GB2135556A - Radio transmitter arrangements - Google Patents

Abstract

In a mobile radio transmitter arrangement there is provided a feedback arrangement between the aerial feed path and an input to the modulator stage (2), for correcting envelope distortion and to prevent overload in the event of aerial mismatch or disconnection. The feedback signal is derived from both forward and reverse directional couplers (6, 7) in the aerial feed path. <IMAGE>

Description

SPECIFICATION Radio Transmitter Arrangements The present invention relates to radio transmitter arrangements.

in particular, although not exclusively, the invention relates to mobile radio transmitter arrangements for VHF communications systems.

According to one aspect of the present invention in a radio transmitter arrangement having an output path for modulated carrier signals for connection to an aerial, there are provided means responsive to modulated carrier signals passing in either direction along said output path to apply a feedback signal to a modulator stage of said transmitter arrangement in such a sense as to tend to correct for envelope distortion introduced between an input to said modulator stage and said output path.

According to another aspect of the present arrangement in a mobile radio transmitter arrangement there are provided first and second directional power couplers connected in the modulated carrier output path of the arrangement to apply to respective demodulators respective fractions of the modulator carrier signal power passing in either direction along said output path, and means to apply a feedback signal to a modulator stage of the transmitter arrangement in dependence upon outputs from one or both of said demodulators in such a sense as to tend to reduce distortion in the modulated carrier signal.

A radio transmitter arrangement in accordance with the present invention will now be described by way of example with reference to the accompanying drawing, which shows part of the transmitter arrangement schematically.

Referring to the drawing the transmitter arrangement, which may form part of a mobile transmitter-receiver equipment for a VHF communications system, comprises a carrierfrequency signal source 1, which may be a frequency synthesizer, a modulator stage 2, by means of which carrier signals from the source 1 are arranged to be amplitude modulated by signals from an error amplifier 3, and a power amplifier 4.The modulated and amplified signals from the output of the amplifier 4 are arranged to be applied to an aerial 5 by way of a reverse directional coupler 6 and a forward directional coupler 7, these couplers 6 and 7 being arranged to divert a fraction of any radio frequency power flowing respectively from or to the aerial 5 to respective envelope detectors or demodulators 8 and 9, the outputs of which are applied by way of a summing circuit 10 and a correction circuit 1 1 to the inverting input of the combining amplifier 3. The correction circuit 11 comprises a differential amplifier 12 having a semiconductor diode 13 arranged in a feedback path to its inverting input to provide a non-linear gain characteristic such as to compensate for imperfect diode characteristics in the envelope detectors 8 and 9.

The arrangement allows the amplifier 4 to be designed for efficiency at the expense of amplitude linearity. The amplifier 4 on its own may introduce gross distortion of the signal envelope. By detecting and feeding back to the inverting input of the amplifier 3 a signal representing the modulation envelope this distortion can be greatly reduced.

It is common in mobile transmitter-receiver arrangements for there to be considerable mismatch at the aerial outlet, or even for the aerial to be disconnected altogether, either during the testing or setting up of the transmitterreceiver arrangement or accidentally, and the inclusion of the reverse power coupler 6 and its envelope detector 8 serves to prevent the overloading of the modulator stage 2 and the amplifier 4 under such conditions.

1. A radio transmitter arrangement having an output path for modulated carrier signals for connection to an aerial, wherein there are provided means responsive to modulated carrier signals passing in either direction along said output path to apply a feedback signal to a modulator stage of said transmitter arrangement in such a sense as to tend to correct for envelope distortion introduced between an input to said modulator stage and said output path.

2. A mobile radio transmitter arrangement wherein there are provided first and second directional power couplers connected in the modulated carrier output path of the arrangement to apply to respective demodulators respective fractions of the modulator carrier signal power passing in either direction along said output path, and means to apply a feedback signal to a modulator stage of the transmitter arrangement in dependence upon outputs from one or both of said demodulators in such a sense as to tend to reduce distortion in the modulated carrier signal.

3. A mobile radio transmitter arrangement in accordance with Claim 2 wherein said feedback signal is applied to said modulator stage by way of a correction circuit arranged to compensate for imperfect rectifier diode characteristics in said demodulators.

4. A mobile radio transmitter arrangement in accordance with Claim 3 wherein said correction circuit comprises an amplifier circuit having a rectifier diode connected in a negative feedback path between an output path and an input path thereof.

5. A radio transmitter arrangement in accordance with any preceding claim in which said feedback signal serves to prevent overloading of at least said modulator stage in the event of mismatch on said output path.

6. A radio transmitter arrangement substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Radio Transmitter Arrangements The present invention relates to radio transmitter arrangements. in particular, although not exclusively, the invention relates to mobile radio transmitter arrangements for VHF communications systems. According to one aspect of the present invention in a radio transmitter arrangement having an output path for modulated carrier signals for connection to an aerial, there are provided means responsive to modulated carrier signals passing in either direction along said output path to apply a feedback signal to a modulator stage of said transmitter arrangement in such a sense as to tend to correct for envelope distortion introduced between an input to said modulator stage and said output path. According to another aspect of the present arrangement in a mobile radio transmitter arrangement there are provided first and second directional power couplers connected in the modulated carrier output path of the arrangement to apply to respective demodulators respective fractions of the modulator carrier signal power passing in either direction along said output path, and means to apply a feedback signal to a modulator stage of the transmitter arrangement in dependence upon outputs from one or both of said demodulators in such a sense as to tend to reduce distortion in the modulated carrier signal. A radio transmitter arrangement in accordance with the present invention will now be described by way of example with reference to the accompanying drawing, which shows part of the transmitter arrangement schematically. Referring to the drawing the transmitter arrangement, which may form part of a mobile transmitter-receiver equipment for a VHF communications system, comprises a carrierfrequency signal source 1, which may be a frequency synthesizer, a modulator stage 2, by means of which carrier signals from the source 1 are arranged to be amplitude modulated by signals from an error amplifier 3, and a power amplifier 4.The modulated and amplified signals from the output of the amplifier 4 are arranged to be applied to an aerial 5 by way of a reverse directional coupler 6 and a forward directional coupler 7, these couplers 6 and 7 being arranged to divert a fraction of any radio frequency power flowing respectively from or to the aerial 5 to respective envelope detectors or demodulators 8 and 9, the outputs of which are applied by way of a summing circuit 10 and a correction circuit 1 1 to the inverting input of the combining amplifier 3. The correction circuit 11 comprises a differential amplifier 12 having a semiconductor diode 13 arranged in a feedback path to its inverting input to provide a non-linear gain characteristic such as to compensate for imperfect diode characteristics in the envelope detectors 8 and 9. The arrangement allows the amplifier 4 to be designed for efficiency at the expense of amplitude linearity. The amplifier 4 on its own may introduce gross distortion of the signal envelope. By detecting and feeding back to the inverting input of the amplifier 3 a signal representing the modulation envelope this distortion can be greatly reduced. It is common in mobile transmitter-receiver arrangements for there to be considerable mismatch at the aerial outlet, or even for the aerial to be disconnected altogether, either during the testing or setting up of the transmitterreceiver arrangement or accidentally, and the inclusion of the reverse power coupler 6 and its envelope detector 8 serves to prevent the overloading of the modulator stage 2 and the amplifier 4 under such conditions. CLAIMS

1. A radio transmitter arrangement having an output path for modulated carrier signals for connection to an aerial, wherein there are provided means responsive to modulated carrier signals passing in either direction along said output path to apply a feedback signal to a modulator stage of said transmitter arrangement in such a sense as to tend to correct for envelope distortion introduced between an input to said modulator stage and said output path.

2. A mobile radio transmitter arrangement wherein there are provided first and second directional power couplers connected in the modulated carrier output path of the arrangement to apply to respective demodulators respective fractions of the modulator carrier signal power passing in either direction along said output path, and means to apply a feedback signal to a modulator stage of the transmitter arrangement in dependence upon outputs from one or both of said demodulators in such a sense as to tend to reduce distortion in the modulated carrier signal.

3. A mobile radio transmitter arrangement in accordance with Claim 2 wherein said feedback signal is applied to said modulator stage by way of a correction circuit arranged to compensate for imperfect rectifier diode characteristics in said demodulators.

4. A mobile radio transmitter arrangement in accordance with Claim 3 wherein said correction circuit comprises an amplifier circuit having a rectifier diode connected in a negative feedback path between an output path and an input path thereof.

5. A radio transmitter arrangement in accordance with any preceding claim in which said feedback signal serves to prevent overloading of at least said modulator stage in the event of mismatch on said output path.

6. A radio transmitter arrangement substantially as hereinbefore described with reference to the accompanying drawing.