Digital Backend Meeting Digital Backend Meeting MPIfR MPIfR Bonn Bonn - - Sep 6, 2004 Sep 6, 2004 Tunable Tunable Digital Filter Digital Filter for the ALMA for the ALMA Correlator Correlator Gianni Comoretto Gianni Comoretto INAF INAF - - Osservatorio di Arcetri Osservatorio di Arcetri Alain Alain Baudry Baudry , Philippe , Philippe Cais Cais , , Benjamin Benjamin Quertier Quertier Observatoire Observatoire de Bordeaux de Bordeaux Andre Andre Gunst Gunst ASTRON ASTRON - - Dwingeloo Dwingeloo
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Tunable for the ALMA Correlator - radionet-eu.org Digital... · for the ALMA. Correlator. Gianni Comoretto. ... •64 channels/IF @ 2GHz BW, 4 polarization ... independently tunable
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• 64 12m dishes on the Atacama highland (5000 m elevation)• 15m -> 12 Km baselines• 9 bands from 32 to 850 Ghz• 2 polarization * 8 GHz instantaneous bandwidth• 3 bit sampler, 2 bit correlator
• 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 => zooming
Problems• Sub-channel stitching and platforming => overlapping sub-
channels• Digital filter architecture => economically implement 32 filters• Multiple quantization effects• Increased data rate to control computer => keep original TM mode
SubchannelSubchannel stitchingstitchingThe problemThe 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
SubchannelSubchannel stitchingstitchingSimulation 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. continuous
• Result from hybrid correlatorcompared to reference spectrum
• Hybrid spectrum both withand without final 2-bitquantization to discriminate effects
Difference 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
QuantizationQuantization effects effects -- LOLOPhase resolution in mixer sin/cos table determines SFDRMixer output requantization determines SFDR and lossFor > 50 dB SFDR at least 6 bits of mixer output are neededSin/cos generation, mixing, requantization can be implemented in a single LUT memory
# Bits Loss SFDR3 3,3% 31 dB4 1,5% 38 dB6 0,9% 52 dB
Performance of quantized mixers(3 bit input, N bit output)
LO leakage very well rejected. Main source of unwanted harmonics is sampler DC offset
Reversing LO and 1st filter LO operates at decimated speed. Full (N bit) multiplication possible1st filter must be recomputed/reloaded every time band changes2nd filter identical