Centimeter Receiver Design Considerations

with a look to the future

Steven White

National Radio Astronomy Observatory

Green Bank, WV

Todd. Hunter, Fred. Schwab. GBT High-Frequency Efficiency Improvements, NRAO May 2009 Newsletter

Performance Limitations

• Surface (Ruze λ/16)– ξ = 50%– 300 µmeters → 63 Ghz

• Atmosphere e-t t = optical depth

• Spill Over Ts

• Pointing

• Receiver Noise Temperature (Amplifier) TR

Frequency Coverage

• 300 Mhz to 90 Ghz

• l: 1 meter to 3 millimeters

• l < 1/3 meter - Gregorian Focus

• l > 1/3 meter - Prime Focus

Gregorian Subreflector

Reflector Feeds

Profile: L (size), S (size), Ka (spacing), KFPA (spacing), Q (spacing) Linear Taper: C, X, Ku, KDesign Parameters: Length (Bandwidth), Aperture (Taper, Efficiency)

GBTα= 15º , Focal Length = 15.1 meters, Dimensions = 7.55 x 7.95 meters

Optimizing G/T

Prime Focus Feed

Cross Dipole 290-395 MHz

Gregorian Feeds

140’ & 300’ Hybrid mode prime focus

KFPA Feed

W band feed

140’ Prime Focus and Cassegrain Feed

S, Ku (2x), L

Radio Source Properties

• Total Power (continuum: cmb, dust)– Correlation Radiometer Receivers (Ka Band)– Bolometers Receivers (MUSTANG)

• Frequency Spectrum (spectral line, redshifts, emission, absorption)– Hetrodyne – Prime 1 & 2, L, S, C, X, Ku, K, Ka, Q

• Polarization (magnetic fields) – Requires OMT – Limits Bandwidth

• Pulse Profiles (Pulsars)

• Very Long Baseline Interferometry (VLBI)– Phase Calibration

Prime Focus Receivers

Receiver Frequency Trec Tsys Feed

• PF1.1 0.290 - 0.395 12 46 K X Dipole

• PF1.2 0.385 - 0.520 22 43 K X Dipole

• PF1.3 0.510 - 0.690 12 22 K X Dipole• PF1.4 0.680 - 0.920 21 29 K Linear Taper

• PF2 0.910 - 1.230 10 17 K Linear Taper

Gregorian ReceiversFrequency Band Wave Guide Band Temperature [GHz] [GHz] [º K]

Trec Tsys

• 1-2 L OMT (Septum) 6 20 • 2-3 S OMT (Septum) 8-12 22 • 4-6 C OMT (Septum) 5 18 • 8-10 X OMT (Septum) 13 27 • 12-15 Ku 12.4 -18.0 14 30 • 18-25 K 18.0 - 26.5 21 30-40 • 22-26 K 18.0 - 26.5 21 30-40 • 26-40 Ka 26.5 - 40.0 20 35-45 • 40-52 Q 33 - 50.0 40-70 67-134 • 80-100 W 75 to 110 ~ 3 10^-16

W/√Hz

Receiver Room Turret

Receiver Room Inside

Polarization Measurements

• Linear– Ortho Mode Transducer – Separates Vertical and Horizontal

• Circular – OMT + Phase Shifter (limits bandwidth)– 45 Twist– Or 90 Hybrid to generate circular from linear

Linear Polarization

Orthomode Transducer

Circular Polarization

A Variety of OMTs

K band OMT

Equivalent Noise

Amplifier Equivalent Noise

Amplifier Cascade

Input Losses

HFET Noise Temperature

Radiometer

Correlation Radiometer (Ka/WMAP)

1/f Amplifier Noise

MUSTANG 1/f Noise

HEMT 1/f Chop RatesAmplifier

(band)

νo

[GHz]

Δνrf

[GHz]

fchop(ε =.1)

[Hz]

Δνrf (ε = .1,

f = 5 Hz)

[GHz]

L 1.5 0.5 0.8 3

C 4.0 1 2 2

X 10 3 7 2

Ka 30 10 80 0.6

Q 45 15 375 0.2

W 90 30 1500 0.1

E.J. Wollack. “High-electron-mobility-transistor gain stability and its design implications for wide band millimeter wave receivers”. Review of Sci. Instrum.

66 (8), August 1995.

A HFET LNA

K-band Map Amplifier

Typical Hetrodyne Receiver

Frequency Conversion

Linearity

Intermodulation

Some GBT Receivers

K band Q band

Ka Band

Receiver TestingDigitial Continuum Receiver

Lband XX (2) and YY (4)

Ku Band

Refrigerator Modulation

Ka Receiver (Correlation)Zpectrometer

Lab Spectrometer Waterfall Plot

MUSTANG Bolometer

Focal Plane Array Challenges

• Data Transmission ( State of the Art)

• Spectrum Analysis ( State of the Art)

• Software Pipeline

• Mechanical and Thermal Design.– Packaging– Weight– Maintenance– Cryogenics

Focal Plane Array Algorithm

• Construct Science Case/Aims• System Analysis, Cost and Realizability• Revaluate Science Requirements → Compromise• Instrument Specifications.

– Polarization– Number of Pixels– Bandwidth– Resolution

K band Focal Plane Array

• Science Driver → Map NH3

– Polarized without Rotation

• Seven Beams → Limited by IF system

• 1.8 GHz BW → Limited by IF system

• 800 MHz BW → Limited by Spectrometer

Focal Plane Coverage

simulated beam efficiencyvs. offset from center

1. Initial 7 elements above 68%beam efficiency (illuminationand spillover)

2. Expandable to as many as61 elements

3. beam efficiency of outermostelements would drop to ~60%.

4. beam spacing = 3 HPBWs

3 HPBW

K Band f = 22 GHz

28"

.177 HPBW/mm

= 13.36 mm

88.9 mm

KBand Focal Plane Array

K Band Single Pixel

Phase Shifter

Thermal Transition

OMT

Feed

Noise Module

HEMT

Isolators

Sliding

Transition

Seven Pixel

What’s next for the GBT?

• A W band focal plane array• Science Case is strong and under development.• Surface is improving• Precision Telescope Control System program is

improving the servo system.• Needs.

– Digital IF system– Backend (CICADA)– Funding (Collaborators)

References

• Jarosik, et al. “Design, Implementation and Testing of the MAP Radiometers”, N. The Astrophysical Journal Supplement, 2003, 145

• E.J. Wollack. “High-electron-mobility-transistor gain stability and its design implications for wide band millimeter wave receivers”. Review of Sci. Instrum. 66 (8), August 1995.

• M. W. Pospieszalski, “Modeling of Noise Parameters of MESFET’s and MODFET’s and Their Frequency and Temperature Dependence.” IEEE Trans. MW Theory and Tech., Vol 37. No. 9

• Norrod and Srikanth, “A Summary of GBT Optics Design”. GBT Memo 155.

• Wollack. “A Full Waveguide Band Orthomode Junction.” NRAO EDIR 303.

• https://safe.nrao.edu/wiki/bin/view/GB/Knowledge/GBTMemos

• https://safe.nrao.edu/wiki/bin/view/Kbandfpa/WebHome

Thank you for you attention!

• Questions?