WO1997031465A2

WO1997031465A2 - Frequency control in an fsk receiver

Info

Publication number: WO1997031465A2
Application number: PCT/GB1997/000478
Authority: WO
Inventors: Robert Edmund Todd
Original assignee: Cedardell Limited
Priority date: 1996-02-21
Filing date: 1997-02-20
Publication date: 1997-08-28
Also published as: AU1805997A; WO1997031465A3; GB9603660D0

Abstract

An RF receiver (1) comprises an antenna (2) for receiving radiated RF signals and for providing an FSK electrical signal. The receiver further comprises a first mixer (31) in which a signal from a local oscillator is mixed with an input signal from the FSK signal to provide an output signal. The mixer is controlled by local oscillator (32). The output from the first mixer is a version of the signal derived from the FSK signal at input (3). The second mixer (33) is controlled by clock signal and outputs a signal which is a frequency shifted version of the signal on output (3). The receiver further comprises detector circuit (36) for receiving a second signal and providing a third electrical signal having an amplitude representing an instantaneous logic level of the digital frequency modulating signal. The receiver further comprises the feedback resistor (12) providing a tuning voltage to the voltage controlled oscillator of the first mixer.

Description

FREQUENCYCONTROL IN AN FSK RECEIVER

The present invention relates to a frequency modulated (FM) detection apparatus and method, and relates particularly to an apparatus and method for detection of digitised FM data.

Radio frequency transmission of digital signals is usually achieved by means of frequency-shift keying (FSK) , in which an RF carrier signal is shifted in frequency between two closely spaced frequencies, respectively above and below the carrier frequency representing the l*s and O's of digital serial bits transmitted. A received FSK signal is detected by providing an electrical signal lowered in frequency from the carrier frequency of the signal by a fixed frequency interval, and providing an output electrical signal having an amplitude representing the frequency deviation of the frequency shifted signal so that the output electrical signal represents the frequency modulating signal.

One of the limitations of existing digital FM detection techniques is that limitations to the rate at which the output electrical signal representing the serial data bits can be generated in turn limit the rate at which transmitted data can be received and processed.

It is an object of the present invention to provide a digital FM detection technique which overcomes the above disadvantage of the prior art.

According to on aspect of the present invention there is provided a receiver for digitised frequency modulated signals, the receiver comprising mixer means for receiving a first electrical signal representing the signal at a carrier frequency and frequency modulated to carry power at first and second frequencies representing respective first and second signal logic levels, wherein the mixer means is controlled by a local oscillator and in use provides a second electrical signal representing the first signal lowered in frequency by a substantially fixed amount; detector means for receiving said signal and providing a third electrical signal having an amplitude representing an instantaneous logic level of the digital frequency modulating signa l ; and feedback means for adjusting the frequency of the local oscillator to adjust the carrier frequency corresponding to the second electrical signal within a range between said first and second frequencies in dependence upon the amplitude of the third electrical signal.

By slightly off-setting the frequency of the carrier signal processed by the receiver from the carrier signal of interest, the sensitivity of the receiver is enhanced as far as detection of the instantaneous logic level is concerned. This in turn increases the speed at which the instantaneous logic level of the incoming signal can be identified which has the advantage of increasing the rate at which incoming data can be received and processed.

Preferably, the receiver is adapted to receive digitised frequency modulated radio frequency signals.

In a preferred embodiment, the feedback means adjusts the frequency of the local oscillator away from the first or second frequency representing the instantaneous logic level of the signal output by the receiver.

In a preferred embodiment, the mixer means includes a voltage influenced oscillator.

The feedback means preferably comprises an impedance connected between an output terminal of the detector means and an input terminal of the voltage influenced oscillator.

The receiver preferably further comprises at least one further mixer means arranged between the mixer means and said detector means for lowering the frequency of the second signal output signal by a substantially fixed interval.

According to another aspect of the invention, there is provided a method of demodulating digitised frequency modulated signals, the method comprising converting an incoming signal into a first electrical signal having a carrier frequency modulated by a transmitted signal to carry power at first and second frequencies representing first and second logic levels of the frequency modulating signal; mixing the first electrical signal with a signal at a reference frequency to provide a second electrical signal representing said first signal lowered in frequency by substantially fixed interval; detecting said second signal to provide a third electrical signal having an amplitude representing the instantaneous logic level of the frequency modulating signal; and adjusting the carrier frequency corresponding to the second signal within a range between said first and second signals in dependence upon the amplitude of the third signal.

The carrier freguency corresponding to the second signal is preferably adjusted away from the first or second frequency representing the instantaneous logic level of the third signal output by the receiver.

Preferably, the signal to be demodulated falls within the radio frequency band.

The method preferably further comprises the step of adjusting a voltage influenced oscillator in dependence upon the amplitude of the third signal.

A preferred embodiment of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawing, in which:

Figure 1 is a schematic diagram of a digitised FM receiver embodying the present invention;

Figure 2 is a schematic block diagram of the FM receiver of Figure 1.

Referring to the figures, an RF receiver 1 comprises an antenna 2 for receiving radiated RF signals and providing an FSK electrical signal along an input lead 3 at a carrier frequency. The signal along lead 3 then passes to an input terminal of an input amplifier 4, the output signal from which passes via an LC network 5 to pin 1 of an FM receiver integrated circuit 6, such as an MC3363 FM receiver manufactured by Motorola.

The signal input at pin 1 of the integrated circuit 6 is input to a first mixer 31 in which a signal from a local oscillator is mixed with the input signal to provide an output signal at pin 23 of the integrated circuit 6 at an intermediate frequency of 10.7 MHZ. The local oscillator 32 of the first mixer 31 includes a voltage influenced oscillator 7 wherein adjustment of the frequency of the voltage controlled oscillator adjusts the point at which the expected incoming frequency should be so that the frequency of the input signal on lead 3 which is mixed with the local oscillator signal and input to the integrated circuit 6 is adjusted. The output from the first mixer is a version of the signal on lead 3, but at 10.7 MHZ.

The output signal of the first mixer 31 passes from pin 23 of the integrated circuit 6 via a ceramic filter 8 and is input via pin 21 of the circuit 6 to a second mixer 33 where it is mixed with the output signal of an oscillator 34 having a reference frequency controlled by a clock signal input via clock terminal 9 to pin 6 of the receiver circuit 6. As a result, the second mixer outputs a signal of frequency 455 KHZ from pin 7 of the circuit 6 which is a frequency - shifted version of the signal on lead 3. The output of the second mixer 34 is then passed via a further ceramic filter 10 and pin 8 of the circuit 6 to a limiter amplifier 35, the output of which is input to a detector circuit 36 comprising a quaderature detector. The detector circuit provides an output signal 37 such that amplitude of the output signal represents the instantaneous logic level of the frequency modulating signal received on lead 3 from antenna 2.

The output of the detector is amplified and input to an FSK comparator 38, the output of which is taken from pin 18 of the integrated circuit 6 to an output terminal 11 of the receiver 1. A feedback resistor 12 is connected between output terminal 11 and pin 27 of circuit 6 to provide a tuning voltage to the voltage controlled oscillator 7 of the first mixer in dependence upon the amplitude of the signal output at terminal 11.

Operation of the device shown in Figure 1 will now be described.

Digitised serial data to be transmitted is frequency odulated onto an RF carrier signal by a FSK technique as will be known to persons skilled in the art. As a result, the RF signal at the carrier frequency is frequency modulated to carry power at first and second closely spaced FSK frequencies, slightly separated from the carrier frequency and respectively higher and lower in frequency. The antenna 2 detects the RF signal and outputs an electrical signal at the carrier frequency along line 3 having the same frequency modulation as the transmitted signal.

After amplification by the input amplifier 4, the input signal is mixed with the signal produced by the first mixer 31, and the input voltage of the voltage controlled oscillator 7 is selected so that the 10.7 MHZ output signal of the first mixer 31 is a frequency-shi ted version of the signal at the carrier frequency of interest.

After amplification and subsequent filtering by ceramic filter 8, the output signal of the first mixer is input to the second mixer 33 to provide a 455 KHZ output signal which is also a frequency shifted version of the signal at the carrier frequency of interest.

After amplification and subsequent filtering by ceramic filter 10, the output signal of the second mixer 33 is input to the limiter 35 and detector 36, and the amplified output of the detector circuit provided at output terminal 11 of the receiver 1 is high or low, depending upon the logic level of the detected frequency modulating signal.

The feedback signal via resistor 12 to pin 27 of the integrated circuit 6 slightly adjusts the operating freguency of the local oscillator of the first mixer so that the 10.7 MHZ output signal of the first mixer corresponds to a carrier frequency slightly off-set from the carrier frequency of interest. However, this offset carrier frequency still lies between the first and second frequencies at which power of the FM signal at the carrier freguency of interest is carried.

As a result, if the instantaneous signal output from terminal 11 corresponds to logic level 0 of the frequency modulating signal , the carrier frequency processed by the integrated circuit 6 is shifted in the direction of the power carrying frequency representing logic level 1 of the frequency modulating signal. In this way, the local oscillator frequency shift caused by a slight shift in incoming frequency will emphasise the change in incoming frequency, thus causing the sensitivity of the final detector stage is increased, which in turn enhances the speed at which the logic level corresponding to the output of the detector stage can be determined.

Similarly, if the instantaneous signal output at terminal 11 corresponds to logic level 1, the passed carrier frequency is slightly shifted towards the power - carrying frequency representing logic level 0, with the result that the sensitivity of the detector is increased.

In other words, data emerging from the comparator 38 is fed back into the local oscillator 7 as soon as it appears. This action drives the local oscillator slightly away from the expected incoming frequency received from the first mixer 31.

If the decoding of the signal was correct, this action will emphasize the data. If on the other hand the decoding of the signal was incorrect, this action will attenuate the data. The action may be viewed as "Intelligent Hysteresis".

It will be appreciated by persons skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.

Although the present invention has been described with reference to radiation falling within the radio frequency band, it is to be understood that the principles can be applied to radiation in any chosen band, for example, optical or ultra violet.

Claims

CLAIMS :

1. A receiver for digitised frequency modulated signals, the receiver comprising first mixer means for receiving a first electrical signal representing the signal at a carrier frequency and frequency modulated to carry power at first and second frequencies representing the respective first and second signal logic levels, wherein the first mixer means is controlled by a local oscillator and the use provides the second electrical signal representing the first signal lowered in frequency by a substantially fixed amoun ; detector means for receiving said second electrical signal and providing a third electrical signal having an amplitude representing an instantaneous logic level of the digital frequency modulating signal; and feed back means for adjusting the frequency of the local oscillator to adjust the carrier frequency corresponding to the second electrical signal within a range between said first and second frequencies in dependence upon the amplitude of the third electrical signal.

2. A receiver according to claim 1 which receiver is adapted to receive digitised frequency modulated radio frequency signals.

3. A receiver according to claim 1 or claim 2, wherein the feed back means adjusts the frequency of the local oscillator away from the first or second frequency representing the instantaneous logic level of the signal output by the receiver.

4. A receiver according to any one of the preceding claims, wherein the mixer means includes a voltage influenced oscillator.

5. A receiver according to any one of the preceding claims, wherein the feed back means comprises an impedance connected between an output terminal of the detector means and an input terminal of the voltage influenced oscillator.

6. A receiver according to any one of the preceding claims, further comprising at least one further mixer means arranged between the mixer means and said detector means for lowering the frequency of the second signal output signal by a substantially fixed interval .

7. A method of demodulating digitised frequency modulated signals, the method comprising converting an incoming signal into a first electrical signal having a carrier frequency modulated by a transmitted signal to carry power at first and second frequencies representing first and second logic levels of the frequency modulating signals; mixing the first electrical signal with a signal at a reference frequency to provide a second electrical signal representing said first signal lowered in frequency by a substantially fixed interval; detecting said second signal to provide a third electrical signal having an amplitude representing the instantaneous logic level of the frequency modulating signal; and adjusting the carrier frequency corresponding to the second signal within a range between said first and second signals in dependence upon the amplitude of the third signal.

8. A method according to claim 7, wherein the carrier frequency corresponding to the second signal is positioned away from the first or second frequency representing the instantaneous logic level of the third signal output by the receiver.

9. A method according to claim 7 or claim 8, wherein the signal to be demodulated falls within the radio frequency band.

10. A method according to any one of claims 7 to 9, further comprising the step of adjusting a voltage influenced oscillator independent upon the amplitude of the third signal.