1. Matt Morgan National Radio Astronomy Observatory
A K-Band Spectroscopic Focal Plane Array for the Robert C. Byrd Green Bank Radio Telescope
8/11/2008
Matt Morgan
National Radio Astronomy Observatory

2. The World's Largest Fully-Steerable Radio Telescope
The GBT has a 100-meter diameter unobstructed aperture using offset Gregorian
optics, and has a 3-foot wide focal plane that lends itself naturally to large-format
focal plane arrays.

3. Why K-Band?
data courtesy of Jay Lockman

4. Why K-Band?
data courtesy of Jay Lockman

5. "Science Case for a K-Band Focal Plane Array for the GBT" Morgan, Pisano, Lockman, Di Francesco, Wagg, Ott
Fiducial Projects:
Physical Properties of Infrared Dark Clouds
Chemistry in Dark Clouds
Nature of Observations
Dark Clouds: maps in NH3 and perhaps other molecules
Chemistry: deep integrations at many frequencies

6. Size of the Focal Plane 36" mounting ring spacing = 3.45" refrigerator
The front-ends will be fed by a hexagonally close-packed array of room-temperature
feedhorns. (Due to thermal and electromagnetic issues, cooling the feedhorns was
considered an unjustifiable trade-off).

7. Baseline Instrument Specifications
Requirement
Frequency Band
GHz (complete K-Band coverage)
Instantaneous RF Bandwidth
1.8 GHz
Number of beams
7 (expandable up to 61)
TRX (each beam, not including sky)
< 25 K (75% of band)*
< 35 K (entire band)
Aperture Efficiency
>55%, any pixel
Polarization
dual, circular (axial ratio <= 1 dB)
Polarization Isolation
>25 dB
Pixel-Pixel Isolation
>30 dB
Headroom
> 30 dB (to 1 dB compression point)
*Should in fact be equivalent to EVLA receiver noise temperatures.

8. System Schematic

9. Bandwidth Limitations
Sub-Assembly
Total Potential Bandwidth
comments
cold-electronics (feed, OMT, LNA...)
>8.5 GHz
performance degrades outside of GHz
warm analog electronics
1.8 GHz (up to 8 dual-polarized beams
limited by existing IF transmission system, requires multiplexing
digital electronics
800 MHz (4 beams)
50 MHz (8 beams)
limited by existing spectrometer

10. An Upgrade Path to Large-Format Imaging Cameras
 The current IF Transmission System and spectrometer are severely limited in the
total IF bandwidth that can be processed simultaneously.
 These systems are not capable of supporting spectroscopic focal-plane arrays
with more than about 10 pixels, at least with bandwidths that are interesting for
radio astronomy.
 If we want the GBT to have large-format focal plane arrays, than we need to
develop both the IF and digital backends as well as the front-ends to use them.
 An upgraded IF Transmission System or Spectrometer with larger numbers of
channels would not be immediately useful by itself without the front-ends to use
them, whereas it appears feasible to develop a small front-end array (<10 pixels)
that is scientifically interesting given the existing backend systems.
 Such an array could take almost immediate advantage of the upgraded backend
systems when they come online.

11. Warm vs. Cooled Feedhorns
Cooling the feedhorn could in theory reduce the receiver noise by 20%, however
this introduces a number of a technical problems for a very large array, including
infrared heat loading, and cryostat cavity effects.

12. Feedhorn Array
OD = 3.4"
A new compact-taper corrugated feedhorn was designed for this project. It
will incorporate a quick-release attachment mechanism for easy insertion
and removal from the array.

13. Beam Spacing and Efficiency for Two Different Size Feeds
Frequency
[GHz]
Beam Spacing (3.4" feed)
[HPBW]
Beam Spacing (2.8" feed) 18
2.3
2.0 22
2.7
2.4 26
3.2
90º Elevation
30º Elevation

14. Measured Feedhorn Patterns and Edge Taper (3.4" feed)
Freq.
(GHz)
H-pln
(dB)
E-pln 18
-11.7
-11.2 19
-12.6
-12.3 20
-13.2
-13.6 21
-13.7
-14.2 22 23
-13.5
-14.0 24 25
-13.0
-13.4 26
-14.3
26.5
-13.9
-14.7
Avg.

15. 22 GHz Telescope Beams in Azimuth (Asym.) Plane HPBW = 34 arcsecX”
Throw
(arcsec)
(HPBW)
3.45 94
2.7
6.9
189
5.5
10.4
284
8.2
13.8
378
11.0
17.3
473
13.7

16. Electromagnetic Components
Existing phase shifter, transition, and OMT designs will be used. Thermal isolation will
be provided by a thermal gap.

17. Internal Noise Source for Calibration
The noise source will be completely internal to the cryostat, eliminating 60 additional
high-frequency thermal transitions.

18. Integrated Cold Noise Source and Cal Coupler
E. Bryerton
 Uses a CHA2092b MMIC amplifier from UMS ($21) as a noise source.
 Flatter output than the existing GBT noise cal using an external diode.

19. EVLA-Type K-Band Amplifier
{{{picture of amp, plot of performance}}}
The existing K-Band amplifier design used by the EVLA will also be used
in this instrument.

20. Dewar Mechanical Considerations
The cryostat and most housings will be made entirely of Aluminum. The top plate of
the dewar which must remain flat will be honeycombed for strength.

21. Dewar Mechanical Considerations
1. thermal gaps
2. circular to square transition
3. phase shifter
4. power combiner
5. OMT
6. noise coupler
7. isolators
8. LNAs
9. waveguide "trombones"
10. downconverters (outside
dewar)
The absence of integration in the cold part of the receiver makes for very long
"rocket-like" pixels.

22. IF Frequency Plan for 7-Pixel Prototype
In order to make use of the existing IF Transmission System and spectrometer,
pixel outputs will be multiplexed two at a time onto each fiber channel.

23. Integrated Downconverter Modules
The IF multiplexing scheme requires two different downconverter types to be built.

24. Integrated Downconverter Module #1

25. M&C Using Inter-Integrated Circuit (I2C) Bus

26. FPA Modes with Existing GBT Spectrometer
Instantaneous bandwidth would improve dramatically with a new spectrometer.
No change is need to the front end!

27. Future Developments
 Larger Focal Plane Arrays will require a new IF Transmission System capable of
handling an order of magnitude larger data volume.
 digital fiber links are most desirable, but several major technical challenges
(RFI, power dissipation, etc.) must be overcome for this to be viable.
 A new spectrometer will also be necessary to adequately process the large
volume of data produced by a fully-populated array.
 Once the backend subsystems are in place, the K-band pixels can be replicated
and the array populated with relatively little risk
 (However, I think it would be prudent to do more integration inside the
cryostat first.)
 Experience gained from this program as well as subsequent upgrades to the
backend subsystems will open the door for heterodyne arrays at other
frequencies. (W-Band?)