Heterodyne Receivers and Arrays

Transcription

1 Heterodyne Receivers and Arrays Gopal Narayanan Types of Detectors Incoherent Detection Bolometers Total Power Detection No phase information used primarily on single-dish antennas Coherent Detection Heterodyne Receivers Frequency Conversion Total Power Detection Spectral Information Preserved Phase Information preserved used in interferometers and single-dish telescopes

2 Modes Electromagnetic Definition Propagating Spatial Distribution of Energy in a transmission line or free space Does not change its spatial distribution as it travels Free Space : Simple transverse expansion, but maintains same shape Bounded Transmission Line : Does not change at all except for getting weaker Electric and Magnetic Fields in a single mode oscillate sinusoidally with time and position according to frequency and wavelength Examples Free-space Gaussian Modes Bounded Media Waveguides Above cutoff frequency, f C propagating modes < f C evanescent modes

3 Rectangular Waveguides Full-height Rectangular waveguide => b= a 2 Single Mode for: λ/2 < a < λ λc = 2a Cutoff Wavelength For example, let a = 2.54 mm (0.1 inches) λc = 2a = 5.08 mm => fc = 59 GHz λ = a = 2.54 mm => fu = 118 GHz Single-mode waveguide for 59 < f < 118 GHz (good for 3mm wavelength band) For f < 59 GHz, waveguide's modes are evanescent For f > 118 GHz, waveguide has more than one mode!

4 Waveguide Lowest Loss (bounded) transmission line At 100 GHz, waveguide loss ~ 5 db/m For f < 26 GHz, waveguide size becomes too large Use coax instead. Coax At 10 GHz, coax loss ~ 2 db/m

5 Feed Horns Feed horns: Transition from waveguide to free-space modes. Couples to telescope Corrugated Horns Vs Smooth Horns: Beam Pattern Symmetry Low Cross-Pol Conical Electric/Magnetic fields in aperture of smooth horn In corrugated horns, boundary conditions different at horn walls Corrugated Conical Pyramidal

6 Definitions Waveguide : Hollow metal pipe in which signal propagates by multiple reflections from walls. Wave propagates in a particular energy distribution called mode RF (Radio Frequency) Amplifier : Device to increase signal power, placed at input of receiver. For f>50 GHz, RF amplifiers have waveguide inputs Mixer : Circuit that combines RF signal (small signal) with a local oscillator (LO) (large signal) and produces an output at lower frequency (IF). LO : Large monochromatic signal (Large voltage swing causes mixer to become non-linear) IF Amplifier : Amplifier that follows mixer. Less expensive. Most of the gain in a typical radio astronomical system Spectrometer : Device that splits up the IF band into its frequency components, i.e. Spectrum

7 Noise BB at 0K All parts of receiver contribute noise Passive (transmission lines, etc.) Active (Mixers, Amplifiers, etc.) Millimeter & Submillimeter wavelengths Usual to characterize noise of devices by Noise Temperature Even applied to noise sources that are not entirely thermal in origin Device Noise Temperature = TN Equivalent Noiseless Device BB at TN K A noisy device acts as if its input is connected to a (virtual) blackbody at a temperature which is the same as the noise temperature of the device usually shown as in the right figure. Device TN

8 Measuring Noise Temperature TIN POUT TIN + TN TN P hot Thot TN Y= = P cold Tcold TN Prop hides Gain (G), frequency bw, etc. Y-Factor Method T hot YT cold TN = Y 1 For example, measure with input Bbs at 290 K (room temperature) and 77 K (LN2), say Y = 2 => TN = 136 K At mm and submm wavelengths, blackbodies are available (eccosorb) Advantages of Y-factor method: Requires no knowledge of G and BW Only linear detectors required Fast and reasonably accurate

9 Quantum Limit for TN Coherent Receiver both amplitude and phase detected Heisenberg Uncertainty Principle! T Q= h 5 K k 100 GHz Rayleigh Jean's Limit, P = kbtb, where B Bandwidth, TB is equivalent blackbody at input IF Power at output of Receivers: PIF = GBk(TR + TB) where, G Gain of receiver TR Equivalent Receiver Noise temperature TB Equivalent BB temperature at input to receiver

10 Types of Receivers 1) f < 200 GHz 2) f > 200 GHz Schottky or SIS eg. SEQUOIA 3) Bolometer Receivers

11 Entire Receiver System

12 Some Examples: SEQUOIA World's fastest imaging heterodyne array at 3mm wavelength Cryogenic Focal Plane array operating at frequencies of GHz 32 pixels in dual-polarized 4 x 4 array. Two dewars with 16 pixels each combined with wire grid Uses InP pre-amplifiers with db gain Two backend spectrometers per pixel, can be independently tuned within 15 GHz Used at the Quabbin 14m telescope as a workhorse instrument for 6 years, will be moved to LMT, once LMT is ready

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14 Redshift Search Receiver for the LMT Next Lecture

15 A 1mm SIS Receiver for the LMT Receiver in the Lab Noise temperature Measurements Single Pixel 1mm SIS Receiver (dual polarization, sideband separation receiver with IF BW of 4-12 GHz) that will commission the 1mm band at LMT

16 Principle of Down-conversion IF = LO RF SSB Receiver: Single Sideband Only one sideband makes it through the receiver. Other (image) sideband rejection (either quasi-optically or at mixer) DSB Receiver: Both Sidebands are superimposed on each other at IF output Sideband Separation Receiver: Both sidebands converted to different IF outputs

17 Mixer Classical Treatment Basically, mixers can be thought of as switches Bsin( RFt) Recall the trigonometric identity: sin sin = cos cos 2 Asin(sin( LOt)Bsin( RFt) Asin( LOt) IF = LO RF Any arbitrary signal can be decomposed to sines by Fourier analysis If RF is small-signal, and LO is large signal (usual case), LO-RF terms dominate Various filters usually kept at the IF side of mixer to eliminate unwanted terms

18 SSB or DSB? SSB Lower Spectral Confusion Lower system noise temperature within a given sideband terminate unwanted SB in a cold load DSB Twice as much spectral data, if care is taken Twice as much continuum power Receiver has fewer components less complexity Sideband Separation Best of both worlds! More recent heterodyne receivers use Sideband separation

19 Noise Temperature Budget LIN LM GIF T POUT TIN Optics TM Mixer TIF IF Chain POUT = GIF(TIF + (1/LM)(TM + (TIN+ T)/LIN)) Receiver Noise Temperature, TR = TIN + LINTM + LINLMTIF For low noise receiver: TIN Low emissivity optics LIN Low loss optics TM Low Noise mixer LM Low conversion loss TIF Low IF Noise Temperature

20 Types of Mixers Name of the Game Nonlinear I-V Curves! 1. Schottky Diode Mixers V I e 1 where = e kt As T α => more non-linearity Schottky no longer competitive with SIS for f < 800 GHz Room temperature mixing possible with Schottky receivers. Used in remote sensing and satellites (like SWAS)

21 2. SIS Mixers Superconductor Insulator Superconductor Physical Temperature < TN typically < 4K I e V Here, α is very large! Lowest noise between GHz Photon assisted Tunneling For Nb SIS junctions, 2 /e = 3mV => Bandgap cutoff frequency given by ev/h = 730 GHz

22 Superconductor Theory BCS (1957) Bardeen, Cooper and Schreifer Theory of Superconductors Electrons in normal conductor repel Electrons in superconductor Cooper pair Cooper pairs act as single boson. All Cooper pairs are in single quantum state (do not obey Pauli exclusion principle) at 0 V bias! Noisy tunneling process at 0 V. Needs to be suppressed by applying a magnetic field that breaks these Cooper pairs. Conventional Electron Tunneling - referred to as quasi-particle tunneling

23 SIS Junction Geometry

24 IF Power With LO power Without LO power cf. CSO Tuning

25 3. Hot Electron Bolometer (HEB) Normal Superconducting Normal Resistive Superconducting Normal Response time and BW are dependent on how quickly hot electrons are moved out of superconductor! Two types of HEBs Phonon cooled HEBs (pheb) thin and long, and diffusion cooled HEBs (dhebs) thick and short HEB Newer technology Can be used between 100 GHz 100 THz! IF BW depends on thermal time constant, τo Lower τo => Higher IF BW

26 Diffusion-cooled HEB vs Phonon Cooled HEB

27 State of Art in Heterodyne Receiver Noise Temperature Zmuidzinas 2002

28 IF Amplifiers Amplify down-converted signal from mixer Since mixers have conversion loss, fairly important to have low IF noise temperature Highest possible gain (to isolate from noise of subsequent stages) Cryogenically cooled => low power dissipation requirement Well-matched to mixer GaAs and InP transistors are used High total power stability MMICs (Monolithic Microwave ICs) often used

29 Local Oscillators Needed for frequency conversion Required power levels varies a few μw to 100s of μw for arrays Narrow linewidth, low amplitude and phase noise, phase locking Frequency agile to cover large RF bandwidths Technologies Solid state oscillators (eg. Gunn) + freq multipliers (made of diode chains) Photonics LO (new technology) Quantum Cascade Lasers (developing)

30 Array Receivers Why heterodyne array receivers? Single pixel SIS receivers are approaching quantum limit (esp. at lower frequencies). Remaining limit is atmospheric Mapping Speed substantially increased with arrays N fold increase in time for an N-element array, also telescope motion is reduced Best use of good weather conditions Mapping consistency reduced systematic effects due to pointing offsets, relative calibration Cons & Challenges Complicated Expensive Tight packing Cryogenic cooling capacity Delivery of LO Power

31 Sideband Separation Mixers Single Sideband Mixers reject noise in image sideband more sensitive! Sideband Separation Mixers more desirable more spectral coverage with no cost in sensitivity Waveguide-based sideband separation scheme less bulky, and allows integration compared to quasioptical methods

32 OMAR Overview 1mm Array Receiver for LMT Dual-polarized 16pixel array RF Bandwidth GHz USB & LSB both available simultaneously 4 12 GHz IF Band Novel Integrated Mixer-Preamplifier Block Eases Integration

33 Focal-Plane Array Assembly 300K horn section 40K horn section 4K horn section MPA (Mixer Preamplifier Blocks) Magnet Assembly G10 Thermal Break IF Outputs LO Splitter Tree

34 UMass SIS Lab Test Station Single-ended SIS mixerreceiver in test dewar Sumitomo SRDK 415DE closed-cycle 4K test-system