GB2076238A

GB2076238A - Radio receiver

Info

Publication number: GB2076238A
Application number: GB8028339A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1980-05-16
Filing date: 1980-09-02
Publication date: 1981-11-25
Also published as: FR2482804A1; NO811615L; SE8103035L; GB2076238B; DE3118852A1; DK215781A

Abstract

This invention relates to a radio receiver for receiving frequency shift keyed radio signals. The receiver has a mixer 2 which receives the transmitted signals, a local oscillator 3 which feeds a mixing signal to the mixer 2 and a detector 7 which is fed from the mixer 2 through a single receiving channel. A chopper 8 applies a chopping signal to the local oscillator 3 to change the frequency of the oscillator between two values none of which lies midway between the two frequencies, the chopping signal being fed also to the detector 7 in synchronism with that applied to the local oscillator 3 to logically compare the chopper signal with the mixer output so that the detector detects the transmitted binary bits. The invention is useful in the fields of paging, telemetry and communication systems. <IMAGE>

Description

SPECIFICATION Radio receiver This invention relates to a radio receiver and in particular to a receiver for receiving fre quency shift keyed (FSK) radio signals.

In FSK systems binary bits are transmitted by transmitting logical" 1" binary bits at one frequency and logical "0" binary bits as another frequency. The binary bits are trans -mitted at a predetermined data rate.

On reception it is necessary to decide whether a received frequency reprsents a "1" or a "0" and a known receiver for achieving this uses two receiving channels.

Each receiving channel includes a mixer and received signals are fed to the mixers in phase quadrature to one another. A local oscillator feeds a mixing signal to each mixer, the mixing signals being at a frequency which is midway between the frequencies transmit- ted for the logical ''1'' and "0". Output signals from the two mixers are filtered, ampli fied and fed to a detector which provides output signals representative of the transmit ted data. Such a two channel system has been described in U.K. Patent Specification No. 1172977.

The use of two separate receiving channels results in duplication of components and in a receiver which is complex and costly.

This invention seeks to provide a radio receiver for receiving frequenc" shift keyed radio signals in which the above mentioned disadvantages are mitigated.

According to this invention there is pro vided a radio receiver for receiving frequency shift keyed (FSK) radio signals in which differ ing binary bits are transmitted at respective ones of two frequencies; the receiver compris ing a mixer for receiving the transmitted sig nals: a local oscillator arranged to feed a mixing signal to the mixer; a detector ar ranyed to be fed with an l.F. signal from the mixer through a single receiving channel: a chopper arranged to apply a chopping signal to the local oscillator to change the frequency oscillator, at a predetermined rate, between two values, none of which lies midway be tween the two frequencies, the chopping sig nal being fed, in operation, to the detector in synchronism with that applied to the local oscillator whereby the detector is operative to detect the transmitted binary bits.

The detector may be a sampling detector in which case the chopping signal is effective to change the sampling rate of the detector, or the detector may be a linear detector in which case the chopping signal is effective to change the frequency of a heterodyning signal which is mixed with the l.F. signal received by the detector.

This invention will now be described further with reference to the drawing which shows a radio receiver in accordance with the invention.

Referring to the drawing frequency shift keyed radio signals are received at an aerial 1 and are fed to one input of a mixer 2, a second input of which receives a mixing signal from a local oscillator 3. Intermediate frequency (I.F.) output signals from the mixer 2 are fed via a low pass filter 4, a limiting amplifier 5 and a high pass filter 6 to one input of a detector 7.

A chopper 8 feeds a chopping signal to a second input of the local oscillator and this chopping signal is effective to cause the local oscillator to switch between two frequencies at the chopping rate, neither of which frequencies is midway between the two transmitted frequencies in the received FSK signal.

The chopper 8 is also connected to feed the chopping signal to the second input of the detector 7 in synchronism with the chopping signal fed to the local oscillator 3. At any instant the mixer 2 will provide either a relatively high frequency tone or a relatively low frequency tone in dependence upon the transmitted data at that instant and the frequency of the local oscillator 3. If the chopper output is such as to set the local oscillator to the higher of its two frequencies and the mixer 2 provides an output which is also the relatively high frequency tone then the frequencies of the local oscillator 3 and the received signal are at their furthest apart.

If the mixer 2 provides an output signal which is at the relatively low frequency tone whilst the local oscillator 3 is at its high frequency then the transmitted frequency of the received signal and the local oscillator frequency are at their closest together.

Feeding the chopping signal from the chopper 8 to the detector 7 in synchronism with that fed to the local oscillator 3 enables the detector 7 to detect the appropriate binary bit values by effectively comparing the chopper signal with the mixer output signal in logical fashion to provide an output signal which is representative of the transmitted data. Binary bit output signals appear at an output terminal 9 of the detector 7.

The switching of the local oscillator 3 offers advantages by effectively time division multiplexing one channel so that it operates as two channels. This results in the use of fewer components and in a less complex system than would be achieved by using more than one channel.

The invention is envisaged as being useful in the fields of paging, telemetry and commu nicaion systems.

I. A radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at respective ones of two frequencies; the receiver compris

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

Claims

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

SPECIFICATION Radio receiver This invention relates to a radio receiver and in particular to a receiver for receiving fre quency shift keyed (FSK) radio signals.

In FSK systems binary bits are transmitted by transmitting logical" 1" binary bits at one frequency and logical "0" binary bits as another frequency. The binary bits are trans -mitted at a predetermined data rate.

On reception it is necessary to decide whether a received frequency reprsents a "1" or a "0" and a known receiver for achieving this uses two receiving channels.

Each receiving channel includes a mixer and received signals are fed to the mixers in phase quadrature to one another. A local oscillator feeds a mixing signal to each mixer, the mixing signals being at a frequency which is midway between the frequencies transmit- ted for the logical ''1'' and "0". Output signals from the two mixers are filtered, ampli fied and fed to a detector which provides output signals representative of the transmit ted data. Such a two channel system has been described in U.K. Patent Specification No. 1172977.

The use of two separate receiving channels results in duplication of components and in a receiver which is complex and costly.

This invention seeks to provide a radio receiver for receiving frequenc" shift keyed radio signals in which the above mentioned disadvantages are mitigated.

According to this invention there is pro vided a radio receiver for receiving frequency shift keyed (FSK) radio signals in which differ ing binary bits are transmitted at respective ones of two frequencies; the receiver compris ing a mixer for receiving the transmitted sig nals: a local oscillator arranged to feed a mixing signal to the mixer; a detector ar ranyed to be fed with an l.F. signal from the mixer through a single receiving channel: a chopper arranged to apply a chopping signal to the local oscillator to change the frequency oscillator, at a predetermined rate, between two values, none of which lies midway be tween the two frequencies, the chopping sig nal being fed, in operation, to the detector in synchronism with that applied to the local oscillator whereby the detector is operative to detect the transmitted binary bits.

The detector may be a sampling detector in which case the chopping signal is effective to change the sampling rate of the detector, or the detector may be a linear detector in which case the chopping signal is effective to change the frequency of a heterodyning signal which is mixed with the l.F. signal received by the detector.

This invention will now be described further with reference to the drawing which shows a radio receiver in accordance with the invention.

Referring to the drawing frequency shift keyed radio signals are received at an aerial 1 and are fed to one input of a mixer 2, a second input of which receives a mixing signal from a local oscillator 3. Intermediate frequency (I.F.) output signals from the mixer 2 are fed via a low pass filter 4, a limiting amplifier 5 and a high pass filter 6 to one input of a detector 7.

A chopper 8 feeds a chopping signal to a second input of the local oscillator and this chopping signal is effective to cause the local oscillator to switch between two frequencies at the chopping rate, neither of which frequencies is midway between the two transmitted frequencies in the received FSK signal.

The chopper 8 is also connected to feed the chopping signal to the second input of the detector 7 in synchronism with the chopping signal fed to the local oscillator 3. At any instant the mixer 2 will provide either a relatively high frequency tone or a relatively low frequency tone in dependence upon the transmitted data at that instant and the frequency of the local oscillator 3. If the chopper output is such as to set the local oscillator to the higher of its two frequencies and the mixer 2 provides an output which is also the relatively high frequency tone then the frequencies of the local oscillator 3 and the received signal are at their furthest apart.

If the mixer 2 provides an output signal which is at the relatively low frequency tone whilst the local oscillator 3 is at its high frequency then the transmitted frequency of the received signal and the local oscillator frequency are at their closest together.

Feeding the chopping signal from the chopper 8 to the detector 7 in synchronism with that fed to the local oscillator 3 enables the detector 7 to detect the appropriate binary bit values by effectively comparing the chopper signal with the mixer output signal in logical fashion to provide an output signal which is representative of the transmitted data. Binary bit output signals appear at an output terminal 9 of the detector 7.

The switching of the local oscillator 3 offers advantages by effectively time division multiplexing one channel so that it operates as two channels. This results in the use of fewer components and in a less complex system than would be achieved by using more than one channel.

The invention is envisaged as being useful in the fields of paging, telemetry and commu nicaion systems.

I. A radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at respective ones of two frequencies; the receiver compris ing a mixer for receiving the transmitted signals; a local oscillator arranged to feed a mixing signal to the mixer; a detector arranged to be fed with an l.F. signal from the mixer through a single receiving channel; a chopper arranged to apply a chopping signal to the local oscillator to change the frequency of the oscillator, at a predetermined rate, between two values, none of which lies between the two frequencies, the chopping signal being fed in operation to the detector in synchronism with that applied to the local oscillator whereby the detector is operative to detector the transmitted binary bits.
2. A radio receiver as claimed in Claim 1 in which the detector is a sampling detector the chopping signal being effective to change the sampling rate of the detector.
3. A radio receiver as claimed in Claim 1 in which the detector is a linear detector the chopping signal being effective to change the frequency of a heterodyning signal which is mixed with the l.F. signal received by the detector.
4. A radio receiver substantially as herein described with reference to and as illustrated in the drawing.