WO2014146706A1

WO2014146706A1 - Filter circuit, measuring device, filtering method and measuring method for blocking undesired signals

Info

Publication number: WO2014146706A1
Application number: PCT/EP2013/055810
Authority: WO
Inventors: Christine JOCHNER; Klaus-Dieter Schwarz
Original assignee: Rohde & Schwarz Gmbh & Co. Kg
Priority date: 2013-03-20
Filing date: 2013-03-20
Publication date: 2014-09-25

Abstract

A filter circuit for filtering a first signal and a second signal from a measurement signal comprises a first filter (22) and a second filter (23) which are connected in series. At least the first filter (22) is a band-stop filter. The first filter (22) and/or the second filter (23) are frequency-adjustable filters.

Description

Filter circuit, measuring device, filtering method and measuring method for blocking undesired signals The invention relates to a filter circuit, a measuring device, a filtering method and a measuring method, which remove undesired signals from a measurement signal.

A great deal of test cases within high-frequency test specifications for mobile telephones requires the

suppression of undesired signals in certain frequency ranges. Depending upon the constellations of the desired signal to the undesired signal, the above-mentioned suppression is conventionally reached by use of low-pass filters, band-stop filters or high-pass filters. A band- stop filter is necessary, when the frequency distance between the desired signal and the undesired signal is smaller than the filter flank of a low-pass filter or of a high-pass filter, since the flanks of a band-stop filter are usually significantly steeper. For different frequency bands used in mobile telecommunications, conventionally a set of band-stop filters with fixed frequency is used. For each frequency band, a band-stop filter is used for suppressing the entire uplink and a second band-stop filter is used for suppressing the complete downlink.

The US-patent application US 2011/0053622 Al shows a filter unit comprising two variable band-stop filters connected in series, which are used for suppressing one undesired signal within a measurement signal. The use of two filters merely enhances the amount of signal

suppression or increases the bandwidth of the one signal, which can be suppressed. The object of the present invention is to provide a filter circuit, a measuring device, a filtering method and a measuring method capable of suppressing at least two different signals within a measuring signal without requiring a great number of individual filters.

The object is solved by the features of claim 1 for the filter circuit and claim 9 for the filtering method. The dependent claims contain further developments.

An inventive filter circuit for filtering a first signal and a second signal from a measurement signal comprises a first filter and a second filter which are connected in series. At least the first filter is a band-stop filter. The first filter and/or the second filter are frequency- adjustable filters. It is therefore possible to remove at least two independent undesired signals from a measuring signal . Advantageously, the second filter is a band-stop filter or a high-pass filter or a low-pass filter. It is therefore very easy to adapt to the actual location of the undesired signals in the frequency domain. Advantageously, the first filter and/or the second filter is a cavity filter or an Yttrium iron garnet filter. The first filter and the second filter then have different maximum power ratings. The properties of different

undesired signals can therefore be taken care of.

Advantageously, a frequency range of the first signal and a frequency range of the second signal are different. The first signal and the second signal are located in

different frequency bands or in a same frequency band. The filter circuit is therefore able to remove the undesired signals even when they do not touch in the frequency range . The at least one adjustable filter is advantageously set up for being adjusted so that the stop-bands of the first filter and of the second filter cumulatively cover the frequency ranges of the first signal and the second signal. A nearly complete suppression of the undesired signals is therefore possible.

Advantageously, the first signal and the second signal are carrier signals, which are directly neighboring in the frequency domain or separated by a frequency gap in the frequency domain. The often occurring scenario of having to remove carrier signals in order to measure spurious emissions can therefore be covered.

An inventive measuring device comprises a filter described above and control means for controlling a frequency adjustment of the at least one adjustable filter. A user can therefore very easily use the filter circuit for different purposes. Advantageously, the measuring device further comprises detection means for detecting a frequency range of the first signal and of the second signal. The control means are then set up for automatically adjusting the frequency of the first filter and/or the second filter based upon the detected frequency range of the first signal and the second signal. A further reduction in user effort and error likeliness is therefore achievable. An inventive filtering method serves the purpose of filtering a first signal and a second signal from a measurement signal, using a first filter and a second filter which are connected in series. At least the first filter is a band-stop filter. The first filter and/or the second filter are frequency adjusted for performing the filtering. It is therefore possible to remove at least two independent undesired signals from a measuring signal. An exemplary embodiment of the invention is now further explained with respect to the drawings, in which

Fig. 1 shows a block diagram of an exemplary embodiment of the inventive measuring device and filter circuit ;

Fig. 2 shows a first test case in a frequency diagram;

Fig. 3 shows a second test case in a frequency diagram;

Fig. 4 shows a third test case in a frequency diagram;

Fig. 5 shows a fourth test case in a frequency diagram;

Fig. 6 shows a fifth test case in a frequency diagram;

Fig. 7 shows a sixth test case in a frequency diagram;

Fig. 8 shows a seventh test case in a frequency

diagram;

Fig. 9 shows an eighth test case in a frequency

diagram, and Fig.10 shows a flow chart of an exemplary embodiment of the inventive filtering method.

First, we demonstrate the construction and general

function of an embodiment of the inventive filter circuit and measuring device along Fig. 1. Along Fig. 2 - 9, different test cases for use with the embodiments of the inventive filter circuit and filtering method are

described. Finally, with regard to Fig. 10, an exemplary embodiment of the inventive measuring method is described. Similar entities and reference numbers in different

Figures have been partially emitted.

Fig. 1 shows an exemplary embodiment of the inventive measuring device 1. The measuring device 1 comprises input-/output-means 10, analog processing means 11, an analog-digital-converter 12, digital processing means 13, display means 14 and control means 15. The input-/output- means 10 are connected to the analog processing means 11, which are connected to the analog-digital-converter 12. The analog-digital-converter 12 is connected to the digital processing means 13, which in turn is connected to the display means 14. The control means 15 are connected to the analog processing means 11, the digital processing means 13 and the display means 14 and control their function .

The analog processing means 11 comprise pre-amplification means 20 and filter means 21. The filter means 21 comprise a first filter 22 and a second filter 23. The pre- amplification means 20 are connected to the filter means

21. More exactly, they are connected to the first filter

22, which is connected in series to the second filter 23. The second filter 23 again is connected to the analog- digital-converter 12. Moreover, the analog processing means 11 can furthermore comprise frequency reduction means either before or after the filter means 21. The frequency reduction means though are not displayed here. The frequency reduction means solve the purpose of

reducing the frequency of the measuring signal, for example by mixing it.

The digital processing means 13 comprise processing means 24, which are connected to the analog-digital-converter 12 and to the control means 15 and to the display means 14. Moreover, the digital processing means comprise detection means 25, which are connected to the control means 15 and to the processing means 24.

A measuring signal is either received by an antenna comprised by the input-/output-means 10 or directly input into the input-/output-means 10 by use of a cable. The input-/output-means 10 hand the signal on to the analog processing means 11, which perform analog processing.

Especially, the signal is handed on to the pre- amplification means 20, which perform a pre-amplification . The resulting amplified signal is passed on to the filter means 21, which perform a filtering. Especially, the signal is handed on to the first filter 22, which hands the filtered signal on to the second filter 23. The resulting filtered signal is passed on to the analog- digital-converter 12, which digitizes it and passes it on to the processing means 24 within the digital processing means 13. The processing means 24 perform a digital processing, for example a demodulation and a decoding.

The demodulated and decoded signal is passed on to the detection means 25, which detect, in which frequency bands or advantageously in exactly which frequency ranges there are undesired signals present within the measuring signal. This information is passed on to the control means 15. The control means 15 are then advantageously set up before adjusting the adjustable filters 22 and 23 so that the detected undesired signals are suppressed within the measuring signal. Moreover, the processing means 24 pass on the measured signal to the display means 14, which display the measured signal.

The selection of the individual filters (band-stop, high- pass, low-pass) depends upon the location of the desired parts of the measuring signal and the location of the undesired signals in the measuring signal within the frequency range. The selection can be automated by using detection means 25 of Fig. 1. In this case, an

advantageous construction could include more than two filters within the analog processing means 11, which can be connected in series in different combinations by the control means 15. Alternatively to detecting the location, the test case presently measured already includes

knowledge about the location of undesired signals.

Moreover, the different filters used can advantageously be of different filter technologies. For example, a cavity filter can be used, when high-power signal components have to be removed. On the other hand an yttrium iron garnet filter can be used, in case of an extremely high necessary filter quality. Also, it is possible to serially connect more than two filters in order to suppress more than two signals from the measuring signal.

For example, the purpose of suppressing certain signals within a measuring signal is to allow a measurement of spurious emissions outside of an allowed emission

bandwidth. In such a measurement, the allowed bandwidth for transmission is removed from the measuring signal, so that the spurious emissions can be measured accurately. Especially in case of several different frequency ranges, at which a device under test is allowed to transmit simultaneously, a suppression of these different signals is necessary. A measuring of spurious emissions can be performed at a frequency, which is lower than the two allowed frequency ranges, which is between the two

frequency ranges or which is above the two frequency ranges. Alternatively, all of these measurements can be performed . In a case, in which only spurious emissions above a certain frequency or below a certain frequency are

relevant, instead of the use of two band-stop filters, the use of a band-stop filter and a high-pass filter or a low- pass filter is advantageous, since all frequency

components in the stop-band of the high-pass filter or the low-pass filter are removed.

Moreover, the inventive filter circuit or filtering method can be used for testing the ability of a device under test to cope with high blocking signals with defined distance to a downlink band. Besides a desired blocking signal, undesired signals are created within a signal generator, for example harmonic, sub-harmonic and non-harmonic signals. Moreover, broadband noise is generated. All these undesired signals are within the downlink band. The downlink band therefore has to be protected by suppressing these undesired signals on the downlink frequency. When carrier aggregation and dual band are used, two downlink bands have to be protected simultaneously. In case of the blocking signal being located at a

frequency lower than both used frequency bands, the carrier within the lower frequency band is suppressed with a band-stop filter, while the carrier within the higher frequency band is suppressed with a low-pass filter.

In case of the blocking signals being located between the two frequency bands, both carriers are suppressed with band-stop filters.

In case of the blocking signal being at a frequency higher than the higher of the frequency bands, the lower

frequency band carrier is blocked by use of a high-pass filter, while the carrier of the higher frequency band is blocked with a band-stop filter.

In a first test case, which is shown in Fig. 2, LTE intra- band contiguous carrier aggregation is shown. A first carrier 32a is located directly neighboring to a second carrier 33a in the frequency range. The carriers 32a, 33a are located within a single frequency band. The first filter used here is an adjustable band-stop filter with a filter curve 30a. The second filter is also an adjustable band-stop filter with a filter curve 31a. The band-stop filters are each configured so that the band-stop range is about as broad as one of the carriers 32a, 33a. The center frequencies of the band-stop filters are adjusted so that each band-stop filter blocks the entire frequency range of one of the carriers 32a, 33a. In case of the band-stop ranges being larger than the frequency-range of each of the carriers 32a, 33a, the band-stop ranges can overlap. In this case, the center frequencies of the band-stop filters are located so that the outer flanks of the band- stop filters lie in the position as depicted in Fig. 2 - directly at the outer borders of the frequency ranges of the carriers 32a, 33a, while the overlap occurs in the area around the frequency at which the carriers 32a, 33a neighbor.

In Fig. 3 a second test case is shown. In this test case WCDMA dual cell HSDPA with two carriers 32b, 33b is shown. The carriers 32a, 33a are located within a single

frequency band. Here the carriers have a significantly lower bandwidth than the band-stop filters as can be seen from the band-stop filter curves 30b, 31b. In this case, the band-stop filters are adjusted so that both carriers 32b, 33b are at least covered by one of the band-stop filters.

In Fig. 4 a third test case is show. Here, LTE intra-band non-contiguous carrier aggregation is depicted. Two carriers 32c, 33c are located within a single frequency band. The carriers 32c, 33c though are not directly neighboring. In this constellation, the center frequency of the band-stop filters is adjusted so that each band stop filter blocks the entire frequency range of a single carrier 32c, 33c. It can be seen from Fig. 4 that the filter curve 30c, which corresponds to a first filter, blocks the carrier 32c, while the filter curve 31c, which corresponds to a second filter, blocks the carrier 33c.

In a fourth test case depicted in Fig. 5, WCDMA NC-4C- HSDPA is shown. In this case, maximally four carriers are located within two sub-blocks. The sub-blocks though are not directly neighboring in the frequency range. In this example, three carriers 32dl, 32d2, 32d3 are located within a first sub-block and are referred to as signal 32d. Moreover, in a second sub-block a carrier 33d is located. It can be seen from Fig. 5 that the filter curve 30d of a first adjustable filter is adjusted so that the signal 32d, which comprises the individual carriers 32dl- 32d3 is completely within the band-stop range. Moreover, the filter curve 31d, which corresponds to a second adjustable band-stop filter covers the second sub-block and therefore completely blocks the carrier 33d. In a fifth test case depicted in Fig. 6, LTE inter-band carrier aggregation is shown. A first carrier 32e is located in one frequency band, while a second carrier 33e is located in another frequency band. This is indicated by the interruption in the frequency-axis. One can see from Fig. 6 that the filter curve 30e of a first adjustable band-stop filter is adjusted so that it blocks the entire first carrier 32e, while a filter curve 31e, which

corresponds to a second adjustable band-stop filter is adjusted so that it completely blocks the entire carrier 33e from the other frequency band.

Moreover, in Fig. 7, a sixth test case showing dual band for carrier HSDPA is depicted. In this case, again four different carriers are allocated within two separate sub- blocks, which are located in different frequency bands.

The carriers 32fl, 32f2 and 32f3 are regarded as a signal 32f and are located in a first sub-block within a first frequency band. This sub-block within this first frequency band is blocked by a first adjustable band-stop filter with the filter curve 30f. A fourth carrier 33f is located in a second sub-block in a completely separate second frequency band. It is blocked by a second adjustable band- stop filter with a filter curve 31f. In the examples shown in Fig. 2-7, two band-stop filters, which each are adjustable, have been used. To simplify the filter construction, in addition to a single adjustable band-stop filter, moreover a high-pass filter or a low- pass filter, which is advantageously adjustable, can be used. Also only one of the filters can be adjustable.

In Fig. 8 two carriers 32g and 33g within two separate frequency bands are shown. The first carrier 32g is suppressed by a first adjustable band-pass filter with the filter curve 30g. The second carrier 33g is suppressed by a low-pass filter with the filter curve 31g.

Moreover, in Fig. 9, two different carriers 32h, 33h within two separate frequency bands are shown. The carrier 32h is suppressed by a high-pass filter with a filter curve 30h, while the carrier 33h is suppressed with an adjustable band-stop filter with a filter curve 31h. Fig. 10 furthermore shows an exemplary embodiment of the inventive filtering method. In a first step 100, frequency ranges of undesired signals within a measuring signal are either input or detected. In case they are input, they are directly input by a user. In case they are detected, the frequency ranges are determined automatically. In a second step 101, at least one filter is adjusted, so that all of the undesired signals are removed from the measuring signal. Optionally, more than one filter is adjusted.

Moreover, optionally in this step also a selection of filters out of a plurality of filters is performed. This is especially relevant, when certain filters can only be adjusted within a certain frequency range and the

undesired signals lie outside of this frequency range. In a third step 102, the measuring signal is filtered using a series connection of the above described filters. In a fourth step 103, a measurement of the measuring signal is performed. The step 103 though is not part of the inventive filtering method, but merely of the

inventive measuring method comprising the inventive filtering method.

The invention is not limited to the examples shown above. Especially, different filter types can be used. Also a different number of filters can be used. The

characteristics of the exemplary embodiments can be used in any advantageous combination.

Claims

1. Filter circuit for filtering a first signal (32a-32h) and a second signal (33a-33h) from a measurement signal, comprising a first filter (22) and a second filter (23) which are connected in series,

wherein at least the first filter (22) is a band-stop filter, and

wherein the first filter (22) and/or the second filter (23) are frequency-adjustable filters.

2. Filter circuit according to claim 1,

characterized in that

the second filter (23) is a band-stop filter or a high- pass filter or a low-pass filter.

3. Filter circuit according to claim 1 or 2,

characterized in that

the first filter (22) and/or the second filter (23) is a cavity filter or an Yttrium iron garnet filter, and the first filter (22) and the second filter (23) have different maximum power ratings.

4. Filter circuit according to any of the claims 1 to 3, characterized in that

a frequency range of the first signal (32a-32h) and a frequency range of the second signal (33a-33h) are

different, and

the first signal (32a-32h) and the second signal (33a-33h) are located in different frequency bands or in a same frequency band.

5. Filter circuit according to any of the claims 1 to 4, characterized in that the at least one adjustable filter (22, 23) is set up for being adjusted so that stop-bands of the first filter (22) and the second filter (23) cumulatively cover the

frequency ranges of the first signal (32a-32h) and the second signal (33a-33h) .

6. Filter circuit according to any of the claims 1 to 5, characterized in that

the first signal (32a-32h) and the second signal (33a-33h) are carrier signals, which are directly neighboring in the frequency domain or separated by a frequency gap in the frequency domain.

7. Measuring device comprising a filter according to any of the claims 1 to 6 and control means (15) for

controlling a frequency adjustment of the at least one adjustable filter (22).

8. Measuring device according to claim 7,

characterized in that

the measuring device further comprises detection means (25) for detecting a frequency range of the first signal (32a-32h) and of the second signal (33a-33h) , and

the control means (15) are set up for automatically adjusting the frequency of the first filter (22) and/or the second filter (23) based upon the detected frequency range of the first signal (32a-32h) and the second signal (33a-33h) .

9. Filtering method for filtering a first signal (32a-32h) and a second signal (33a-33h) from a measurement signal, using a first filter (22) and a second filter (23) which are connected in series, wherein at least the first filter (22) is selected to be band-stop filter, and

wherein the first filter (22) and/or the second filter (23) are adjusted in frequency for performing the filtering.

10. Filtering method according to claim 9,

characterized in that

the second filter (23) is selected to be a band-stop filter or a high-pass filter or a low-pass filter.

11. Filtering method according to claim 9 or 10,

characterized in that

the first filter (22) and/or the second filter (23) is selected to be a cavity filter or an Yttrium iron garnet filter, and

the first filter (22) and the second filter (23) are selected to have different maximum power ratings.

12. Filtering method according to any of the claims 9 to 11,

characterized in that

a frequency range of the first signal (32a-32h) and a frequency range of the second signal (33a-33h) are selected to be different, and

wherein the first signal (32a-32h) and the second signal (33a-33h) are located in different frequency bands or in same frequency band.

13. Filtering method according to any of the claims 9 to 12,

characterized in that

the at least one adjustable filter (22, 23) is adjusted that stop-bands of the first filter (22) and the second filter (23) cumulatively cover the frequency ranges of the first signal (32a-32h) and the second signal (33a-33h) .

14. Filtering method according to any of the claims 9 to 13,

characterized in that

15. Measuring method for measuring a measuring signal comprising the steps of a filtering method according to any one of the claims 9 to 14,

wherein the measuring method further comprises

- detecting a frequency range of the first signal (32a- 32h) and of the second signal (33a-33h) , and

- automatically adjusting the frequency of the first filter (22) and/or the second filter (23) based upon the detected frequency range of the first signal (32a-32h) and the second signal (33a-33h) .