Tunable Digital Filter for the ALMA Correlator - RadioNet

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Tunable Digital Filter for the ALMA Correlator - RadioNet

Digital Backend MeetingMPIfR Bonn - Sep 6, 2004Tunable Digital Filterfor the ALMA CorrelatorGianni ComorettoINAF - Osservatorio di ArcetriAlain Baudry, , Philippe Cais,Benjamin QuertierObservatoire de BordeauxAndre GunstASTRON - Dwingeloo


NRAO filterboard (onechannel) on test fixtureDigital Backend Meeting MPIfR Bonn - Sep 6, 2004


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004ALMA Correlator (2)Hybrid correlator (F-X-F) architecture• Digital filterbank to split the input bandwidth into 32 sub-channels• One or more correlator planes analyze each sub-channel• 8192 usable spectral channels (for all polarizations)• Each sub-channel can be independently tuned => zoomingProblems• Sub-channel stitching and platforming => overlapping subchannels• Digital filter architecture => economically implement 32 filters• Multiple quantization effects• Increased data rate to control computer => keep original TM mode


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Hybrid correlatorGeneral architecture• Hybrid correlator concept:• Filterbank to adapt inputbandwidth to correlatorspeed• Conventional correlatorto analyze the output ofeach filter• Increased resolution &flexibility• Sub-bands must be joinedtogether2 GHz bandwidth32 x62.5 MHz bw


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004General problems with hybrid correlators●Edge effects important● aliasing & filter rolloff● different response for real andimaginary components● different "barycenter shift"● phase errors near band edge●To reduce effects:● Fourier transform on semi-integerchannel freqs.● Band filter: Ntaps=2xNchans.4Ktaps, but should increase withresolution● Correct for channel shift


Example simulation.Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Nonoverlapping subchannelscorrelated pseudo-randomnoise with defined amplitudeand phase spectrumprocessed through hybrid,nonoverlapping correlator,with 64 pts/sub-bandGreen: Original spectrumRed: continuous sub-bandspectrumx: spectral pointsPhase cancels at band edges


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Subchannel edge errorsRed: corrected for amplituderesponseblue: corrected for channelbarycenter shiftCorrecting only for amplituderesponse leaves errors of ~25%and ~10 deg. in phaseDynamic range limited to ~15dBCorrecting for channel shiftimproves by a factor of 2Error is confined on 2 edgechannels.


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Overlapping subchannels• An overlap of 1-2 channels improve dramatically the spectralaccuracy• Some band lost at the edges of the whole 2 GHz band• In principle can be done with polyphase, but requires change ofrate in the data clock or radix-17 FFT• Adopted solution: complete digital receiver with tunable LO• filter requirements less stringent (2Ktaps)• further simplified using a 2-stage FIR• Amplitude accuracy ~0.2%• Phase ~0.5 deg


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Subchannel stitchingThe problem- Subchannels in the hybrid correlator are separatelydigitized- Quantization errors are different for different subchannels- Multiple quantizations present in the systemThus:Sub-channel alignment problems- Must measure total power in each subchannel- Level-dependent quantization correction- Validate with simulations


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Subchannel stitchingSimulation data setSimulation data set• Gaussian noise: completely stochastic signal to simulate real data• Independent uncorrelated noise and correlated signal• Variable phase relation between the two signals• Maximum correlation coefficient ~0.3• 65 Msample: 16 ms of real data• Reference cross spectrum obtained using a 2048 ch. continuouscorrelator


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004• Result from hybrid correlatorcompared to referencespectrum• Hybrid spectrum both withand without final 2-bitquantization to discriminateeffectsSubchannel stitchingResult analysisDifference within noise except corresponding to strong linesNo difference except excess noise in spectrum with 2-bit quantization35 dB of measured dynamic range, despite unrealistc strong spectral lines


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Tunable digital filter• Conceptually a SSB digital receiver• Complex LO and mixer• Low pass filter for band selection• Implementation using a 2-stage complex low-pass filter• 1 st stage: minimum selectivity required for correct decimation• 2 nd stage operates at 125 MHz, determines bandshape & final width


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Tunable digital filter architecture (1)Signal processingTest signal with random noise +2 strong spectral lines Input band rotated in order tohave the desired frequencyaround zero First filter: complex low-passfew taps, wide transitionregionstap encoding with 8 bitrejection ~ 47-50 dB


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Tunable digital filter architecture (2) Resampling to final frequencytransition regions aliasedback, but passband is clean Second filter:equivalent number of tapsmultiplied by decimationfactor.Compensates for 1 st filterrolloff Conversion to real output Total power measurement andre-quantization


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Implementation parameters• Local• 1• 2Oscillator:• 16 bit (5+11) frequency register, 30 KHz step, 0-2 GHz range• 9 bit sin/cos table implemented in 512x6 LUT (block RAM)• output 6 bit: 0.9% loss, > 50 dB spurious free dynamic rangest stage filter:• symmetric 128 taps, implemented with 64 LUTs (each branch)• 8 bits frozen coefficients (LUT optimization) ~ 48 dB rejection• output re-quantized to 8 bits (0.2% sensitivity loss)• 1:32 decimationnd stage filter:• symmetric 64 taps (loadable)• implemented with 16 9x9 bit multipliers (each branch)• 9 bits coefficients: >40 dB rejection, 0.3 dB peak-to-peak ripple• Real output, quantized to 2 bits


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Quantization effects - LO Phase resolution in mixer sin/cos table determines SFDR Mixer output requantization determines SFDR and loss For > 50 dB SFDR at least 6 bits of mixer output are needed Sin/cos generation, mixing, requantization can be implemented ina single LUT memory LO leakage very well rejected.Main source of unwantedharmonics is sampler DC offsetPerformance of quantized mixers(3 bit input, N bit output)# Bits Loss SFDR3 3,3% 31 dB4 1,5% 38 dB6 0,9% 52 dB


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Filter response 8 bit taps in 1 st filter and 9 in 2 nd adequate for 47 dBminimum stopband rejection


In-band rippleDigital Backend Meeting MPIfR Bonn - Sep 6, 2004Filter responseComposite stopband rejection(8+9 bit)2 nd FIR tap width: 9 bit10 bitmany bits


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Operation modes• Basic mode: 62.5 MHz band, 32independently tunable filters• Bypass mode: each filter outputs omeof the 32 input samples, requantizedto 2 bit• Reduced bandwidth: 31.25 MHz (tobe used with correlator oversamplemode) - 27.3 MHz usable BW• 4x2 and 4x4 bit modes for increasedsensitivity


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Filter board implementation• Tunable filter board iscompatible with existingfilter board• Interface chip (CPLD2)for programming andpersonality download• Delay with 3 XilinxSpartan FPGAs – 8µsdelay range• 32 filters implementedon 16 Altera StratixFPGAs• Final version usingAltera 1S40 Hardcopy• Signal distribution usingpoint-to-point 1.8V• Estimated powerconsumption: ~60W40W for Hardcopy


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Pre-prototype board• 6 channels (3 FPGAs)• Delay and distributionlogic identical to finalboard• Single-ended and LVDSinterconnections• Selectable I/O voltage• A/D and D/A converter+ speed-up memory• Standalone tests or usedwith test fixture


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004• Pre-prototype board built andunder test• Test fixture from NRAO forsignal generation and analysis– Digital noise + 2 tones– A/D and D/A converter +speed-up memory– Socket for ALMA sampler– ALMA correlator chip– C167 controller– Control program on PC• Prototype board designed andawaiting test resultsDesign status


Digital Backend Meeting MPIfR Bonn - Sep 6, 2004Alternate design - undecimated LOReversing LO and 1 st filterLO operates at decimated speed. Full (N bit) multiplication possible1 st filter must be recomputed/reloaded every time band changes2 nd filter identical


Same test signalDigital Backend Meeting MPIfR Bonn - Sep 6, 2004Alternate design signal processing1 st filter: complex coefficientsbandpass. Frequency rotated version ofprototype LPF Coefficient truncation cannotbe optimized for all freqs.Decimation produces desired bandin an arbitrary positionComplex LO/mixer recoverscorrect frequency orderingSuccessive processing identical

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