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Title:
TRANSMITTER, TELECOMMUNICATION TERMINAL AND METHOD USING THE TRANSMITTER
Document Type and Number:
WIPO Patent Application WO/2007/074411
Kind Code:
A2
Abstract:
A transmitter comprising a quadrature mixer circuit (14) having at least two inputs (20, 22) for receiving a local oscillation signal and an input signal having their highest power peaks in a power density spectrum located at frequencies LO and IF, respectively, and at least one output (24) for providing an in-phase component Ims of a mixed signal, the mixed signal having its higher power peak in a power density spectrum located at frequency LO + IF where a component at frequency LO - IF is rejected. The transmitter further comprises a filter (16) for reducing a parasitic effect of the local oscillation harmonic frequency situated at 3*LO. The quadrature mixer circuit comprises a further output (26) for providing a quadrature-phase component Qms of the mixed signal, and wherein the filter is a complex filter connected to both quadrature mixer circuit outputs (24, 26) and having an output (90) for outputting the signal to be transmitted, the complex filter being adapted to discriminate positive frequencies from negative frequencies using both components Ims and Qms, and wherein the complex filter is set to specifically reject at least a negative frequency which is equal to -3*LO + n*IF, where n is an integer.

Inventors:
DELBECQ DOMINIQUE (FR)
GERME MICHEL (FR)
LEBAILLY GUILLAUME L (FR)
LECACHEUR NICOLAS L (FR)
Application Number:
PCT/IB2006/054781
Publication Date:
July 05, 2007
Filing Date:
December 12, 2006
Export Citation:
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Assignee:
NXP BV (NL)
DELBECQ DOMINIQUE (FR)
GERME MICHEL (FR)
LEBAILLY GUILLAUME L (FR)
LECACHEUR NICOLAS L (FR)
International Classes:
H04B1/26; H04B15/00; H04B1/16
Foreign References:
EP0216803A11987-04-08
US20010016016A12001-08-23
EP1133051A12001-09-12
US5999804A1999-12-07
Attorney, Agent or Firm:
DE JONG, Dirk (IP Department HTC 60 1.31 Prof Holstlaan 4, AG Eindhoven, NL)
Download PDF:
Claims:

CLAIMS

1. A transmitter comprising: a quadrature mixer circuit (14) having at least two inputs (20, 22) for receiving a local oscillation signal and an input signal having their highest power peaks in a power density spectrum located at frequencies LO and IF, respectively, and at least one output (24) for providing an in-phase component I m8 of a mixed signal, the mixed signal having its higher power peak in a power density spectrum located at frequency LO + IF where a component at frequency LO - IF is rejected, and a filter (16) for reducing a parasitic effect of the local oscillation harmonic frequency situated at 3 *LO, wherein the quadrature mixer circuit comprises a further output (26) for providing a quadrature-phase component Qm 8 of the mixed signal, and wherein the filter is a complex filter connected to both quadrature mixer circuit outputs (24, 26) and having an output (90) for outputting the signal to be transmitted, the complex filter being adapted to discriminate positive frequencies from negative frequencies using both components Ims and Q ms , and wherein the complex filter is set to specifically reject at least a negative frequency which is equal to -3*LO + n*IF, where n is an integer.

2. A transmitter comprising: - a quadrature mixer circuit (14) having at least two inputs (20, 22) for receiving a local oscillation signal and an input signal having their highest power peaks in a power density spectrum located at frequencies LO and IF, respectively, and at least one output (24) for providing an in-phase component I m8 of a mixed signal, the mixed signal having its higher power peak in a power density spectrum located at frequency LO - IF where a component at frequency LO + IF is rejected, and a filter (16) for reducing a parasitic effect of the local oscillation harmonic frequency situated at 3*LO,

wherein the quadrature mixer circuit comprises a further output (26) for providing a quadrature-phase component Qm 8 of the mixed signal, and wherein the filter is a complex filter connected to both quadrature mixer circuit outputs (24, 26) and having an output (90) for outputting the signal to be transmitted, the complex filter being adapted to discriminate positive frequencies from negative frequencies using both components Ims and Q ms , and wherein the complex filter is set to specifically reject at least a negative frequency which is equal to -3*LO + n*IF, where n is an integer.

3. A transmitter according to any one of the preceding claims, wherein the quadrature mixer circuit (14) comprises four Gilbert cell mixers (44, 46, 74, 76), each Gilbert cell mixer having a current switching stage (150, 166, 194, 196) series connected with a transconductance amplifier stage (152, 180), at least one of the transconductance amplifier stages being common to two different Gilbert cell mixers.

4. A transmitter according to claim 3, wherein the Gilbert cell mixers are

Gilbert cell double-balanced mixers.

5. A transmitter according to any one of the preceding claims, wherein the complex filter (16) comprises at least one notch filter.

6. A transmitter according to any one of the preceding claims, wherein n is included between -10 and +10.

7. A telecommunications terminal having a transmitter according to any one of the preceding claims.

8. A method to transmit a signal using a transmitter according to any one of claims 1 to 6, wherein the method comprises the step (120) of filtering the outputs of the quadrature cell mixer circuit with the complex filter, which is able to discriminate positive frequencies from negative frequencies using both components Im 8 and Q ms to obtain the signal to be transmitted, and wherein the complex filter is set to specifically reject at least a negative frequency which is equal to -3*LO + n*IF, where n is an integer.

Description:

Transmitter, telecommunication terminal and method using the transmitter

FIELD OF THE INVENTION

The present invention relates to a transmitter, a telecommunication terminal and a method using the transmitter.

BACKGROUND OF THE INVENTION

There exist transmitters comprising: a quadrature mixer circuit having at least two inputs to receive a local oscillation\ signal and an input signal having their highest power peaks in a power density spectrum located at frequencies LO and IF, respectively, and at least one output to output an in- phase component I m8 of a mixed signal, the mixed signal having its higher power peak in a power density spectrum located at frequency LO + IF in which the component at frequency LO - IF is rejected, and a filter designed to reduce the parasitic effect of the local oscillation harmonic frequency at 3*L0. Mixers are non- linear devices used in systems to multiply one frequency by another. The multiplication principle is the following.

Let us assume that the following signals a and b are to be multiplied: a = AsinJβ^ + φJ and b = Bsin(β) 2 t+φ 2 ) where:

- Q) 1 is the pulsation of the signal of frequency LO, and

- Q) is the pulsation of the signal of frequency IF

The resulting multiplied signal will be: a*b = ABsin(Q) 1 t-i-φ 1 ) .sin(Q) 2 t-i-φ 2 )

This can be written as follows: AB [008(( ( X )1 +O) 2 V + (φ i 2 )) - cos(( COl - (J ) 2 V - (φ i 2 ))]

2

Thus, the outputted mixed signal has one frequency component at LO + IF and one frequency component at LO - IF. Generally, only one of these frequency components is to be transmitted and the other frequency component is to be cancelled or rejected. The cancelled or rejected frequency component is called "spurious" or "image".

Quadrature mixer circuits use in-phase and quadrature-phase components of the local oscillation signal and the input signal to reject the spurious. The quadrature- phase component is equal in magnitude to the in-phase component but delayed in time by 90°, where 360° is equal to a period of the component.

Qadrature mixer circuits may be image-reject mixers as e.g. an image-reject mixer is disclosed in Fig.l of US 2003/0143966.

Generally, the local oscillation signal has a high amplitude and the quadrature mixer circuit perceives this local oscillation signal as a square wave. This means that the treated local oscillation signal also has harmonic frequencies at a frequency equalling 3*LO, 5*LO and so on. Note that throughout this description, the symbol « * » indicates a multiplication. The harmonic frequency at 3*LO is particularly harmful and should be rejected. In fact, the harmonic frequency at 3*LO is combined by the mixer with the input signal frequency IF and results in a parasitic spectrum line at frequency 3*LO - IF and/or 3*LO + IF. In the transmitter, the mixed signal is outputted to an amplifier. The applicant has noted that the amplifier - which saturates - generates intermodulation products at its output: 3*LO +_IF with 2(LO + . IF). This results in noise at frequency LO + n*IF that is very difficult to cancel, where n is a positive or negative integer generally included between -3 and +3.

Solutions to specifically reduce the parasitic effect of the 3*LO harmonic frequency have already been proposed. US 2004/0116093 discloses a quadrature mixer circuit having LC filters to cancel the noise at 3*LO ± IF. However, the LC filter is not very efficient because the frequency which should be filtered, i.e. 3*LO ± IF, is quite close to the frequency LO ± IF to be transmitted. Furthermore, the filter needs an inductance that is area - consuming.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a transmitter having a filter to reduce theparasitic effect of the local oscillation harmonic frequency at 3*LO, which is simpler to design and therefore less expensive. The invention is defined by the independent claims. Dependent claims describe advantageous implementations. It is provided a transmitter comprising: a quadrature mixer circuit (14) having at least two inputs (20, 22) for receiving a local oscillation signal and an input signal having their highest power peaks in a power density spectrum located at frequencies LO and IF, respectively, and at least one output (24) for providing an in-phase component I m8 of a mixed signal, the mixed signal having its higher power peak in a power density spectrum located at frequency LO + IF where a component at frequency LO - IF is rejected, and a filter (16) for reducing a parasitic effect of the local oscillation harmonic frequency situated at 3 *LO, wherein the quadrature mixer circuit comprises a further output (26) for providing a quadrature-phase component Qm 8 of the mixed signal, and wherein the filter is a complex filter connected to both quadrature mixer circuit outputs (24, 26) and having an output (90) for outputting the signal to be transmitted, the complex filter being adapted to discriminate positive frequencies from negative frequencies using both components Ims and Q ms , and wherein the complex filter is set to specifically reject at least a negative frequency which is equal to -3*LO + n*IF, where n is an integer.

A negative frequency is equal to a positive frequency but delayed in time by 90°. A definition of negative frequencies as well as complex filters can be found in the following article: "Complex Signal Processing is Not Complex" by Kenneth W. Martin, IEEE Transactions on Circuits and Systems - 1: Regulars Papers, Vol. 51, No. 9, September 2004.

Recall that complex filters use cross coupling between the real and imaginary signal paths in order to implement asymmetrical (in the frequency domain) filters having transfer functions that do not have the conjugate symmetry of real filters. In a complex representation, the 3.LO + n*IF harmonic frequency results in a spectrum line at negative frequency -3*LO + n*IF. Said negative frequency 3*LO + n*IF is spaced apart in the complex representation from frequency LO + IF or

LO - IF by a frequency band δBF1 that is roughly equal to 4.LO because frequency IF is much smaller than frequency LO. If a non-complex filter, that is not able to discriminate positive from negative frequencies, was used, then the non-complex filter would have to be designed to reject the positive frequency +3*L0 + n*IF. This positive frequency is spaced apart from frequency LO + IF or LO - IF only by a frequency band δBF2 that is roughly equal to 2*L0. As the frequency band δBF1 is twice larger than the frequency band δBF2, the complex filter is easier to design and implement than a non-complex filter.

The embodiments of the above transmitter may comprise one or several of the following features: the quadrature mixer circuit comprises four Gilbert cell mixers, each Gilbert cell mixer having a current switching stage series connected with a transconductance amplifier stage, at least one of the transconductance amplifier stages being common to two different Gilbert cell mixers, - the Gilbert cell mixers are Gilbert cell double-balanced mixers, the complex filter comprises at least one notch filter, and n is included between -10 and +10.

The above embodiments of the transmitter present the following advantages: they have a transconductance amplifier that is common to two Gilbert cell mixers thereby saving components and lowering the manufacturing price, and by using Gilbert cell double-balanced mixers, the suppression of spurious products is improved.

The invention relates also to a telecommunication terminal having the above transmitter. These and other aspects of the invention will be apparent from the following description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.l is a schematic diagram of a terminal having a transmitter with a quadrature mixer circuit,

Fig.2 is a flowchart of a method of transmitting a signal with the transmitter ofFig.l,

Figures 3 A to 3E are schematic complex representations of density power spectrums of signals at different points in the transmitter of Fig.1,

Fig.4 is a schematic diagram of an improved embodiment of the transmitter ofFig.l, Fig.5 is a schematic diagram of a current switching stage used in the transmitter of Fig.4, and

Fig.6 is a schematic diagram of a transconductance amplifier stage of the transmitter of Fig.4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig.l shows a telecommunications terminal 2 having a transmitter 4. For example, terminal 2 is a mobile phone.

In the following description, functions or constructions well-known by a person of ordinary skill in the art are not described in detail. For simplicity, in Fig.1 , only the elements necessary to understand the invention are shown.

Transmitter 4 has an input 6 to receive an input signal of frequency IF and an output 8 to output a mixed signal to be transmitted over the air. Output 8 is connected to an antenna 10. In the following description, when it is recited that a signal has a frequency

F, this means that its highest power peak in a power density spectrum is located at frequency F. In other words, a signal of frequency F encompasses signals having only frequency F as well as signals having frequency F and some other less powerful frequency components like harmonic frequencies. The mixed signal corresponds to the input signal but with a translated frequency. Hereinafter, for illustration purposes, the translated frequency is equal to IF + LO, where LO is the frequency of a local oscillation signal.

Frequency LO is much greater than frequency IF. For example, frequency LO is at least eight times greater than frequency IF. Transmitter 4 has a local oscillator 12 that outputs the local oscillation signal at frequency LO.

Transmitter 4 also has: a quadrature mixer circuit 14 to multiply the input signal by the local oscillation

signal, a complex filter 16 to filter the output of quadrature mixer circuit 14 to reject or cancel a negative frequency component at - (3*LO - IF), and an amplifier 18 to amplify the output of filter 16 so as to obtain the signal to be transmitted over the air.

The output of amplifier 18 is connected to output 8 through other well- known electronic components that are not shown in Fig.1 , for simplicity.

More precisely, quadrature mixer circuit 14 has one input 20 to receive the local oscillation signal and one input 22 to receive the input signal. For example, the input signal is a baseband signal. Circuit 14 has also two outputs 24 and 26 to output an in-phase component Im 8 and a quadrature-phase component Qm 8 of the mixed signal, respectively. « In-phase » and « quadrature-phase » are well-known terms. For a more precise definition of these terms, reference is made to the previously referenced IEEE article, for example. Components I ms and Q ms have a frequency IF + LO in which the spurious components at frequency LO - IF have been cancelled or rejected.

Circuit 14 includes an image-reject mixer 30 that outputs component I m8 and an image-reject mixer 32 that outputs component Q ms .

Mixer 30 has two inputs 34 and 36 connected to inputs 20 and 22 to receive the local oscillation signal and the input signal, respectively. Mixer 30 has also one output 38 connected to output 24. Mixer 30 includes two quadrature splitters 40 and 42, an in-phase mixer 44, a quadrature-phase mixer 46 and a quadrature-signal combiner 48.

Splitter 40 has one input connected to input 34 to receive the local oscillation signal and two outputs 50 and 52. Outputs 50 and 52 output an in-phase component LO I and a quadrature-phase component LO Q of the local oscillation signal.

Similarly, splitter 42 has one input connected to input 36 to receive the input signal and two outputs 54 and 56. Outputs 54 and 56 output an in-phase component IF I and a quadrature-phase component IF Q of the input signal. In-phase mixer 44 multiplies in-phase components LO I and IF I. Mixer 44 therefore has two inputs connected to outputs 50 and 54, respectively. Mixer 44 has also one output 58 to output the result of the multiplication of the in-phase components.

Quadrature-phase mixer 46 multiplies the quadrature-phase components LO Q and IF Q. Mixer 46 therefore has two inputs connected to outputs 52 and 56, respectively. Mixer 46 has also one output 60 to output the result of the multiplication of the quadrature-phase components. Combiner 48 combines the multiplication results outputted by mixers 44 and

46 so as to cancel the spurious at frequency LO - IF. Combiner 48 has two inputs connected to outputs 58 and 60, respectively. Combiner 48 has also one output connected to output 38 to output component I m8 .

Image-reject mixer 32 also includes two quadrature splitters 70 and 72, two mixers 74 and 76 and one combiner 78.

Splitter 70 has one input connected to input 20 and two outputs 80 and 82. Outputs 80 and 82 provide components LO I and LO Q.

Splitter 72 has one input connected to input 22 and two outputs 84 and 86 to output components IF I and IF Q, respectively. Outputs 80 and 86 are connected to inputs of mixer 74, so that mixer 74 multiplies components LO I and IF Q.

Outputs 82 and 84 are connected to inputs of mixer 76, so that mixer 76 multiplies components LO Q and IF I. Except these differences, the other features of mixer 32 are identical to those of mixer 30 and are not described here in further detail. Mixers 44, 46, 74 and 76 can be any kind of well-known mixers.

Complex filter 16 uses components Im 8 and Q ms to discriminate positive frequencies from negative frequencies. This implies that the transfer function of filter 16 has at least one complex coefficient. Such complex filters are described in the above referenced IEEE article. For example, in this embodiment, filter 16 is a notch filter designed to reject only negative frequency -3*LO + IF.

Filter 16 has an output 90 to output a filtered in-phase component (IF + LO) I of the mixed signal. Here, for illustration purposes, a filtered quadrature- phase component (IF + LO) Q of the mixed signal outputted by filter 16 is not used.

Amplifier 18 has an input connected to output 90 and an output 92 connected to output 8. Output 92 outputs the amplified, filtered in-phase component of the mixed signal at frequency LO + IF.

The operation of the transmitter 4 will now be described with reference to Fig.2. Initially, in step 100, circuit 14 receives the local oscillation signal and the input

signal. In step 102, quadrature splitters 40, 42, 70 and 72 build the in-phase and quadrature-phase components LO I, LO Q, IF I and IF Q.

In step 104, mixers 44, 46, 74 and 76 multiply two by two the components LO I, LO Q, IF I and IF Q. More precisely, in step 104, the following operations 106, 108, 110 and 112 are performed in parallel by mixers 44, 46, 74 and 76, respectively:

- in operation 106, mixer 44 multiplies components LO I and IF I,

- in operation 108, mixer 46 multiplies components LO Q and IF Q,

- in operation 110, mixer 74 multiplies components LO I and IF Q, and - in operation 112, mixer 76 multiplies components LO Q and IF I.

In step 114, combiner 48 adds the outputs of mixers 44 and 46 so as to build components I ms in which the spurious at frequency LO - IF is rejected.

In parallel, in step 116, combiner 78 adds the outputs of combiners 74 and 76 so as to obtain components Q ms in which the spurious at frequency LO - IF is rejected.

Subsequently, in step 120, filter 16 rejects the frequency component at frequency -3*LO + IF, using components Im 8 and Q ms .

Then, in step 122, the filtered signal is amplified by amplifier 18 before being outputted through output 8. The outputted signal is then transmitted to antenna 10. For example, the signal outputted at output 8 is a radio frequency signal RF that can be radiated in the air by the antenna 10. The frequency of this radio frequency signal is equal to LO + IF.

Figs.3 A to 3E show a complex representation of power density spectrums of the input signal, the local oscillation signal, the mixed signal outputted by circuit 14, the mixed signal outputted by filter 16 and the mixed signal outputted by amplifier 18, respectively.

Fig.3A and Fig.3B show that the input signal and the local oscillation signal have only one power peak at frequencies IF and LO, respectively.

Fig.3C shows that at the output of circuit 14, the highest power peak is at frequency LO + IF. Fig.3C shows also that there exist spurious at frequencies

-3.LO + IF and 5LO + IF. Thus, in a complex representation, the noise produced by the harmonic frequency at 3.LO is located in the negative frequency and spaced apart from frequency LO + IF by a frequency band equal to 4.LO. Note that the graph of Fig.3C

can be obtained only if in-phase and quadrature-phase components of the mixed signal are used. In a real representation that is not obtained from in-phase and quadrature- phase components, the parasitic spectrum line generated by the 3.LO harmonic frequency is located at frequency 3*LO - IF. Fig.3 D shows that at the output of filter 16, the power of frequency

-3.LO + IF has greatly been reduced, whereas the powers of frequencies LO + IF and 5.LO + IF have not been modified.

Fig.3E shows that frequency -3*L0 + IF, that has not been completely cancelled by filter 16, is combined with harmonic frequency 2(LO + IF) and results in a parasitic spectrum line at frequency LO - 3*IF. However, the power of this parasitic spectrum line is small by comparison to the power of frequency LO + IF, because the power of frequency -3.LO + IF has greatly been reduced by filter 16. For example, the power of the noise at frequency LO - 3.IF is at least five times smaller than the power of frequency LO + IF. Fig.4 shows in more detail a specific embodiment of circuit 14. In Fig.4, only mixers 44, 46, 74 and 76 are shown in detail. In this embodiment, mixers 44, 46, 74 and 76 are Gilbert cell double-balanced mixers.

Mixers 44, 46, 74 and 76 have the same structure, and only the structure of mixer 44 will be described in detail hereinafter. Mixer 44 has a current switching stage 150 connected in series with a transconductance amplifier stage 152.

Stage 150 is driven by differential components LO I and LO In. The component LO In is equal to component LO I multiplied by -1.

Stage 150 has two inputs 154 and 156 connected to a potential Vcc through resistors 158 and 160.

Inputs 154 and 156 are also connected to corresponding inputs 162 and 164 of a current switching stage 166 of mixer 46.

These connections between inputs 154, 156 and inputs 160, 162 add the current of both mixers 44 and 46 and implement the function of combiner 48. Filter 16 is connected to an extremity of resistors 158 and 160 so as to receive differential components I ms and I msn . Component Im 8n is equal to component Im 8 multiplied by -1.

Stage 150 has also two outputs 170 and 172 connected to corresponding inputs 174 and 176 of stage 152.

Stage 152 is driven by differential components IF I and IF In. Component IF In is equal to component IF I multiplied by -1.

An output of stage 152 is connected to a reference potential like ground.

Stages 150 and 152 will be described in more detail in Figs. 5 and 6, respectively.

Mixer 46 includes resistor 160, stage 166 and transconductance amplifier stage 180.

Mixer 74 includes a resistor 190 connected to potential Vcc, and a current switching stage 194 that is connected with transconductance amplifier stage 180. Mixer 76 includes a resistor 192, and a current switching stage 196 that is connected with transconductance amplifier stage 152.

Transconductance amplifier stage 180 is driven by differential components IF Q and IF Qn, where IF Qn is equal to component IF Q multiplied by -1.

Stage 194 is driven by differential components LO I and LO In, where component LO In is equal to component LO I multiplied by -1.

Stage 196 is driven by differential components LO Q and LO Qn, where LO Qn is equal to component LO Q multiplied by -1.

Mixers 76 and 74 are interconnected in a similar way as mixers 44 and 46 so as to implement combiner 78. Transconductance amplifier stage 180 has a structure identical to transconductance amplifier stage 152, and current switching stages 194 and 196 have structures identical to stage 150.

Fig.5 shows current switching stage 150 in more detail. Stage 150 has two pairs of transistors, respectively, 200, 202 and 204, 206. The emitters of transistors 200, 202 are connected to output 170 and the emitters of transistors 204 and 206 are connected to output 172.

Collectors of transistors 200 and 204 are connected to input 154. Collectors of transistors 202 and 206 are connected to input 156. The bases of transistors 200 and 206 are driven by component LO I. The bases of transistors 202 and 204 are driven by component LO In.

Fig.6 shows the transconductance amplifier stage 152 in more detail.

Stage 152 has two transistors 210 and 212, collectors of which are connected to inputs 174 and 176, respectively. The emitters of transistors 210 and 212 are

connected to ground. The base of transistor 210 is driven by component IF I and the base of transistor 212 is driven by component IF In. As a result, the voltage applied to the bases of transistors 210 and 212 is transformed into a corresponding current that flows through transistors 210 and 212. The mixers of Fig.4 operate like a well-known Gilbert cell double-balanced mixer, so that a further explanation will not be given. It should be noticed that contrary to conventional Gilbert cell double-balanced mixers, here, stages 152 and 180 are common to two different Gilbert cell double-balanced mixers and that the inputs of the current switching stages of two adjacent mixers are connected so as to combine the signal o f the two adj acent mixers .

Many other embodiments are possible. For example, the embodiment of Fig.4 may be simplified if differential signals are not used.

A current source may be added in the transconductance amplifier stages 152 or 180 (reference is made for example to Fig.9 of US 2004/0116093). Resistors may also be added in the structure of Fig.6 to improve the linearity.

The embodiment of Fig.1 may be realized using mixers, other than Gilbert mixers. For example, to implement mixers 44, 46, 74 and 76, the following well-known mixers can be used: single diode mixers, double diode mixers, single-balanced two- diode mixers, single-ended junction field-effect transistor (JFET) mixers, dual-gate metal-oxide semiconductor field-effect transistor (MOSFET) mixers, single-balanced Gilbert cell mixers and so on.

The complex filter 16 may be designed to reject frequencies, other than frequency -3*LO + IF. More generally, filter 16 can be designed to reject frequency - 3*LO + n*IF, where n is a positive or a negative integer. The most useful value for n is +1 or -1. However, values of n between -10 and +10 may be of interest for special applications and values of n chosen in the group {-3; -1; +1; +3} are preferred.

Filter 16 may also be designed to simultaneously reject several frequency components. To this end, filter 16 may comprise a plurality of series connected notch filters, each notch filter being designed to specifically reject one frequency. Filter 16 may also be a complex band-pass filter designed to reject any frequencies other than frequency LO + IF.

Finally, transmitter 4 has been described in the special case where the mixed signal to be output is at frequency IF + LO, whereas frequency LO - IF is rejected.

However, the above teachings also apply to a transmitter that outputs a mixed signal at frequency LO - IF, whereas the spurious at frequency LO + IF is rejected. To this end, combiners 48 and 78 can be implemented using subtracters instead of adders.

It is remarked that the scope of protection of the invention is not restricted to the embodiments described herein. Neither is the scope of protection of the invention restricted by the reference numerals in the claims. The word "comprising" does not exclude other parts than those mentioned in the claims. The word "a(n)" preceding an element does not exclude a plurality of those elements. Means forming part of the invention may both be implemented in the form of dedicated hardware or in the form of a programmed purpose processor. The invention resides in each new feature or combination of features.