'"'"'s rule for this particular receiver.

4. A system comprising:

an audio source for supplying stereo audio signals having left and right channels at an output jack, said output jack having at least firsts, second and third terminals, said audio source having a signal ground conductor coupled to said first terminal, said left and right channel audio signals coupled to said second and third terminals, respectively;

a stereo RF transmitter having an RF output at which said RF transmitter supplies as an output signal a modulated RF stereo signal and having an audio plug sized to mate with said output jack, said RF transmitter having a signal ground conductor and having left and right channel inputs coupled to said second and third terminals of said audio plug, respectively, by first and second coils of a three coil common mode RF choke having first, second and third coils wound on the same magnetically permeable core so as to have identical high impedances at the frequency of said transmitter and identical low impedances at audio frequencies, said audio plug having a first second and third terminals which make electrical contact with said first second and third terminals, respectively, of said output jack when said audio plug is mated with said output jack, said first terminal of said audio plug coupled to said RF output through said first coil of said three coil common mode RF choke;

an antenna comprising said signal ground conductor of said audio source as one element of said antenna and said signal ground conductor of said RF transmitter as another element of said antenna, said antenna having a complex impedance which has a real component and an imaginary component;

and wherein said first coil of said common mode RF choke functions to electrically isolate the signal ground conductors of said RF transmitter and said audio source at the frequency of said RF carrier but electrically couple said signal ground conductors of said RF transmitter and said audio source via a low impedance path at audio frequencies, said first coil of said common mode RF choke also serving to cancel at least part of said reactive component of said antenna impedance so as to transform the impedance of said dipole antenna to an impedance closer to a purely real impedance with a smaller reactive component;

and further comprising means coupled to said RF output for transforming said impedance of said antenna, as modified by said first coil of said RF choke, to an impedance which is a closer match to the complex conjugate of the output impedance of said RF transmitter at said RF output.

5. A system for use with an audio source which supplies stereo left and right channel audio signals at an output jack, said output jack having first, second and third terminals, said audio source having a signal ground conductor coupled to said first terminal, said audio signals coupled to said second and third terminals, respectively, comprising:

an RF transmitter having an RF output at which said RF transmitter supplies as an output a modulated RF stereo signal and having an audio plug sized to mate with said output jack, said RF transmitter having a signal ground conductor and having an audio input comprising left and right audio channel inputs, said audio plug having a first terminal which makes electrical contact with said first terminal of said output jack when said audio plug is mated with said output jack, said first terminal of said audio plug coupled to said RF output via a first coil of a three coil common mode RF choke, said audio plug having second and third terminals coupled to the left and right channel inputs of said audio input via second and third coils of said RF choke, and said second and third terminals of said audio plug making electrical contact with said second and third terminals of said output jack when said output jack and said audio plug are mated;

a dipole antenna comprising said signal ground conductor of said audio source as one element of said dipole and said signal ground conductor of said RF transmitter as another element of said dipole antenna, said dipole antenna having a complex impedance which has a real component and an imaginary component;

and wherein said first coil of said RF choke coupled between said first terminal of said audio plug of said RF transmitter and said signal ground conductor of said RF transmitter functions to electrically isolate the signal ground conductors of said RF transmitter and said audio source from each other at the frequency of said RF carrier but to electrically couple said signal ground conductors of said RF transmitter and said audio source via a low impedance path at audio frequencies, and and also serves to cancel at least part of said reactive component of said complex impedance of said dipole so as to transform the impedance of said dipole antenna to an impedance closer to a purely real impedance with a smaller reactive component;

and further comprising impedance transformation means coupled to said RF output for transforming said impedance of said dipole antenna, as modified by said first coil of said RF choke, to an impedance which is a closer match to the complex conjugate of the output impedance of said RF transmitter at said RF output.

6. The system of claim 5 further comprising a receiver mounted on a headphone, said receiver tuned to the frequency of said RF transmitter, said receiver having an IF section comprised of two cascaded piezoelectric resonators or other IF filters and an amplifier coupled to amplify the signal passing through said piezoelectric resonators or other IF filters to make up for at least some of the loss caused by the addition of a second piezoelectric resonator, the use of two cascaded piezoelectric resonators increasing the selectivity of said receiver.

7. The system of claim 6 wherein each of said piezoelectric resonators or other IF filters has a passband bandwidth selected such that the net bandwidth of the receiver is set in accordance with Carson'"'"'s Rule based upon the FM frequency deviation of said transmitter.

8. A system for use with an audio source for supplying audio signals at an output jack, said output jack having at least first and second terminals, said audio source having a signal ground conductor coupled to said first terminal, said audio signals coupled to said second terminal, comprising:

an RF transmitter having an RF output at which said RF transmitter supplies as an output a modulated RF carrier and having an audio plug sized to mate with said output jack, said RF transmitter having a signal ground conductor and having an audio input, said audio plug having a first terminal which makes electrical contact with said first terminal of said output jack when said audio plug is mated with said output jack, said first terminal of said audio plug coupled to said RF output, said audio plug having a second terminal coupled to the audio input, and said second terminal of said audio plug making electrical contact with said second terminal of said output jack when said output jack and said audio plug are mated;

a dipole antenna comprising said signal ground conductor of said audio source as one element of said dipole and said signal ground conductor of said RF transmitter as another element of said dipole antenna, said dipole antenna having a complex impedance which has a real component and an imaginary component;

and further comprising isolation means coupled between said first terminal of said audio plug of said RF transmitter and said signal ground conductor of said RF transmitter for electrically isolating the signal ground conductors of said RF transmitter and said audio source from each other at the frequency of said RF carrier but for electrically coupling said signal ground conductors of said RF transmitter and said audio source via a low impedance path at audio frequencies, and for cancelling at least part of said reactive component of said antenna impedance so as to transform the impedance of said dipole antenna closer to a purely real impedance with a smaller reactive component;

and further comprising impedance transformation means coupled to said RF output for transforming said impedance of said dipole antenna, as modified by said isolation means, to an impedance which is a closer match to the complex conjugate of the output impedance of said RF transmitter at said RF output, and

wherein said impedance transformation means is an autotransformer having one coil with a tap along said coil coupled to said RF output of said RF transmitter and having one end of said coil coupled to signal ground conductor of said RF transmitter and having the other end of said coil coupled to said dipole antenna.

9. A portable RF transmitter unit for use with a portable audio unit including an audio circuit having a ground conductor and outputting a stereo audio signal comprising left and right channel signals and an output jack having first, second and third terminals connected to said ground conductor and said left and right channel audio signals, respectively, of the audio unit, the RF transmitter unit comprising:

an input plug having first, second and third terminals, the input plug being sized to mate with the output jack of the audio unit such that first, second and third terminals of the output jack contact the first, second and third terminals, respectively, of the input plug, respectively, when the input plug and output jack are mated; and

an RF stereo transmitter having left and right channel audio input terminals and first and second RF output terminals, the left and right channel audio input terminals of the RF transmitter being connected to the second and third terminals of said input plug through second and third coils of a three coil common mode RF choke for receiving the left and right channel audio signals when the input plug and output jack are mated, the first RF output terminal of said transmitter being connected to said first terminal of the input plug such that when the input plug and output jack are mated the ground conductor of the audio unit acts as one radiating element of an antenna, the second RF output terminal being connected to a ground conductor of the RF transmitter, said ground conductor of said RF transmitter being coupled to said first terminal of said input plug through said first coil of said three coil common mode RF choke, said first, second and third coils of said common mode RF choke having identical high impedances at the frequency of said transmitter and having identical low impedances at audio frequencies, said ground conductor of said RF transmitter acting as the other radiating element of the antenna.

10. The RF transmitter unit according to claim 9 further comprising a battery coupled to said ground conductor, wherein the ground conductor of the RF transmitter further comprises conductive circuit components of the RF transmitter such as the battery.

11. The apparatus of claim 9 further comprising an autotransformer coupling said first RF output terminal of said RF transmitter to said ground conductor of said audio unit through said input plug and said output jack, for providing a closer impedance match between the output impedance of said RF transmitter and the input impedance of said antenna, said autotransformer having one coil with a tap along said coil, said tap coupled to said first RF output terminal of said RF transmitter and having one end of said coil coupled to said ground conductor of said RF transmitter and having the other end of said coil coupled to said ground conductor of said audio unit.

12. The RF transmitter unit according to claim 9, further comprising a step down transformer connected between the first RF output terminal and the ground terminal of the input plug to more closely match the output impedance of the RF transmitter with the impedance of the antenna.

13. A portable DC powered RF transmitter unit for use with an audio unit including an audio circuit which supplies as an output an audio signal and including an output jack having an audio output terminal for receiving the audio output signal and a ground terminal connected to a ground trace of the audio unit, the RF transmitter unit comprising:

a housing made of any material including metal;

an input plug having an audio input terminal and a ground terminal, the input plug being sized to mate with the output jack of the audio unit such that the audio output and ground terminals of the output jack contact the audio input and ground terminals of the input plug, respectively, when the input plug and output jack are mated;

an RF transmitter having an input terminal and first and second RF output terminals and having a ground trace, the input terminal of the RF transmitter being connected to the audio input terminal of the input plug when the input plug and output jack are mated, said connection being via a frequency deviation control network comprised of a voltage divider which divides the voltage of the audio signal down to a lower voltage and presents the reduced voltage audio signal at a node and a preemphasis circuit comprising a resistor and capacitor in parallel coupled to said node and functioning to limit excursions of said audio signal such that said input terminal of said RF transmitter receives an audio signal which has been suitably conditioned to limit the resulting deviation of the FM signal generated by said RF transmitter to the standard 75 kHz peak deviation used by FM broadcast receivers so as to prevent distortion of the transmitted audio in a standard broadcast band FM receiver, the first RF output terminal of said transmitter being connected to the ground terminal of said input plug such that when said input plug and output jack are mated, the ground trace of the audio unit acts as one radiating element of an antenna, the second RF output terminal being connected to ground trace of the RF transmitter unit, the ground trace of the RF transmitter acting as the other radiating elements of the antenna; and

an RF choke connected so as to isolate the two radiating elements of the antenna from each other at the transmitting frequency of the RF transmitter but to act as a low impedance path between the ground traces at audio frequencies.

14. The RF transmitter unit according to claim 13 wherein said RF transmitter is an FM stereo transmitter and wherein said audio unit generates a stereo audio signal having left and right channel audio signals, and wherein said input plug and output jack include terminals to transfer both the right channel and left channel audio signals to right and left audio inputs of said RF transmitter, respectively, each audio signal being coupled from said input plug to an audio input of said RF transmitter through a deviation control network having the structure defined in claim 13.

15. The RF transmitter unit according to claim 14 further comprising an autotransformer coupling said first RF output terminal of said RF transmitter to said ground trace of said audio unit through said input plug and said output jack, for providing a closer impedance match between the output impedance of said RF transmitter and the input impedance of said antenna, said autotransformer having one coil with a tap along said coil, said tap coupled to said first RF output terminal of said RF transmitter and having one end of said coil coupled to said ground trace of said RF transmitter and having the other end of said coil coupled to said ground trace of said audio unit.

16. The RF transmitter unit according to claim 13, further comprising a step down transformer coupling said first RF output terminal and said ground trace of said audio unit through the ground terminal of the input plug and the appropriate terminal of said output jack on said audio unit to more closely match the output impedance of the RF transmitter with the input impedance of the antenna having as its radiating elements the ground traces of said audio unit and said RF transmitter.

17. A portable RF transmitter, comprising:

a portable battery operated audio source having an output jack and having a signal ground conductor coupled to a first terminal of said output jack, said audio source having right and left stereo audio channels which output signals and which are coupled to second and third terminals of said output jack;

a portable, battery operated RF transmitter positioned within a housing from which extends an audio plug sized to mate with said output jack and having first, second and third terminals which electrically connect to said first, second and third terminals, respectively, of said output jack when said audio plug is inserted into said output jack, said transmitter having a signal ground conductor coupled via a radio frequency choke to said signal ground conductor of said audio source through said first terminals of said output jack and said audio plug when said output jack and audio plug are mated, said radio frequency choke having a high impedance at approximately 100 MHz but approximately zero impedance at audio frequencies, said transmitter having an RF output comprising an RF ground output terminal coupled to said signal ground conductor of said transmitter and an RF output terminal coupled to said first terminal of said audio plug so as to be connected to said signal ground conductor of said audio source when said output jack and audio plug are mated, whereby said signal ground conductor of said RF transmitter and said signal ground conductor of said audio source act as dipole antenna elements of a radiating antenna and further comprising a broadband stepdown impedance matching transformer having a single winding with centertap and operating as an autotransformer, said centertap coupled through a D.C. blocking capacitor to said RF output, and wherein one end of said single winding is coupled to the signal ground conductor of said RF transmitter and the other end of said single winding is coupled through said audio plug and said output jack to the signal ground conductor of said audio source and wherein the number of turns from said centertap of said transformer single winding to the end of said winding coupled to said signal ground conductor of said RF transformer is approximately half the number of turns from said end of said winding coupled to said signal ground of said audio source to said end coupled to said signal ground conductor of said RF transformer.

18. A portable RF transmitter, comprising:

a portable battery operated audio source having an output jack and having a signal ground conductor coupled to a first terminal of said output jack, said audio source having right and left stereo audio channels which output signals and which are coupled to second and third terminals of said output jack;

a portable, battery operated RF transmitter positioned within a housing from which extends an audio plug sized to mate with said output jack and having first, second and third terminals which electrically connect to said first, second and third terminals, respectively, of said output jack when said audio plug is inserted into said output jack, said transmitter having a signal ground conductor coupled via a radio frequency choke to said signal ground conductor of said audio source through said first terminals of said output jack and said audio plug when said output jack and audio plug are mated, said radio frequency choke having a high impedance at approximately 100 MHz but approximately zero impedance at audio frequencies, said transmitter having an RF output comprising an RF ground output terminal coupled to said signal ground conductor of said transmitter and an RF output terminal coupled to said first terminal of said audio plug so as to be connected to said signal ground conductor of said audio source when said output jack and audio plug are mated, whereby said signal ground conductor of said RF transmetter and said signal ground conductor of said audio source act as dipole elements of a radiating antenna and further comprising a broadband stepdown impedance matching autotransformer having a single winding with a centertap, said centertap coupled to said RF output, and wherein one end of said single winding is coupled to the signal ground conductor of said RF transmitter through a D.C. blocking capacitor having a low impedance at the transmit frequency of said RF transmitter the same said end of said single winding also coupled to a source of D.C. voltage, and wherein the other end of said single winding is coupled through said audio plug and said output jack to the signal ground conductor of said audio source through a D.C. blocking capacitor.

19. A portable RF transmitter, comprising:

a portable battery operated audio source having an output jack and having a signal ground conductor coupled to a first terminal of said output jack, said audio source having right and left stereo audio channels which output signals and which are coupled to second and third terminals of said output jack;

a portable, battery operated RF transmitter positioned within a housing from which extends an audio plug sized to mate with said output jack and having first, second and third terminals which electrically connect to said first, second and third terminals, respectively, of said output jack when said audio plug is inserted into said output jack, said transmitter having a signal ground conductor coupled via a radio frequency choke to said signal ground conductor of said audio source through said first terminals of said output jack and said audio plug when said output jack and audio plug are mated, said radio frequency choke having a high impedance at approximately 100 MHz but approximately zero impedance at audio frequencies, said transmitter having an RF output comprising an RF ground output terminal coupled to said signal around conductor of said transmitter and an RF output terminal coupled to said first terminal of said audio plug so as to be connected to said signal ground conductor of said audio source when said output jack and audio plug are mated, whereby said signal ground conductor of said RF transmitter and said signal ground conductor said audio source act as dipole antenna elements of a radiating antenna and further comprising;

a broadband stepdown impedance matching transformer having primary and secondary windings, one end of said primary winding coupled to said RF output, and the other end of said primary winding coupled to the signal ground conductor of said RF transmitter through a D.C. blocking capacitor having a low impedance at the transmit frequency of said RF transmitter, the end of said primary winding coupled to said RF transmitter signal ground conductor also coupled to a source of D.C. voltage, and wherein one end of said secondary winding is coupled through said audio plug and said output jack to the signal ground conductor of said audio source and the other end of said secondary winding coupled to the signal ground conductor of said RF transmitter.