GB2086158A

GB2086158A - Radio receiver

Info

Publication number: GB2086158A
Application number: GB8027511A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1980-08-23
Filing date: 1980-08-23
Publication date: 1982-05-06
Also published as: FR2489059A1; NO152190C; NO152190B; DK155066C; NO812770L; DE3132233C2; SE8104972L; SE452832B; HK48485A; GB2086158B; FR2489059B1; DE3132233A1; DK155066B; DK365181A

Abstract

This invention relates to a radio receiver for receiving frequency shift keyed radio signals in which differing binary bits are transmitted at respective ones of two frequencies. The receiver has a mixer (2) which receives the transmitted signals and a local oscillator (3) which feeds a mixing signal to the mixer and is switchable between two frequencies neither of which lies midway between the two transmitted frequencies. A controlling switch (6) is fed with signals derived from the mixer (2) and switches the local oscillator (3) so as to maintain the frequency of the signal derived from the mixer (2) at a predetermined value. The invention is particularly useful in pagers. <IMAGE>

Description

SPECIFICATION Radio receiver This invention relates to a radio receiver and in particular to a receiver for receiving frequency shift keyed (FSK) radio signals. In FSK systems binary bits are transmitted by transmitting logical one binary bits at one frequency and logical nought binary bits at another frequency. The binary bits are transmitted at a predetermined data rate.

On reception it is necessary to decide whether a received frequency represents a "one" or a "nought" 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 transmitted for the logical "one" and "nought". Output signals from the two mixers are filtered, amplified and fed to a detector which provides output signals representative of the transmitted 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.

In United Kingdom Patent Application No. 8016220 and Application No. 8028339 which claims priority from Application No. 801 6220 there is described a radio receiver for receiving frequency shift keyed radio signals in which only a single receiving channel is used.

In the invention of the above-mentioned applications provides a radio receiver for receiving frequency shift keyed (F.SK) radio signals in which differing binary bits are transmitted at respective ones of two frequencies; the receiver comprising 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 midway 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 detect th e the transmitted-binary bits.

At any instant the mixer 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. If the chopper output is such as to set the local oscillator to the higher of its two frequencies and the mixer provides an output which is also the relatively high frequency tone then the frequencies of the local oscillator and the received signal are at their furthest apart. If the mixer output is at the relatively low frequency whilst the local oscillator is at its high frequency then the transmitted frequency of the received signal and the local oscillator frequency are at their closest together.

The detector is operative to compare the chopper signal with the mixer output in logical fashion and provides an output signal which represents the transmitted data.

The above described single channel receiver provides an advantage in switching the frequency of the local oscillator in that a single channel is effectively time division multiplexed so that it operates as two channels. This provides a significant reduction in cost and complexity.

A problem arises with the above described single channel receiver when operation is required at higher data rates. It is essential for the receiver to operate satisfactorily that the mixer provides an output which is one or other of two distinct tones.

As the data rate is increased a point is reached when the mixer output ceases to be a discrete tone and becomes a band of frequencies.

This invention seeks to provide a receiver for receiving FSK signals which utilises a single receiving channel and in which the above mentioned disadvantage is mitigated.

According to this invention there is provided 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 comprising a mixer for receiving the transmitted signals; a local oscillator switchable between two frequencies neither of which is midway between the two transmitted frequencies, the oscillator being arranged to feed a mixing signal to the mixer; and a controlling switch responsive to signals derived from the mixer for switching the oscillator to maintain the frequency of the signals derived from the mixer at a predetermined value.

The controlling switch may include a frequency detector for sensing the frequency of the signals derived from the mixer and in one embodiment of the invention the frequency detector is a digital frequency detector.

Aflip-flop may be provided coupled to the frequency detector and operative to change state each time the frequency detector senses that the frequency of the signals derived from the mixer has changed from the predetermined value.

The flip-flop may be coupled to the local oscillator to effect switching of the local oscillator at each change of state of the flip-flop.

An exemplary embodiment of the invention will now be described 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 which is switchable between two frequencies.

Output signals from the mixer 2 are fed via a low pass filter 4 and a limiting amplifier 5 to a controlling switch 6 which is coupled to the local oscillator 3 to effect switching thereof and to an output terminal 7.

The signal received by the aerial 1 will be at one or other of two frequencies namely a relatively high frequency or a relatively low frequency in dependence upon whether the transmitted data is a logical one or nought. The received signals on being fed to the mixer 2 are mixed with a local oxcillator signal fed from the mixer 3. The local oscillator 3 is switchable between two values one of which is a relatively high frequency and the other a relatively low frequency. Both of these frequencies lie between the two transmitted frequencies.

Assume that the high frequency is received at a particular instant at the aerial 1 and is fed to the mixer 2 and that the local oscillator 3 is set to its low frequency value. Under these circumstances the mixer 2 will provide a relatively high frequency tone, referred to as the "high tone" should either the frequency of the received signals change to the lower operative frequencies or that of the local oscillator change to the high value then the output signals provided by the mixer 2 will change to a relatively low frequency output tone referred to as the "low tone".

The controlling switch 6 which is fed with signals from the mixer 2 is operative to sense whether the mixer 2 provides the high tone or the low tone output and controls the local oscillator to maintain the output signals from the mixer 2 at the frequency of the high tone.

The controlling switch 6 comprises a digital frequency detector 8 which is fed with output signals from the mixer 2 via the low pass filter 4 and the limiting amplifier 5, and a flip-flop 9 which is controlled by the output state of the digital frequency detector 8. The flip-flop 9 provides a switching output which is operative to set the local oscillator to one or other of its two values.

In operation the digital frequency detector 8 detects whether the output signals fed from the mixer 2 are at the high tone frequency and if this is the case the flip-flop 9 is maintained in its existing state so as to maintain the local oscillator 3 at the mixing frequency so as to continue producing the high tone. Should the transmitted data change in logical state so that the frequency fed to the mixer 2 changes the output signals provided by the mixer 2 will change to the low tone. This change is detected by the digital frequency detector 8 which provides an output signal to cause the flip-flop 9 to change state and this change of state is fed as a logical switching signal to the local oscillator 3.

The switching signal fed from the flip-flop 9 causes the local oscillator 3 to change state to restore the mixer output signal to the high tone.

The restoration of the high tone is sensed by the digital frequency detector 8 which is inoperative to maintain the flip-flop 9 in its existing state and consequently to maintain the local oscillator 3 at its existing operating frequency.

Since the flip-flop 9 is caused to change state each time the received data changes logical state then the logical output state of the flip-flop 9 effectively follows the logical state of the transmitted data which is recovered by providing an output terminal 7 coupled to the output of the flip-flops 9.

The receiver of the present invention is capable of operating at high data rate because the local oscillator 3 is now only switched when there is a change in state of the transmitted data and the whole receiver is effectively just following the logical state of the transmitted data. The problems encountered in the above described Prior Art arrangement due to high speed chopping of the local oscillator are now avoided.

The invention has been described by way of example only and modifications may be made without departing from the scope of the invention.

The controlling switch 6 need not be exactly in the form described but any suitable controlling switch may be employed which is operative to switch the local oscillator in dependence upon the frequency of the signal received by the switch. The detector 8 need not be a digital frequency detector but any suitable detector such as a linear detector, a frequency discriminator or a pulse counting detector may be used.

Claims

1. 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 comprising a mixer for receiving the transmitted signals; a local oscillator switchable between two frequencies neither of which is midway between the two transmitted frequencies, the oscillator being arranged to feed a mixing signal to the mixer; and a controlling switch responsive to signals derived from the mixer for switching the oscillator to maintain the frequency of the signal derived from the mixer at a predetermined value.

2. A radio receiver as claimed in claim 1 in which the controlling switch includes a frequency detector for sensing the frequency of the signals derived from the mixer.

3. A radio receiver as claimed in claim 2 in which the frequency detector is a digital frequency detector.

4. A radio receiver as claimed in claim 2 or 3 in wich a flip-flop is coupled to the frequency detector and is operative to change state each time the frequency detector senses that the frequency of the signals derived from the mixer has changed from the predetermined value.

5. A radio receiver as claimed in claim 4 in which the flip-flop is coupled to the local oscillator to effect switching of the local oscillator at each change of state of the flip-flop.

6. A radio receiver substantially as herein described with reference to and as illustrated in the drawing.