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Receiver Systems Alex Dunning. The Basic Structure of a typical Radio Telescope CSIRO. Receiver Systems for Radio Astronomy ReceiverConversionDigitiser.

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Presentation on theme: "Receiver Systems Alex Dunning. The Basic Structure of a typical Radio Telescope CSIRO. Receiver Systems for Radio Astronomy ReceiverConversionDigitiser."— Presentation transcript:

1 Receiver Systems Alex Dunning

2 The Basic Structure of a typical Radio Telescope CSIRO. Receiver Systems for Radio Astronomy ReceiverConversionDigitiser Signal Processing / Correlator Captures and amplifies the incoming radiation Filters and reduces the frequency of the microwave signal Converts the analog signal to a digital bit stream Divides signal into frequency bins and forms correlation products between signals

3 They are much the same CSIRO. Receiver Systems for Radio Astronomy

4 Radiotelescope Receivers CSIRO. Receiver Systems for Radio Astronomy

5 The Receiver CSIRO. Receiver Systems for Radio Astronomy On the outside...

6 The Receiver CSIRO. Receiver Systems for Radio Astronomy On the inside...

7 The Australia Telescope Receivers CSIRO. Receiver Systems for Radio Astronomy 10cm-25cm 12mm-18.7mm 6mm-10mm O 2 absorption 2.8mm-3.5mm 4.6cm-6.7cm 3.2cm-3.7cm 2.5cm-7cm Future upgrade L/S C/X K Q W 1:1.25 bandwidth 1:1.65 bandwidth 1:2.5 bandwidth

8 Where do they go? CSIRO. Receiver Systems for Radio Astronomy

9 At the focus of course

10 Waveguides CSIRO. Receiver Systems for Radio Astronomy Replace cables at high frequencies Operate like optical fibres for microwaves Only work over a limited frequency range Can support signals with two polarisations

11 Receiving the signal – Feed horns CSIRO. Receiver Systems for Radio Astronomy Feed Signal Waveguide output Captures the focused microwaves into a waveguide output

12 Feed Horns CSIRO. Receiver Systems for Radio Astronomy

13 Coupling noise into the System CSIRO. Receiver Systems for Radio Astronomy Feed Signal Noise source Coupler 7mm waveguide coupler Noise coupled in through small holes Noise coupled in through vane 21cm waveguide coupler 12mm noise source

14 Separating Polarisations – Orthomode Transducers (OMTs) CSIRO. Receiver Systems for Radio Astronomy Polariser Feed Signal Noise source Coupler Pol B Pol A Separates incoming signal into two linear or circular polarisations Linear OMTs are more effective over broad frequency bands (usually) 12mm Orthomode transducer 4cm Orthomode transducer

15 Separating Polarisations – Orthomode Transducers (OMTs) CSIRO. Receiver Systems for Radio Astronomy

16 Low Noise Amplifiers (LNA) CSIRO. Receiver Systems for Radio Astronomy Polariser Feed Signal Noise source Coupler LNA To conversion System Pol B Pol A High Electron Mobility Transistor

17 CSIRO. Receiver Systems for Radio Astronomy ….so though receiver topologies can be quite varied I am saying that this is a pretty typical structure of our receivers …………and the 3/7/12 mm systems reflect this.

18 CSIRO. Receiver Systems for Radio Astronomy

19 What is the rest of the stuff? CSIRO. Receiver Systems for Radio Astronomy Whats this?

20 Electronics CSIRO. Receiver Systems for Radio Astronomy Supplies and monitors all amplifier voltages and currents Monitors system temperatures and pressures

21 Cryogenics CSIRO. Receiver Systems for Radio Astronomy 15K section 80K section Helium Lines Helium Compressor Helium Refrigerator Refrigerator in the Parkes 12mm receiver Cold finger

22 CSIRO. Receiver Systems for Radio Astronomy Copper Radiation Shield 80K Helium Refrigerator cold finger Vacuum Dewar Thermal Isolation waveguide 15K section Low Noise Amplifiers Gap

23 CSIRO. Receiver Systems for Radio Astronomy ….but why do we need to cool our receivers at all? …………well first

24 How weak is the signal? CSIRO. Receiver Systems for Radio Astronomy Your Hand 1.38× 10 -23 W Hz -1 K -1 × 300K × 2 × 10 9 Hz = 8 × 10 -12 W 10Jy radio source 10 × 10 -26 W m -2 Hz -1 × 300m 2 × 2 × 10 9 Hz = 6 × 10 -14 W Mobile Phone 1W Mobile Phone on the moon 1W ÷ 4π (3.8×10 8 m) 2 ÷ 5×10 6 Hz 10Jy Effective area of an Australia telescope dish Bandwidth of an Australia telescope digitiser Boltzmann's constant Lunar Distance 3G transmit bandwidth

25 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 CSIRO. Receiver Systems for Radio Astronomy

26 Reduce noise by cooling CSIRO. Receiver Systems for Radio Astronomy Electronic device generates a signal Cold stuff (liquid nitrogen)

27 The Conversion System CSIRO. Receiver Systems for Radio Astronomy Contains: more amplification band defining filters frequency conversion level adjustment signal detection band shaping Frequency Convertion Signal Filter To Digitiser Amplifier Level Adjustment

28 Filters CSIRO. Receiver Systems for Radio Astronomy High Pass Filter Low Pass Filter Band Pass Filter 21cm band filter Slow roll off where possible so you can push the band edges Hard roll off where necessary to stop strong interference

29 Mixing it down – Frequency Conversion CSIRO. Receiver Systems for Radio Astronomy Signal 1 Signal 2 Signal 1 × Signal 2 Mixer (Multiplier) Frequency Power Frequency Power cos(ω 1 t)cos(ω 2 t)=½[cos((ω 1 +ω 2 )t)+ cos((ω 1 -ω 2 )t)] ΔfΔfΔfΔf

30 Mixing it down – Frequency Conversion CSIRO. Receiver Systems for Radio Astronomy Signal 1 Signal 2 Mixer (Multiplier) Frequency Power Frequency Power cos(ω 1 t)cos(ω 2 t)=½[cos((ω 1 +ω 2 )t)+ cos((ω 1 -ω 2 )t)] ΔfΔfΔfΔf Low pass filter

31 Mixing it down – Frequency Conversion CSIRO. Receiver Systems for Radio Astronomy Signal 1 Mixer (Multiplier) Frequency Power Frequency Power ΔfΔfΔfΔf Local Oscillator f lo cos(ω 1 t)cos(ω LO t) ½cos[(ω 1 -ω LO )t] Upper Side Band (USB)

32 Mixing it down – Frequency Conversion CSIRO. Receiver Systems for Radio Astronomy Signal 1 Mixer (Multiplier) Frequency Power Frequency Power ΔfΔf ΔfΔf Local Oscillator f lo cos(ω 1 t)cos(ω LO t) ½cos[(ω LO -ω 1 )t] Lower Side Band (LSB)

33 Mixing it down – Frequency Conversion CSIRO. Receiver Systems for Radio Astronomy Signal 1 Mixer (Multiplier) Frequency Power Frequency Power ΔfΔfΔfΔf Local Oscillator f lo Band pass filter

34 Single Sideband Mixers CSIRO. Receiver Systems for Radio Astronomy Signal 22cos(ω 1 t) 2cos(ω 1 t) 2sin(ω 1 t) sin[(ω 1 - ω LO )t] (USB) -sin[(ω LO - ω 1 )t] (LSB) cos[(ω 1 - ω LO )t] (USB) cos[(ω LO - ω 1 )t] (LSB) -cos[(ω 1 - ω LO )t] (USB) cos[(ω LO - ω 1 )t] (LSB) 0(USB) 2cos[(ω 1 - ω LO )t] (LSB)

35 Single Sideband Mixers CSIRO. Receiver Systems for Radio Astronomy Signal 22cos(ω 1 t) 2cos[(ω LO - ω 1 )t] (USB) 0(LSB) Signal Local Oscillator Lower sideband Upper sideband

36 Attenuators – The Volume Knob CSIRO. Receiver Systems for Radio Astronomy Allow the signal level to be varied May be several in the system Usually set automatically 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!

37 CSIRO. Receiver Systems for Radio Astronomy Of course real systems are a little more complicated..... They usually contain multiple conversions and many amplification and filter stages.... But thats the gist of it.

38 Things to remember CSIRO. Receiver Systems for Radio Astronomy Sometimes local oscillators leak if you look deep enough you might find one! Single sideband mixers can result in signals turning up at the wrong frequency, albeit at a very low level. Make sure your attenuators are set right. Too high and the system noise increases. Too low and you may distort your signal.

39 Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: enquiries@csiro.au Web: www.csiro.au Thank you CSIRO Astronomy and Space Science Alex Dunning RF Engineer Phone: 02 9372 4346 Email: alex.dunning@csiro.au Web: www.csiro.au/org/CASS


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