Receiver Systems

Alex Dunning

The Basic Structure of a typical Radio Telescope

Antenna

Receiver

Conversion

Digitiser

Signal Processing / Correlator

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

CSIRO. Receiver Systems for Radio Astronomy Radiotelescope Receivers

CSIRO. Receiver Systems for Radio Astronomy Radio Receivers

“A radio receiver converts signals from a radio antenna to a usable form” Wikipedia

CSIRO. Receiver Systems for Radio Astronomy Ours look more like this...

• Captures the signal reflected from the antenna • Amplifies the signal

Compact Array 3/7/12mm Receiver

CSIRO. Receiver Systems for Radio Astronomy Or this...

Parkes 10/50cm Receiver

CSIRO. Receiver Systems for Radio Astronomy Or this...

ASKAP Phased Array Receiver

CSIRO. Receiver Systems for Radio Astronomy Some even look like this...

Allen Telescope Log Periodic Receiver

CSIRO. Receiver Systems for Radio Astronomy The Receiver

On the outside...

Vacuum Dewar

Feed Horns

CSIRO. Receiver Systems for Radio Astronomy The Receiver

On the inside...

Ortho-Mode Amplifiers Transducers

CSIRO. Receiver Systems for Radio Astronomy

system temperature minimum detectable T flux S  sys Ae  RF Integration Time Effective Collecting Area Observing Bandwidth

CSIRO. Wide Band Receiver Upgrade of the CASS Compact Array The Australia Telescope Receivers

Current upgrade

C/X

L/S K Q W

7cm

3.5mm

25cm

6.7cm

3.7cm

10mm

18.7mm

absorption

2.5cm

10cm

3.2cm

6mm

4.6cm

2.8mm 12mm

1:2.5 bandwidth 1:1.65 bandwidth 1:1.25 bandwidth

CSIRO. Receiver Systems for Radio Astronomy Parkes Receiver Bands

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

CSIRO. Receiver Systems for Radio Astronomy In a prime focus...

The Receiver goes here

CSIRO. Receiver Systems for Radio Astronomy In a Cassegrain system...

The Receiver goes here

CSIRO. Receiver Systems for Radio Astronomy Receiving the signal – Feed horns

Feed

Signal Captures the focused microwaves into a waveguide output

Waveguide output

CSIRO. Receiver Systems for Radio Astronomy Feed Horns

  E   0  B  0 B  E   t E  B   J    0 0 0 t

CSIRO. Receiver Systems for Radio Astronomy 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

CSIRO. Receiver Systems for Radio Astronomy A Tale of Two Feedhorns

Corrugated Smooth Walled

E-Field At Feed mouth

300 300

250 250

200 200 X and Y Feed

150 150 Gain Patterns Gain

100 100

50 50

0 0 -25 -20 -15 -10 -5 0 5 10 15 20 25 -25 -20 -15 -10 -5 0 5 10 15 20 25 Theta [deg] Theta [deg]

CSIRO. Receiver Systems for Radio Astronomy  



    sin 1  d d

CSIRO. Receiver Systems for Radio Astronomy Coupling noise into the System

Feed Coupler Signal

Noise source

Noise coupled 7mm waveguide coupler in through small holes

Noise coupled in through vane

21cm waveguide coupler 12mm noise source

CSIRO. Receiver Systems for Radio Astronomy Separating Polarisations –

Ortho-mode Transducers (OMTs) 3mm Ortho-mode transducer

Pol A Feed Coupler Polariser 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

CSIRO. Receiver Systems for Radio Astronomy Separating Polarisations – Ortho-mode Transducers (OMTs)

CSIRO. Receiver Systems for Radio Astronomy Low Noise Amplifiers (LNA)

Pol A Feed Coupler Polariser LNA Signal To conversion Noise source System Pol B LNA

High Electron Mobility Transistor (HEMT)

CSIRO. Receiver Systems for Radio Astronomy Why is the first Low Noise Amplifier so important?

T2 T3 T4 Tsystem  T1     GainLNA GainLNA G2 GainLNA G2 G3

T 1 T2 T3

Feed

Signal LNA Second Third Stage Stage Amplifier Amplifier

Pol A Feed Coupler Polariser LNA Signal To conversion Noise source System Pol B LNA

….so although receiver topologies can be quite varied I’m saying that this is a pretty typical structure of our receivers

…………and the Compact Array 3/7/12 mm systems reflect this.

CSIRO. Receiver Systems for Radio Astronomy

CSIRO. Receiver Systems for Radio Astronomy The Phased Array Approach

CSIRO. Receiver Systems for Radio Astronomy Getting more from each antenna

• Simple Receiver Collects

4/20 CSIRO. Getting more from each antenna

• Simple Receiver Collects

• Phased Array Feed • collects more (~every /2)

4/20 CSIRO. Getting more from each antenna

• 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

LNA + Conversion + Digital beamformer Filtering

Weighted (complex) sum of inputs

5/20 CSIRO. Connected Chequerboard Array

• Complete sampling of focal region fields • Digital beamforming

LNA + Conversion + Digital beamformer Filtering

Weighted (complex) sum of inputs

6/20 CSIRO. Single Beam Excitation of a Phased Array

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

What’s this?

CSIRO. Receiver Systems for Radio Astronomy Electronics

• Supplies and monitors all amplifier voltages and currents

• Monitors system temperatures and pressures

CSIRO. Receiver Systems for Radio Astronomy Cryogenics

15K section

80K section

Helium Compressor Cold finger

Refrigerator in the Parkes 12mm receiver Helium Lines Helium Refrigerator

CSIRO. Receiver Systems for Radio Astronomy Gap Thermal Isolation waveguide

Vacuum Dewar 15K section

Low Noise Helium Refrigerator Amplifiers cold finger

Copper Radiation Shield 80K

CSIRO. Receiver Systems for Radio Astronomy

….but why do we need to cool our receivers at all?

…………well first

CSIRO. Receiver Systems for Radio Astronomy How weak is the signal? Effective area of Parkes telescope dish 10Jy radio source → 10 × 10-26 W m-2Hz-1 × 1900m2 × 1 × 109 Hz -13 = 2× 10 W Bandwidth of Digital Filter Bank 3 Boltzmann's constant Your Hand → 1.38× 10-23 W Hz-1K-1 × 300K × 1 × 109 Hz = 4 × 10-12 W

Mobile Phone → ≈ 1W

Lunar Distance 3G transmit Mobile Phone on the moon→ bandwidth ≈ 1W ÷ 4π (3.8×108m)2 ÷ 5×106Hz ≈ 10Jy

CSIRO. Receiver Systems for Radio Astronomy

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 Reduce noise by cooling

Electronic device generates a signal

Cold stuff (liquid nitrogen)

CSIRO. Receiver Systems for Radio Astronomy CSIRO Astronomy and Space Science Alex Dunning RF Engineer

Phone: 02 9372 4346 Email: [email protected] Web: www.csiro.au/org/CASS

Thank you