Backend electronics for radioastronomy G. Comoretto.
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Transcript of Backend electronics for radioastronomy G. Comoretto.
Backend electronics for Backend electronics for radioastronomyradioastronomy
G. Comoretto
Data processing of a Data processing of a radioastronomic signalradioastronomic signal
Receiver (front-end) Separates the two polarizations Amplifies the signal by ~108 Limits the band to a few GHz Translates the sky frequency to a more manageable
range The resulting signal is then processed by a back end
Electric field E(t)
Power density S(f)to backend
Data processing of a Data processing of a radioastronomic signalradioastronomic signal
Measure S as a function of time, frequency, polarization status, baseline Total power Polarimetry Spectroscopy Interferometry Pulsar (search and timing)
Record the instantaneous field E(t) for further processing
VLBI/ Remote interferometry Radio science
Composite of the above (e.g. spectropolarimetric interferometry)
Signal conversionSignal conversion
IF output may be too wide Difficulties of building wideband backends Necessity of having several spectral points across
the IF bandwidth (e.g. for Faraday rotation) Interest in a specific spectral region (e.g. line
spectroscopy) Necessity to avoid contaminated portion of the IF
band Baseband converters (BBC): select a portion of the IF
bandwidth and convert it to frequencies near zero
Each BBC followed by a specific backend (total power, polarimeter, spectrometer, VLBI channel....)
Simplest observable: total integrated flux over the receiver bandwidth
Filter: selects the frequency band of interest Square law detector: diode (simpler, wideband) or
analog multiplier (more accurate, expensive, band limited)
Integrator: sets integration time: time resolution vs. ADC speed
ADC: converts to digital. Integrator & ADC are often implemented as a voltage-to-frequency converter & counter
Total powerTotal power
Sensitivity:
= integration time f = bandwidth or frequency resolution S = total (receiver dominated) noise
For modern receivers, 1/f gain noise dominant for t > 1-10 s
need for accurate calibration & noise subtraction Added mark Correlating receiver On-the fly mapping Wobbling optics
Total powerTotal power
PolarimetryPolarimetry
Dual polarization receiver: vertical/horizontal or left/right
Cross products give remaining Stokes parameters
Instrumental polarization: 30dB = 0.1%
Bandwidth limited by avaliable analog multipliers
Need for coarse spectroscopic resolution (Faraday rotation)
SpectroscopySpectroscopy Acousto-optic spectrometer:
signal converted to acoustic waves in a crystal diffraction pattern of a laser beam focussed on a CCD amplitude of diffracted light proportional to S(f)
Large bandwidth, limited (1000 points) resolution Rough, compact design All parameters (band, resolution) determined by
physical design => not adjustable
AOS Array for Herschel - HiFiAOS Array for Herschel - HiFi
LiNb cell with 4 acoustic channels Instantaneous band: 4x1.1 GHz (4 – 8 GHz) Resolution : 1 MHz
Spectroscopy – Digital Spectroscopy – Digital correlatorcorrelator
Digital spectrometers: Bandwidth determined by sampling frequency
Max BW technologically limited, currently to few 100MHz Reducing sampling frequency decreases BW = > increased
resolution Autocorrelation spectrometers (XF)
Compute autocorrelation function: Fourier transform to obtain S(f) Frequency resolution:
Signal quantized to few bits (typ. 2) Complexity proportional to N. of spectral points
Spectroscopy – FFT Spectroscopy – FFT spectrometerspectrometer
FFT spectrometers: Compute spectrum of finite segment of data
Square to obtain power and integrate in time
Complexity proportional to log2(N) => N large Requires multi-bit (typ. 16-18 bit) arithmetic Easy to implement in modern, fast FPGA, with HW
multipliers Slower than correlator, but keeping pace Polarimetric capabilities with almost no extra cost
Spectroscopy – FFT Spectroscopy – FFT spectrometerspectrometer
Poly-phase structure: multiply (longer) data segment with windowing function => very good control of filter shape
Very high dynamic range (106-109) => RFI control
InterferometryInterferometry
Visibility function: <E1(t)*E2(t+)> Computed at distant or remote location: need for
physical transport of the radio signal Directly connected interferometers Connected interferometers with digital samplers
at the antennas and digital data link E-VLBI: time-tagged data over fast commercial
(IP) link Conventional VLBI: data recorded on magnetic
media Accurate phase and timing control
InterferometryInterferometry Visibility computed on dedicated correlator or FFT
processor Conventional correlator scales as (number of antennas)2
FFT (FX) scales as N Must compensate varying geometric delay:
Varying sampler clock Memory based buffer, delay by integer samples Phase correction in the frequency domain
Due to frequency conversion, varying delay causes “fringe frequency” in the correlation
ALMA correlator (1 quadrant)
Digital vs. Analog BackendDigital vs. Analog Backend All backend functions can be performed on a digital
signal representation Current programmable logic devices allow to implement
complex functions on a single chip Digital system advantages:
predictable performances – easy calibration high rejection of unwanted signals - RFI Better performances, filter shapes etc. Easy interface with digital equipments
Example of a general-purpose full digital backend
Digital vs. Software Digital vs. Software BackendBackend
Software backends (e.g. SW correlator) becoming possible e.g Blue Chip IBM supercomputer viable as LOFAR
correlator Most Radio Science processing done on software
Computing requirements scale as a power of the BW Dedicated programmable logic still convenient 1 FPGA: 50-500 MegaOPS, ~16 FPGA/board MarkIV correlator (in FX architecture): 1.7 TeraOPS EVLA Correlator: 240 TeraOPS
Digital Backend: ExamplesDigital Backend: Examples
ALMA Digital filterbank:
2 GHz IF input 32x62.5 MHz
independently tunable BBC
General purpose board, can be configured to implement 16 FFT spectropolarimeters @ 125 MHz BW each
Digital Backend: ExamplesDigital Backend: Examples
VLBI dBBC:
1 GHz IF input 250 MHz output bandwidth Directly interfaces with E-VLBI
BEE2 Berkeley system
1 GHz IF input General purpose board, with library of
predefined components System design and validation using MATLAB