GB2488510A

GB2488510A - Up converter for telecommunications receiver

Info

Publication number: GB2488510A
Application number: GB1022117.4A
Authority: GB
Inventors: Michael Deeney
Original assignee: Elonics Ltd
Current assignee: Elonics Ltd
Priority date: 2010-12-31
Filing date: 2010-12-31
Publication date: 2012-09-05
Also published as: GB201022117D0

Abstract

A up-converter system is provided for generating a low-IF output from a zero-IF input in a digital TV radio tuner. In an embodiment, the up-converter includes a passive current mode 3-phase double quadrature mixer 34 driven by 3-phase quadrature local oscillator inputs, a polyphase filter 36 to reject 7th order harmonics, and a polyphase filter 38 to reject residual adjacent (e.g. N-1, N-2) channels. The double quadrature mixer 34 receives a zero-IF signal having a wanted channel and unwanted adjacent channels. The mixer 34 up-converts the zero-IF signal and selectively attenuates negative third and negative seventh order harmonic components of the upconverted signal. It also selectively attenuates negative third and positive fifth order harmonic components. The polyphase filter 38 attenuates adjacent channels occupying the negative frequency range corresponding to the positive frequency range of the wanted channel in the upconverted signal. An anti-alias filter 40 rejects higher order harmonics generated in the mixing process. The method provides the desirable properties of a zero-IF tuner, in particular relaxed image rejection requirement, in a tuner that provides the low-IF output required by digital TV baseband circuits.

Description

Upconverter and method of upconversion The present invention relates to the field of signal processing, in particular upconverters for telecommunications receivers, such as television tuners, for providing an output baseband signal to a baseband processor.

BACKGROUND

Radio frequency (RF) tuners are essential components in a host of applications. One such application is that of digital TV. A common method of receiving and processing digital TV signals is via an antenna followed by a set-top box (STB) which feeds its output to a television. The STB function can be divided into two parts, radio and processing. The radio tuner receives all the TV channels, tunes to one particular channel, and outputs a baseband signal. The baseband function demodulates and processes this output in order to extract programme data suitable for input to the television.

The tuner can be implemented in many different ways, both in terms of architecture and technology implementation. For example, in terms of technology, traditionally tuners for TV applications have consisted of a number of discrete components integrated into a module (the so-called "can tuner"). In recent years, there has been a move to higher levels of integration, primarily using integrated circuits (lCs). These lCs can be implemented using different semiconductor materials (e.g. silicon, gallium arsenide etc.) and using different types of devices internally (CMOS, bipolar, BiCMOS primarily). In terms of the architecture of the radio, the most popular has traditionally been the super heterodyne receiver, which translates, in one or more stages, the RF frequencies to a lower intermediate frequency (IF). This IF is low enough to be used by the baseband circuitry to accomplish the baseband processing. An alternative approach is to use a zero-IF (ZIF) architecture which translates the RF input directly to dc (direct current). This approach has traditionally had a number of drawbacks (such as dc offset), but these have been largely addressed through the use of integrated circuit techniques. The advantages of zero-IF over super-heterodyne include increased integration and fewer off-chip components.

On the other hand, the majority of STB baseband lOs use a low-IF, or near-zero IF (NZIF), input, therefore in such an application the tuner must have a low-IF output. Achieving a low-IF output with sufficient dynamic range to accommodate potentially very large adjacent channel interferers is not a trivial task. One of the more difficult requirements in the European terrestrial digital TV (DVB-T) standard to meet is to maintain the image rejection of the receiver to greater than 60dB -if this is not the case, the adjacent channel interferer can fold directly into the wanted band such that low power wanted signals cannot then be detected. In practice, complex on-chip calibration schemes are usually required to achieve this level of image rejection.

For example, Figure 1 illustrates a hybrid silicon tuner described by V. Fillatre et al. in "A SiP Tuner with Integrated LC Tracking Filter for both Cable and Terrestrial TV Reception" in 2007 IEEE International Solid-State Circuits Conference, pp208-209. This tuner generates a low-iF output using a single-conversion architecture. In order to attain the 60dB of image rejection, a double quadrature mixer, an IF polyphase filter and an IIQ mismatch calibration scheme are used.

Furthermore, a CMOS low-IF tuner for digital TV is disclosed by Supisa Lerstaveesin et al. in "A 48-860MHz CMOS Low-IF Direct-Conversion DTV Tuner", IEEE Journal of Solid-State Circuits, Vol. 43, No. 9, September 2008, pp2013-2024. This tuner achieves a low-IF output by using a combination of analogue and digital techniques. Firstly, the RF input is downconverted to a low-IF, which is digitized by Analogue-digital converters (ADC5). Channel filtering and IIQ calibration is then carried out digitally, with the final digital output converted back to an analogue signal using a Digital-analogue converter (DAC) for interfacing to the baseband.

In contrast to the low-IF mode of such approaches, in the zero-IF mode the image rejection requirement is much more relaxed (-30-40dB, depending on the modulation scheme used), because the (large) adjacent channel interferer does not fall at the image frequency and hence does not fold in-band. These levels of image rejection can be achieved in a receiver without the need for calibration.

It would be advantageous to use the desirable properties of the zero-IF tuner, in particular the much more relaxed image rejection requirement, and incorporate it into a system that provides the low-IF output required by digital TV baseband integrated circuits.

According to a first aspect of the present invention, there is provided an upconverter for a telecommunications receiver, the upconverter comprising: -an input for receiving a zero-IF signal comprising a wanted channel occupying a positive frequency range and unwanted adjacent channels; -a single sideband double quadrature upconversion mixer operable to upconvert the zero-IF signal to provide an upconverted signal; -a polyphase filter operable to receive the upconverted signal and to attenuate portions of adjacent channels occupying a negative frequency range corresponding to the positive frequency range of the wanted channel in the upconverted signal; and -a low-IF output operable to output the filtered upconverted signal, wherein the upconversion mixer and polyphase filter are configured to selectively attenuate negative third and negative seventh order harmonic components of the upconverted signal.

Preferably, the upconversion mixer is configured to selectively attenuate negative third and positive fifth order harmonic components of the upconverted signal.

Preferably, the upconversion mixer comprises a 3-phase upconversion mixer, for driving by 3-phase quadrature local oscillator inputs.

Preferably, the upconverter further comprises a second polyphase filter operable to attenuate a seventh order harmonic component of the upconverted signal.

According to a second aspect of the present invention, there is provided a method of upconversion comprising: -receiving a zero-IF signal comprising a wanted channel occupying a positive frequency range and unwanted adjacent channels; -upconverting the zero-lF signal using a double quadrature upconversion mixer to provide an upconverted signal; -attenuating portions of adjacent channels occupying a negative frequency range corresponding to the positive frequency range of the wanted channel in the upconverted signal using a polyphase filter; and -outputting the filtered upconverted signal, wherein the steps of upconverting and attenuating comprise selectively attenuating the negative third and negative seventh order harmonic components of the upconverted signal.

Preferably, the step of upconverting comprises selectively attenuating the negative third and positive fifth order harmonic components of the upconverted signal.

Preferably, the method further comprises attenuating a seventh order harmonic component of the upconverted signal.

According to a third aspect of the present invention, there is provided a low-IF tuner comprising a zero-IF tuner and the upconverter according to the first aspect.

LIST OF FIGURES

Figure 1 illustrates a prior art (Fillatre et al) schematic implementation of hybrid tuner with low-IF output; Figure 2 illustrates a set top box system having a digital TV radio tuner in accordance with an embodiment of the present invention; Figure 3 illustrates an upconverting system, in accordance with an embodiment of the present invention, for translating zero-iF input to low-IF output; Figures 4a-b illustrate the double quadrature mixer with 3-phase mixing; Figures 5a-c illustrate a typical wanted signal and unwanted adjacent channel signals at several stages in analogue filter and upconversion process; Figures 6a-d illustrate wanted and unwanted channels after upconversion, folding effects and use of a polyphase filter in accordance with an embodiment of the present invention; Figures 7a-e illustrate folding effects of unwanted adjacent channels in upconversion to various intermediate frequencies (IF) and two different channel bandwidths; Figure 8 illustrates harmonic effects using single sideband mixing; and Figure 9 is a flowchart illustrating a method according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention provides integration of a SSB upconversion mixer in a system for generating a low-IF output suitable for meeting the specifications for digital TV (and specifically the DVB-T standard).

Figure 2 illustrates a set top box system having a digital TV radio tuner in accordance with an embodiment of the present invention. The set top box system comprises a digital TV radio tuner, itself comprising a zero-IF tuner and an upconverter 24, together with a DVB-T baseband processing IC 28. In operation, the zero-IF tuner 20 outputs a zero IF-signal 22 to the upconverter 24. The upconverter 24 outputs a low-IF signal 26 to the DVB-T baseband IC 28.

Figure 3 illustrates a system, corresponding to the upconverter 24 shown in Figure 2, for generating a low-IF output from a zero-IF input in a digital TV radio tuner. This embodiment comprises a 3-phase single-sideband (SSB) double quadrature upconversion mixer 34 (described in more detail with reference to Figures 4a and 4b) (with I and Q buffer amplifiers), a passive polyphase filter 38 (with I and Q buffer amplifiers) to reject any residual adjacent channel interferers, and a polyphase filter 36 (with I and 0 buffer amplifiers) with an anti-alias filter 40 to reject higher order harmonics generated in the mixing process.

In this embodiment, the upconverter comprises: (i) A passive current mode 3-phase double quadrature mixer 34 driven by 3-phase quadrature local oscillator inputs. The double quadrature mixer has an in put for receiving a zero-IF signal comprising a wanted channel and unwanted adjacent channels. It is a single sideband upconversion mixer and it upconverts the zero-IF signal to provide an upconverted signal. The double quadrature mixer is configured to selectively attenuate negative third and negative seventh order harmonic components of the upconverted signal. it is also configured to selectively attenuate negative third and positive fifth order harmonic components of the upconverted signal.

(ii) A polyphase filter 36 to reject 7th order harmonics.

(iii) A polyphase filter 38 to reject residual adjacent (e.g. N-I, N-2) channels. The polyphase filter receives the upconverted signal and attenuates adjacent channels occupying the negative frequency range corresponding to the positive frequency range of the wanted channel in the upconverted signal.

(iv) An anti-alias filter 40 to reject higher order harmonics generated in the mixing process.

The upconverter provides a low-IF output of the filtered upconverted signal.

The components of the system presented in Table 1, including double quadrature upmixing and the use of polyphase filters to reject unwanted negative frequencies, are used to address the requirements of low IF digital television tuners by attenuating folded first order signals or fundamental signals and attenuating harmonic signals.

Attenuate Attenuate Attenuate Attenuate folded 3rd +5th -7th signals harmonic harmonic harmonic Polyphase filter Y Upconverting with Y Y double quadrature combined with polyphase filter Upconverting with three Y Y phase mixing Polyphase filter Y Table 1: Functions of the upconverter components (Y=Yes) In contrast with the prior art described in Fillatre et al, which inherently has as its output a low-IF output, the upconverter of this embodiment is added at the output of a zero-IF tuner. Because the system is based on zero-IF, an IF polyphase filter is not required in the zero-IF tuner, rather the IF filter is implemented as a real low pass filter in the zero-IF tuner. Also, as the image rejection requirement is not as stringent as in a single stage low-IF receiver, a single, rather than a double quadrature downconversion only is required in the zero-IF tuner and no l/Q (in-phase/quadrature) calibration is needed.

Figure 4a illustrates the upconversion mixer 34 of Figure 3, but in more detail. In-phase and quadrature zero-IF inputs, I_IF and Q_IF respectively, are received by the upconversion mixer 34 from the IF strip 32. In order to implement three-phase mixing, each of the four mixers receives three separate clocks (labelled <2>, ci,> and <0>, or <2. .0>), either with an in-phase local oscillator l_LO<2. .0> or quadrature local oscillator Q_LO signal. Thus each mixer's clock input is a three-line bus.

Figure 4b illustrates the timing of the clocks l_L0c0>, l_LO<1>, l_LOc2>, 0_LOcO>, Q_LOcl> and Q_L0c2> used for 3 phase mixing. The three LO signals of different phases are thus used to replicate a pseudo sine wave.

Embodiments of the present invention address a number of the challenges associated with the design of low IF television tuners as the mixing and filtering stages in the solution address the problem real use cases presented by adjacent channels.

In a television tuner, in addition to the wanted channel signal (0), there can be a number of adjacent channel signals (N-I, N-2, etc. and Ni-I, N+2, etc.) which can be problematic for the tuner. In a zero-IF tuner these may be dealt with through good linearity and low pass filtering, however in a low IF upmixing approach there can be several harmonic and folding effects which will restrict the linearity performance of the whole tuner and S filtering system.

Figure 5a illustrates the presence of these signals at the input of the IF stage of a tuner chip, assuming the tuner is using a direct conversion architecture. The wanted signal has an 8MHz bandwidth (BW). The adjacent unwanted lower frequency channels are the N-i signal, N-2 signal, etc. The adjacent unwanted lower frequency channels are the N-i-i signal, N-'-2 signal, etc. channels beyond N±2 are not shown for simplicity.

Figure Sb shows these signals at following a typical low pass filtering IF gain stage. The unwanted adjacent channels are reduced but can still be of a significant size, unless a very high order filter is used, with unacceptable associated area and power costs.

Figure Sc shows how these signals may be upconverted to a desired IF, here 4.5MHz. However after upconverting, the wanted signal may become corrupted by unwanted mixing effects. These are illustrated in Figures 6b and 7a-e and discussed below.

FOLDING

Figure 6a shows the upconverted wanted and unwanted channel signals, as shown in Figure Sc. It is a problem with the low IF approach that the adjacent channel may occupy the equivalent negative frequency range of the wanted signal. This is shown in Figure 6b. For example, the wanted signal in Figure 6a lies between 0.5MHz and 8.5MHz. The equivalent negative frequency range, -0.5MHz to -8.5MHz is occupied by the N-i and ii N-2 channels. Without deliberate signal processing of the I and Q paths, the circuitry cannot discern between positive and negative frequencies and the unwanted signals in the negative domain will "fold over" around the 0MHz axis to interfere with the wanted signal, as shown in Figure 6b by the hatched rectangles labelled N-i' and N-2'.

In order to address this problem, a polyphase filter (38 in Figure 3) is used to attenuate the N-i and, to a lesser extent, the N-2 signals. This is represented in Figure 6c, which shows a simplified illustration of i 0 polyphase filter response providing rejection of negative frequencies.

In Figure 6c, the polyphase filter is shown schematically by the dashed line as attenuating out to -8.5MHz. A narrower bandwidth than that would be more practical, as shown by the dotted line, which shows attenuation iS out to 8.SM, but to a lesser extent than shown by the schematic dashed line. For the dotted line, most of the attenuation is limited to the -0.5MHz to -4MHz range, which covers the most problematic section of the adjacent channel (N-i). To achieve a wider bandwidth illustrated schematically by the dashed line would require greater area and power to be spent in the filter. This also means that in Table i, while it is true that the double quadrature with polyphase will attenuate the -3rd harmonic to some degree, with a polyphase filter corresponding to the dotted line, most of the attenuation is achieved through the 3 phase mixing.

The result of the filtering is illustrated in Figure 6d, which shows upconverted wanted channel signal (0) with N+I-i & N --1-2 adjacent channels, with the folding effect shown when using such a polyphase filter.

Although folding still takes place, the unwanted signal is at a low enough power that it does not present an issue by interfering with the wanted signal centred on 4.5MHz.

The proportion of adjacent unwanted channel signals N-I, N-2, N-3, etc. interfering with the wanted channel signal (0) depends on the particular IF and channel width used. Figures 7a-e illustrate folding effects of unwanted adjacent channels in upconversion to various intermediate frequencies (IF) and two different channel bandwidths.

Figure 7a, illustrates the extreme case with an 8MHz channel bandwidth, with the wanted channel (0) centred on an IF of 4.0MHz. The folding leads to entirely N-i' unwanted channel interference in the wanted channel (0).

Figure 7b, corresponding to Figure 6b, shows an 8MHz channel bandwidth, with the wanted channel (0) centred on an IF of 4.5MHz. The folding leads to mainly N-i' interference in the wanted channel (0), with iS some N-2'.

Figure 7c shows an 8MHz channel bandwidth, with the wanted channel (0) centred on a higher IF of 8.0MHz. With this ratio of channel bandwidth to IF, the folding leads to only N-2' interference in the wanted channel (0).

Figure 7d again shows an 8MHz channel bandwidth, with the wanted channel (0) centred on a still higher IF of 9.0MHz. With this ratio of channel bandwidth to IF, the folding leads to mainly N-2' interference in the wanted channel (0), but now with some N-3'.

Figure 7e shows a further example, with a 4MHz channel bandwidth, with the wanted channel (0) centred on a more standard IF of 4.5MHz. With this ratio of channel bandwidth to IF (being the same as shown in Figure 7d), the folding again (as per Figure 7d) leads to mainly N-2' interference in the wanted channel (0), but with some N-3'.

HARMONICS

In addition to folding effects on the upconverted signal, the input signal may be upconverted by harmonics of the local oscillator LO. The use of single sideband mixing results in positive frequencies from the 1st, 5th, 9th, etc. harmonics and negative frequencies from the 3rd, 7th, etc. harmonics.

Figure 8 illustrates harmonic effects using single sideband mixing. From top to bottom, zero-IF, upconverted first order or fundamental signals mixed with the LO and signals mixed with the -3rd, +5th and -7th harmonics respectively are shown. Figure 8 also illustrates the frequency range A where harmonics interfere directly with the wanted channel and range B where harmonics can fold over to interfere with the wanted channel. Thus adjacent channels converted by these harmonics can lie on (range A), or fold over (from range B) onto the wanted signal.

To mitigate interference issues with the adjacent channels of -3rd and -7th order harmonics, the embodiment uses a double quadrature mixing topology. This technique, when combined with a polyphase filter (for example 38 in figure 3) allows attenuation of the negative frequency components and therefore the 3rd and 7th harmonics.

In order to mitigate interference issues with the adjacent channels of -3rd and +5th order harmonics, the three phase mixing architecture is used.

Using the pseudo sine wave rather than square wave switching reduces the 3rd and 5th order harmonic effects.

Although double quadrature and three-phase mixing are used to address interference issues, an additional unwanted effect arises when the output signal is to be sampled by an ADC, as is the case in the majority of demodulator chips used with TV tuners. In this case, frequencies above half the sampling rate of the ADC will alias back into the wanted frequency domain and distort the signal.

A low pass anti-aliasing filter can be used, as is the case in the embodiment of Figure 3, with anti-alias filter 40, however if some of the harmonic components are still high power, a high order anti-aliasing filter would be required, and would have a significant power and area impact on the chip. Alternatively, the polyphase filtering used to reject negative frequencies illustrated in Figure 6 could be extended to reject frequencies further out in a single stage, however this comes at a great area and performance cost. In an embodiment the negative 7th order harmonic component is specifically addressed with an additional polyphase filter (36 in Figure 3). This approach limits the size of the anti-aliasing filter required.

Harmonic components of frequencies higher than this will have already been significantly attenuated by the preceding zero-IF strip low pass filter so further attenuation is not required.

Figure 9 illustrates a method of upconversion according to an embodiment of the present invention. The flowchart show the steps as follows: 902: Receiving a zero-IF signal comprising a wanted channel and unwanted adjacent channels.

904: Upconverting the zero-IF signal, for example using a 3-phase double quadrature upmixer, to provide an upconverted signal. This step may include selectively attenuating the negative third and negative seventh order harmonic components of the upconverted signal. This step may also include selectively attenuating the negative third and positive fifth order harmonic components of the upconverted signal.

906: Attenuating, for example using a polyphase filter, adjacent channels occupying the negative frequency range corresponding to the positive frequency range of the wanted channel in the upconverted signal.

908: A further filtering step, for example using another polyphase filter, is attenuating a seventh order harmonic component of the upconverted signal.

910: Outputting the filtered upconverted signal.

ADVANTAGES

Embodiments of the present invention provide low-IF output without the need for complex calibration schemes or high power-consuming Analogue-Digital converters (ADC5).

Embodiments of the present invention maintain the advantages of a zero-IF receiver but with a low-IF output.

Embodiments of the present invention provide a low cost CMOS tuner with suitable output for addressing the digital TV set top box market.

Embodiments of the present invention provide lower complexity than other approaches for generating the low-IF output.

Further modifications and improvements may be added without departing from the scope of the invention herein described.

Claims

Claims 1. An upconverter for a telecommunications receiver, the upconverter comprising: -an input for receiving a zero-IF signal comprising a wanted channel occupying a positive frequency range and unwanted adjacent channels; -a single sideband double quadrature upconversion mixer operable to upconvert the zero-IF signal to provide an upconverted signal; -a polyphase filter operable to receive the upconverted signal and to attenuate portions of adjacent channels occupying a negative frequency range corresponding to the positive frequency range of the wanted channel in the upconverted signal; and -a low-IF output operable to output the filtered upconverted signal, wherein the upconversion mixer and polyphase filter are configured to selectively attenuate negative third and negative seventh order harmonic components of the upconverted signal.
2. The upconverter of claim 1, wherein the upconversion mixer is configured to selectively attenuate negative third and positive fifth order harmonic components of the upconverted signal.
3. The upconverter of claim 2, wherein the upconversion mixer comprises a 3-phase upconversion mixer, for driving by 3-phase quadrature local oscillator inputs.
4. The upconverter of any previous claim, further comprising a second polyphase filter operable to attenuate a seventh order harmonic component of the upconverted signal.
5. A method of upconversion comprising: -receiving a zero-IF signal comprising a wanted channel occupying a positive frequency range and unwanted adjacent channels; -upconverting the zero-IF signal using a double quadrature upconversion mixer to provide an upconverted signal; -attenuating portions of adjacent channels occupying a negative frequency range corresponding to the positive frequency range of the wanted channel in the upconverted signal, using a polyphase filter; and -outputting the filtered upconverted signal, wherein the steps of upconverting and attenuating comprise selectively attenuating the negative third and negative seventh order harmonic components of the upconverted signal.
6. The method of claim 5, wherein the step of upconverting comprises selectively attenuating the negative third and positive fifth order harmonic components of the upconverted signal.
7. The method of claim 5 or claim 6, further comprising attenuating a seventh order harmonic component of the upconverted signal.
8. A low-IF tuner comprising a zero-IF tuner and the upconverter of any of claims 1 to 4.