Upgrade of the VLBA C-Band Feed & ReceiverDesign Review

Presented by S durand Designed by Bob Hayward and:Sivasankaran Srikanth (Feed Design)

Hollis Dinwiddie (Mechanical Design)Everett Callan (Master Receiver Builder)

Gordon Coutts & Craig Hennies (OMT Testing)Marian Pospieszalski & the CDL Amplifier Group

Pat Madigan & the VLA Machine ShopMichael Hedrick & the Green Bank Machine Shop

21 April 2011

• Astronomers were eager to:• Access the Methanol Maser at 6,668 MHz GHz• Access to OH at 6016-6049 MHz• Carry out continuum observations with two IF’s separated

by several GHz to allow the effects of the atmosphere to be subtracted out

Example : IF-AC=4.0-4.5 and ID-BD=7.5-8.0 GHz

• Limitations of the existing C-Band system:• Mediocre sensitivity due to old GAsFET LNA’s• Narrow bandwidth due to:

– Septum Polarizer (4.5-5.2 GHz)– LNA’s (4.5-5.2 GHz)– Warm IF (4.5-5.2 GHz)– T105 Conversion (4.5-5.2 GHz)

Why Upgrade the VLBA C-band ?

T(Rx) for Old VLBA vs. New EVLA Rx’sAverage of 8 Dual-Channel Receivers

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.00

5

10

15

20

25

30

35

40

45

50

Receiver Noise Temperature - VLBA vs. EVLA Receivers

VLBA RxEVLA Rx

Frequency (GHz)

Noi

se T

empe

ratu

re (K

)

Required Specifications

• To modify the VLBA Receiver to give us the RF performance that is as close to that achieved by an EVLA Receiver as possible.

Sri’s VLBA 4-8 GHz Feed Design

• Profiled Corrugated Horn• Four machined sections • Ring-Loaded Mode Converter• Plugs into old feed location• Total Length = 37.5”• Outer Diameter = 18.5”• Weight = 55 lbs

Installation of the New Feed at VLBA Pie Town• Done with Feed S/N 01• And Unmodified C-Band Rx• Uses a Transition Plate so an

old Receiver can be attached to a new Feed and thus allow installation of lateral support brackets and turnbuckles.

• Luckily feed is small enough to fit through the hatch (i.e., a crane is not required for hoisting it over the dish).

Existing VLBA C-Band Receiver

SeptumPol

RCP

LCP

LNA

LNA

PamtechCTB1107

PamtechCTB1107

Omni-Spectra2089-6203-00

T

-30 dB

Narda4014C-30

AtlanticMicrowaveAMC 0935

GAsFET4.5-5.2 GHz

GAsFET4.5-5.2 GHz

TNoiseDiode+20-10 dB

High-Cal( Not implemented on VLBA)

TNoiseDiode-6 dB

Omni-Spectra2020-6617-10

Om

ni-Spectra2020-6616-06

PulseCal

TCal

PA

Reactel6B1-4850-700S12

4.5-5.2 GHz

MiteqAMF-2B-4552

4.5-5.2 GHz

DitomD3I4080-14-8 GHz

DitomD3I4080-14-8 GHz

PA

Reactel6B1-4850-700S12

4.5-5.2 GHz

MiteqAMF-2B-4552

4.5-5.2 GHz

DitomD3I4080-14-8 GHz

DitomD3I4080-14-8 GHz

T

Narda4014C-30

-30 dB

TCal

& PCal

TCal

& PCal

PCal

The Cryogenic Pamtech Isolators were a surprise. There was no mention of them in the VLBA Technical Report #3. There must have been added after the 1st prototype Receiver was built.

Keep

New

N/A

Toss

Key:Cryo

Upgraded VLBA C-Band ReceiverO

MT

T

-30 dB

RCP

LCP

TCal

TCal

& PCal

PCal

LNA

LNA

Hyb

rid

Dorado3ICC60-1

Dorado3ICC60-1

Mac

tech

CA

7205

U

In HEMT4-8 GHz

InP HEMT4-8 GHz

-30 dB

T

Narda4014C-30

Narda4014C-30

Omni-Spectra2089-6203-00

T

NoiseDiode

-10 dB

Noise/ComNC3205-G

10 dB

Omni-Spectra2020-6617-10

PulseCal

TCal

& PCal

RCP4-8 GHz

Out

PA

TTEK5221-4/8G

4-8 GHz

CiaoCA48-281G=26dB

DitomD3I4080-14-8 GHz

DitomD3I4080-14-8 GHz

PA

TTEK5221-4/8G

4-8 GHz

CiaoCA48-281G=26dB

LCP4-8 GHz

Out

DitomD3I4080-14-8 GHz

DitomD3I4080-14-8 GHz

Note that we leave out the High-Cal feature, which has never been used on the VLBA

Keep

New

N/A

Toss

Key:Cryo

Modifications & Problems• Making the modifications to the RF path of the receiver were

relatively straight forward.• However, cooling the beast was a problem.

– The old VLBA receiver had its Septum Polarizer tied to the 50°K Stage. – We wanted the new OMT to be tied to the 15°K Stage since it has more

resistive losses. • Hollis spent quite a while fighting to minimize the final

temperature that the 15°K Stage reached.– It was hard to get it much below 30°K and the temperature was very

dependent on how many other receivers were on the same Helium line.– Required reducing the weight of the OMT as well as adding Space

Blankets around the 50°K Radiation Shield.• It was also decided that the Thermal Gap assembly used in the

EVLA design would need to be improved.

EVLA-StyleOMT &

Thermal Gap

Includes:

“Old” Thermal Gap+

Heavy Blocks+

Absorber Strip

AN72 Absorber wrapped around

Thermal Gapto prevent

Cavity Resonances.

VLBA-StyleOMT &

Thermal GapIncludes:

New Circular Thermal Gapwith Choke Ring and long standoffs

+“Swiss Cheese” Blocks

to reduce thermal mass+

Mount for Hybridmakes for an integrated

Circular Polarizer

Recent Cool-down Results• Thanks to Hollis’ new Thermal Gap Assembly, the

Model 22 is now cooling the RF Tree in the Prototype Receiver down to acceptable temperature levels.

• The 15°K Cold Stage temperature is also less susceptible to variations from other fridges being connected to the Helium lines:– ~12-14°K when alone– ~14-16°K with one Model 350 on the loop– ~16-18°K with two Model 350 on the loop

• The 2 x Model 350 scenario is what occurs on a VLBA Antenna.

• VLBA C-Bands typically 15-25°K

VLBA C-Band Prototype (VC#11)

4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.00

5

10

15

20

25

30

35

40

45

50

Receiver Noise Temperature on VLBA C-Band #11-RCPUnmod (Circ) & Modified (Linear) Rx vs. EVLA Rx C#16 (Circ)

(RHH : 13 April 2011)

VLBA C#11, Unmodified, CircularEVLA C#16, Old T-Gap, CircularVLBA C#11, New T-Gap, Linear

Frequency (GHz)

Rec

eive

r T

empe

ratu

re (K

)Early RF Sensitivity Tests on VC#11Old EVLA vs. New VLBA Thermal Gap

Using the new T-Gap assembly with its proper Choke Ring yields a vast improvement over that

achieved on the EVLA with the old T-Gap design.

VLBA C-Band ReceiverOld vs. Interim vs. New Configuration

C

RCP

LCP

RCP

LCP

Old T1054.5-5.2 GHzConverter

Module

4.5-5.2GHz

L104 #2

IF OutputA & C

500-1000 MHz4.5-5.2 GHz

C

RCP

LCP

RCP

LCP

New T4054 - 8 GHzConverter

Module

4.0-8.0GHz

IF OutputA, B, C & D

500-1000 MHz

4.0-8.0 GHz

L104 #2

L104 #3

Old C-Band Rx with Old T105

Modified C-Band Rx with New T405

C

RCP

LCP

RCP

LCP

Old T1054.5-5.2 GHzConverter

Module

4.5-5.2 GHz

4.0-8.0GHz

IF OutputA & C

500-1000 MHz4.0-8.0 GHz

(From old Rx) L104 #2

4.5-5.2GHz

Modified C-Band Rx with Old T105 SMA

Downconverter Design3900-5900 and 5600-7900MHz

Front Panel

Front Panel

Front Panel

Front Panel

3400 to 8600 MHz

512 to 1024 MHz3900 to 5900

5600 to 7900

Conclusions• Thanks to the new Thermal Gap design, the

upgraded VLBA C-Band is now superior to the EVLA receiver.

• Expect T(Rx) < 10°K from 4.2-8.0 GHz• It is now probably the most sensitive

wideband system in the world at this frequency.

Questions ?