Heterodyne Receivers and Heterodyne Receivers and ArraysArrays

Gopal Narayanangopal@astro.umass.edu

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

Modes – Electromagnetic DefinitionModes – 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, fC

propagating modes➔ < f

C

evanescent modes

Rectangular WaveguidesRectangular Waveguides

“Full-height” Rectangular waveguide =>

b=a2

Single Mode for: /2 < a < λ λ

λC = 2a Cutoff Wavelength

For example, let a = 2.54 mm (0.1 inches)λ

C = 2a = 5.08 mm => f

C = 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 evanescentFor f > 118 GHz, waveguide has more than one mode!

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.

CoaxAt 10 GHz, coax loss ~ 2 db/m

Feed HornsFeed Horns

Feed horns: Transition from waveguide to free-space modes. Couples to telescope

Conical Corrugated Conical

PyramidalCorrugated Horns Vs Smooth Horns:

● Beam Pattern Symmetry● Low Cross-Pol

Electric/Magnetic fields in aperture of smooth horn

In corrugated horns, boundary conditions different at horn walls

DefinitionsDefinitions

● 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

NoiseNoise

● 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

BB at 0 K

Device Noise Temperature

= TN BB at

TN K

Noiseless DeviceEquivalent

TN

DeviceA 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.

TN

Measuring Noise TemperatureMeasuring Noise Temperature

TIN

POUT

∝ TIN + T

N

Y-Factor Method

Y=Phot

Pcold

=ThotTN

TcoldTN

TN=Thot−YTcold

Y−1

Prop hides Gain (G), frequency bw, etc.

For example, measure with input Bbs at 290 K (room temperature) and 77 K (LN

2), 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

Quantum Limit for TQuantum Limit for TNN

Coherent Receiver – both amplitude and phase detected

Heisenberg Uncertainty Principle!

TQ=hk

≃5K

100GHz

Rayleigh Jean's Limit, P = kBTB,

where B – Bandwidth, TB is equivalent blackbody at input

IF Power at output of Receivers:

PIF = GBk(T

R + T

B)

where,

G – Gain of receiver

TR – Equivalent Receiver Noise temperature

TB – Equivalent BB temperature at input to receiver

Types of ReceiversTypes of Receivers

1) f < 200 GHzeg. SEQUOIA

2) f > 200 GHz

Schottky or SIS

3) Bolometer Receivers

Entire Receiver SystemEntire Receiver System

Some Examples: SEQUOIASome Examples: SEQUOIA

World's fastest imaging heterodyne array at 3mm wavelength

● Cryogenic Focal Plane array operating at frequencies of 85 – 115.6 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 35-40 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

Redshift Search Receiver for the LMT

Next Lecture

A 1mm SIS Receiver for the LMTA 1mm SIS Receiver for the LMT

Receiver in the LabNoise 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

Principle of Down-conversionPrinciple 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

Mixer – Classical TreatmentMixer – Classical Treatment

Basically, mixers can be thought of as switches

Bsin(RF

t)Asin(sin(

LOt)Bsin(

RFt)

Asin(LO

t)Recall the trigonometric identity:

sinsin=cos −−cos

2 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

SSB or DSB?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

Noise Temperature BudgetNoise Temperature Budget

LIN

TIN

TM

TIF

LM

GIF

Optics Mixer IF Chain

T POUT

POUT

= GIF(T

IF + (1/L

M)(T

M + (T

IN+ T)/L

IN))

Receiver Noise Temperature, TR = T

IN + L

INT

M + L

INL

MT

IF

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

Types of MixersTypes of Mixers

Name of the Game – Nonlinear I-V Curves!

1. Schottky Diode Mixers

I∝eV−1

where=

ekT

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)

2. SIS Mixers2. SIS Mixers

Superconductor – Insulator – SuperconductorPhysical Temperature < T

N typically < 4K

I∝eV

Here, α is very large! Lowest noise between 50 – 1000 GHz

Photon assisted Tunneling

For Nb SIS junctions, 2∆/e = 3mV => Bandgap cutoff frequency given by eV/h = 730 GHz

Superconductor TheorySuperconductor 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

SIS Junction GeometrySIS Junction Geometry

With LO power

Without LO power

IF Power

cf. CSO Tuning

3. Hot Electron Bolometer (HEB)3. Hot Electron Bolometer (HEB)

Normal NormalSuperconductingSuperconducting

Superconducting Normal

Resistive

● HEB – Newer technology● Can be used between 100

GHz – 100 THz!● IF BW depends on thermal

time constant, τo

● Lower τo => Higher IF BW

● 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

Diffusion-cooled HEB vs Phonon Diffusion-cooled HEB vs Phonon Cooled HEBCooled HEB

State of Art in Heterodyne State of Art in Heterodyne Receiver Noise TemperatureReceiver Noise Temperature

Zmuidzinas 2002

IF AmplifiersIF 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

Local OscillatorsLocal 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)

Array ReceiversArray ReceiversWhy 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

Sideband Separation MixersSideband 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 quasi-optical methods

OMAR OverviewOMAR Overview

● 1mm Array Receiver for LMT

● Dual-polarized 16-pixel array

● RF Bandwidth 200 – 280 GHz

● USB & LSB both available simultaneously 4 – 12 GHz IF Band

● Novel Integrated Mixer-Preamplifier Block Eases Integration

Focal-Plane Array AssemblyFocal-Plane Array Assembly

300K horn section

40K horn section 4K horn section

MPA (Mixer Pre-amplifier Blocks)

LO Splitter Tree

IF Outputs

G10 ThermalBreak

Magnet Assembly

UMass SIS Lab Test StationUMass SIS Lab Test Station

Sumitomo SRDK 415DE closed-cycle 4K test-system

Single-ended SIS mixer-receiver

in test dewar