GB2545915A

GB2545915A - Audio coupling device for radios

Info

Publication number: GB2545915A
Application number: GB1523069.1A
Authority: GB
Inventors: Polkinghorne Martyn
Original assignee: J3m Tech Ltd
Current assignee: J3m Tech Ltd
Priority date: 2015-12-30
Filing date: 2015-12-30
Publication date: 2017-07-05
Also published as: GB201523069D0

Abstract

An audio coupling device 100 that provides an interface between audio equipment 101 and a radio receiver 103 to allow the electrical output of the audio equipment 101 to be listened to with the radio receiver 103. The audio coupling device 100 comprises an input stage for receiving an electronic signal from audio equipment 101, an oscillator circuit for generating a radio frequency electronic signal, a modulator connected to the audio input stage and to the oscillator circuit, the modulator arranged to combine the signal from the input stage and the signal from the oscillator circuit, and a radio frequency output stage for directly coupling an output signal into the aerial 104 of the radio receiver 103. The direct coupling is preferably by means of an electrical output attachable by wire 105 to an external antenna 104 of the radio receiver 103 or an aerial socket of the radio receiver 103. Alternatively, the direct coupling is by means of an inductive coupling coil for coupling the output signal directly to a wound coil-type antenna of the radio receiver 103.

Description

AUDIO COUPLING DEVICE FOR RADIOS BACKGROUND Field of the Invention

The invention is in the field of accessories for radio receivers. The invention relates to devices that provide an interface between audio equipment and radio receivers and allow the electrical output of the audio equipment to be listened to with the radio receiver.

Description of Related Art

Vintage radio sets have a decreasing number of radio stations available to tune in to. Typically these sets are arranged to pick up AM radio signals in the Long Wave (153 - 259 kHz), Medium Wave (526 - 1606 kHz) and Short Wave (2.3-26 MHz) bands. However, AM broadcasts are being superseded by FM and digital broadcasts at VHF (87-108 MHz) because of the better sound quality available with these technologies. Despite these technical developments, there is a requirement for older AM radio sets to remain in service, mainly because their owners cherish these older radio sets and still wish to use them. A solution is to provide a device that can receive an electrical audio input signal from, for example, the electrical output of an FM or digital radio, cassette, CD or MP3 player and transmit this signal at a wavelength that can be received by the older radio set. These devices are known for FM radio sets but are less common for AM radio sets because of strict regulations around transmission of AM signals: FM signals do not propagate far, but for the lower frequencies in the AM band there is a greater risk of interference with other transmissions. All radio transmitters require careful and reliable control of the frequency of the carrier signal to prevent interference in other wavebands. The required level of frequency control adds complexity and therefore expense to the device.

The object of the invention is to overcome these problems.

SUMMARY OF THE INVENTION

An audio coupling device is provided that can couple directly into a radio receiver. In an embodiment of the invention an audio coupling device for coupling an audio signal into a radio receiver via the aerial of the radio receiver is provided, the coupling device comprising an audio input stage for receiving an electronic signal from audio equipment, an oscillator circuit for generating a radio frequency electronic signal, a modulator connected to the audio input stage and to the oscillator circuit, the modulator arranged to combine the signal from the input stage and the signal from the oscillator circuit, and a radio frequency output stage, where the output stage includes a direct coupler for directly coupling an output signal from the device into the aerial of the radio receiver. The direct coupler could be an electrical output attachable by a wire to an external antenna of the radio or an aerial socket of the radio receiver or an inductive coupling coil for coupling the output signal directly to a wound coil-type antenna of the radio receiver. Direct coupling of the output signal rather than transmitting the signal means that the power radiated by the coupling device is substantially less than a device that transmits a signal. For example, a transmitting device could typically transmit at a power of 10 mW, whereas the output power of the present direct coupling device is less than 1 mW; furthermore this power is directly provided to the receiving radio set and therefore the radiated signal is minimal.

The oscillator circuit may be an LC oscillator, comprising at least one inductor and at least one capacitor. Direct coupling of the device to the radio set has the advantage that, because there is no risk of interference with other broadcasts, the frequency of the radiated signal from the device does not have to be carefully and reliably controlled. Therefore a less accurate but less complex oscillator can be used, such as an LC oscillator, for example a Colpitts oscillator. Importantly, the device dispenses with the use of a quartz crystal to generate the carrier wave; crystals can consume a lot more power than an LC oscillator and can radiate power themselves. Furthermore, the components of the device of the present invention do not need to be tuned during manufacture; the tuning can be carried out by the user at the radio receiver and therefore complex and expensive tuning circuits can be omitted. User responsibility for tuning at the radio receiver means that no set-up tuning of components is required during manufacture to compensate for variations in individual components, and this leads to cost savings.

Low power output also means low power consumption and therefore the device can use an internal power supply such as a battery; typical operating voltage can be less than 5v.

The audio signal input stage can include automatic gain control to prevent over-modulation of the output signal. Mixing circuitry may be provided to convert a stereo signal to a mono signal. The output frequency of the oscillator circuit may be in the AM band of 150 kHz to 26 MHz. The modulator may be a Gilbert cell and can be implemented as discrete components. A plurality of inductors may be arranged to be switchable into the oscillator circuit to provide a simple control the output frequency of the oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an embodiment of the invention in use as an interface between an audio device and a radio set with an external antenna.

Figure 2 shows an embodiment of the invention in use as an interface between an audio device and a radio set without an external antenna.

Figure 3 is a block diagram showing the elements of an embodiment of the invention.

Figure 4 is a schematic diagram showing the components of an LC oscillator used to generate the carrier wave in an embodiment of the invention.

Figure 5 is a schematic diagram showing the components of the modulator used to combine the carrier wave and signal and modulating signal in an embodiment of the invention.

Figure 6 is a schematic diagram showing the components of the automatic gain control in an embodiment of the invention.

Figure 7 is a diagram showing an example of an inductive coupling coil printed on a circuit board.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The term direct coupling is used herein to mean either electrical connection with a wire or inductive coupling between coils. The terms radio set and radio receiver are used interchangeably and refer to a device for receiving and amplifying RF signals and converting these to an audio signal that can be listened to by a use either directly via a loudspeaker or indirectly via headphones.

An embodiment of the invention is a coupling device that allows a conventional AM radio receiver to receive an audio signal from an external source and use the full capabilities of the radio receiver, such as the tuning controls, to receive this signal. In this way, an AM receiver can still be used to receive and amplify audio signals, even if AM broadcast signals are unavailable. The coupling device provides an interface between an audio device and an AM radio receiver.

Figure 1 shows the device in use as an interface between an audio device and a radio set with an external antenna. An embodiment of the coupling device 100 is located near a conventional radio set 103. The coupling device 100 receives an input from a conventional audio device 101, such as an MP3 player, CD player, FM radio, digital radio etc. The audio device 101 is connected to the coupling device 100 by cable 102. In an embodiment, the output of the coupling device 100 is an RF signal that can be directly provided to an antenna 104 of the radio set 103 by a cable 105. The cable 105 may be clipped onto the antenna 104 or alternatively, if the radio has an antenna socket (not shown), connected directly into the antenna socket.

In some applications, for example the AM receiver 106 shown in Figure 2, the radio 106 may not have an external aerial input for connection to the coupling device 100. In this instance, when a direct electrical connection is not possible to the AM receiver 106 then loose electromagnetic coupling from an inductive coil in the coupling device 100 to a component such as a frame aerial or ferrite rod aerial in the AM receiver 106 is also possible. In this arrangement, the radiated power is negligible and below the relevant regulatory limits. Radios with no external antenna will typically use an internal frame aerial or ferrite rod aerial to receive incoming signals. In this case, the coupling device 100 can be connected using a coupling coil forming a loosely coupled RF transformer with the receiving coil of the frame or ferrite rod aerial inside the AM radio 106. In the alternative embodiment shown in Figure 2 a radio receiver 106 has no external aerial or socket, but instead has an internal antenna. The coupling device 100 outputs an RF signal via a coupling coil, which can couple the signal directly to the internal aerial of the radio set 106. The coupling device 100 does not transmit a signal to the radio 106, but inductively couples into the aerial, therefore the coupling device 100 should be placed in close proximity to the radio set 106. As the coupling device 100 is moved away from the radio set 106 the signal decays rapidly. The coupling is operating in the near-field or nonradiative region.

The coupling device 100 enables an AM receiver to receive a suitable signal by direct electrical connection to the aerial socket or RF receiver input rather than receiving a radiated signal. In passing the AM signal to the receiver by an electrical rather than radiated signal, the device 100 is not an AM transmitter and thereby is not subject to the constraints of an RF transmitter. A block diagram of the architecture of the coupling device 100 is shown in Figure 3. Input stage 301 is arranged to receive an electronic signal from the audio device 101. The audio input source can be either a stereo or mono signal. For a stereo signal an audio mixer stage 302 is connected to the input stage 301 to convert the stereo signal into a mono signal. The mixer stage 302 is connected to an automatic gain control (AGC) circuit 303, described in more detail below. The output of the AGC 303 is connected to one of two inputs of an AM modulator 305. The other input of the AM modulator is connected to the output of a local oscillator circuit 304, also described in more detail below. The output of the AM modulator 305 is connected to an RF drive stage 306. The RF drive stage 306 includes an output to a connecting cable 105 which can be connected to the aerial connector of a medium wave AM receiver 103. Alternatively, the RF drive stage 306 can include an inductive coil, for coupling to an internal antenna of radio set 106.

OSCILLATOR

The local oscillator 304 is used to generate an ac signal with a frequency within the AM broadcast band (500 to 1700 kHz), which provides the RF input for the modulator stage 305. An example circuit for the oscillator 304 is shown in Figure 4. In this embodiment the oscillator is an LC oscillator arranged in a Colpitts configuration with the frequency controlled by the values of inductor L1 and capacitors C1 and C2. Q1 is an n-channel junction FET which provides the gain element for the oscillator with feedback provided by capacitor C3. The output level from the oscillator is controlled by the potential divider formed by resistors R2 and R3.

Radio transmitters, regardless of their power output, must have a carefully managed output bandwidth to prevent interference with other radio channels. The oscillator setup, which provides the RF carrier signal, is definitive in managing the spread of output frequency bandwidth. The common choice generating a carrier signal is a quartz crystal oscillator because the frequency output can be tightly defined. However this adds extra cost and complication, since the oscillation frequency of a crystal is typically an order of magnitude, or more, higher than the medium wave band frequency. Therefore some form of frequency conversion, such as a phase locked loop (PLL) is required. Commercial crystal oscillator modules with a frequency converter such as a PLL consume more power and are more expensive to produce than an LC oscillator. They can also radiate more power than an LC oscillator. A viable LC oscillator setup in a radio transmitter includes elements to control the output frequency; this is because the tolerances of the inductors and capacitors of the LC oscillator, which may be +/- 5% can introduce an error in output bandwidth of +/- 10%. To reduce this variation to an acceptable level, filters and/ or tuning circuits are required. These circuits are individually adjusted during manufacture for each device to ensure consistency; this tuning step adds complexity and cost to the production process. In the present coupling device 100, which does not transmit a signal but couples directly into a radio receiver, no complex frequency management or tuning is necessary. Any variation in output frequency due to component variations, thermal effects or any other reason, can be compensated for at the radio receiver; the end user simply tunes the radio receiver to the output frequency of the coupling device 100. Therefore a very simple oscillator circuit, for example the LC oscillator, can be used. This means that manufactured devices 100 have high reproducibility at low cost because there is no need to tune each device on the production line. The absence of complex filtering and tuning circuitry leads to power savings; the coupling device can run at an operating voltage of less than 5v, which means that an on-board battery can be used.

MODULATOR

The AM modulator circuit 305 is shown in more detail in Figure 5. In this embodiment the modulator circuit 305 is an analogue Gilbert multiplier cell formed by transistors Q1-Q8 and has two inputs that are combined by the cell 305; the first input is connected to the output of the oscillator circuit 304 to provide the R.F. signal to be modulated and is marked RFJnput in Figure 5. The second input is connected to the output of the AGC circuit 303 for the audio path and is marked Modulationjnput in Figure 5. The modulator 305 can provide a differential output signal between out_p and out_n. Each of these outputs can be used independently to drive the RF output stage.

The single-ended RF signal from the oscillator 304 is a.c. coupled onto the multiplier inputs formed by the matched npn transistors of Q3/Q4 and Q6/Q7. The d.c. operating point is set by the potential dividers formed by R7/R8 and R4/R5.

The signal from the AGC 303 to the Modulatorjnput is a.c. coupled onto the source-degenerated differential npn pair formed by Q5/Q8. The amplitude of the input signal together with the value of R13 controls the modulation index. The d.c. operating point of this stage is controlled by the potential dividers formed by R9/R11 and R6/R10.

The operating current for the multiplier cell is set by the current sources formed by Q1 and Q2 with the bias being set by Q9 and the respective values of the resistors R15-R17.

The AGC is itself needed to accommodate the wider range of input signals to the device to provide a well-controlled signal amplitude to the modulation input of the Gilbert multiplier cell. In this way the modulation level of the AM modulator can be controlled. This is important since over-modulation produces distortion and under modulation effectively reduces the signal level detected at the receiver. This configuration allows this to be achieved simply with no manual adjustment to compensate for variation in the circuit component values.

The output level of the oscillator is less critical in this configuration so the oscillator can be relatively simple, and amplitude control of the oscillator output is less critical.

In this embodiment, the Gilbert multiplier cell is constructed using a discrete implementation, i.e. not embedded within a custom made integrated circuit. Pairs of matched transistors are used but these are provided in commonly available arrays, e.g. on a Harris Semiconductor CA3046 chip. These are standard components, which reduces the cost because many companies make transistor array components; they are important for achieving miniaturisation in many electronic products so are produced in huge volume at low cost.

COUPLING

The coupling device 100 can be connected to a radio receiver either with a direct electrical connection or inductively coupled. A coil is provided in the coupling device 100 for inductive coupling. A suitable coupling coil can be formed using a wiring layer within the printed circuit board used to implement the AM modulator. A possible implementation is shown in Figure 7 where the inductive coil 701 is located on one side of the printed circuit board 700 with the circuit components on the other. The spiral inductor 701 is connected to the output of the RF drive stage 306. The coil 701 shown is not a perfectly symmetrical coil because it is arranged to fit around existing vias and tracking. This will degrade the performance of the coil slightly but it can perform well enough in this application to avoid the additional cost and complexity of having an inductive coil as an additional component. Typical card dimensions supporting the coil are 85mm x 55mm, but many other dimensions would be suitable. Alternatively a separate coil of wound copper wire could be used.

Inductive coupling works in the near field or non-radiative region, meaning that the energy stays within a short distance of the coil. The range is short and depends on the size and shape of the coil. The fields and thus the power coupled decreases rapidly with distance, so if the distance between the coupling device coil 701 and the radio receiver coil is large then little power will be coupled between the coils.

The distinction should be made between radiating and coupling the output power of the device 100 to a radio set; radiating power from one device to another requires a high field strength and limits are set by regulations for how high the field strength can be without a licence because of possible interference with other transmissions. The relationship between field strength E, at a distance r from an antenna transmitting power P is given by the following equation, assuming that the power falls off with distance according to the inverse square law and assuming an impedance of free space of 120π ohms:

Electromagnetic compatibility regulations (CISPR in Europe, FCC in the US for example) state a maximum field strength for electrical devices. This could be, for example 50 mV/m, typically measured at 3 metres from the device. Using the above equation, the power P of the transmitter would need to be max 0.75mW to remain within the regulations. However, this is for an idealised antenna; to radiate this power efficiently, an antenna would have to be around the same dimension as the wavelength of the signal; for AM signals of, for example 1 MHz, the wavelength is 300 metres. In an embodiment of the present invention, the power at the output is under 1mW because of careful design of the circuitry. Since the wire or coil of the present device is of the order 50 cm, only a fraction of the output power could be radiated. Therefore the field strength radiated by the invention is comfortably below regulatory levels. An embodiment of the present invention operates by coupling power between devices, where the field strength beyond the coupled devices is low.

If the device is placed in close proximity to the AM radio then sufficient coupling can exist to couple a signal from the device to the AM radio whilst still complying with the relevant electromagnetic compatibility standards and regulations.

The output of the coupling device 100 can be switched between the electrical output and the coupling coil depending on which mode of operation is desired. The use of the multiplier circuit provides differential outputs which allow one of the outputs to be used in single-ended mode for the electrical output and the other differential output could also be used in single-ended mode to drive the coupling coil. In this way, both the electrical output and coupling coil can be driven without requiring any switching arrangement.

AGO

This input signal is processed by an Automatic Gain Control (AGC) 303, which provides an output signal of known amplitude to provide the modulation source for the AM modulator 305. The amplitude of the AGC output 303 is used to control the modulation index and to avoid over-modulation within the AM modulator stage 305.

The AM modulator 305 is driven by the AGC 303 and local oscillator 304 outputs to provide an amplitude-modulated signal suitable for reception by the AM receiver. The output is used by an RF drive stage 306 to provide a signal which can be electrically coupled into the AM radio receiver. This signal needs to be sufficient to overwhelm any radio signal on a similar frequency. However it also needs to be low enough in amplitude so as not to cause distortion within the AM receiver or to generate a significant radiated component.

An example circuit for the AGC 303 is shown in Figure 6. The audio input signal is a.c. coupled through C3 onto the load resistor R7. The input voltage across R7 is attenuated by the potential divider formed by R6 and the drain resistance of the junction p-channel FET J1. This signal is then a.c. coupled onto the non-inverting input of the operational amplifier U1 which forms a fixed gain amplifier with a gain controlled by the ratio of R4 to R1 with the input bias controlled by the potential divider R5/R2.

The AGC function is provided by a.c. coupling the output of U1 and using this to drive the base of an npn transistor, Q1. When the output signal amplitude approaches the Vbe of Q1, this transistor starts to turn on which pulls down the gate of J1. Consequently this will reduce the drain resistance of J1 which will act to reduce the input signal to the amplifier U1 and hence its output voltage.

It follows that this circuit operates as an automatic gain control regulating the output amplitude, the potential divider R9/R10 allows that output amplitude to be attenuated from the level of the Vbe for Q1. A capacitor C4 is used to control the stability of the feedback loop within the AGC.

RF DRIVE

The RF drive stage 306 is connected to the output of the modulator 305 and can be as simple as an ac coupling capacitor which couples to the output. A more sophisticated stage could be provided, for example to drive the coupling loop and the electrical output if required.

To operate the coupling device by direct electrical connection to the radio set, the user connects an external audio device 101 to the coupling device 100 using cable 102 and connects the coupling device 100 to radio set 103 with a cable 105. The three devices are then powered up, so that an electrical signal with a stereo audio signal is output from the audio device 101. The coupling device 100 receives the audio signal at its input 301. Audio mixer 302 converts the stereo signal to a mono signal. The signal is then processed by the AGC to keep it within predefined limits. The limited audio signal is then combined with the output from the oscillator 304 by the modulator 305 which uses the audio signal to modulate the carrier signal from the oscillator 304. The AM signal is then sent to the RF drive stage 306 and then to the output socket 307 of the coupling device 100. Cable 105 transfers the AM signal from the coupling device 100 to the external antenna 104 of the radio set 103. The user is informed of the approximate carrier frequency of the coupling device 100, which can be present on the device or in accompanying written material. The user then tunes the radio set 103 to this frequency and finds the exact frequency by trial and error in the way that a radio set would normally be tuned to a broadcast station.

To operate the coupling device by inductive coupling to the radio set, the user connects an external audio device 101 to the coupling device 100 using cable 102 and locates the coupling device 100 in close proximity to radio set 103, rather than using a cable to connect to the radio set.

When the signal is electrically coupled into the aerial connector of the AM receiver the signal amplitude is designed to be significantly stronger than any received radio signal. However if there is a strong local source of RF interference then the ability to adjust the local oscillator frequency can be useful in allowing a quieter portion of the AM band to be selected for use. In addition, the AM bands of radio receivers can vary, and an adjustable frequency is useful to allow the device to be used with a wide variety of receivers. An LC oscillator in the Colpitts configuration allows the carrier frequency to be changed simply by changing the value of the inductor. For example, by switching between 3 inductors, 3 different carrier frequencies can be provided.

Claims

1. An audio coupling device for coupling an audio signal into a radio receiver via the aerial of the radio receiver, the device comprising an audio input stage for receiving an electronic signal from audio equipment, an oscillator circuit for generating a radio frequency electronic signal, a modulator connected to the audio input stage and to the oscillator circuit, the modulator arranged to combine the signal from the input stage and the signal from the oscillator circuit, and a radio frequency output stage including a direct coupler for directly coupling an output signal into the aerial of the radio receiver.

2. An audio coupling device in accordance with claim 1, wherein the direct coupler is an electrical output attachable by a wire to an external antenna of the radio or an aerial socket of the radio receiver.

3. An audio coupling device in accordance with claim 1, wherein the direct coupler is an inductive coupling coil for coupling the output signal directly to a wound coil-type antenna of the radio receiver.

4. An audio coupling device in accordance with claim 1, wherein the direct coupler includes both an electrical output and a coupling coil.

5. An audio coupling device in accordance with claim 3 or claim 4, wherein the circuitry of the device is mounted on a printed circuit board and wherein the inductive coupling coil is printed on the printed circuit board.

6. An audio coupling device in accordance with any preceding claim, wherein the oscillator circuit is an LC oscillator, comprising at least one inductor and at least one capacitor.

7. An audio coupling device in accordance with claim 6, wherein the LC oscillator is a Colpitts oscillator.

8. An audio coupling device in accordance with any preceding claim, wherein the modulator is arranged to modulate the amplitude of the signal from the oscillator circuit with the signal from the audio input stage.

9. An audio coupling device in accordance with any preceding claim, wherein the device has an internal power supply such as a battery.

10. An audio coupling device in accordance with any preceding claim, wherein the audio signal input stage includes automatic gain control.

11. An audio coupling device in accordance with any preceding claim, wherein mixing circuitry is provided to convert a stereo signal to a mono signal.

12. An audio coupling device in accordance with any preceding claim, wherein the output frequency of oscillator circuit is in the range 150 kHz to 26 MHz.

13. An audio coupling device in accordance with any preceding claim, wherein the modulator is a Gilbert cell.

14. An audio coupling device in accordance with any preceding claim, wherein the Gilbert cell comprises discrete components

15. An audio coupling device in accordance with claim 6 to 14, wherein a plurality of inductors are arranged to be switchable into the oscillator circuit to control the output frequency of the oscillator.