Abstract:
In a telecommunications transmitter and receiver two IQ-mixers of the same type are operated in 180 degrees phase shift. In both the transmitter and receiver, an input signal is coupled to each IQ-mixer and the output signal of the mixers are combined so as to cancel unwanted error components in each individual output signal. In the transmitter, residual carrier signals are cancelled, and in the receiver, DC-offsets are cancelled. In both, the wanted signals are added, doubling the amplitude of the resultant output signal leading to four times more output power and improved dynamic range.

Description:
TECHNICAL FIELD 
   The present invention relates to a telecommunications receiver and a telecommunications transmitter. 
   BACKGROUND OF THE INVENTION 
   In the field of receiver and transmitter technology for mobile telecommunications to date, only receivers incorporating fixed analog intermediate frequency (IF) stages have been found suitable for use in base stations. This is because of the large range of power levels which must be handled, and also a requirement for a low level of blocking. The range of power levels is often called the dynamic range and is usually quantified as the ratio between strongest and weakest usable power levels. Blocking is, of course, the problem of a weaker signal being drowned out by a stronger signal. 
   Direct conversion receivers with zero or near-zero IF stages are becoming available. However they are only suitable for use in mobile user terminals but not base stations because power range and blocking requirements of mobile user terminals are much less demanding than those of base stations. Direct down-conversion receivers are known to suffer from the problem of direct current (DC) offsets in their IQ-mixers, which limits the useful dynamic range. Incidentally, IQ-mixers are also known in the art as I/Q modulators and quadrature mixers, where I refers to the in-phase component of a signal and Q refers to the quadrature phase component. 
   A so-called Othello chipset for mobile user terminals is known, which incorporates a zero IF direct conversion receiver for triple-band operation (GSM 900, 1800, 1900 MHz bands). A chipset for mobile user terminals is also known which operates with a near zero IF, namely 100 kHz. Such chipsets use Variable Gain Amplifiers (VGA) as part of an automatic gain control (AGC) loop to cope with the dynamic range requirements of the mobile user terminal. To compensate for the direct current (DC) offset, averaging over a long time period is used to estimate the DC-offset correction to be applied. 
   The DC-offset in a direct down-conversion receiver limits performance. Accurate detection is not possible when the DC-offset is stronger than the wanted signal as occurs with weak input signals. In consequence, the dynamic range of the analog to digital converter (ADC) used to sample the in-phase (I) and quadrature phase (Q) signal components is often insufficient; the dynamic range then being the ratio of the strongest received signal to the DC-offset rather than to the weakest received signal. 
   As regards transmitters, direct up-conversion can be used, although a high level of carrier signal suppression is often required. Direct up-conversion modulators each consisting of an IQ mixer are available in the marketplace. However, the powers used in base stations are often near to the minimum powers which these modulators can handle, i.e. the “noisefloor”. Such modulators typically offer carrier signal suppression in the order of 35 dB without tuning. However, to improve carrier suppression, a manual or automatic tuning process is used involving e.g. adjustment of variable resistors and/or capacitors so as to compensate for slight differences in gains and delays between the I and Q branches of the IQ mixer. 
   The limited carrier suppression capability of known direct up-conversion mixers limits their performance. Amplitude modulation applied to the I and Q input signals is limited by the carrier signal suppression because the minimum amplitude of each of the I and Q signals equals the level of the carrier after suppression. Also the suppressed carrier generates some distortion, called error vector magnitude (EVM), of the output signal. This distortion adds to the wanted signal as a rotating phasor or a displacement in the IQ-plane. 
   SUMMARY OF THE INVENTION 
   Embodiments of receivers and transmitters in accordance with the present invention realize that two circuits of the same type, and possibly integrated on the same chip, normally behave very similarly. In these embodiments, using two IQ-mixer stages, which are operated in 180 degrees phase shift, results in the error components of the output signals being cancelled. Embodiment of the receivers and transmitters in accordance with the present invention thus preferably involve the use of two IQ-mixers of the same type in order to cancel out unwanted components of output signals. In the receivers, DC-offsets are cancelled. In the transmitters, residual carrier signals are cancelled. Also, for both transmitters and receivers, the wanted signals are added which doubles the amplitude of the output signal and leads to four times more output power, thus also giving an improved dynamic range on the output. 
   An embodiment of the present invention is a telecommunications receiver having first and second IQ mixers of the same type. The second mixer is provided an input signal in antiphase to the phase of the received signal input to the first mixer. Each mixer outputs an I signal and a Q signal. The I signal from the second mixer is in phase or antiphase to the I signal from the first mixer, and the Q signal from the second mixer is in phase or in antiphase to the Q signal from the first mixer. The I and Q signals each include a respective DC offset component. A first output signal is produced, which is the sum of the two I signals when the two I signals are in opposite phases or the difference between the two I signals when they are of the same phase, such that the DC offset components of the I signals at least partially cancel. Also a second signal is produced being the sum of the two Q signals when the two Q signals are of opposite phase or the difference between the two Q signals when they are of the same phase, such that the DC offset components of the Q signals at least partially cancel. 
   Another embodiment of the present invention is a telecommunications transmitter having first and second IQ mixers of the same type. Each mixer is provided with an I signal and a Q signal. The I signal input to the second mixer is in phase with the I signal input to the first mixer. The Q signal input to the second mixer is in phase with the Q signal input to the first mixer. The second mixer provides an output signal in antiphase to an output signal provided by the first mixer, each output signal including a residual carrier component. A signal for transmission is provided by combining the two output signals by phase shifting one of the signals by 180 degrees and adding this to the other output signal, such that the residual carrier components at least partially cancel. 
   Embodiments of the present invention can also provide significant other advantages. As regards receiver embodiments, DC offsets are cancelled in real time, independent of the signal strength, frequency, or the level of the local oscillator (LO) input signals to the mixers. A further advantage is that any feedthrough of the local oscillator signal in the reverse direction is cancelled in the 180° power combiner at the input to the receiver. 
   As regards transmitter embodiments, carrier suppression is improved by cancelling out the individual residual carrier signals, e.g. residual local oscillator signals, of each IQ-mixer in the 180° combiner. Furthermore, the noisefloor, i.e. minimum signal power to be handled, is improved by 3 dB as the noise of both IQ-mixers is not correlated, but the wanted signals processed by the two mixers are correlated. 
   As regards further advantages of embodiments of both receivers and transmitters, device dependant effects, such temperature drifts, are fully compensated for as they are the same for the two IQ-mixers of the same type. A further advantage of receiver embodiments is that a need of the prior art to tune gain and delay differences between I and Q branches (as discussed above) is avoided. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating a first embodiment of a receiver in accordance with the invention; 
       FIG. 2  is a diagram illustrating a second embodiment of a receiver; 
       FIG. 3  is a diagram illustrating a third embodiment of a receiver; 
       FIG. 4  is a diagram illustrating a first embodiment of a transmitter in accordance with the invention; and 
       FIG. 5  is a diagram illustrating a second embodiment of a transmitter. 
   

   DETAILED DESCRIPTION 
   Embodiments of receivers are described below. Conversion is direct from radio frequency (RF) to baseband (i.e. Direct Current). Embodiments of transmitters are then described involving baseband to RF conversion, also without an intermediate frequency (IF) conversion stage. 
   A First Receiver 
   In a first receiver embodiment  1  as shown in  FIG. 1 , the input signal A is amplified by an amplifier DRV and split into two paths  10 , 12 , a path  10  with no phase shift and a path  12  with a phase shift of 180 degrees. One way to achieve the phase shift is to use a 180° Hybrid  14 , to ensure that the signal levels are the same for both paths  10 , 12 . 
   Signals on each of the paths  10 , 12  are then processed by a respective IQ mixer  16 , 18 , each of which uses two LO-signals, one  22  with 0° phase shift and one  23  with 90° phase shift. The IQ-mixers  16 , 18  for both paths  10 , 12  are identical in structure. Each mixer  16 , 18  incorporates a splitter  20  and signal inputs  22 , 23  from a local oscillator (LO, not shown). Both IQ-mixers are implemented on the same integrated circuit chip. The length of the connectors  38  from the 180° Hybrid  14  to each splitter  20  is the same. 
   Due to imperfections of the IQ-mixers  16 , 18  an unwanted DC offset appears at the outputs of the IQ-mixers  16 , 18 . Each I output of IQ-mixer  2  is 180° phase shifted compared to the corresponding I output of IQ-mixer  1 . However, the DC offsets from the two IQ-mixers  16 , 18  are not shifted in phase. The same applies for the Q outputs also. 
   All the outputs are cross-connected to adder stages  24 , 25 , 26 , 27 , the I and Q components with the same phase being connected so that they add. At each adder, one of the two inputs has a DC offset shifted by 180° relative to the other. Accordingly, the DC offsets are cancelled by the adders. 
   After some amplification by differential variable gain amplifiers (denoted  30 , 31  in  FIG. 1  for I signals, denoted  30 ′, 31 ′ in  FIG. 1  for Q signals) and baseband filtering by low pass filters (denoted  28 , 29  in  FIG. 1  for I signals, denoted  28 ′, 29 ′ in  FIG. 1  for Q signals), the I and Q signals (I signal  32  and Q signal  34 , each having in phase and anti-phase components) are sampled by an analog to digital converter (ADC)  36  and then demodulated and decoded by processing stages (not shown). In this example, the ADC has a baseband input power of 10 dBm maximum. 
   Another effect of imperfections of IQ-mixers is the feedthrough of LO-signal to the input of the IQ-mixer. In the described embodiment, these residual signals would arrive with the same level back at the output of the 180° Hybrid  14 , which splits the input signal. In this case it operates in an opposite manner such that the residual signals are added at the input to the 180° Hybrid  14 , but as they are 180° shifted in phase the two residual signals cancel. 
   All of the cancellation effects work in real time. 
   A Second Receiver 
   A second receiver embodiment  201  is shown in  FIG. 2 . This receiver has essentially the same structure and function as the receiver shown in  FIG. 1  except that the input signal A′ is directed to one path  210  (so there is no 180° Hybrid). A reference signal B which is zero is provided instead to the other path  212  so that the second IQ-mixer  218  is used to generate a reference DC offset which is then used for cancellation purposes as in the receiver in  FIG. 1 . As cancellation of effects which are dependent of the input signal level are not taken into account, the performance of this embodiment may be lower than the receiver in  FIG. 1 . 
   Other Receivers 
   A third receiver  301  is shown in  FIG. 3 . In this receiver, the I and Q signals from each IQ-mixer  316 ,  318  are provided as unbalanced signals to the differential variable gain amplifiers  330 . Specifically, the 0° phase I component from IQ-mixer  316  and the 180° phase I component from IQ-mixer  318  are passed through a respective differential variable gain amplifier  330  and low pass filter  328  then through a further respective differential variable gain amplifier  331  and further low pass filter  329  to provide the I signal  332  at baseband frequency. In similar fashion, the 0° phase Q component from IQ-mixer  316  and the 180° phase Q component from IQ-mixer  318  are passed through a respective differential variable gain amplifier  330 ′ and low pass filter  328 ′ then through a further respective differential variable gain amplifier  331 ′ and further low pass filter  329 ′ to provide the Q signal  334  at baseband frequency. Adders (see reference numerals  24 , 25 , 26 , 27  in  FIG. 1  for comparison) are thus not required. As any DC offset is of 0° phase in the I and Q components provided by the IQ-mixers  316 , 318 , each differential variable gain amplifier  330 , 330 ′ only amplifies the difference between its two inputs, the DC offset in the two inputs being cancelled by this operation. 
   In another receiver embodiment (not shown) otherwise similar to the first receiver shown in  FIG. 1 , the amplifier DRV is a differential amplifier having input signals +A and −A. 
   A First Transmitter 
   As shown in  FIG. 4 , in a first transmitter embodiment, I and Q analog signals are generated by an encoder (not shown) connected to a digital to analog converter (DAC)  50 . After amplification by amplifiers  52  and lowpass filtering by filters  54 , the I and Q signals both in phase and in antiphase are fed to two identical IQ mixers  56 , 58 . The signal inputs  60  are connected in reverse order at IQ-mixer  2  (reference numeral  58 ) compared to IQ mixer  1  (reference numeral  56 ) as shown in the Figure. Each mixer  56 , 58  includes a combiner  420  and signal inputs  422 , 423  from a local oscillator (not shown). 
   At the output  62  of IQ-mixer  2  (reference numeral  58 ) the signal is shifted in phase by 180° compared to the output  64  of IQ-mixer  1  (reference numeral  56 ). The signal at each output  62 ,  64  includes a residual carrier signal. The residual carrier signal arises in particular due to crosstalk from the LO as a result of some capacitive coupling, further due to some DC at the signal inputs  60  in consequence of tolerances in the generation and handling of in-phase and in antiphase I and Q signals, and due to some impedance mismatches. The residual carrier signal at each of the outputs  62 ,  64  of the IQ-mixers  56 ,  58  have the same phase. In consequence, at the 180° hybrid  64 , which acts as a combiner, the wanted signals are added and the unwanted residual carrier signals are cancelled, the resultant signal being provided at the output  66 . 
   Other Transmitters 
   In another embodiment (not shown), another source also having balanced (i.e. symmetric) outputs is used in place of the encoder and DAC  50  in the transmitter configuration shown in  FIG. 4 . 
   In another embodiment as shown in  FIG. 5 , an I and Q signal source with unbalanced (i.e. unsymmetric) outputs is used instead. The signal source is a digital to analog converter (DAC)  550 . In this embodiment, only in-phase, i.e. 0 degrees, I and Q signal components are provided by the DAC  550 . After amplification  552  and low pass filtering  554 , the signal components are provided to each IQ mixer  556 , 558 , the antiphase (i.e. 180 degrees) input ports of which are grounded (ground  551 ). 
   Abbreviations 
   
       
       ADC Analog to digital converter 
       AGC Automatic gain control 
       DC Direct current 
       I In-phase 
       IF Intermediate frequency 
       LO Local oscillator 
       Q Quadrature phase 
       RX Receiver 
       TX Transmitter 
       VGA Variable gain amplifier