Wideband frequency signal digitizer

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Wideband frequency signal digitizer MOTOROLA, INC. C. 100050

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WIDEBAND FREQUENCY SIGNAL DIGITIZER AND METHOD

Field of the Invention 5 The present invention relates to multi-channel digital transceivers, and more particularly, to a wideband frequency signaldigitizer and a method of efficiently digitizing wideband frequencysignals. 10 Background of the Invention

There are numerous advantages to implementing a radiocommunication system using digital techniques. Notably, there is ' " ' »ι|·"ι,*ιιη**,ι,&gt;|· ft.....,.η i.n.Rrfrrf......nrrMnwiwniwwtotTOwmi enhanced system capacity, reduced noise, and reduced hardware and15 associated power consumption. There has been proposed several digital radio communication systems.

Fundamental to the digital radio communication system is the requirement that the received analog radio signal be digitized. Thewell known Nyquist criteria provides that such digitization is 20 accomplished with minimal error at about twice the bandwidth of theanalog signal. In United States Patent No. 5,251,218 a methodologytypical of the prior art is disclosed for digitizing an analog radiofrequency signal in accordance with this principle. It will beappreciated, however, where the radio signal occupies a large 25 bandwidth, ADCs capable of operation at very high sampling ratesare required. Such devices, to the extent they are available, areexpensive and often suffer reduced performance, i.e., havesignificant distortion and increased power consumption whenoperated at high sampling rates. 2

The spectrum allocated to radio communication systems istypically large with respect to the requirements for digitizing. Insome radio communication systems, however, although the desiredsignal occupies a large bandwidth, not all of the bandwidth is 5 occupied by signals of interest. In cellular radio telephone communication systems, for example, the communication bandwidthis not contiguous. The cellular A-band, for example, is allocated abandwidth of 12.5 megahertz (MHz). Spectrally, however, the entireA-band covers 22.5 MHz of bandwidth in two discontinuous 10 portions. In order to digitize the A-band, one would need an ADCcapable of operating, according to Nyquist criteria, at least at 45MHz or 45 million samples per second (Ms/s), and more reliably at56Ms/s.

Therefore, there is a need for a device for digitizing wideband 15 frequency band signals which is does not require high samplingrates, and does not significantly increase the amount of hardwarerequired for the communication system.

Brief Description of the Drawings 20 FIG. 1 is a block diagram representation of a widebandfrequency signal digitizer in accordance with a preferredembodiment of the present invention; FIG. 2 is a block diagram representation of a wideband25 frequency signal digitizer in accordance with another preferred embodiment of the present invention; FIG. 3 is a block diagram representation of a widebandfrequency signal digitizer in accordance with another preferredembodiment of the present invention; 3 FIGs. 4A-4B spectrally illustrate the processing of a widebandfrequency signal in accordance with a preferred embodiment of thepresent invention; and FIGs. 5A-5H spectrally illustrate the processing of a widebandfrequency signal in accordance with another preferred embodimentof the present invention.

Detailed Description of the Preferred Embodiments A wideband frequency signal digitizer and method for digitizinga wideband frequency signal provide for optimally positioning asegment of the wideband frequency signal within a Nyquist band ofan analog-to-digital converter. Remaining segments of the widebandfrequency signal are closely positioned relative to the first segmentsuch that the entire wideband frequency signal is easily digitizedusing a single or multiple analog-to-digital converters operating atreduced sampling rates while concomitantly reducing or eliminatingundesirable spurious signals from the resulting digitized signal.

The following detailed description is presented with reference todigitizer and method for efficiently and accurately digitizing the splitportions of the cellular communication system A-band frequencyband. It will readily appreciated by one of ordinary skill in the art,however, that the present invention has application to digitizing anywideband signal occupying continuous or discontinuous spectrum.Moreover, while the present invention is described as operating ontwo segments of the wideband signal, the present invention is equallyapplicable to a wideband frequency signal separated into a pluralityof segments and processed via a plurality of signal paths.

Referring to Fig. 1, a wideband frequency signal digitizer 10 inaccordance with a preferred embodiment of the present invention is 4 shown. An analog signal is received at antenna 12 and is signalconditioned through filters 14 and 18 and amplifier 16 as is knownin the art. The conditioned analog signal is communicated to mixer20 where it is mixed with a signal from local oscillator 22. Thisconverts, or frequency translates, the received and conditioned signalto an intermediate frequency (IF) signal.

The translated (IF) signal is then communicated to splitter 24where the translated signal is split into a first segment and a secondsegment. The second segment is filtered through filter 26 and mixedwith a second local oscillator 28 signal in mixer 30. The secondsegment is then filtered in filter 31 and communicated to summer 34.The first segment signal is filtered through filter 32 and is alsocommunicated to summer 34. The first and second segments aresummed and then digitized through analog-to-digital converter 36 ata sampling frequency /s·

The operation of mixers 20 and 30 is to frequency translate thesegments of the wideband frequency signal such that it can bedigitized This is illustrated in and the operation of digitizer 10described with reference to FIGs. 4A and 4B. The spectrum 400illustrated in FIG. 4A is typical of the signal received at antenna 12for the cellular A-band after processing through filters 14 and 18and amplifier 16. The spectrum 400' illustrated in FIG. 4Brepresents the spectrum of FIG. 4A after processing through mixers20 and 30. The spectrum 400' is translated to an IF frequency whichis within a Nyquist band of the analog-to-digital converter. Thewider portion, 402 of the spectrum 400' is positioned closelyadjacent the sampling frequency f$. The narrow portion 404 ofspectrum 400' is split from spectrum 400' and processed as aseparate segment. The result of mixer 30 is to translate the secondsegment 404 of the wideband frequency signal to a position nearlyadjacent the first segment 402, as can be seen. The first and second 5 segments 402 and 404 so positioned may then be digitized with asingle ADC at a sampling rate slightly greater than the totalbandwidth of the first and second segments. That is the minimumsampling rate: /s = 2 * (BWw + BWn) MHz (a) where BWw, BWn are as shown and where a separation band BWg406 is provided between the first and second segments 402 and 404for filtering. The first and second segments may only be placed asclosely adjacent as is possible without portions of the first and secondsegments falling within the transition regions of the filters.

The transition region of the analog filter is illustrated in FIG.4A. The transition region begins at the edge of the band segmentand extends to a point, "A". Point "A" represents an attenuationpoint which, in the preferred embodiment, is approximately 80decibels (dB), which is defined as the "alias point", i.e., the point atwhich signals at frequencies falling outside of the filtered regionwould produce undesirable aliases in the digitized spectrum.

With reference now to FIG. 2, a second embodiment of awideband frequency digitizer 100 according to the present inventionis shown. Signals are received at antenna 112 and are processedthrough filter 114 and amplifier 116. The signal is split in splitter118 into first and second segments which are communicated to firstand second signal paths 120 and 122, respectively. The first segmentis filtered through filter 124 and is mixed with a local oscillator 128signal in mixer 126. The mixed first segment signal is then filteredthrough filter 130 and is digitized in ADC 132 at a first samplingrate, fs. The digitized first segment is then filtered through digitalfilter 134 and is communicated to summer 150.

The second segment of the signal, communicated along signalpath 122, is filtered through filter 136 and mixed with a localoscillator 140 signal in mixer 138. The signal is then filtered again 6 through filter 142 and digitized in ADC 144 at a sampling rate of/s/2. The resulting digital signal is then digitally filtered throughdigital filter 146 and interpolated to fs and high pass filtered ininterpolator/filter 148. The resulting signal is then communicated to 5 summer 150 where it is summed with the digitized first segment ofthe signal yielding the entire digitized signal.

Digitizer 100 is preferable were the second segment of thesignal has bandwidth smaller than the transition region and less thanhalf the bandwidth of the first segment. This is illustrated and the 10 operation of 100 will be described with reference to FIGs. 5A-5H.The left and right sides of FIGs. 5A-5H illustrate separately theprocessing of a received signal by digitizer 100 as occurs alongsignal paths. FIGs. 5A and 5D illustrate separate segments 502 and 504 of a 15 received signal. With reference to FIG. 5A, segment 502 is theresult of processing the signal along first signal path 120 throughfilter 130. Segment 502 is then digitized by ADC 132 at a samplingrate fs resulting in the digital signal portions 506 illustrated in FIG.5B. Sampling rate fs is chosen as approximately 2.5 times the 20 bandwidth of segment 502. These signal portions are then digitalfiltered through filter 134 as shown in FIG. 5C to remove anyundesirable frequency components from the digital signals.

Segment 504 is the result of processing the signal along thesecond signal path 122 through filter 142. Segment 504 is then 25 digitize through ADC 146 at a sampling rate of /s/2 resulting in thedigital signal portions 508 illustrated in FIG. 5E. The digital signalportions 508 are first digitally filtered through digital filter 146 toremove undesirable signal components. Next, the digital signalportions 508 are interpolated up to a rate of fs and digitally filtered 30 in interpolator/filter 148 as illustrated in FIG. 5F to produce a digital signal portion 510 shown in FIG. 5G. Digital signal portion 7 510 is summed with digital signal portions 506 in summer 150resulting in the digital signal spectrum shown in FIG. 5H.

The present invention advantageously combines analog filteringprior to digitizing and followed by digital filtering of split segmentsof a signal to be digitized. Digital filtering offers the advantage ofallowing the digitized signal portions to be positioned closelyadjacent spectrally for reducing sampling frequency and data rates.

With reference now to FIG. 3, a third embodiment of awideband frequency digitizer 200 according to the present inventionis shown. Digitizer 200 includes two signal paths 220 and 222 whichare generally equivalent to those of digitizer 100 with the processingof the signal after the ADC being modified. Signals are received atantenna 212 and are processed through filter 214 and amplifier 216.The signal is split in splitter 218 into first and second segmentswhich are communicated to the first and second signal paths 220 and222, respectively. The first segment is filtered through filter 224and is mixed with a local oscillator 228 signal in mixer 226. Themixed first segment signal is then filtered through filter 230 and isdigitized in ADC 232 at a first sampling rate, f$. The digitized firstsegment is then filtered through digital filter 234, interpolated by 3in interpolator 236, low pass filtered through digital filter 238 anddecimated to 1.5 /s in decimator 240 then communicated to summer250.

The second segment of the signal, communicated along signalpath 222, is filtered through filter 242 and mixed with a localoscillator 246 signal in mixer 244. The signal is then filtered againthrough filter 248 and digitized in ADC 252 at a sampling rate offs/2. The resulting digital signal is then low pass filtered throughdigital filter 254 and interpolated to 1.5 fs in interpolator 256 andhigh pass filtered in filter 260. The resulting signal is thencommunicated to summer 250 where it is summed with the digitized 8 first segment of the signal yielding the entire digitized signal at 1.5/s·

Digitizer 200 is preferred where the second, smaller bandsegment is greater than the transition region but less than the half thebandwidth of the first segment. Where the second, smaller bandsegment is greater than the half the first band segment, digitizer 200is modified slightly. The second signal is digitized at the samplingfrequency fs. As will be further appreciated, the interpolators 236and 256 and decimator 240 are not required.

Digitizing the second segment under Nyquist criteria wouldsuggest a sampling rate approximately 2 - 2.5 times the bandwidth ofthe second segment. However, in the present invention, the samplingrate is advantageously chosen as fs/2 which is easily generated fromfs and will not introduce harmonics into band. This sampling rate ischosen even where fs/2 or fs is higher than is required by Nyquistcriteria for the second segment. Local oscillator frequency selectionis straight forward, and the frequencies are chosen such that thebands are positioned closely adjacent, spectrally, without overlap asshown in FIGs 4A-4B and 5A-5H. Providing digital filteringsimplifies isolating the band segments allowing the segments to beplaced very close together.

The preferred embodiments of the present invention werepresented with reference to digitizing a frequency band having twosegments. It should be understood, however, that a widebandfrequency where the wideband frequency can be divided into anumber of segments, can be digitized in accordance with the presentinvention. For example, digitizer 10 is applicable where thesegments can be mixed closely adjacent each other in a single Nyquistband. Digitizers 100 or 200 are applicable where the segments cannot be mixed to within a single Nyquist band by combining a numberof signal paths equal to the number of segments to digitize. 9

The foregoing invention provides for digitizing widebandsegments at lower sampling frequencies otherwise required. This isaccomplished without introducing undesirable clock frequencies orharmonics into band. The scope and true spirit of the invention will 5 be readily appreciated from the foregoing discussion the subjoinedclaims.

What is claimed is: