GB2146876A

GB2146876A - FSK radio receiver

Info

Publication number: GB2146876A
Application number: GB08324666A
Authority: GB
Inventors: Peter Edward Chadwick; Philip Albert Kenneth Moon
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1983-09-14
Filing date: 1983-09-14
Publication date: 1985-04-24
Also published as: DE3431731A1; SE8404304L; GB2146876B; GB8324666D0; JPS6085651A; SE8404304D0; FR2551937A1

Abstract

The receiver has a mixer (2) which receives the transmission frequencies (f and f+ DELTA f). A local oscillator (3) feeds a mixing signal to the mixer (2) and is switchable between two frequencies neither of which lies midway between the two transmission frequencies, the switching being controlled by the output. The low or band mass filtered signal to detector 8 thus comprises one of four frequencies which fall in respective upper and lower bands, which bands are distinguished by the detector to provide the binary output at (7). The local oscillator frequency is switched to its alternative frequency either when an upper band frequency is detected or alternatively in the absence of such a frequency. The receiver thus requires only one filter, and one mixer, with no quadrature local oscillator. <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" 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.

2076238 there is described a radio receiver for receiving frequency shift keyed radio signals in which only a single receiving channel is used.

The invention of the above-mentioned application provides 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 arranged to feed a mixing signal to the mixer, a detector arranged to be fed with an I.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 the 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 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.

In our British Patent Application Number 8027511 there is described a receiver for receiving FSK signals which utilises a single receiving channel to overcome the before mentioned difficulties. That receiver comprises 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.

This receiver has been found to operate quite satisfactorily but it does require that the oscillator is switched between two frequencies which are spaced equidistantly one each side of the arithmetic mean of the transmitted frequencies.

The present invention seeks to provide a receiver in which such equidistant spacing is not required.

According to the invention there is provided a radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at a different one of two transmission frequencies, the receiver comprising a mixer having a signal input for receiving the transmission frequencies, an input for a local oscillator and an output, a local oscillator having an output coupled with the local oscillator input of the mixer, which local oscillator has a switching signal input and is switchable between two frequencies which are offset relative to the arithmetic mean of the two transmission frequencies by different amounts, a detector having an input coupled with the output of the mixer and a switching output coupled with the switching input of the local oscillator which detector is arranged to determine when the output from the mixer contains a frequency which is the difference between one of the two transmission frequencies and one of the local oscillator frequencies or the other of the transmission frequencies and the other of the local oscillator frequencies and to provide a switching signal for switching the local oscillator to its alternative frequency either when such a frequency is detected, or alternatively in the absence of detection of such a frequency, whereby the switching signal follows the transmitted binary bits and is coupled to a receiver output to provide a signal representative of the transmitted binary information.

The receiver may include muting means responsive in the absence of a received signal to inhibit switching of the local oscillator frequency.

Alternatively, the switching signal may be coupled to the receiver output via switch means and the receiver may include a muting means responsive in the presence of a received signal to close the switch and to open the switch in the absence of a received signal.

In orderto reduce the risk of adjacent channel interference the local oscillator frequencies may each be in a frequency band the extremities of which are defined by the two transmission frequencies.

The receiver may include a filter coupled between the output of the mixer and the input of the detector and having a pass band arranged to permit passage of the frequencies to be detected and to reject higherfrequencies. The filter may be a low pass filter.

The detector may include a frequency detector circuit for sensing the frequency of the signals derived from the mixer and in one embodiment of the invention the frequency detector circuit 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-fiop.

In order that the invention and its various other preferred features may be understood more easily, some embodiments thereof will now be described, byway of example only, with reference to the drawings, in which: Figure 1 is a block schematic diagram of a radio receiver constructed in accordance with the invention, Figure 2 shows the distribution of signal and oscillator frequencies of the arrangement of Figure 1 on a frequency scale, together with the mixer output frequencies, Figure 3 shows the relative position of wanted frequencies and unwanted adjacent channel frequencies of the arrangement of Figure 1 on a frequency scale, Figure 4 is a block schematic diagram of an alternative configuration of a radio receiver constructed in accordance with the invention, Figure 5 is a block schematic diagram of yet another configuration of radio receiver constructed in accordance with the invention, and Figure 6 is a block schematic diagram similar to Figure 5 but showing an alternative inhibit configuration.

In the drawing of Figure 1, frequency shift keyed radio signals f and f+Af, corresponding to digital modulation in dependence upon whether the transmitted data is a logical nought or logical one, are received at an aerial 1. These signals are fed to one input of a mixer 2, a second input of which receives a mixing signal from a local oscillator3 which is switchable between two frequencies a relatively low frequency fO, and a relatively high frequency fo2 each of which lies between the frequencies f and f+Af but neither of which lies midway therebetween as can be seen from Figure 2 Output signals from the mixer 2 are fed via a low pass filter 4 and a limiting amplifiers to a detector 6 which is coupled both to the local oscillator 3, to effect switching thereof, and to an output terminal 5 In the presence of a signal the output of the mixer will contain the frequencies f, -f, f02-f, f+Af-f01, f+Af-f02. 0, and f02 are not symmetrically disposed around the arithmetical mean off and f+Af and accordingly the frequencies fo2-f and f+Af-0, are not equal. Likewise, the frequencies f,, -f and f+Af-f2 are not equal. The frequencies f+Af and f+Af-f01 are higher frequencies and f01-f and f+Af-fo2 are lower frequencies. These are distinguished as "high tones" and "low tones" respectively.

The low pass filter is of a bandwidth sufficient to pass both low and high tones without distortion whilst signals in adjacent channels produce in the mixing process frequencies which are higher than the high tone which are rejected by the low pass filter 4. This is shown in Figure 3.

The filter 4 provides the selectivity of the receiver and being a low pass filter is relatively simple to produce. It need not be a true low pass filter as the lowest frequency it requires to pass is f01 -f. This means that capacitive coupling can be employed. In addition signal to noise ratio can be improved because of the reduction of noise bandwidth. The inciusion of what is effectively a band pass filter has the advantage of reducing the deterious effects of flicker

( 1 ) noise produced in the mixer.

There are many types of detector circuits 8 suitable for use in the detector 6 which will occur to those skilled in the art. For example a phase lock loop tone detector circuit arranged to lock each high tone may be employed. A suitable device is the National Semiconductors LM567 tone detector integrated circuit. Other possibilities are linear detector circuits, pulse counting detector circuits which provide a voltage which varies in dependence upon the pulse count or even a frequency discriminator circuit such as the Foster Seeley or Travis. An even more convenient arrangement for realisation in integrated circuit form is a digital form of frequency detection circuit employing counters and timing to detect the difference between high and lowtones.

The detector circuit may include a trigger circuit e.g. a flip-flop circuit 9 (as shown in Figure 1) or a Schmitt Trigger to provide a switching signal forthe local oscillator from an analogue signal.

The configuration of Figure 1 may be arranged to operate as follows. If we assume that the incoming signal is, at a particular instant in time, at frequency f, and the local oscillator 3 at frequency f02 then a high tone results at the input to the detector. The output from the detector can be arranged to provide a voltage which does not cause the flip flop 9 to change its state and accordingly the local oscillator is maintained at frequency f02 when this high tone is present. If the incoming signal changes to frequency f+Af, the output from the mixer becomes a low tone value.The frequency discriminator function of the detector is such that the detector output will change to a different voltage and this will cause the flip flop to change its state and cause the oscillator to change to frequency f01. The output from the detector becomes a high tone again, the detector voltage changes back to its original voltage and the system will stay in this condition until a further change in input frequency occurs. Thus the switching output from the flip flop circuit 9 of the detector 6 will be stepped between two levels in sympathy with the frequency of the incoming signal and reproduces the binary information transmitted in the FSK mode. This information is coupled to an output 7.

The local oscillator 3 may be of a type that is constrained to operate only at one of two different frequencies by virtue of its design. One possibility of such an oscillator is a multivibrator with switched timing currents which are controllable by the output from the detector. Another possibility is the use of a trigger circuit eg a bistable circuit to provide predetermined bias levels for changing bias on a variable capacity diode in the oscillator circuit in dependence upon detection of a high tone or low tone.

As f,, and f02 are not symmetrically disposed about the arithmetic mean off and f+Af, then the frequencies fo2-f and f+nf-f,, are not equal.

Similarly, frequencies f01 -f and f+Af-fo2 are not equal.(See Figures 2). However provided that the frequency detector is such as to be able to discriminate between high tone and low tone values the system will function.

Although the previous description of operation has referred to maintaining a high tone value at the input to the detector it will be appreciated that the arrangement could be designed to maintain a low tone value. Such a configuration is intended to fall within the scope of this invention. Maintenance or detection of the high tone value is advantageous in some circumstances for example where we employ a tone detector. Such a detector requires a finite number of cycles for adequate detection and accordingly the higher tone will be responded to more quickly.

Although the preferred arrangement employs local oscillator frequencies which are located within a band the extremities of which are defined by the transmission frequencies, so as to provide good adjacent channel rejection, itwill be appreciated that they could be located outside this band. In addition the local oscillator frequencies may be both higher than or both lower than this frequency band.

Adjacent channel rejection may not be a problem in some applications eg paging if a limited number of channels are required.

Another embodiment of the invention is shown in Figure 4. This embodiment comprises an arrangement similar to Figure 1 but in which the low pass filter is replaced by a band pass filter 10 which is arranged to pass the high tones and reject the low tones. A simple detector circuit 11 can now be employed in the detector 6 to provide an output voltage which switches a bistable trigger circuit 9 to its opposite state when a high tone is detected. The trigger circuit serves to switch the local oscillator3 to its alternative frequency so restoring low tone output from the mixer.

An alternative arrangement which operates similarly employs a band pass or low pass filter to select only the low tone. Switching is in this case effected each time a low tone is detected. Both such arrangements are considered to fall within the scope of this invention.

Yet another arrangement is shown in Figure 5.

This arrangement is similar to that of Figure 4.

However, the detector 12 is arranged to trigger the trigger circuit 9 when no high tone is detected.

Accordingly the circuit strives to switch to a high tone condition. In the absence of a received signal the oscillator would continually switch between the two frequencies. However to avoid this an inhibit circuit 13 is provided which detects whether there is a signal output from the band pass filter and if there is not it provides an inhibit signal to the trigger circuit 9 to prevent switching. A suitable circuit is a simple level detector or rectifier.

An alternative inhibit arrangement is shown in Figure 6 where switch 14 is provided between the local oscillator 3 and the input to the mixer 2. The switch is arranged to be closed in response to detection of a signal by the inhibit circuit 13.

It will be appreciated that although the arrangements of Figures 5 and 6 have been described as detecting the absence of a high tone, a similar arrangement could be employed to detect the absence of a low tone. Both such arrangements are considered to fall within the scope of this invention.

Some advantages that can be obtained by the arrangements described as compared with quadrature channel FSK receivers are: 1/Only one filter and mixer are required.

2/ No requirementforquadrature local oscillator signals to drive two mixers.

3/ Power consumption can be reduced.

4/Considerable simplification of circuitry and potential for reduction in space requirement particularly advantageous for fabrication in integrated circuits.

The system will find application in pocket radio paging applications, radio control and radio communications and is of special benefit where power consumption must be low.

Claims

1. A radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at a different one of two transmission frequencies, the receiver comprising a mixer having a signal input for receiving the transmission frequencies, an input for a local oscillator and an output, a local oscillator having an output coupled with the local oscillator input of the mixer, which local oscillator has a switching signal input and is switchable between two frequencies which are offset relative to the arithmetic mean of the two transmission frequencies by different amounts, a detector having an input coupled with the output of the mixer and a switching output coupled with the switching input of the local oscillator which detector is arranged to determine when the output from the mixer contains a frequency which is the difference between one of the two transmission frequencies and one of the local oscillator frequencies or the other of the transmission frequencies and the other of the local oscillator frequencies and to provide a switching signal for switching the local oscillator to its alternative frequency when such a frequency is detected, whereby the switching signal follows the transmitted binary bits and is coupled to a receiver output to provide a signal representative of the transmitted binary information.

2. A radio receiver for receiving frequency shift keyed (FSK) radio signals in which differing binary bits are transmitted at a different one of two transmission frequencies, the receiver comprising a mixer having a signal input for receiving the transmission frequencies, an input for a local oscillator and an output, a local oscillator having an output coupled with the local oscillator input of the mixer, which local oscillator has a switching signal input and is switchable between two frequencies which are offset relative to the arithmetic mean of the two transmission frequencies by different amounts, a detector having an input coupled with the output of the mixer and a switching output coupled with the switching input of the local oscillator which detector is arranged to determine when the output from the mixer contains a frequency which is the difference between one of the two transmission frequencies and one of the local oscillator frequencies or the other of the transmission frequencies and the other of the local oscillator frequencies and to provide a switching signal for switching the local oscillator to its alternative frequency in the absence of detection of such a frequency, whereby, in the presence of a signal, the switching signal follows the transmitted binary bits and is coupled to a receiver output to provide a signal representative of the transmitted binary information.

3. A radio receiver as claimed in claim 2, including muting means responsive in the absence of a received signal to inhibit switching of the local oscillator frequency.

4. A radio receiver as claimed in claim 2, wherein the switching signal is coupled to the receiver output via switch means and the receiver includes a muting means responsive in the presence of a received signal to close the switch and to open the switch in the absence of a received signal.

5. A radio receiver as claimed in any one of the preceding claims, wherein the local oscillator frequencies are each in a frequency band the extremities of which are defined by the two transmission frequencies.

6. A radio receiver as claimed in any one of the preceding claims, including a filter coupled between the output of the mixer and the input of the detector and having a pass band arranged to permit passage of the frequencies to be detected and to reject higher frequencies.

7. A radio receiver as claimed in claim 6, wherein the filter is a low pass filter.

8. A radio receiver as claimed in any one of the preceding claims, in which the detector comprises a digital frequency detector.

9. A radio receiver as claimed in any one of the preceding claims, in which 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.

10. A radio receiver as claimed in claim 9, 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.

11. A radio receiver substantially as described herein with reference to the drawings.