FIELD OF THE INVENTION

The field of the invention relates to a frequency converter for shifting a frequency of a received signal.

BACKGROUND

Wireless communication signals are transmitted in different frequency channels. The signals are usually converted on receipt to a lower intermediate frequency band. A mixer with a local oscillator may be used in the conversion of the signal to this intermediate frequency band. The local oscillator may be configured with a frequency that is higher than the signal, in which case the conversion is called high side injection, or it may be configured with a lower frequency in which case it is termed low side injection. In addition to shifting the frequency of the desired input signal other signals will also be frequency shifted by this mixer and some of these may be shifted to the same intermediate frequency band as the desired signal. This provides noise and undesired signal in the intermediate frequency band.

In a satellite communication system, the received signal from the satellite is small and in many cases below the thermal noise of the receiver. Where such a signal is shifted to an intermediate frequency band, care must be taken that other signals that are shifted to the same intermediate frequency band are not unduly large, otherwise the desired signal will be lost in the noise from the other signals.

It would be desirable to provide a system for shifting the frequency of an input signal in a controlled manner that enabled noise effects to be reduced

SUMMARY

A first aspect provides a converter for shifting a frequency of a received input signal to form an output signal in a predetermined intermediate frequency band said converter comprising: a local oscillator configured to generate a signal at a selected frequency; determining circuitry configured to determine current conditions; control circuitry configured to control said selected frequency output by said local oscillator to be higher or lower than a frequency of said received input signal; said control circuitry being configured to select said higher or lower frequency for said local oscillator in dependence upon said current conditions determined by said determining circuitry; and at least one frequency shifting element comprising at least one mixer for mixing said signal from said local oscillator with said input signal to form said output signal in said predetermined intermediate frequency band.

The current conditions may comprise at least one of: a strength of signals in at least one noise forming frequency band, said noise forming frequency band comprising a frequency band that contributes to noise in said predetermined intermediate frequency band, a quality of an output signal output by said converter, and a proximity of said input signal to an edge of a predefined bandwidth that said converter is configured to receive signals within.

It was recognised that when performing a frequency shift of a desired input signal, the circuitiy shifting the desired signal also shifts signals in other frequency bands, and where these are shifted to the same frequency band as the desired input signal, that is to the predetermined intermediate frequency band, then these signals will act as noise signals. It was also recognised that the input signal maybe shifted to the predetermined intermediate frequency band by using a local oscillator with an output frequency that is either higher or lower than the input signal, and that the selection of one or the other will change the frequency band(s) that is shifted to the predetermined intermediate frequency, that is the noise forming frequency band(s). Thus, one way of reducing the noise may be to make the converter configurable and able to select either a higher or lower frequency of the local oscillator depending on current conditions, the frequency selected being that deemed to generate the least noise in the output signal. Such a configurable converter is provided with determining circuitry operable to determine the current conditions, these conditions being indicative of the amount of noise that is, or may be, produced at the intermediate frequency band were the higher or lower frequency of the oscillator to be selected and control circuitry configured to be responsive to these determinations to select the corresponding higher or lower frequency of the local oscillator that it deems will reduce the noise in the output signal. In this way, the noise levels in the output signal may be controlled to some extent by controlling the frequency of the local oscillator.

The input signal may be a communication signal transmitted in a channel, the channel having a predetermined frequency band, the centre band of the channel being the frequency considered.

The control circuitry may be configured to control said selected frequency output by said local oscillator to be higher or lower than a frequency of said received input signal by an amount equal to the predetermined intermediate frequency, the predetermined intermediate frequency being the central frequency of the predetermined intermediate frequency band.

In some embodiments, said current conditions comprise said strength of signals in said at least one noise forming frequency band, said control circuitry being responsive to said determining circuitry determining that current conditions indicate that signals within said at least one noise forming frequency band would be weaker were said higher frequency band to be selected to control said local oscillator to generate said higher frequency and being responsive to said determining circuitry determining that current conditions indicate that signals within said at least one noise forming frequency band would be weaker were said lower frequency band to be selected to control

The current conditions determined by the determining circuitry may comprise the strength of signals within potential noise forming frequency bands, these bands being dependent upon the frequency of the local oscillator. Thus, the determining circuitry may determine the signal strength in one or more frequency bands that would contribute to noise in the predetermined intermediate frequency band were the local oscillator to have a certain output frequency, and the signal strength in one or more frequency bands that would contribute to noise in the predetermined intermediate frequency band were the local oscillator to have a different output frequency. From this a desirable output frequency of the local oscillator may be selected such that the signal strength in the noise forming frequency band is reduced.

The current conditions considered by the determining circuitry may also comprise the quality of the output signal. The quality of the output signal is an indication of the amount of noise in the output signal and may be indicative of the strength of any noise signal that is being shifted to the intermediate frequency. Thus, the quality of the output signal may also be used as a factor in deciding what output frequency to select for the local oscillator. In this regard, where the noise is unduly large or increases, then the control circuitry may be configured to change the selected local oscillator frequency to toggle to the other of high or low side injection and determine if the quality of the signal improves.

An other of the current conditions that may be considered by the determining circuitry is how close to an edge of a predefined bandwidth the input signal is within. In this regard where the input signal is close to the edge of the predefined bandwidth it may be preferable for the noise forming band(s) to be within the predefined bandwidth and the control circuitry may select the local oscillator output frequency accordingly.

In some embodiments, said at least one noise forming frequency band comprises an image forming frequency band, said image forming frequency band comprising a frequency band displaced from a frequency of said local oscillator by a same amount and in a different direction to a frequency band of said received input signal.

The input signal may be within a frequency band and it is the centre frequency of th frequency band that is considered when considering the displacement of the image forming frequency band from this signal.

In some embodiments the image forming frequency band is a band displaced from said received input signal by an amount that is twice the predetermined intermediate frequency.

The selected frequency of said local oscillator is displaced from said input signal by said predetermined intermediate frequency.

In some embodiments, one of said determining circuitry or control circuitry is configured when said determining circuitry is unable to detect a signal strength in a potential noise forming frequency band to treat said signal strength of said potential noise forming frequency band as a high value.

As noted previously the converter may be configured such that the output frequency of the local oscillator is selected such that the signal strength in the noise forming frequency band(s) is reduced. Where the signal strength in a potential noise forming frequency band is unknown, then it may be desirable not to select a local oscillator output frequency band that renders this band a noise forming frequency band and thus, the determining or control circuitry may treat this unknown value as though it were a high value. In this way, the selection of the local oscillator output frequency is selected to preferentially select a noise forming frequency band of a known strength as the noise forming frequency band.

In some embodiments, said determining circuitry is configured to determine a strength of signals in said at least one noise forming band by one of: monitoring a strength of received signals in said at least one noise forming band; and receiving an input signal indicating a strength of signals in said at least one noise forming band.

The strength of signals in the noise forming bands may be detected by the determining circuitry or may be determined from information received regarding signals transmitted on other neighbouring channels.

In some embodiments, the converter is for converting said input signal to baseband, said converter comprising a second local oscillator configured to generate a signal at a second predetermined frequency; and said at least one frequency shifting element comprising a two stage frequency shifting element, said first stage comprising said at least one mixer for mixing said signal from said local oscillator with said input signal to form said output signal in said predetermined intermediate frequency band and said second stage being configured to shift said signal within said predetermined intermediate frequency band to baseband, said second stage comprising at least one mixer for mixing said signal at said predetermined intermediate frequency band with said signal from said second local oscillator to form said output signal at baseband.

In some embodiments, said determining circuitry is configured to determine from said current conditions a strength of signals generated by said first stage converter at harmonics of said second predetermined frequency generated by said second oscillator each of said harmonics of said second predetermined frequency comprising one of said at least one noise forming frequency bands, said control circuitry being configured to consider a strength of signals from at least some of said plurality of noise forming frequency bands prior to controlling said local oscillator to select said lower or higher frequency than a frequency of said received input signal.

Where there is a second stage in the conversion process there may be additional noise forming frequency bands and these may be related to the harmonics of the second local oscillator, these noise forming bands may be termed harmonic fold-back or harmonic mixing bands. This is because noise or unwanted signals in these bands are folded back to the frequency of interest by mixing with the harmonics of the local oscillator. When selecting the desired frequency of the local oscillator the signal strength in these bands maybe considered along with the signal strength in the image forming bands.

In some embodiments, said converter is configured to generate an in-phase and quadrature output signal at baseband, wherein said first stage of said at least one two stage frequency shifting element comprising phase shift circuitry for generating two versions of said local oscillator signal said two versions being phase shifted with respect to each other, and mixers for mixing said two versions of said local oscillator signal with said input signal to form first and second intermediate frequency signals; said second stage of said two stage frequency shifting element comprising further phase shift circuitry configured to generate two versions of said second local oscillator signal that are phase shifted with respect to each other and further mixers for mixing said first and second intermediate frequency signals with each of said two versions of said second local oscillator signals; and combining circuitiy for combining said first and second intermediate frequency signals mixed with said two versions of said further local oscillator signals to generate said in-phase and quadrature output signals; wherein said control circuitiy is configured to control at least one of said further phase shift circuitry and said combining circuitiy to change a polarity of one version of said further local oscillator signals when controlling said local oscillator to switch between said higher and lower frequency.

By suitable configuration of the second stage of the two stage frequency shifting element using phase shift circuitry to generate in-phase and quadrature signals, this circuitry can be configured to not only act to shift the frequency to baseband but also to suppress signals from the image noise forming frequency band. This second stage is sometimes termed the image reject mixer and acts to filter out the image signal. Where the converter is configured to switch from high to low side injection, then suitable adjustment of this stage is required as the position of the image and desired signal reverses relative to the frequency band output by the first local oscillator. This can be addressed by switching the polarity of one of the versions of the further local oscillator signals. This may be done by simply multiplying one of the versions of the signal by -1, alternatively it may be done by phase shifting one of the signals by 180°.

The phase shift between the two versions of the local oscillator signals is substantially 90° so that an in-phase and quadrature signals are generated.

In some embodiments, said control circuitry is configured in response to at least one of: said determining circuitiy determining that a quality of an output signal output by said converter is below a predetermined level; a signal strength in at least one of said noise forming frequency bands has changed by more than a predetermined amount; a predetermined time has elapsed; to control said local oscillator to change from generating said selected frequency at either said higher or lower frequency than said input signal frequency to generating said selected frequency at the other of said lower or higher frequency and to determine whether said quality of said output signal improves and where so to maintain said updated generated frequency and where not to revert to the previous generated frequency.

The control circuitry maybe configured to change between high or low side injection when conditions change. The change in conditions may be detected by detecting a change in conditions such as a quality of the output signal, the strength of signals in neighbouring channels and in particular the noise forming bands, and/ or it may be done periodically. The control circuitry may be configured to determine if the change has improved the quality of the signal and if so to maintain the changed state and if not to revert to the previous state.

A second aspect provide a receiver comprising a converter according to a first aspect, said receiver being configured to receive signals within a predefined frequency band; said control circuitry being configured to control said local oscillator to generate a frequency that is lower than said input signal in response to said determining circuitiy determining that said frequency of said input signal is close to a higher end of said predefined frequency band and to control said local oscillator to generate a frequency that is higher than said input signal in response to said determining circuitiy determining that said frequency of said input signal is close to a lower end of said predefined frequency band.

Where the converter is in a receiver, the receiver may be configured to receive signals within a predefined frequency band. For example, where the receiver is used in SATCOM communications, then if the band is a band reserved for those communications the expected level of signal within this band will be known and will be small, while outside of the band signal levels may be veiy different, and may be unknown. In such a case it may be advantageous to ensure that the image forming band is within the predefined band. Thus, low or high side injection may be selected where the input signal is towards the edge of the predefined band. Close to an edge of said predefined band may be within two times the intermediate frequency of a boundary of the predefined band.

A third aspect provides a phased array antenna comprising a plurality of receiver elements configured to receive signals within said predefined frequency band, said phased array antenna comprising said converter according to a first aspect, said converter being configured to shift a frequency of said received signals.

A phased array antenna has a plurality of antenna elements and adjusts the relative phase of the signals received at each of the antenna elements to steer the focus of the antenna electronically rather than by mechanically moving the antenna. Embodiments are particularly applicable for such antenna where the signal received may not be strong and noise in neighbouring bands may be an issue.

Where the receiver is used in an element of a phased array antenna then it may be advantageous to switch between low and high side injection of the different elements in a coordinated way and if it is determined that a better signal quality has been achieved using the other side injection then this is maintained and if not, then the control circuitry controls the local oscillator to revert to the original frequency. In this way the antenna can be updated as conditions change in a coordinated, controlled way, and with the effects of the changes being monitored.

In some embodiments, said converter comprises a plurality of said phase shift elements, each of said plurality of receiver elements being configured to transmit a received input signal to a respective one of said plurality of phase shift elements.

The input signals received from each of the receiver elements may be frequency shifted individually, alternatively the signals maybe combined and the combined signal frequency shifted.

Input signals referred to herein maybe communication signals, the communication signals may be continuous signals defined by channels, with any part of a continuous signal received on a same channel comprising a part of the same communications signal.

A fourth aspect provides a method for shifting a frequency of a received input signal to form an output signal in a predetermined intermediate frequency band said method comprising: controlling a local oscillator to generate a signal at a selected frequency; determining current conditions, said current conditions comprising at least one of: a strength of signals in at least one noise forming frequency band, said noise forming frequency band comprising a frequency band that contributes to noise in said predetermined intermediate frequency band, a quality of an output signal output by said converter, and a proximity of said input signal to an edge of a predefined bandwidth that said converter is configured to receive signals within; controlling local oscillator to set said selected frequency to be higher or lower than a frequency of said received input signal in dependence upon said current conditions determined in said determining step; and frequency shifting said input signal by mixing said signal from said local oscillator with said input signal to form said output signal in said predetermined intermediate frequency band.

In some embodiments, said current conditions comprise said strength of signals in said at least one noise forming frequency band, and where said step of determining determines that said current conditions indicate that signals within said at least one noise forming frequency band would be weaker if said higher frequency band were to be selected controlling said local oscillator to generate said higher frequency and where said step of determining determines that said current conditions indicate that signals within said at least one noise forming frequency band would be weaker were said lower frequency band to be selected controlling said local oscillator to generate said lower frequency.

In some embodiments, in response to said determining step being unable to detect a signal strength in a potential noise forming frequency band, treating said signal strength as having a high value.

In some embodiments, said method determines a strength of signals in said at least one noise forming band by one of: monitoring a strength of received signals in said at least one noise forming band; and receiving an input signal indicating a strength of signals in said at least one noise forming band.

In some embodiments, said method converts said input signal to baseband, said method generating a signal at a second predetermined frequency at a second local oscillator; and performing said frequency shifting step in two stages, said first stage comprising mixing said signal from said local oscillator with said input signal to form said output signal in said predetermined intermediate frequency band , and said second stage comprising mixing said signal at said predetermined intermediate frequency band with said signal from said second local oscillator to form said output signal at baseband. In some embodiments, said step of determining comprises determining from said current conditions a strength of signals generated by said first stage converter at harmonics of said second predetermined frequency generated by said second oscillator each of said harmonics of said second predetermined frequency comprising one of said at least one noise forming frequency bands, and considering a strength of signals from at least some of said plurality of noise forming frequency bands prior to controlling said local oscillator to select said lower or higher frequency than a frequency of said received input signal.

In some embodiments, said method generates an in-phase and quadrature output signal at baseband, wherein said first stage of said frequency shifting comprises generating two versions of said local oscillator signal said two versions being phase shifted with respect to each other, and mixing said two versions of said local oscillator signal with said input signal to form first and second intermediate frequency signals; said second stage of said two stage frequency shifting comprising generating two versions of said second local oscillator signal that are phase shifted with respect to each other and mixing said first and second intermediate frequency signals with each of said two versions of said second local oscillator signals; and combining said first and second intermediate frequency signals mixed with said two versions of said further local oscillator signals to generate said in-phase and quadrature output signals; and changing a polarity of one version of said further local oscillator signals when controlling said local oscillator to switch between said higher and lower frequency.

In some embodiments, said method is responsive to determining at least one of: a quality of an output signal output by said converter is below a predetermined level; a signal strength in at least one of said noise forming frequency bands has changed by more than a predetermined amount; and a predetermined time has elapsed; to control said local oscillator to change from generating said selected frequency at either said higher or lower frequency than said input signal frequency to generating said selected frequency at the other of said lower or higher frequency and to determine whether said quality of said output signal improves and where so to maintain said updated generated frequency and where not to revert to the previous generated frequency.

A fifth aspect provides a computer program comprising computer readable instructions which when executed by a processor associated with a frequency converter are operable to control said processor and frequency converter to perform steps in a method according to a fourth aspect. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 schematically shows the position of image forming bands in the Ku band for the case where either high side or low side injection is selected;

Figure 2 schematically shows a receiver comprising a frequency shifter according to an embodiment;

Figure 3 schematically shows a receiver comprising a frequency shifter according to a further embodiment;

Figure 4 schematically shows a flow diagram illustrating steps in a method according to an embodiment;

Figure 5 schematically shows the position of noise folding bands and the effect of the noise when either high or low side injection is selected;

Figure 6 shows a phased array antenna according to an embodiment; and Figure 7 shows a phased array antenna according to a further embodiment.

DESCRIPTION OF THE EMBODIMENTS

One or more embodiments of the present invention relate to frequency converter within a receiver that may be within a phased array antenna, particularly a phased array antenna system/ panel that can be used to provide an improved gain and signal - to-noise ratio when sending and receiving electromagnetic communications signals. The phased array antenna system may comprise one or more phased array antenna panels that automatically focus their beam of maximum gain to a transmitter or receiver of electromagnetic communications signals such as a satellite.

In a frequency conversion receiver system, such as a dual conversion receiver system where the intermediate frequency is not baseband, the position of the first local oscillator LO frequency Flo as compared to the Received input signal RF frequency Frf defines several properties of the system.

Where the position of Flo is above Frf there is High-Side Injection (HSI) means the LO frequency is higher than the received RF frequency Frf. Where there is what is termed low-Side Injection (LSI) when the LO frequency is lower than the received RF frequency Frf.

A receiver usually has a Mixer that converts Frf to an intermediate frequency (IF) Fif that is then processed by either an analog or a digital circuit. Mixer inherently converts two signals to the same IF band, one is the wanted signal at Frf and the other one is called Image (IM) at Fim. The image signal is formed by signals outside of the receiving band that are themselves shifted to the intermediate frequency band.

IM frequency is dependent on the relation between LO and Frf :

For example: Frf= 12GHz, Fif= 100MHz for a high-side inject system /O=12.IGHZ-> Fim= 12.2GHz; this means the mixer will convert the content around 12GHz and 12.2GHz to 100MHz for a low-side inject system /o=n.9GHz-> Fim= 11.8GHz; this means the mixer will convert the content around 12GHz and 11.8GHz to 100MHz

Figure 1 schematically shows how the output frequency of the local oscillator in a frequency converter such as a mixer affects the position of the image forming frequency band that is shifted to the same output frequency as the signal, that is the intermediate frequency band.

Looking at the left-hand side of Figure 1, a signal at 10.8 GHz is shifted to an intermediate frequency of 100 MHZ using an oscillator that is offset by 100 MHZ from the input signal sigi. In the case that the local oscillator is offset in a lower frequency direction, it outputs a signal at 10.7 GHZ and the image forming band IM_obi, whose signal will also be displaced to the intermediate frequency of 100 MHZ is at 10.6GHZ. In the case that the local oscillator outputs a signal offset in the other direction at 10.9 GHz, then the image forming band IM_ibi will be at 11GHz. In the left-hand example using the higher frequency local oscillator, that is using high side injection will provide an image forming band of reduced signal strength and thus, reduced noise will be generated at the intermediate frequency by the shifting of this signal.

Looking at the right-hand example, the opposite is true and low side injection LSI will generate less noise and is preferable. In the example of Figure 1 two signals sigi, sig2 are shown at either end of the Ku band, this is the predefined band in which satellite communication signals are transmitted. Within this band signals are received from satellites and thus, their strength is generally low. Thus, where the two signals are at either end of the band as in this example, one image will always fall outside the regulated band and this may be larger. In some cases, the circuitry may not be able to determine the strength of signals outside of the band. The circuitry may be configured to select high or low side injection, either in response to determining the relative strengths of the two image bands, or simply in order to ensure the image forming band is within the Ku band.

In summary, Sigi and Sig 2 are two wanted communication signals that are located in the lower end and upper end of the Ku band respectively. If HSI is used i.e. LOh is used on the first mixer, for sigi, the image will be located within the Ku band as noted by IM_ini. This signal is the weaker and thus, preferred of the two potential noise generating signal. However, the image for sig2 were high side injection to be selected, is outside the band (denoted by IM_ob2 ) and thus, for image sig2 low side injection is preferable.

If LSI is used i.e. L01 is used, sigi will see an out of band image i.e. IM_obi while sig2 encounter an in-band image IM_ib2.

This example shows that to have image always within the band, configurable HSI /LSI system should be used.

Such systems require configurable LO allocation. There may also be a requirement to select a suitable image reject filter where a system is configured to switch between high side injection and low side injection.

In this regard, in a receiver system, there should be either an Image-Reject (IR) filter before the mixer or the received image is removed after the mixing in the intermediate frequency IF stage. In this regard where the frequency converter is a two stage converter then the second mixer set up in the intermediate frequency stage can be designed to also filter the image; such mixer topology is called image reject mixer. In most modern receivers, quadrature mixers are used to implement quadrature downconversion to facilitate the rejection of the image band. Having In-phase (I) and Quadrature (Q) signals makes it easier for image rejection in the IF stage. In reality, image rejection can not be done perfectly and there is always residue of the image in the band of interest. Hence the system Noise Figure (NF), Signal to Noise Ratio (SNR) and EVM (error vector magnitude) are degraded. 3odB rejection is a typical value that can be achieved in a modern process unless calibration is added.

The image frequency always contains thermal noise; however it can also contain unwanted signal such as communication signals that would act as jammers for the wanted signal. It would be desirable if Fim could be chosen in a way that there is no or very small signal in that region. If the signal in the image band is smaller than or comparable to the received signal, IR filtering of 3odB or similar could be good enough for many applications as the ratio of the wanted to unwanted signal would be more than 3odB. That is why a system with a non-zero IF frequency i.e. a dual conversion system, allows the LO frequency to be chosen in a way that avoids or at least reduces the possibility of there being a big jammer in the IM band.

In applications such as SATCOM communications, the received signal from the satellite is very small and in most case below the thermal noise of the system. Hence if the IM band is within the SATCOM defined band, it is guaranteed that the image is mostly occupied by thermal noise. For example, for Ku band receivers where the band is defined from 10.7 GHz to 12.75 GHz by FCC or other regulators in different regions, if Fim is within the regulated Ku band, it will only contain signals comparable with the wanted signal. Hence a typical IR filter may be enough to minimize or at least reduce the effect of IM noise and signal. On the other hand, if Fim falls outside the Ku band, there is no guarantee that the level of the unwanted signal is small. As the level of signals in the Ku band is very small and in most case below the KT noise, if Fim is outside the band it could be much higher than the wanted signal and hence a typical IR of 3odB or similar will not be able to sufficiently reduce or minimize the SNR or NF (noise figure) degradation. Hence it is desirable to ensure that the IM is within the regulated band.

Figure 2 shows a frequency converter circuit within a receiver according to an embodiment. The input signal 10 is received at antenna 5 and transmitted to a first set of mixers 20A, 20B which apply a selected frequency from a first local oscillator LOi to the input signal. Phase shift circuitry 30 applies a 90° phase shift to the local oscillator signal that it receives such that there is a 90 ⁰ phase shift to the signal mixed at mixer 20B compared to the signal at mixer 20A. In this example the mixers 20A, 20B convert the input signal to an intermediate frequency and this is passed through a low pass filter to reduce noise and then through a variable gain amplifier. In other embodiments the filters may be band pass filters or there may be no filters and the amplifier may also have different forms.

In this embodiment, the converter is a two-stage converter that converts the signal in the first stage to an intermediate frequency and then in the second stage to baseband. The architecture is based on weaver architecture and uses phase shifting of the signals at the two local oscillators and appropriate combining of the phase shifted signals to repress the image signal and output the wanted signal, the so-called image reject mixer configuration.

In the second stage a further local oscillator LO2 provides the required frequency for shifting the signal to baseband. There is phase shift circuitry 60 to generate two versions of the LO2 signal that are phase shifted with respect to each other by 90 ⁰ . These signals along with the two signals output by the first stage are then applied to a set of four mixers 22 which shift the signals from the intermediate frequency down to the baseband. The signals from the different mixers 22 are then combined at combining circuitry 28 to form in-phase I_BB output signals at output 40 and quadrature Q_BB output signals at output 50. The quadrature signal has a 90° phase shift when compared to the output signal 40. Again there may be a low pass filter to remove noise and unwanted high frequency signals. The phase shift circuitry 60 is controlled by control circuitry 80 to switch the value of the polarity of one of the versions of the LO2 signals depending on whether high side or low side injection has been selected. In this regard the phase shift circuitry 60 generates two versions of the LO2 signal the two versions being phase shifted with respect to each other by 90 ⁰ , and when switching between LSI and HSI the polarity of one of these signals is switched, in some cases by phase shifting that version of the signal by 180°, while in others by multiplying one of the signals by -1. This step is done as in this embodiment the second stage of the converter is configured to act as an image reject mixer to repress the image signal. Where the system switches between high side and low side injection the position of the image and wanted signals reverse and in order for the second stage to continue to repress the image signal rather than repressing the wanted signal following this switch the polarity of one of the versions of the second oscillator signals output by the phase shifting circuitry 60 is changed.

In this embodiment the converter circuitry comprises a frequency shifting element or circuitry that comprises the mixers 20, 22, determining circuitry 70 and control circuitry 80, the converter also comprises local oscillators L01 and LO2 and phase shift circuitry 30, 60. Determining circuitry 70 determines current conditions while control circuitry 8o controls the local oscillators and in particular controls the frequency of the first local oscillator LOi such that either high side or low side injection is selected. Control circuitry 8o also controls the phase shift circuitry 6o phase shifting the signal from the second local oscillator such that it is compatible with the selection of the high or low side injection

In some embodiments the frequencies of the first and second local oscillators are selected so that the frequency of the second local oscillator is a fraction of the frequency of the first local oscillator LOi for ease of implementation.

Determining circuitry 70 determines current conditions which may include the quality of the output signal, the position of the signal relative to the edge of a predetermined band and/or the strength of signal within potential noise forming bands, such as the image forming band, whose location depends on whether high or low side injection is selected. Depending on these values, the selection of high or low side injection may be made or may be changed by toggling from one to the other.

Looking at Figure 1 for example there are two possible image forming bands associated with sig 1 and they are on the low side IM_OB1 and on the high side IM_IB1. As in this example we are at the edge of the Ku band the signals outside of the Ku band are much higher than those within it, that is the image forming band IM_OB1 is much higher than the image forming band IM_IB1. Thus, in this embodiment the determining circuitry will determine either that image forming band IM_OB1 is higher than the image forming band IM_IB1 or simply that the input signal is close of the edge of the Ku band and will select high side injection and will set the frequency of the local oscillator to be to.qghz. This will mean that the image forming band is at uGhz rather than at io.6Ghz. In this way, the image forming band has a far lower strength signal and the noise intermediate frequency signal is reduced.

Figure 3 shows a further embodiment where the local oscillator 2 is not a sine wave but is a pulse wave.

Figure 4 shows a flow diagram illustrating steps in a method according to an embodiment. In the initial step S10 an input signal is received at a phased array antenna. It is then determined that step D5 is a signal is close to the edge of the KU band and if it is, it is then determined at step D15 whether the signal is close to the lower boundary of the KU band. If it is then high side injection is selected whereas if it is not then it is close to the upper boundary and low side injection is selected.

If it is determined at step D5 that the signal is not close to the edge of the KU band then at step D25 it is determined if the signal strength in the lower frequency image band is greater than the signal strength in the higher frequency image band. In this regard, these image bands are on either side of the input signal at a distance of two times the intermediate frequency from the input signal band. If the signal strength in the lower image frequency band is higher than the signal strength in the higher image frequency band then high side injection is selected whereas if the lower frequency image band is less than the higher frequency image band then low side injection is selected.

Once a particular injection side has been selected then the frequency converter will operate in this way. The control circuitry may be configured to determine whether this is still the preferable injection side from time to time. This may be done periodically, it maybe done in response to signals received that neighbouring channel conditions are changing and/ or it may be done as it is in this example by determining at step D35 whether the quality of the output signal has fallen below a predetermined value. If it is determined that the injection side should be changed, then at step S40 the control circuitry is configured to control the local oscillator to change the frequency such that it changes to the other injection side. It is then determined at step D45 if the quality of the output signal has improved. If it has not then at step S50 the method reverts to the original injection side whereas if it has improved it stays on the new injection side. Step D35 may be repeated where the output signal falls again below a predetermined value.

Figure 5 shows the effect of the signal strength in “noise folding bands” on the resultant signal and how with suitable selection of either high or low side injection the noise due to this may be reduced. Noise forming bands, also termed harmonic fold-back or harmonic mixing bands are frequency bands located at a harmonic of the second local oscillator LO2 frequency, which corresponds to the intermediate frequency, that is the difference in frequency between the signal and the first local oscillator. Frequency shifting the signal to the intermediate frequency band shifts signals in these harmonic fold-back bands to harmonics of the LO2 frequency and when in the second stage of the frequency shift the signals are shifted to baseband these signals are folded onto the baseband signal.

In the example of Figure 5, the noise folding bands with a significant signal are on the higher frequency side of the signal, and thus, where high side injection is selected these are shifted to lower order harmonics of the second local oscillator and thus, have a significant strength when shifted to baseband. Were the LSI to be selected, then they are present but as higher order harmonics and thus, their strength when shifted to baseband is significantly reduced, making the selection of LSI in this case prefereable. Figure 6 shows a phased array antenna 100 according to an embodiment. There are a plurality of antenna elements 5, and each is associated with frequency shifting circuitry 90 that comprises the mixers, filters and amplifiers of Figure 2 or 3. There are the two local oscillators L01 and LO2 and a controller 85 which comprises the control circuitry 80 and determining circuitry 70 of Figures 2 or 3. The controller controls the local oscillators and the phase shift circuitry in the frequency shifting circuitry 90 so that either high or low side injection is selected. As this is a phased array antenna each of the elements 90 also has additional phase shift circuitry associated with it that is also controlled by controller 85 to focus the antenna.

Figure 7 shows an alternative embodiment of the phased array antenna too where the frequency shift down conversion happens after signals from the individual antenna elements are coherently added up following the phase shift to focus the signal. In this embodiment each of the antenna elements 5 is associated with phase shift circuitry 92 controlled by controller 85 which phase shift the signals received from the individual antennas to focus the beam. The phase shifted signals are coherently added up to form a combined signal and in this embodiment it is this combined signal that is downconverted to baseband at circuitry 94. Circuitry 94 corresponds to the mixers, filters and amplifiers of Figures 2 or 3. The phased array antenna also comprises the two local oscillators L01 and LO2 and the controller 85 which in addition to comprising control circuitry for controlling the phase shift circuitry 92 to focus the beam comprises the control circuitry 80 and determining circuitry 70 of Figures 2 or 3. The controller 85 controls the local oscillators and the phase shift circuitry in the circuitry 94 so that either high or low side injection is selected.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.