Radio receiver

Abstract

A radio receiver comprising: an antenna for receiving a radio frequency signal amplitude modulated with an audio frequency signal; a digitizer for periodically sampling the radio frequency signal and generating a digital reception signal representative of the amplitude of the radio frequency signal; and a demodulator for demodulating the digital reception signal to generate a representation of the audio frequency signal.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application
Q=A·cos(θ_b)
where A is the instantaneous value of the audio component and θ_b is the phase angle of the beat frequency. From the identity:

$\mathrm{Tan}\left(\theta \right)=\frac{\mathrm{Sin}\left(\theta \right)}{\mathrm{Cos}\left(\theta \right)}$
the tangent of the beat frequency angle can be calculated at any instant, allowing the audio component A to be cancelled when θ_b is in the first quadrant. Since:
|Sin(α)|=|Sin(−α)|
and
|Cos(α)|=|Cos(−α)|
the tangent of the beat frequency can be generally evaluated by:

$\mathrm{Tan}\left({\theta }_b\right)=\frac{A·\mathrm{Sin}\left({\theta }_b\right)}{A·\mathrm{Cos}\left({\theta }_b\right)}$
In order to avoid very large results, the denominator should be selected to be larger than the respective numerator to ensure that the result is a number between 0 and 1. This can be achieved by swapping the numerator and denominator depending upon their respective values. The swapping procedure is mathematically valid since:

$\mathrm{Tan}\left({\theta }_b\right)=T_{\theta }=\frac{I}{Q}\dots \left(Q>I\right)$ $\mathrm{Tan}\left({\theta }_b\right)=T_{\theta }=\frac{Q}{I}\dots \left(I>Q\right)$
Thus the beat frequency can be calculated in all four quadrants without any alteration to the mathematical principle. The arctangent can then be calculated giving the raw angle of the beat frequency. Given that the beat frequency is sinusoidal, the instantaneous value of the beat can be found. Form this a correction tone P_θ can be generated by:

$P_{\theta }=\frac{1}{\mathrm{Cos}\left[{\mathrm{Tan}}^{-1}\left(T_{\theta }\right)\right]}$
which can be multiplied by the I or Q signal to cancel the beat tone.

Therefore, the following algorithm can be implemented in the beat frequency canceller 12 to cancel out the beat frequency:

• • 1. Read I and Q signals.
  • 2. Calculate moduli of I and Q signals
  • 3. If modulus of I is greater than modulus of Q then let T_θ equal modulus of Q divided by modulus of I, otherwise let T_θ equal modulus of I divided by modulus of Q
  • 4. Evaluate correction tone P_θ for calculated T_θ
  • 5. Evaluate output signal by multiplying P_θ by I or Q signal
    In step 4 a look-up table may be used to determine the correction tone. In step 5 it is preferred that the larger of I and Q is multiplied by the correction tone.

In the beat frequency canceller 12 the software reads the two partially filtered signals from the digital hardware by which the high speed filtering of blocks 10 and 11 is performed and completes the filtering operation. It then applies the I and Q signals to the beat frequency cancellation algorithm. After finding the moduli of I and Q, and performing the appropriate division, the software then uses a lookup table to calculate the beat frequency correction data (i.e. reciprocal of the cosine of the arctangent of the ratio). The correction factor is then multiplied by either I or Q (which ever is the greater) to give audio output.

The BFC algorithm described above is based on an assumption that the beat frequency is sinusoidal. It is desirable for the beat frequency waveform to be as sinusoidal as possible, to minimise the formation of additional harmonic products. Because the downconversion is performed at 1-bit resolution—effectively square wave—harmonics may occur. The harmonic content of a square wave consists of linearly attenuated odd harmonics, i.e. ⅓^rd of third harmonic, ⅕^th of the 5^th harmonic, etc. The product of two of these is odd harmonics but at inverse square amplitudes, i.e. 3rd harmonic at 1/9^th amplitude. The resultant signal has the form of a triangle-wave. The BFC algorithm then actually ‘generates’ a waveform at 4 times the beat frequency on top of the audio because of the disparity between the I and Q waveforms and a sine-wave. In reality, both waveforms are not square waves at the RF frequency. The off-air received RF input is noise shaped and over-sampled and the local oscillator has some noise shaping too. If necessary, the shape of the waveform produced by the local oscillator may be improved by applying over-sampling and/or noise shaping techniques to the local oscillator.

By digitising the received radio frequency signal itself, i.e. before any downconversion has been performed, the receiver of FIG. 1 avoids the need for many discrete analogue components that are required in traditional receiver designs, for example in their RF and IF sections and conversion sections.

The filtering and/or the beat cancellation sections of the receiver of FIG. 1 may be used advantageously in a receiver that does not digitise at RF, but in which digitisation is performed at IF or baseband.

The output at 13 of beat canceller 12 is a fully demodulated digitised signal at audio frequency. That signal is passed to audio unit 14 which can process the signal as required, for example to alter its volume or tone or to perform frequency equalisation. The signal is then converted to the analogue domain by D-to-A converter 15, amplified by amplifier 16 and passed to loudspeaker 17. A user input device 18 such as a keypad allows a user to enter the frequency that is to be received. The keypad input is interpreted by keypad port 19 and passed to output port 20 which sends a signal at 21 to control the local oscillator 5 to oscillate at the frequency entered by the user. Loudspeaker 17 may be replaced by headphones, or the output signal may be passed to a recorder.

The amplifier 16 may be a class D amplifier and it may receive a class D input.

A number of the units shown in FIG. 1 are preferably provided on a common integrated circuit. In one preferred embodiment, the units 12, 14, 15, 19, 20 and the low rate halves of filters 10 and 11 are preferably implemented on a common integrated circuit. That circuit may comprise a digital processor capable of executing software to perform the functions of units 10, 11, 14 and 15 and input and output ports to perform the functions of units 19 and 20 and to link it to the high rate halves of filters 10 and 11 and to amplifier 16. If desired, the entire system of FIG. 1, with the exception of the loudspeaker 17, the antenna 1 and the input device 18 could be formed on a single integrated circuit.

The applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any of the present claims. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A radio receiver, configured for operation in a radio system in which a plurality of radio channels are transmitted at frequencies spaced apart by a predetermined channel spacing, the radio receiver comprising:

an antenna configured to receive a radio frequency signal amplitude modulated with an audio frequency signal;

a digitiser configured to sample the radio frequency signal and to generate a digital reception signal representative of the amplitude of the radio frequency signal;

a demodulator configured to demodulate the digital reception signal to generate a representation of the audio frequency signal, wherein the demodulator comprises a local oscillator configured to generate a first local oscillator signal at a reception frequency of the radio receiver, a first mixer configured to mix the first local oscillator signal with the digital reception signal to form a first demodulated signal, and a second mixer configured to mix a second local oscillator signal with the digital reception signal to form a second demodulated signal, wherein the second local oscillator signal is at the reception frequency of the radio receiver and having a quadrature phase relationship with the first local oscillator signal, wherein the demodulator further comprises a filter configured to filter the demodulated signals to thereby generate first and second filtered demodulated signals, the filter having a peak response in the region of the reception frequency of the radio receiver and having nulls in its response in a region of frequencies spaced apart from the reception frequency of the radio receiver by the predetermined channel spacing; and

a beat cancellation device configured to form a beat-cancelled signal by processing the first filtered demodulated signal and/or and the second filtered demodulated signal to at least partially cancel from at least one of the first filtered demodulated signal and/or and the second filtered demodulated signal a beat frequency resulting from the mixing of the local oscillator signals with the digital reception signal,

wherein the radio receiver is configured for operation in a radio system in which a plurality of radio channels are transmitted at frequencies spaced apart by a predetermined channel spacing, and the demodulator further comprises a filter configured to filter the first demodulated signal, the filter having a peak response in the region of the reception frequency of the receiver and having nulls in its response in a region of frequencies spaced apart from the reception frequency of the receiver by a channel spacing.

2. A radio receiver as claimed in claim 1, wherein the digital reception signal is a single-bit signal.

3. A radio receiver as claimed in claim 2, wherein the digitiser comprises a single-bit digitiser.

4. A radio receiver as claimed in claim 2, wherein the digitiser comprises a sigma-delta modulator.

5. A radio receiver as claimed in claim 1, wherein the first local oscillator signal is a single-bit signal.

6. A radio receiver as claimed in claim 5, wherein the first mixer comprises an exclusive OR gate having two inputs and configured to receive the digital reception signal at one of its inputs and the first local oscillator signal at the other of its inputs.

7. A radio receiver as claimed in claim 1, further comprising a user input device for input of a user reception frequency, and a local oscillator controller responsive to the user input device for controlling the local oscillator to generate the first local oscillator signal at the user reception frequency.

8. A radio receiver as claimed in claim 1, wherein the second local oscillator signal is generated by the local oscillator.

9. A radio receiver as claimed in claim 1, wherein the beat cancellation device is configured to repeatedly perform:

determining a modulus of the first filtered demodulated signal;

determining a modulus of the second filtered demodulated signal;

comparing the modulus of the first filtered demodulated signal with the modulus of the second filtered demodulated signal;

determining a quotient by dividing the greater lesser of the moduli of the first filtered demodulated signal and the second filtered demodulated signal by the lesser greater of the moduli of the first filtered demodulated signal and the second filtered demodulated signal;

determining a cancellation factor having the value of the reciprocal of the cosine of the arctangent of the quotient; and

forming the beat-cancelled signal by multiplying one of the first and second filtered demodulated signals by the cancellation factor.

10. A radio receiver as claimed in claim 9, wherein said one of the first and second filtered demodulated signals multiplied by the cancellation factor is the one of the first and second filtered demodulated signals having the greater magnitude.

11. A radio receiver as claimed in claim 9, wherein determining the cancellation factor having the value of the reciprocal of the cosine of the arctangent of the quotient comprises retrieving from a stored look-up table the cancellation factor corresponding to the determined quotient.

12. A radio receiver as claimed in claim 1, wherein the beat cancellation device comprises is part of a digital processor configured to form the beat-cancelled signal.

13. A radio receiver as claimed in claim 1, further comprising a user input device configured for user selection of one of a plurality of reception frequencies spaced apart by the predetermined channel spacing.

14. A radio receiver as claimed in claim 1, wherein the digitiser is configured to periodically sample the radio frequency signal at more than twice the reception frequency of the radio receiver.

15. A radio receiver for operation in a radio system in which a plurality of radio channels are transmitted at frequencies spaced apart by a predetermined channel spacing, the radio receiver comprising:

an antenna for receiving a radio frequency signal;

a demodulator for demodulating a signal derived from the radio frequency signal to form a demodulated signal; and

a filter for filtering the demodulated signal, the filter having a peak response in a region of a reception frequency and having nulls in its response in a region of frequencies spaced apart from the reception frequency by the predetermined channel spacing, and the filter being configured to decimate the demodulated signal in hardware, and being configured to filter the decimated signal in software at an audio sample rate.

16. A radio receiver as claimed in claim 15, wherein the radio frequency signal is amplitude modulated with an audio frequency signal.

17. A radio receiver as claimed in claim 15, further comprising a digitiser for periodically sampling the radio frequency signal and generating as a digital reception signal representative of the amplitude of the radio frequency signal.

18. A radio receiver as claimed in claim 17, wherein the digitiser comprises a single-bit digitiser.

19. A radio receiver as claimed in claim 17, wherein the digitiser comprises a sigma-delta modulator.

20. A radio receiver as claimed in claim 17, wherein the demodulator comprises a local oscillator configured to generate a first local oscillator signal at the reception frequency of the radio receiver, and a mixer configured to mix the first local oscillator signal with the digital reception signal to form a first demodulated signal.

21. A radio receiver as claimed in claim 20, wherein the first local oscillator signal is a single-bit signal.

22. A radio receiver as claimed in claim 21, wherein the mixer comprises an exclusive OR gate having two inputs and configured to receive the digital reception signal at one of its inputs and the first local oscillator signal at the other of its inputs.

23. A radio receiver as claimed in claim 20, further comprising a user input device for input of a user reception frequency, and a local oscillator controller responsive to the user input device and configured to control the local oscillator to generate the first local oscillator signal at the user reception frequency.

24. A radio receiver as claimed in claim 23, wherein the user input device is configured to receive an input selecting one of the plurality of radio channels and the local oscillator controller is configured to, on the basis of the selected channel, cause the local oscillator to generate the first local oscillator signal at one of the frequencies spaced apart by the predetermined channel spacing.

25. A radio receiver as claimed in claim 15, wherein the attenuation of the filter at frequencies spaced above and below the reception frequency by the predetermined channel spacing is more than 10⁸ times the attenuation of the filter at the reception frequency.

26. A radio receiver as claimed in claim 15, wherein the attenuation of the filter at frequencies spaced above and below the reception frequency by the predetermined channel spacing is more than 10¹¹ times the attenuation of the filter at the reception frequency.

27. A radio receiver as claimed in claim 15, wherein the predetermined channel spacing is 8.820 kHz.

28. A radio receiver as claimed in claim 15, wherein the filter comprises a cascaded integrator comb filter.

29. A radio receiver as claimed in claim 15, wherein the filter is a third-order filter.

30. A radio receiver as claimed in claim 15, wherein the audio sample rate is 44.1 kHz.

31. A method for cancelling a beat frequency use in a radio receiver configured to receive a radio signal, forming a first demodulated signal by mixing a signal derived from the received radio signal with a first local oscillator signal, and forming a second demodulated signal by mixing the signal derived from the received radio signal with a second local oscillator signal having a quadrature relationship with the first local oscillator signal, the method comprising repeatedly performing:

determining the modulus of the first demodulated signal;

determining the modulus of the second demodulated signal;

comparing the modulus of the first demodulated signal with the modulus of the second demodulated signal;

determining a quotient by dividing the greater lesser of the moduli of the first demodulated signal and the second demodulated signal by the lesser greater of the moduli of the first demodulated signal and the second demodulated signal;

determining a cancellation factor having the value of the reciprocal of the cosine of the arctangent of the quotient; and forming a beat-cancelled signal by multiplying one of the first and second demodulated signals by the cancellation factor.

32. A method as claimed in claim 31, wherein determining the cancellation factor comprises retrieving from a stored look-up table the cancellation factor corresponding to the quotient.

33. A method as claimed in claim 31, wherein the signal derived from the received radio signal is formed at a sampling frequency greater than a reception frequency of the received radio signal.

34. A method as claimed in claim 31, wherein the signal derived from the received radio signal is noise-shaped.

35. A method as claimed in claim 31, wherein the first and second demodulated signals are radio frequency signals.

36. A method as claimed in claim 31, wherein the first and second demodulated signals are intermediate frequency signals.

37. A method as claimed in claim 31, wherein the first and second local oscillator signals are single-bit signals and the method further comprises forming first and second single-bit demodulation signals by mixing the signal derived from the received radio signal with the first and second single-bit local oscillator signals respectively, and decimating the first and second single-bit demodulation signals to form the first and second demodulated signals.

38. A method as claimed in claim 31, wherein the first and second demodulated signals are eight-bit signals.

39. A method as claimed in claim 31, wherein said one of the first and second demodulated signals multiplied by the cancellation factor is the one of the first and second demodulated signals having the greater magnitude.

40. A method as claimed in claim 31, wherein the method is performed by a beat cancellation device and wherein forming the beat-cancelled signal by multiplying one of the first and second demodulated signals by the cancellation factor comprises a digital processor of the beat cancellation device forming the beat-cancelled signal.

41. A method for use in a radio receiver configured to receive a radio signal, the method comprising:

forming a first demodulated signal by mixing a signal derived from the received radio signal with a first local oscillator signal;

forming a second demodulated signal by mixing the signal derived from the received radio signal with a second local oscillator signal having a quadrature relationship with the first local oscillator signal;

evaluating a trigonometrical relationship between signals derived from the first and second demodulated signals to determine a beat frequency;

generating a correction factor from a trigonometric function of the determined beat frequency; and

applying the correction factor to the first demodulated signal or the second demodulated signal to thereby cancel the beat frequency from the first demodulated signal or the second demodulated signal.

42. A radio receiver as claimed in claim 20, wherein the local oscillator is further configured to generate a second local oscillator signal at the reception frequency of the radio receiver, and a mixer is configured to mix the second local oscillator signal with the digital reception signal to form a second demodulated signal, wherein the second local oscillator signal has a quadrature phase relationship with the first local oscillator signal.