EP0992108A2

EP0992108A2 - If-receiver

Info

Publication number: EP0992108A2
Application number: EP99911996A
Authority: EP
Inventors: Hermana W. H. De Groot; Marc V. Arends
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1998-04-23
Filing date: 1999-04-20
Publication date: 2000-04-12
Also published as: JP2002519997A; WO1999056388A2; WO1999056388A3

Abstract

IF receivers are known which receive an antenna signal and normally supply an IF signal. A problem can arise resulting in a phase and or frequency error as result from for example adjacent channel. To solve this problem a correction device is proposed that corrects the phase and/or frequency error in the I and/or Q signals.

Description

IF-Receiver.

The invention relates to an IF -receiver as described in the preamble of Claim 1. The invention further relates to a correction device to be used in such an IF receiver.

From GB-A-2215544 an IF receiver and correction system are known which uses special test signals to determine correction factors. During a certain time these correction factors are assumed to be constant. After some time new test signals are applied to the IF tuner to determine new correction factors. For example to be cast by temperature changes, power supply changes, changes in the magnitude and form of the input signal and aging of the components. One of the disadvantages of the known IF receiver is that such a IF receiver needs test signals or reference signals (for example a carrier tone) that has to be known beforehand, otherwise correction becomes much less accurate or even impossible. A further disadvantage of the known IF receiver is that the known IF receiver cannot cope with frequency dependency without introducing enormous arithmetic complexity in the algorithm.

It is inter alia an object of the invention to provide an IF receiver and a correction device that overcomes the drawbacks of the prior art.

To this end a first aspect of the invention provides an IF receiver as defined in Claim 1. A second aspect of the invention provides a correction device as defined in Claim 4. Analog quadrature receivers are used to get rid of the necessary image filtering before mixing down to an IF frequency. This is done by having an in-phase path, called I, and a quadrature path called Q. The I and Q paths are supposed to have the same amplitude, but 90° degrees phase difference. With analog mixers there always will be a mis-match in amplitude and an error in-phase difference. In a practical analog front end, the maximum error is about ldB in amplitude and two degrees in phase between the I and Q path. For example the amplitude in the I signal is 10% larger than the Q signal and the phase difference between them is 89 degrees in stead of 90 degrees. This error will lead to a so-called frequency cross talk component, that is an attenuated version of the original signal, merit and complex communicated around 0 Hz. In a zero IF system, the frequency can still has a component that falls back into the wanted signal bound, comes from the wanted signal itself. Because it is attenuated by 25- 35 dB, depending on the amount of phase and amplitude error, this usually does not give any problems. In a known IF system however the frequency cross talk component that falls back into the wanted signal band, comes from one (or more) of the adjacent channels. This signals in these adjacent channels might be stronger (up to 65 dB, depending on the system) than the wanted signal is. This gives rise to frequency cross talk component within the wanted signal band, that can be up to 35 dB stronger than the wanted signal itself. In these case correction of the phase and amplitude error is necessary. A correction device comprising for example the Hilbert transform correction is able to correct amplitude and phase errors without any test or reference signals. It is independent of the corrector of the modulated signal.

Embodiments of the invention are defined in the dependent claims.

The invention and additional features, which may optionally be used to advantage, will be apparent from and elucidated with reference to examples described below hereinafter and shown in the Figures. Herein shows:

Figure 1 a block schematic example of an IF receiver with correction device. Figure 2 a block schematic example of the correction device in more detail.

Figure 3 the idea transfer of complex filters, whereby Figure A shows the pass filter for positive frequencies and Figure B the path filter for negative frequencies.

Figure 4 an example of the frequency spectra at different points in the correction device.

Figure 1 shows an IF tuner IFT of the low IF type. At an input I the IF tuner receives an input signal comprising the wanted signal together with a lot of adjacent signals and so-called blocked signals from an (not shown) antenna. The total signal is mixed down in respectively mixer MIX1 and mixer MIX2. The mixer MIX1 supplies an in phase signal Ii to an A/D converter ADl the output of the A D converter ADl is supplied to an input of the correction device CD. The other mixer MIX2 supplies a signal Iq to a second A/D converter AD2. The A D converter AD2 supplies a digital signal to the second input of the correction device CD. The signals Ii, and Iq can be supplied each via filters to the correction device. The output of the correction device is coupled to an output O of the IF tuner. The correction device corrects the phase and amplitude error.

Figure 2 shows an example of a correction device CD2 having a first input 121 for receiving a signal 112 and a second input 122 for receiving a signal IQ2. Both inputs are coupled to adaptive amplitude correction means AAC for correcting the amplitude. The output of the adaptive amplitude correction means is mixed to the signal I with the mixer MIXCD1. The output of the mixer is supplied to a first path filter FI for passing positive frequencies with imaginary coefficients and a second filter F2 for passing negative frequencies with imaginary coefficients the input 122 is coupled to a third filter F3 for passing positive frequencies with imaginary coefficients and is further coupled to a fourth filter F4 for passing negative frequencies with real coefficients. The output of the first and third filter FI, F3 are subtracted in a subtracter. The output of the second and fourth filter F2 and F4 are coupled to an adder ADD1. The output of the subtracter SUB1 and the output of the adder ADD1 are supplied to phase correction means FC. The output of the phase correction FC is supplied to a mixer MIXCD2 which mixer receives at the other input the output signal of the adder ADD1. The output of the subtracter SUB1 and the output of the mixer MIXCD2 are supplied to a subtracter SUB2 for supplying at its output the difference. The output of the subtracter SUB2 is coupled with the output with the correction device CD2. The coefficients of the pass filters F1-F4 can be chosen as follows: coefficients FI- coefficients F4, and coefficients F2= - coefficients F3.

Figure 3 shows the ideal transfer of the complex filters, whereby in Figure 3 a the filter for positive frequencies is shown and Figure 3b the filter for negative frequencies is shown.

Figure 4 shows examples of the frequency spectra on different stages of the correction device between brackets the number of the stage is given (see Figure 2). The continuous lines give the wanted frequency spectrum and the dashed lines give the unwanted frequency spectra. Figure 4b shows an example of a frequency spectrum after the positive path filter FI. All positive signal components of the original spectrum (Figure 4a) are maintained and are also merit to negative side of the spectrum. In Figure 4 only an overdue of the absolute magnitude of the frequency component is given. In Figure 4c an example of a frequency spectrum after the. negative filter F4 is given. By using the exactly merit transfer of the two filters and by using the real branch of the positive filter and the imaginary branch of the negative filter are the signals on point 3 and 4 no longer phase shifted to each other. By determining the signal magnitude on point 3 and 4 of figure 2 it can be concluded that the unwanted signal is much bigger than the wanted signal if the signal magnitude on point 4 is much bigger than the signal magnitude on point 3. This magnitude determination can for example be done by using a RMS value determination or power determination. When the signal magnitude on point 4 is much bigger than on point 3 a further correction is necessary. The correction vector can be calculated by division of the two signal magnitudes as a good estimation of the actual frequency cross talk. The Q branch on point 4 in Figure 2 which contains the unwanted signal can be weakened in such a way that the weakened version corresponds with the unwanted frequency cross-talk components in the I-branch point 3 in Figure 2 after filtering. These weakening factor is called Z. If the original phase difference between I and Q is smaller than 90 degrees (3-5) has to be taken because the unwanted crosstalk has the same sign as the unwanted signal. If the original phase difference I and Q is greater than 90 degrees (3+5) has to be taken because the unwanted cross talk has the inverse sign of the unwanted signal. Because at forehand it is not clear whether the phase difference is greater or smaller than 90 degrees both possibilities have to be carried out.

Above an If receiver and a correction device according to the invention have been described on the basis of an example. The man skilled in the art will be well aware of a lot of variations falling within the scope of the invention concerned.

Claims

CLAIMS:

1. IF receiver comprising an input for receiving an input signal, mixing means for mixing the input signal into a first and a second signal, A/D converter means for converting the input signals into digital signals, and an output characterized in that the IF receiver comprises a correction device for correcting the digital signals.

2. IF receiver according to Claim 1 characterized in that the correction device comprises adaptive amplitude correction means.

3. IF receiver as claimed in Claim 1 characterized in that the correction device comprises mixing means for mixing the output signal of the adaptive amplitude correction means with one of the input signals.

4. IF receiver according to claim 3, characterized in that the correction device comprises filter means coupled to the inputs for filtering the signals comprising the Hilbert transform.

5. IF receiver according to claim 4, characterized in that the correction device comprises adaptive frequency cross talk suppression means.

6. Correction device for use in an IF receiver according to claim 1.