GB2305037A

GB2305037A - Frequency modulation circuit

Info

Publication number: GB2305037A
Application number: GB9518332A
Authority: GB
Inventors: Eyal Fayneh; Jossie Bercovich; Gadi Shirazi; Doron Shalev; Zami Schwartzman
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1995-09-08
Filing date: 1995-09-08
Publication date: 1997-03-26
Anticipated expiration: 2015-09-08
Also published as: GB2305037B; GB9518332D0; GB2346023A; GB0005930D0

Abstract

A frequency-modulation circuit (102) for indirect generation of a frequency-modulated signal includes a frequency generating element for providing a low-frequency reference signal (113) and a frequency control circuit for frequency modulating a received audio signal (124) onto the low-frequency reference signal (113) to provide a low frequency frequency-modulated signal (207). A frequency conversion circuit provides a final frequency frequency-modulated signal (126) by multiplying the low frequency frequency-modulated signal (207) with the low-frequency reference signal (113) to provide a baseband representation of the low frequency frequency-modulated signal. The baseband representation of the low frequency frequency-modulated signal is multiplied to an unmodulated final frequency signal to provide a final frequency frequency-modulated signal (126). A full-duplex radio architecture using a single synthesiser and the frequency modulation circuit is provided. The initial stage of frequency modulation may include a phase locked loop (PLL) or a Direct Digital Synthesizer (DDS).

Description

FREQUENCY MODULATION CIRCUIT AND METHOD OF OPERATION Field of the Invention This invention relates to radio transceivers employing frequency modulation techniques. The invention is applicable to, but not limited to, full duplex frequency modulated radio transceivers using indirect methods for generating frequency modulated signals.

Background of the Invention Operation in full duplex mode of mobile radios is usually implemented by alternately switching an audio attenuator in and out of the transmitter and the receiver paths. In particular, the speaker of the mobile radio is attenuated whilst transmitting and the microphone attenuated whilst receiving. The above operation needs a high level of isolation between the transmitter and the receiver paths to avoid interference being generated due to signal regeneration.

In full duplex mobile radios with say, phone interconnect, the attenuation of transmitter and receiver signals is difficult as the radio transmits and receives signals concurrently. In order to achieve such a high level of isolation a dual synthesiser approach or a side-step oscillator is typically used. This solution is both costly and requires additional space in the layout of the transceiver circuitry. In addition, problems such as lock time, frequency spurs and frequency modulation (FM) noise are more prevalent. A hands-free mode of operation for full duplex radios is frequently offered to mobile radio users with the microphone being installed on, say, a vehicle sun visor and the loudspeaker being installed in the vehicle somewhere near the driver.

Thus it is desirable to have a modulator circuit for operation in a FM full-duplex radio that uses a single synthesiser but still provides a high level of isolation between the transmitter and the receiver paths, and in particular for operation of the mobile radio in a hands-free mode.

Summarv of the Invention In a first aspect of the present invention, a frequency-modulation circuit for indirect generation of a frequency-modulated signal, is provided.

The frequency-modulation circuit comprises a frequency generating element, e.g. a single synthesiser or direct digital synthesiser, for providing a lowfrequency reference signal and a frequency control circuit, e.g. a phase locked loop circuit, for frequency modulating a received audio signal onto the lowfrequency reference signal to provide a low frequency frequency-modulated signal. A frequency conversion circuit provides a final frequency frequencymodulated signal, wherein the frequency conversion circuit multiplies the low frequency frequency-modulated signal with the low-frequency reference signal to provide a baseband representation of the low frequency frequencymodulated signal, the baseband representation of the low frequency frequency-modulated signal is multiplied to an unmodulated final frequency signal to provide a final frequency frequency-modulated signal.

In this manner, a frequency modulated signal is generated indirectly.

The audio signal is modulated onto the signal to be transmitted, at a much later stage than current approaches for generating frequency modulated signals. Advantageously, for full duplex operation in a mobile radio, an unmodulated carrier signal is then used in the receiver path to down-convert received frequency modulated signals, thereby reducing the effect of audio regeneration in the receiver.

In a second aspect of the present invention, a full-duplex radio for transmitting and receiving concurrent frequency modulated signals, is provided. The full-duplex radio uses an indirect technique for generating frequency modulated signals and includes a plurality of receiver sub-circuits operably coupled and arranged for receiving a radio frequency input signal and for providing a baseband output signal. At least one of the plurality of receiver sub-circuits is a frequency conversion element for down-converting the radio frequency input signal to a lower frequency. The full-duplex radio circuit further includes a plurality of transmitter sub-circuits operably coupled and arranged for receiving a baseband input signal and an unmodulated radio frequency signal.A first transmitter sub-circuit of the plurality of transmitter sub-circuits provides a frequency modulated radio frequency output signal. A frequency generating element is operably coupled to the frequency conversion element for concurrently providing an unmodulated radio frequency signal to the frequency conversion element and to the first transmitter sub-circuit.

In this manner, a full duplex radio uses a single synthesiser arrangement to generate indirect frequency modulation signals and concurrently receive frequency modulation signals without having an audio regeneration path.

In a third aspect of the present invention, a method for indirectly generating a frequency modulated signal, is provided. The method includes the steps of modulating an audio signal onto a low frequency signal for generating a low frequency frequency-modulated signal and multiplying the low frequency frequency-modulated signal with the low frequency signal to generate a baseband representation of the low frequency frequencymodulated signal. The baseband representation of the low frequency frequency-modulated signal is multiplied with a final frequency signal to provide a final frequency frequency-modulated signal.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Descrintion of the Drawings FIG. 1 is a block diagram of a prior art full duplex frequency modulated (FM) transceiver architecture employing a single synthesiser.

FIG. 2 is a block diagram of a full-duplex frequency modulation radio architecture employing a single synthesiser and indirect FM according to a preferred embodiment of the invention.

FIG. 3 is a block diagram of an indirect FM modulator integrated circuit (IC) according to the preferred embodiment of the invention.

FIG. 4 is a flow chart for generating frequency modulated signals according to the preferred embodiment of the invention.

Detailed Description of the Drawings Referring first to FIG. 1, a block diagram of a prior art full duplex frequency modulated (FM) transceiver architecture employing a single synthesiser is shown. The full duplex FM transceiver comprises a microphone 10, a synthesiser element 12 a power amplifier 14, a duplexer module 16, an antenna port 18, a front end filter 20, a mixer 22, an intermediate frequency (IF) processing element 24 and a loudspeaker 26.

An acoustic feedback path 28 and an audio regeneration path 30 are also shown.

In the operation of the transmitter path, an audio signal from the microphone 10 is input into the synthesiser element 12. The synthesiser element 12 modulates the audio signal with a final frequency signal (tone) to provide a modulated carrier signal. The modulated carrier signal is input to both the power amplifier 14 in the transmitter path and the mixer 22 in the receiver path. The power amplifier 14 amplifies the modulated carrier signal, inputs it to the duplexer module 16 and transmits the amplified modulated carrier signal through the antenna port 18.

In the operation of the receiving path, a signal is received in the antenna port 18, input into the duplexer module 16 and then into the front end filter 20. The filtered received signal is then input into the mixer 22 and mixed with the modulated carrier signal. The output from the mixer 22 is a modulated intermediate frequency (IF) signal. The IF signal is input to the IF processing element 24 and then output as an audio signal and fed into the loudspeaker 26. A problem arises with this design as an acoustic feedback pathl8 is then created from the loudspeaker 26 to the microphone 10 causing audio regeneration and howling in the radio via the audio regeneration path 30.

Referring now to FIG. 2, a block diagram of a full-duplex frequency modulation radio architecture employing a signal synthesiser and indirect FM technology is shown, according to the preferred embodiment of the invention. The full-duplex FM radio transmits and receives concurrent frequency modulated signals using an indirect technique for generating frequency modulated signals. The full-duplex FM radio includes a plurality of receiver sub-circuits operably coupled and arranged for receiving a radio frequency input signal and for providing a baseband output signal, wherein at least one of the receiver sub-circuits is a frequency conversion element for converting the radio frequency input signal to a lower frequency. The fullduplex FM radio further includes a plurality of transmitter sub-circuits operably coupled and arranged for receiving a baseband input signal and an unmodulated radio frequency signal wherein a first transmitter sub-circuit of the plurality of transmitter sub-circuits provides a frequency modulated radio frequency output signal. A frequency generating element is operably coupled to the frequency conversion element for concurrently providing an unmodulated radio frequency signal to the frequency conversion element and the unmodulated radio frequency signal to the first transmitter subcircuit.

The full duplex FM transceiver of FIG. 2 comprises a microphone 122, a synthesiser element 100, an audio signal 124 an unmodulated carrier signal 128, an indirect FM modulator integrated circuit (IC) 102, a reference oscillator 112, a low frequency reference signal 113, a final frequency frequency-modulated signal 126, a power amplifier 104, a duplexer module 106, an antenna port 108, a front end filter 110, a mixer 114, an intermediate frequency (IF) element 116, a loudspeaker 118 and an acoustic feedback 120.

In operation, describing first the transmitter operation, an audio signal 124 from the microphone 122 is fed into an indirect FM modulator IC 102. An unmodulated carrier signal 128 is output from the synthesiser 100 and a low reference frequency signal 113, e.g. 2.1 MHz, is output from the reference oscillator 112 to the indirect FM modulator IC 102. The indirect FM modulator IC 102 modulates the audio signal from the microphone 122 with the unmodulated carrier signal 128 to generate a final frequency frequency-modulated signal 126. The final frequency frequency-modulated signal 126 is then input to the power amplifier 104. The power amplifier 104 amplifies the final frequency frequency-modulated signal 126 and inputs amplifier modulated carrier signal to the duplexer module 106 and the modulated carrier signal is transmitted from the antenna port 108.

In the operation of the receiving path, a received signal is received at the antenna port 108 and input to the duplexer module 106. The output from the duplexer module 106 is input into the front end filter 110 and the filtered received signal input to the mixer 114 where it is mixed with the unmodulated carrier signal 128. The down-converted output signal from the mixer 114 is fed into an IF element 116 generating an audio level signal that is input to the loudspeaker 118. In conventional frequency modulation circuits, when the level of isolation between the transmitter and receiver path is poor, an acoustic feedback path 120 is created from the loudspeaker 118 to the microphone 122. However, in the preferred embodiment of the present invention, it does not cause any audio regeneration because an unmodulated carrier signal is used in the receive path.

Referring now to FIG. 3, a block diagram of the indirect FM modulator integrated circuit (IC) 102, according to the preferred embodiment of the invention, is shown. The frequency-modulation circuit for indirect generation of a frequency-modulated signal includes a frequency generating element for providing a low-frequency reference signal and a frequency control circuit, e.g. a phase locked loop, for frequency modulating a received audio signal onto the low-frequency reference signal in order to provide a low frequency frequency-modulated signal. The frequency modulation circuit further includes a frequency conversion circuit for providing a final frequency frequency-modulated signal, wherein the low frequency frequency-modulated signal is multiplied with the low-frequency reference signal to provide a baseband representation of the low frequency frequency-modulated signal.The baseband representation of the low frequency frequency-modulated signal is then multiplied to an unmodulated final frequency signal to provide a final frequency frequency-modulated signal.

The indirect FM modulator IC 102 of FIG. 3, includes a frequency control circuit, e.g. a phase locked loop (PLL), having a mixer-X5 200, a narrow band PLL filter 202, a first adder 204 and a voltage controlled oscillator (VCO) 206. The frequency control circuit outputs a signal a(t) 207, to a mixer-X1 209 and a mixer-X2 210. A reference oscillator 112 provides a low-frequency reference signal 113 and is also coupled to mixer-X1 209.

Mixer-X1 209 outputs a low frequency frequency-modulated signal b(t) 213.

A phase shifter 205 is operably coupled to the mixer-X2 210 to provide a low frequency frequency-modulated signal c(t) 211. Low pass filters 212 and 222, are operably coupled to mixer-X1 209, mixer-X2 210, mixer-X3 218 and mixer-X4 214. A phase shifter 220 is operably coupled to mixer-X4 214 and an unmodulated carrier signal 128, to provide a signal e(t) 215. A nonphase-shifted signal, signal d(t) 217, is combined with the phase-shifted signal e(t) 215 at a second adder 216 to provide a final frequency frequencymodulated signal 126. The indirect FM modulator IC 102 receives an audio signal 124 which is input to the first adder 204.

In operation, the low-frequency reference signal 113 and the audio signal 124 are input to the indirect FM modulator IC 102 and combined by the mixer-X5, the narrow band PLL filter 202, the first adder 204 and the VCO 206 to produce the signal a(t) 207.

The signal a(t) equation is: a(t) = A COS(wrt + 0(t)) and ae(t)=k S(t)dt where: a)r is a stable reference frequency, k is a PLL modulation constant, S(t) is the audio signal 124.

The signal a(t) 207 is down converted to baseband in-phase and quadrature signals by mixing the baseband signal with the low-frequency reference signal 113 in two double balanced mixers (X1) 209 and (X2) 210, where one baseband signal path is first input to the phase shifter 205, to produce two signals, a baseband representation of the low frequency frequency-modulated signal b(t) 213 and a second baseband representation of the low frequency frequency-modulated signal c(t) 211.

The signal equations are: b(t) = cos(AO(t))+cos(2rt+AO(t)) and and c(t) = -sin(##(t))-sin(2#rt+##(t)) The low pass filters 222 and 212 remove the high parts of the low frequency frequency-modulated signals b(t) 213 and c(t) 211 providing the baseband parts of the signals which are fed into the two double balanced mixers (X3) 218 and (X4) 214 of the frequency conversion circuit. The baseband part of the low frequency frequency-modulated signals b(t) 213 and c(t) 211 are upconverted by mixing the signals with an unmodulated carrier signal 128 in the two double balance mixers (X3) 218 and (X4) 214 wherein the phase shifter 220 provides the unmodulated carrier signal 128 at X4 in quadrature to X3, e.g. d(t) 217 and signal e(t) 215.The mixers (X3) 218 and (X4) 214 are summed by the second adder 216 to produce a final frequency frequency-modulated signal 126. Instead of a narrow band PLL filter 202, any low frequency FM modulator such as a digital direct synthesiser (DDS) can be used.

The signal equations are: d(t) = cos(wot - Ao(t)) + cos(o)ot + 0(t)) and e(t) = cos(#ot + A0(t)) - cos(wot - Ae(t)) The final frequency frequency-modulated signal 126 equation is: XFM(t) = cos(coot - ##(t)).

Referring now to FIG. 4, a flowchart is shown for generating frequency modulated signals, according to the preferred embodiment of the invention. Audio signals are modulated onto a low frequency signal to generate a low frequency frequency-modulated signal, as in step 300. The low frequency reference frequency-modulated signal is multiplied to the low frequency signal to generate a baseband representation of the low frequency frequency-modulated signal, as shown in step 302. The higher frequency component of the multiplied low frequency frequency-modulated signal is filtered out as shown in step 304. The baseband representation of the low frequency frequency-modulated signal is then multiplied with a final frequency signal to provide a final frequency frequency-modulated signal, as in step 306.

In this manner, full deviation frequency modulation is generated at a final frequency. Advantageously, the indirect FM modulation technique, which can be embedded into a single IC allows a full duplex vehicular speaker phone (VSP) operation with a single synthesiser radio. Cost and real estate associated with the second synthesiser or side step oscillator is reduced. In addition, the indirect FM modulation technique, according to the preferred embodiment of the invention, generates full deviation FM modulation on the final frequency without any of the problems associated with frequency multiplication methods.

Claims

1. A frequency-modulation circuit for indirect generation of a frequencymodulated signal, the frequency-modulation circuit comprising: a frequency generating element for providing a low-frequency reference signal; a frequency control circuit for frequency modulating a received audio signal onto the low-frequency reference signal to provide a low frequency frequency-modulated signal; and a frequency conversion circuit for providing a final frequency frequency-modulated signal, wherein the frequency conversion circuit multiplies the low frequency frequency-modulated signal with the lowfrequency reference signal to provide a baseband representation of the low frequency frequency-modulated signal, the baseband representation of the low frequency frequency-modulated signal being multiplied to an unmodulated final frequency signal to provide a final frequency frequencymodulated signal.

2. The frequency-modulation circuit of claim 1, wherein the frequency generating element is a direct digital synthesiser.

3. The frequency-modulation circuit of claim 1, wherein the frequency generating element is a single synthesiser.

4. The frequency-modulation circuit of claim 1, wherein the frequency control circuit is a phase locked loop.

5. A full-duplex radio for transmitting and receiving concurrent frequency modulated signals, the full-duplex radio using an indirect technique for generating frequency modulated signals and comprising: a plurality of receiver sub-circuits operably coupled and arranged for receiving a radio frequency input signal and for providing a baseband output signal, wherein at least one of the plurality of receiver sub-circuits is a frequency conversion element for converting the radio frequency input signal to a lower frequency; a plurality of transmitter sub-circuits operably coupled and arranged for receiving a baseband input signal and an unmodulated radio frequency signal and wherein a first transmitter sub-circuit provides a frequency modulated radio frequency output signal; and a frequency generating element operably coupled to the frequency conversion element for concurrently providing an unmodulated radio frequency signal to the frequency conversion element and to the first transmitter sub-circuit.

6. A method for generating frequency modulated signals, the method comprising the steps of: modulating an audio signal onto a low frequency signal for generating a low frequency frequency-modulated signal; multiplying the low frequency frequency-modulated signal with the low frequency signal to generate a baseband representation of the low frequency frequency-modulated signal; and multiplying the baseband representation of the low frequency frequency-modulated signal with an unmodulated final frequency signal to provide a final frequency frequency-modulated signal.

7. A full-duplex frequency modulation radio substantially as described herein with respect to FIG. 2.

8. A frequency-modulation circuit substantially as described herein with respect to FIG. 3.