Detector Systems Graeme Carrad November 2011
The Basic Structure of a typical Radio Telescope Antenna Receiver Conversion Digitiser Signal Processing / Correlator
They are much the same CSIRO.
Radiotelescope Receivers
Radio Receivers e A radio receiver converts signals from a radio antenna to a usable form Wikipedia
Ours look more like this... Captures the signal reflected from the antenna Amplifies the signal Compact Array 3/7/12mm Receiver
Or this... Parkes 10/50cm Receiver
Or this... ASKAP Phased Array Receiver
Some even e look like this... Allen Telescope Log Periodic Receiver
The Receiver e On the outside... Vacuum Dewar Feed Horns
The Receiver e On the inside... Amplifiers Ortho-Mode Transducers
The Receiver e On the inside... Amplifiers Ortho-Mode Transducers
What is the signal like? Charged particles change their state of motion when they interact with energy A change in state of motion gives rise to an EM wave Matter is made of huge numbers of charged particles receiving energy being jostled and the radiation consists of unrelated waves at all frequencies and by analogy with the acoustic case it is called NOISE. There is a general background and areas of enhanced radiation and energy
system temperature minimum detectable flux Effective Collecting Area S S A e T sys RF Integration Time Observing Bandwidth CSIRO.
The Australia a Telescope escope Receivers e Current upgrade L/S C/X K Q W 10cm-25cm 4.6cm-6.7cm 3.2cm-3.7cm 2.5cm-7cm 12mm-18.7mm 6mm-10mm O2 a absorption 2.8mm-3.5mm 1:2.5 bandwidth 1:1.65 bandwidth 1:1.25 bandwidth CSIRO.
Parkes Receiver e Bands CSIRO.
Where do they go?
In aprime focus... The Receiver goes here
In a Cassegrain a system... The Receiver goes here
Receiving the signal Feed horns Feed Signal Captures the focused microwaves into a waveguide output Waveguide output
Feed Horns E 0 B 0 B E t B 0J 0 0 E t
Detour: Waveguides Replace cables at high frequencies Operate like optical fibres for microwaves Only work over a limited frequency range Can support signals with two polarisations
Coupling noise into the System Feed Coupler Signal Noise source Noise coupled in through small holes 7mm waveguide coupler Noise coupled in through vane 21cm waveguide coupler 12mm noise source
Separating Polarisations Ortho-mode Ot ode Transducers s (OMTs) 3mm Ortho-mode transducer Feed Coupler Polariser Pol A Signal Noise source Pol B Separates incoming signal into two linear or circular polarisations Linear OMTs exhibit higher polarisation purity over broad frequency bands (usually) 12mm Ortho-mode transducer 4cm Ortho-mode transducer
Separating Polarisations Ortho-mode Ot ode Transducers s (OMTs)
Low Noise ose Amplifiers (LNA) Feed Coupler Polariser Pol A LNA Signal Noise source Pol B LNA To conversion System High Electron Mobility Transistor (HEMT)
Pol A Feed Coupler Polariser LNA Signal Noise source Pol B LNA To conversion System.so although receiver topologies can be quite varied I m Im saying that this is a pretty typical structure of our receivers and the Compact Array 3/7/12 mm systems reflect this.
The Phased Array Approach
Getting more from each antenna a Simple Receiver Collects 4/20 CSIRO.
Getting more from each antenna a Simple Receiver Collects Phased Array Feed collects more (~every /2) 4/20 CSIRO.
Getting more from each antenna a Simple Receiver Collects Phased Array Feed collects more (~every /2) allows corrections 4/20 CSIRO.
Connected Array Start with a simple array of dipoles Join them together Digital beamformer LNA + Conversion + Filtering Weighted (complex) sum of inputs 5/20 CSIRO.
Connected Chequerboard Array Complete sampling of focal region fields Digital beamforming Digital beamformer LNA + Conversion + Filtering Weighted (complex) sum of inputs 6/20 CSIRO.
Single Beam Excitation of a Phased Array 0.4 0.3 0.2 0.1 x (m) 0 0.1 0.2 0.3 0.4 0.4 0.3 0.2 0.1 0 0.1 0.2 0.3 0.4 y (m)
What is the rest of the stuff? What s this? What s this?
Electronics cs Supplies and monitors all amplifier voltages and currents Monitors system temperatures es and pressures
Cryogenics 15K section 80K section Cold finger Helium Compressor Refrigerator in the Parkes 12mm receiver Helium Refrigerator Helium Lines
Gap Thermal Isolation waveguide Vacuum Dewar 15K section Helium Refrigerator cold finger Low Noise Amplifiers Copper Radiation Shield 80K
.but why do we need to cool our receivers at all? well first
How weak is the sg signal? Effective area of Parkes telescope dish 10Jy radio source 10 10-26 W m -2 Hz -1 1900m 2 1 10 9 Hz = 2 10-13 W Bandwidth of Digital Filter Boltzmann's constant Your Hand 1.38 10-23 W Hz -1 K -1 300K 1 10 9 Hz = 4 10-12 W Bank 3 Mobile Phone 1W Lunar Distance Mobile Phone on the moon 1W 4π (3.8 10 8 m) 2 5 10 6 Hz 10Jy 3G transmit bandwidth
Like your hand all the components in the receiver system contribute a thermal noise signal which masks the astronomical signal we are trying to observe. By cooling the receiver we reduce these thermal sources of noise and improve the sensitivity of the receiver by 7-10 times.
Why is the first Low Noise Amplifier so important? T system T 1 T2 Gain LNA T Gain 3 LNA G 2 Gain LNA T4 G 2 G 3 T 1 T 2 T 3 Feed Signal LNA Second Third Stage Stage Amplifier Amplifier
Reduce noise by cooling Electronic device generates a signal Cold stuff (liquid nitrogen)
The Basic Structure of a typical Radio Telescope Antenna Receiver Conversion Digitiser Signal Processing / Correlator
The Conversion o System Signal Amplifier Filter Frequency Conversion Level Adjustment To Digitiser Contains: more amplification band defining filters frequency conversion level adjustment signal detection band shaping
Filters High Pass Filter Low Pass Filter Band Pass Filter Hard roll off where necessary to stop strong interference Slow roll off where possible so you can push the band edges 21cm band filter
Mixing gtdo it down Frequency Conversion o Mixer (Multiplier) Signal 1 Signal 1 Signal 2 Signal 2 cos(ω 1 t)cos(ω 2 t)=½[cos((ω 1 +ω 2 )t)+ cos((ω 1 -ω 2 )t)] Powe r Powe er Δf Frequency Δf Frequency
Mixing gtdo it down Frequency Conversion o Mixer (Multiplier) Signal 1 Signal 2 Low pass filter cos(ω 1 t)cos(ω 2 t)=½[cos((ω 1 +ω 2 )t)+ cos((ω 1 -ω 2 )t)] Powe r Powe er Δf Frequency Δf Frequency
Mixing gtdo it down Frequency Conversion o Mixer (Multiplier) Signal 1 Local Oscillator cos(ω 1 t)cos(ω LO t) ½cos[(ω 1 -ω LO )t] r Powe f lo Upper Side Band (USB) Powe er Frequency Frequency Δf Δf
Mixing gtdo it down Frequency Conversion o Mixer (Multiplier) Signal 1 Local Oscillator cos(ω 1 t)cos(ω LO t) ½cos[(ω LO -ω 1 )t] r Powe Lower Side Band (LSB) f lo Powe er Δf Frequency Δf Frequency
Mixing gtdo it down Frequency Conversion o Mixer (Multiplier) Signal 1 Band pass filter Local Oscillator r Powe f lo Powe er Δf Frequency Δf Frequency
Attenuators The Volume ou Knob Allow the signal level to be varied May be several in the system Usually set automatically ti Just like some other systems if you turn the signal down too far all you get is noise and if you turn it up to far you get distortion!
Of course real systems are a little more complicated... They usually contain multiple conversions and many amplification and filter stages... But that s the gist of it.
The Basic Structure of a typical Radio Telescope Antenna Receiver Conversion Digitiser Signal Processing / Correlator
Sampling Digitise the signals to transport them off to signal processing electronics. Some bandwidth defining filter is normally involved.
Sampling
Correlator A signal processor that can take different forms but there is always a data time series which is compared (correlated) with itself or another antenna s signal, and a Fourier transform is applied to generate a power spectrum. Some averaging or integration normally follows White noise doesn t correlate but a signal buried in the noise does!
Raw noise data
Output
CABB board Full BW (2GHz) Digital Filter Bank All lengths available Current maximum is 4k channel 68% slices, 60% RAM, 50% DSPs BCC FPGA PCI interface firmware that interfaces to all FPGAs and sensors (V,A,T) Data Router simulation of ATCA mesh in Simulink Data distribution worked out. Also contains Fine Delay firmware which is complete Fringe Stopping in development Coarse delay firmware Rear Transition Module (RTM) for 9-bit ADC Zoom DFB The PCB is 26 layers and has thousands of components Correlator firmware
CABB board
CSIRO Astronomy and Space Science Graeme Carrad Assistant Director -Engineering Phone: 02 9372 4305 Email: graeme.carrad@csiro.au Web: www.csiro.au/org/cass Thank you Contact Us Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: enquiries@csiro.au Web: www.csiro.au
To measure the radiation we observe it for an interval long compared to most of the fluctuations and find the mean average power over the interval. Each observation will fluctuate about the true mean and this limits the sensitivity. A rough estimate of the size of the fluctuations: Random fluctuating ti quantity restricted t to bandwidth Df is equivalent to a sequence of Df independent values in 1 sec. Averaging a sequence over t seconds means t* Df values Fluctuations in the mean of n independent readings ~ n -1/2 so our mean power fluctuations will be DP/P ~ (t* Df) -1/2
DP ~ P/(t* P / Df) 1/2 or but the components in the signal path contribute to P because they are matter with thermal energy. P=Psig+Prec Psig Prec So the components contribution masks the signal. It is like trying to measure the change in water level of a swimming pool when dropping a child in during free-for-all time at a swimming carnival