Wideband frequency signal digitizer and method

(54) (56)

PATENT OWNER Motorola Inc., Schaumburg Ill US

INVENTOR

REPRESENTATIVE TITLE

CALLED PUBLICATIONS:

Robert C Elder, McHenry IL US

Ehrner & Delmar Patentbyrå AB Broadband frequency signal digitizer and method for digitizing a broadband frequency signal (57)

WO Al 9 421 049 (H03M 1/12), WO Al 9 421 071 (H04L 27/00) SUMMARY:

to digitize multiple tapes with you. bredcandsfrekvenssig.nal. Digicaliserarer. (10) allows for optimal pcsitior.erir.g of a segment (402) hes broadbancsfrequ.sigr.alen within a Nyctist band hes an aralog-digitalcmvandlare (3c). The remaining segments (402) of the broadband frequency signal are positioned close relative to the first segment so that the entire broadband frequency signal (400) is easily digitized using one or more analog-digital converters (35) while at the same time reducing or eliminating unwanted signals.

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

520 982

SUMMARY

Ξη broadband frequency digitizer (10) to digitize multiple bands of a broadband frequency signal. The digitizer (10) allows for optimal positioning of a segment (402) of the broadband frequency signal within a Nyquist band of an analog-to-digital converter (36). The remaining segments (402) of the broadband frequency signal are positioned close relative to the first segment so that the entire broadband frequency signal (400) is easily digitized using a single or multiple analog-to-digital converter (36) while at the same time reducing or eliminating unwanted signals.

Publish image = Fig. 1

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Field of the Invention

The present invention relates to multi-channel digital transmitters, and more particularly to a broadband frequency signal digitizer to efficiently digitize broadband frequency signals. Background of the invention

There are several benefits to implementing a radio communication system through the use of digital technology. In particular, it results in increased system capacity, reduced noise, and reduced hardware and associated power consumption. Several different digital radio communication systems have been proposed.

Basic to the digital radio communication system is the requirement that the received analog radio signal be digitized. The well-known Nyquist criterion results in such digitization being achieved with minimal error at about twice the bandwidth of the analog signal. U.S. Patent No. 5,251,218 discloses a typical prior art method for digitizing an analog radio frequency signal in accordance with this principle. However, it should be noted that ADCs that have the capacity to operate at very high sampling rates are required as the radio signal occupies a large bandwidth. Such devices, to the extent that they are available, are expensive and often suffer from reduced performance, i.e., have significant distortion and increased power consumption when operated at high sampling rates.

The spectrum allocated to radio communication systems is typically large in terms of digitalization requirements. However, in some radio communication systems, the entire bandwidth is not occupied by interesting signals, although the desired signal takes up a large bandwidth. For example, in cellular radio telephone communication systems, the communication bandwidth is not contiguous. example

520 982 the cellular A band ar assigns a bandwidth of

12.5 MHz. Spectral, however, covers the entire A band

22.5 MHz bandwidth in two non-adjacent portions. To be able to digitize the A band, you need an ADC that has the capacity to operate, according to the Nyquist criterion, at least 45 MHz or 45 million samples per second, and more reliably at 56 million samples per second.

Thus, there is a need for a device for digitizing broadband frequency band signals which do not require high sampling rates, and which does not significantly increase the amount of hardware required for the communication system.

Brief description of the drawings

Fig. 1 is a block diagram of a broadband frequency signal digitizer in accordance with a preferred embodiment of the present invention;

Figure 2 is a block diagram of a broadband frequency signal digitizer in accordance with another preferred embodiment of the present invention;

Fig. 3 is a block diagram of a broadband frequency signal digitizer in accordance with another embodiment of the present invention;

Figures 4A-4B show spectrally the processing of a broadband frequency signal in accordance with a preferred embodiment of the present invention; and Figures 5A-5H spectrally show the processing of a broadband frequency signal in accordance with another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A broadband frequency signal digitizer for digitizing a broadband frequency signal provides optimal positioning of a segment of the broadband frequency signal within a Nyquist band of a broadband frequency signal.

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the band frequency signal is positioned near relative to the first segment so that the entire broadband frequency signal

520 982 are readily digitized using a single or multiple analog-to-digital converter that operates at reduced sampling rates while reducing or eliminating unwanted signals from the resulting digitized signal.

The following detailed description is presented with reference to the digitizer to efficiently and safely digitize the sub-portions of the A-frequency band of the cellular communication system. It will be readily apparent to those skilled in the art that the present invention can be applied to digitize any broadband signal that occupies continuous or discontinuous spectra. In addition, the present invention is equally applicable to a broadband frequency signal which is separated into a plurality of segments and processed via a plurality of signal paths, although the present invention is described as acting on two segments of the broadband signal.

In Fig. 1, a broadband frequency signal digitizer 10 is shown in accordance with a preferred embodiment of the present invention. An analog signal is received at antenna 12 and signal processed via filters 14 and 18 and amplifier 16 as is known in the art. The processed analog signal is transmitted to mixer 20 where it is mixed with a signal from local oscillator 22. This converts, or frequency translates, the received and processed signal into an intermediate frequency signal (IF signal).

The translated signal (IF signal) is then transmitted to splitter 24 where the translated signal is divided into a first segment and a - 4 - X 4 1

x. xx aa v xa xxxg ^ x and mixed with a second local oscillator signal (from local oscillator 28) in mixer 30. The second segment is then filtered in filter 31 and transmitted to summers via filter J2

The first segment signal is filtered

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520 The 982 1 r - i • - 4 segments are summed and then digitized via analog digital converter 36 at a sampling frequency f_s.

The function of the mixers 20 and 30 is to frequency translate the segments of the broadband frequency signal so that it can be digitized. This is shown and the operation of digitizer 10 is described with reference to Figures 4A and 4B. The spectrum 400 shown in Fig. 4A is typical of the signal received at antenna 12 of the cellular A band after processing via filters 14 and 18 and amplifiers 16. The spectrum 400 'shown in Fig. 4B represents the spectrum of Fig. 4A after processing via the mixes 20 and 30. The spectrum 400' is translated into an IF frequency which is within a Nyquist band of the analog-to-digital converter. The wider portion 402 of the spectrum 400 'is positioned adjacent to the sampling frequency of_s. The narrow portion 404 of spectrum 400 'is separated from spectrum 400' and treated as a separate segment. The purpose of the mixer 30 is to translate the second segment 404 of the broadband frequency signal into a position that is adjacent to the first segment 402, as can be seen. The first and second segments 402 and 404 can then be digitized with a single ADC at a sampling rate slightly higher than the total bandwidth of the first and second segments when positioned in this way. The minimum sampling rate is:

f_s = 2 * (BW_w + BW_n) MHz (a) where BW_w, BW_n are as shown and where a separation band BWg 406 is provided between the first and second segments 402 and 404 for filtration. The first and second segments can only be placed as close to each other as possible without portions of the first and second segments falling within the transition regions of the filters.

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4A. The transition region begins at the edge of the band segment extending to a point A. Point A represents 520 982^:: ·; : ···: i * »» ST * '5' “produces a damping point which, in the preferred embodiment, is about 80 dB, which is defined as the folding point, i.e. the point at which signals at the frequency falling outside the filtered region would generate unwanted folds in the digitized spectrum.

Fig. 2 shows a second embodiment of a broadband frequency digitizer 100 in accordance with the present invention. Signals are received at antenna 112 and processed via filters 114 and amplifiers 116. The signal is divided into dividers 118 into first and second segments which are transmitted to first and second signal paths 120 and 122. The first segment is filtered through filter 124 and mixed with a local oscillator signal ( from local oscillator 128) in mixer 126. The mixed first segment signal is then filtered via filter 130 and digitized in ADC 132 at a first sampling rate, e.g._s. The digitized first segment is then filtered through the digital filter 134 and transferred to summers 150.

The second segment of the signal transmitted along signal path 122 is filtered through filter 136 and mixed with a local oscillator signal (from local oscillator 140) in mixer 138. The signal is then filtered again through filter 142 and digitized in ADC 144 at a sampling rate of_s/ 2. The resulting digital signal is then digitally filtered through digital filter 146 and interpolated to f_s and high-pass filtered in interpolator / filter 148. The resulting signal is then transmitted to sumer 150 where it is summed with the first digitized segment of the signal giving the entire digitized signal.

Digitizer 100 is preferred since the second segment of the signal has a bandwidth less than the transition region and less than half the bandwidth of that signal;

ο first segment. This is shown and the operation of digitizer 100 will be described with reference to Figures 5A-5H. The left and right sides of Figures 5A-5H

520 982 separately shows the processing of a received signal by means of digitizer 100 which occurs along signal paths.

Figures 5A and 5D show separate segments 502 and 504 of a received signal. Segment 502 is the result of processing the signal along the first signal path 120 via filter 130, see Fig. 5A. Segment 502 is then digitized using ADC 132 at a sampling rate f_s resulting in the digital signal portions 506 shown in Figure 5B. Sampling speed f_s is selected as approximately 2.5 times the bandwidth of segment 502. These signal portions are then digitally filtered via filter 134 as shown in Figure 5C to remove any unwanted frequency components from the digital signals.

Segment 504 is the result of processing the signal along the second signal path 122 via filter 142. Segment 504 is then digitized via ADC 146 at a sampling rate of_s/ 2 resulting in the digital signal portions 508 shown in Fig. 5E. The digital signal portions 508 are first digitally filtered through the digital filter 146 to remove unwanted signal components. Then, the digital signal portions 508 are interpolated up to a speed of f_s and digitally filtered in interpolator / filter 148 as shown in Figure 5F to generate a digital signal portion 510 shown in Figure 5G. The digital signal portion 510 is summed with the digital signal portions 506 in the sumer 150 resulting in the digital signal spectrum shown in Fig. 5H.

The present invention advantageously combines analog filtering before digitization followed by digital filtering of divided segments of a signal to be digitized. Digital filtering offers the advantage of allowing the digitized signal portions to be positioned near adjacent spectrals

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520 982 lying invention. Digitizer 200 comprises two signal paths 220 and 222 which are substantially equivalent to those of digitizer 100 with modified processing of the signal after the ADC. Signals are received at antenna 212 and processed via filter 214 and amplifier 216. The signal is divided into divider 218 into first and second segments which are transmitted to first and second signal paths 220 and 222, respectively. The first segment is filtered via filter 224 and mixed with a local oscillator signal (from local oscillator 228) in mixer 226. The mixed first segment signal is then filtered through filter 230 and digitized in ADC 232 at a first sampling rate._s.

The digitized first segment is then filtered through digital filter 234, interpolated by 3 in interpolator 236, low pass filtered through digital filter 238 and decimated to 1.5_s in decimers 240 and then transferred to summers 250.

The second segment of the signal transmitted along signal path 222 is filtered through filter 232 and mixed with a local oscillator signal (from local oscillator 246) in mixer 244. The signal is then filtered again via filter

248 and digitized in ADC 252 at a sampling rate of f_s/ 2. The resulting low signal digital signal is then filtered through digital filter 254 and interpolated to 1.5 µm._s in interpolator 256 and high pass filtered in filter 260. The resulting signal is then transmitted to sumer 250 where it is summed with the digitized first segment of the signal which gives the entire digitized signal at 1.5_s.

Digitizer 200 is preferred as the second, smaller band segment is larger than the transition region but less than half the bandwidth of the first segment. When the second, smaller band segment is larger than half the first band segment, digitizer 200 is slightly modified. The second signal is digitized at the sampling frequency f_s.

As will be seen later, the interpolators are not

236 and 256 and decimers 240 necessary.

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Digitization of the second segment under the Nyquist criterion would indicate a sampling rate of approximately 2-2.5 times the bandwidth of the second segment. In the present invention, however, sampling rates are advantageously selected as f_s/ 2 which is easily generated from f_s and that will not introduce harmonics in the band. This sampling rate is also selected when f_s/ 2 or f_sis higher than required by the Nyquist criterion for the second segment. The local oscillator frequency selection is simple, and the frequencies are selected such that the bands are positioned near adjacent spectrally without overlap as shown in Figures 4A-4B and Figures 5A-5H. Providing digital filtering simplifies isolation of the band segments which allows the segments to be placed very close to each other.

The preferred embodiments of the present invention are presented with reference to digitization of a frequency band having two segments. However, it should be noted that a broadband frequency where the broadband frequency can be divided into a number of segments can be digitized in accordance with the present invention. For example, digitizer 10 is applicable where the segments can be blended close to each other in a single Nyquist band. The digitizers 100 or 200 are applicable where the segments cannot be mixed within a single Nyquist band by combining a number of signal paths equal to the number of segments to be digitized.

The above-mentioned invention enables the digitization of broadband segments at lower sampling frequencies than is otherwise required. This is accomplished without the introduction of unwanted clock frequencies or harmonics in the bands. The scope of the invention will readily be apparent from the foregoing discussion and the appended claims.

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