Superheterodyne radio frequency receivers are well known to persons skilled in the art. In such receivers, an incoming radio frequency signal from an antenna is mixed in a mixer with an adjustable local oscillator signal such that the output signals from the mixer are the same as the incoming radio frequency signal, and frequency-shifted versions of the incoming signal at the local oscillator frequency, at the sum of the frequencies of the incoming signal and local oscillator frequencies, and at the difference between the incoming signal and local oscillator frequencies (lcnown as the intermediate frequency). This mixing step is carried out because the signal at the lower, intermediate frequency is much more straightforward to process than the incoming signal at the higher, input frequency. Known superheterodyne receivers are typically designed to operate at a chosen intermediate frequency, so that various components can be standardised, and the frequency of the local oscillator signal to be mixed with the incoming signal is adjusted to adjust the frequency of the incoming signal of which a frequency shifted version is output from the mixer at the intermediate frequency.

However, known receivers of this type suffer from the drawback that because the mixer outputs signals at the sum and difference frequencies of the signals input to it, for any given intermediate frequency the mixer, as well as selecting the input signal of interest, also selects an image signal differing in frequency from the local oscillator frequency by the intermediate frequency, and from the frequency of the selected signal by twice the intermediate frequency.

For example, a commonly used intermediate frequency is 10. 7 MHz, and in order to select signals in the ISM (industrial, scientific, medical) band centred on 433 MHz, the local. oscillator of a receiver is tuned to frequencies around 492. 3 M : Hz. Hoeer, at such local oscillator frequencies, image signals from the Tetra band are also selected, but because the signals on the Tetra band are often broadcast at considerable higher power than the signals in the ISM band, it is difficult to separate the signal of interest from the image signal.

Attempts have been made to overcome this problem by subjecting the input frequency signal to frequency-sensitive amplification or filtering. However, this is very difficult to achieve, since the"roll off'characteristic of most available filters is too slow and insufficiently accurate. It is also known to overcome this problem in the prior art by filtering the intermediate frequency signal and subjecting the intermediate frequency signal to further mixing to improve the separability of the signal of interest from the image signal. However, this so-called dual conversion process suffers from the disadvantage that not only does the process not entirely remove the image frequency, only enabling it to be partially removed, but also makes the receiver apparatus more complex, requiring more supporting circuitry and the use oftwo filters (one for each stage of the conversion process). This significantly increases the cost of the receiver, since the two filters can cost up to 60% of the cost of the complete receiver.

Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.

According to an aspect of the present invention, there is provided a method of processing radio frequency signals, the method comprising- (i) mixing a selected input signal, representing a modulating signal modulated onto a carrier . signal, with a local oscillator signal at a first local oscillator frequency, differing in frequency from said selected input signal by a predetermined amount, to produce a first intermediate signal, at a predetermined intermediate frequency, containing a frequency shifted version of said selected input signal and a frequency shifted version of a first further input signal; (ii) detecting said frequency shifted version of said first further input signal in said first intermediate signal; (iii) changing the frequency of said local oscillator signal to a second local oscillator frequency, differing in frequency from said selected input signal by said predetermined amount, to produce a second intermediate signal, at said predetermined intermediate frequency, containing a frequency shifted version of said selected input signal and a frequency shifted version of a second further input signal; (iv) detecting said frequency shifted version of said second further input signal in said second intermediate signal ; (v) setting said local oscillator to said first or said second local oscillator frequency; and (vi) obtaining the modulating signal of said selected input signal from the selected first or second intermediate signal subsequently to setting of said local oscillator to said first or said second local oscillator frequency.

By setting the local oscillator to first or second local oscillator frequencies differing in frequencies from the selected input signal by the same amount, this provides the advantage that the image signal causing less disturbance to the selected input signal can be chosen, while only one mixer stage is necessary. This provides the further advantage that selectivity of the selected input signal is improved without significant increase of the cost of receiver apparatus.

Said first and second local oscillator frequencies are preferably adjustable.

According to another aspect of the invention, there is provided a radio frequency receiver apparatus comprising :- (i) mixer means for mixing a selected input signal, representing a modulating signal modulated onto a carrier signal, with a local oscillator signal at a first or second local oscillator frequency, said first and second local oscillator frequencies each differing in frequency from said selected input signal by a predetermined frequency difference, to produce respective first and second intermediate signals at a predetermined intermediate frequency equal to said predetermined frequency difference, containing a frequency shifted version of said selected input signal together with a frequency shifted version of respective first or second further input signals ; (ii) detector means for detecting said frequency shifted versions of said first or second further input signals in said first and second intermediate signals respectively ; selector means for setting said local oscillator to said first or second local oscillator frequency ; and (iv) demodulator means for deriving the modulating signal of said selected input signal from said first or second intermediate signal subsequently to setting of said local oscillator to said first or second local oscillator frequency.

The apparatus may further comprise tuner means for adjusting said first and second local oscillator frequencies.

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 drawings in which:- Figure 1 is a schematic diagram of a superheterodyne radio frequency receiver apparatus embodying the present invention; and Figure 2 is a schematic illustration of radio frequency signals selected by a method embodying the present invention.

Referring to Figure 1, a superheterodyne radio frequency receiver 1 receives a radio frequency input signal, representing a modulating signal modulated onto a carrier signal in a manner which will be familiar to persons skilled in the art, from an antenna 2 and amplifies the signal by means of an amplifier 3. The amplified signal is then input to a mixer 4, where it is mixed with a signal of adjustable frequency from a local oscillator 5 to produce output signals at the frequency of the incoming input signal, the local oscillator frequency, and at frequencies-being the sum and difference of the input signal and local oscillator frequencies. The signal at the intermediate frequency, i. e. the difference in frequency between the incoming input signal and the local oscillator frequency, is typically 10. 7 MHz or 455 KHz, as will be familiar to persons skilled in the art.

The signal at the intermediate frequency is then amplified by an amplifier 6 and filtered by a crystal filter 7 and input to a demodulator or detector 8 which derives the modulation signal of the input signal. The demodulated modulation signal output from demodulator 8 is then amplified by amplifier 9 and produces an output signal at output terminal 10.

The frequency of the local oscillator 5 can be adjusted in order to adjust the frequency of the incoming input signal selected to be output from the mixer 4. For any given intermediate frequency, the signal output from mixer 4 at the intermediate frequency contains frequency shifted versions of the two input signals received from antenna 2 differing in frequency from the local oscillator frequency by the intermediate frequency (i. e. one of lower frequency than the local oscillator signal and one of higher frequency). A controller 11 checks the demodulated signal in demodulator 8 for the presence of unwanted image signals, and sends a signal to the local oscillator 5 causing the frequency of the local oscillator 5 to change to the other frequency differing from the input signal frequency of interest by an amount equal to the intermediate frequency. The controller 11 then again checks the demodulated signal in demodulator 8 for the presence of unwanted image signals, and the local oscillator 5 is set to the frequency producing less unwanted image signals.

Referring to Figure 2, an illustration of the process of the present invention is shown. For the purpose of illustration, it is assumed that the intermediate frequency is 1 MHz although it will be appreciated by persons skilled in the art that in reality the intermediate frequency would usually be 10.7 MHz or 455 KHz. In order to select input signal A at 400 MHz, the local oscillator can be set to 399 MHz (i. e. at position B). However, this creates the problem that interfering signal C at 398 MHz is also selected by the mixer and interferes with signal A of interest to the extent that it becomes difficult to separate the signal of interest A from the unwanted signal C. On detection of the presence of signal C in demodulator 8, control unit 11 switches the frequency of local oscillator 5 from 399 MHz to 401 MHz (i. e. to position D).

Signal A of interest is again selected at the intermediate frequency, but because no other signals appear at 402 MHz, the selection of undesirable image signals is avoided.

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.