Abstract
1
IF/LO Thermal Controller for SMA Antennas D. Y. Kubo a , S.W. Chang a , B. N. Koge a , R. Wilson b a Academia Sinica, Institute for Astronomy & Astrophysics, Hilo, HI b Harvard-Smithsonian Center for Astrophysics, Cambridge, MA Abstract As part of an ongoing effort to refine the SMA's technical performance we investigated the YIG oscillator's phase to temperature coefficient. Measured lab data shows approximately 1 degree phase per degree C change in YIG oscillator temperature. A typical post YIG multiplier value for observation at 345 GHz is 48, thus a 1 degree YIG phase change results in a nontrivial 48 degrees at the sky frequency. The YIG oscillator temperature varies quite slowly with diurnal variations and weather, and somewhat quickly (~15 minutes) with YIG tuning voltages. The present method to mitigate this phase movement is through regular (~20 minutes) astronomical calibration on bright celestial sources. This method, through the use of interpolation between calibrations, is used to remove the phase variations from a multitude of sources, one being the antenna YIG oscillator. We have developed an active fan speed controller to maintain a constant temperature within the IF/LO electronics enclosure located in the antenna cabin. After struggling with oscillation issues we have obtained some impressive temperature stability results. We are currently in the process of replicating the design for all 8 antennas. 1. IF Functional Description and Impact on Phase Stability The IF path which carries the 4 to 6 GHz signal begins within the antenna cryostat just after the SIS mixer (lower left of Fig 1 ). This signal is leveled then optically modulated for transmission over fiber to the main control building. The received optical signal is demodulated to electrical, leveled to accommodate for optical loss variations, then passes through the 1 st down conversion which subdivides the IF signal into six blocks. A 2 nd down conversion further subdivides the IF into 24 chunks each centered at 153 MHz and with a bandwidth of 82 MHz (total bandwidth of 1968 MHz). Each of these 24 chunks are fine leveled to accommodate for channel slope and sky variations then digitized using 2-bit samplers operating at 208 Msps (upper right of Fig 1 ). Phase movements or drifts of the IF signal do not scale with LO frequency. We have shown that the IF drift is much less than 1 degree and is therefore negligible even without the aid of thermal stabilization. Thermal stabilization does, however, improve the gain stability of the amplifiers and continuum detectors. 2. LO Functional Description and Impact on Phase Stability The LO system begins with a 10 MHz crystal oscillator phase locked to a GPS reference. A set of ~109 MHz and 200 MHz references are generated then optically modulated for transmission over fiber to the antenna. A common tunable LO (6.0 – 8.5 GHz) denoted as MRG YIG-1 PLL in the lower right of Fig 1 is optically modulated and power divided to each of the 8 antennas. Each antenna receives these LOs on fibers A and C and are demodulated to electrical then leveled to accommodate for optical loss variations. The antenna YIG-1 PLL phase locks to the 200 MHz and LO-1 to produce an output at either LO-1 +/- 200 MHz. Following the YIG-1 output is a harmonic mixer which produces multiple harmonics (M) and mixes them with the Gunn oscillator output operating in the 80 to 120 GHz band. The Gunn output is followed by a final fixed multiplier whose value is denoted as N. Table I provides example LO frequencies for some typical tuning frequencies. It becomes obvious from the table that a small movement in the antenna YIG phase translates to a non-trivial phase movement at the final LO. For example, a 1.0 degree phase movement at the antenna YIG output translates to 32, 48, and 84 degrees at the final LO for 230, 345, and 690 GHz, respectively. Regular calibration is clearly necessary to avoid errors in the position measurement of astronomical sources and to a reduction of the image fidelity. This calibration, however, includes phase corrections for a multitude of sources and is performed only periodically at intervals of approximately 20 minutes. Fig 2. Antenna IF/LO enclosure attached to cabin wall. Left photo – front cover removed, right photo – internal covers removed. Enclosure size is 36 x 24 x 9 inches (91.4 x 61.0 x 22.9 cm), weight is ~135 LBS (61 Kg). Fig 1. Smithsonian Submillimeter Array RF System Block Diagram MRG YIG ANTENNA YIG M GUNN N FINAL LO M*N 6.825 GHz 7.025 GHz 16 112.509 GHz 2 225.018 GHz 32 6.877 GHz 7.077 GHz 16 113.341 GHz 3 340.023 GHz 48 7.947 GHz 8.147 GHz 14 114.167 GHz 6 685.002 GHz 84 Table I. Typical Post Antenna YIG Multiply Ratios 3. Fan Controller Functional Description A photo of the fan controller assembly is shown in Fig 3. The assembly is entirely self contained, including its own power supply, and can easily be installed onto the IF/LO enclosure by replacing the stock cover. The reference thermistor cable (not shown) extends from the controller board and attaches to the center of the enclosure base plate (just below the lower left corner of YIG-1 in Fig 2). The thermistor (negative temperature coefficient resistor) is biased to produce a voltage which is proportional to temperature. This voltage is amplified, filtered then fed to a commercial Microchip TC648 fan speed controller. The TC648 provides pulse width modulation (PWM) of the +12VDC supply to the intake and exhaust fans. PWM is preferred over linear voltage control to prevent stalling of the fan blades at low rotational speeds. The temperature set point can be adjusted from the outside via a feed-through trimmer potentiometer. Fig 3. Fan controller assembly integrated into enclosure cover. Left and right photos are front and rear views, respectively. The intake baffle was added to prevent excessive cooling of the upper assemblies, the mixing fans were added to prevent thermal oscillation. 4. Fan Controller Performance Fig 4 shows the performance of the fan speed controller mounted on the IF/LO enclosure for antenna 5 (acc5) over a five day interval. The other 7 antennas are unregulated. Note that in addition to a smaller diurnal variation, antenna 5 also operates several degrees lower than the other antennas because of the active cooling. A baffle was added to the upper intake fan to prevent excessive cooling of the upper two assemblies. We encountered thermal oscillation with a very long period of ~40 minutes and finally resolved it with the use of 4 internal mixing fans. These mixing fans are run at 100% speed and circulate the air in a counter-clockwise direction as viewed from the front. One difficulty that we are presently encountering is the determination of the optimum temperature set point which appears to be different for different antennas. The solution may have to be empirically derived separately for each antenna. Fig 4. Comparison of IF/LO enclosure temperatures for all 8 antennas over a five day interval. 5. Future Plans The next level of thermal stabilization will involve the development of a complementary heater for the YIG oscillator component itself. The YIG oscillator produces a varying amount of heat which is a function of tuning frequency. The YIG frequency is often changed in the middle of the night during the change over from one science track to the next and results in a slow change in the overall assembly temperatures. M IRRO R 1 2 3 4 5 6 PO LAR IZATIO N SPLITTER SIS ISO LATO R / SIS BIAS HEMT 1ST STAG E (65K) 3RD STAG E (4K) 2ND STAG E (14K) RECEIVER SET A (4 IN SER TS,1 SHOW N) RECEIVER SET B (4 IN SE R TS,N O N E SHOW N) xN GUNN OSC. xM fwd F(s) fine tune Fo FIN AL LO ( S K Y + / -5 G H z) course tune servo CRYOSTAT LO CATED O N O PTICS CAG E IF-1 O U T (5 G H z) ~109 M Hz REFERENCE -30 dBm YIG -1 OSC. fwd F(s) fine tune Fin -Fo = 200 M Hz course tune FORx FORx FO Tx 1550 nm 1310 nm D IPLEXER 200 M Hz REFERENCE 200 M Hz 109 M Hz AN TEN N A IF/LO Box +5 dB m (nom) +20 dBm (m ax) FO Tx 10 M Hz PLL 100 M Hz FO Tx 1550 nm W DM FORx 1310 nm . . . 6-W AY PD 4-W AY PD . . . SAM PLER 2-BITS x4 208 M Hz LENS 2ND STAG E AM P ISO LATO R 2ND STAG E AM P W DM 6.2 to 8.3 G H z LO 52 M H z C LO CK 6 BLO CKS Fc = 1 G Hz (328 M Hz EACH) 4 CHUNKS /BLO CK Fc = 153 M Hz (82 M Hz EACH) 2N D LO SYNTH. 724 M Hz 806 1276 1194 TO TAL O F 6 X 4 = 24 CHUNKS PER R EC EIVER IF 1ST LO SYNTH. 3180 M Hz 3508 3836 6164 6492 6820 (flat) (flat) (oval) (oval) TO CORRELATOR ~109/200 M Hz IF ANTENNA CO NTRO L BLDG -7 dBm (nom ) +4 dBm (nom ) 1310 nm +2 dBm nom +11 dBm (nom ) O PTIC AL SPLITTER 10-W AY PD AN T-1 P D TO R C VR -B YIG -2 (H AR D W ARE SHO WN O NLY FO R RCVR-A) -3 dBm (-10.5 dB) +4 dBm (nom ) 1310 nm (-0.5 dB) 1310 nm ORTEL ORTEL ORTEL (-0.5 dB) -5 dBm (nom ) 1550 nm LASERTRON ho rn harmonic mixer fixed m ultiplier fw d = LO rvrs = {F gunn -(M x LO )}= ~109 M Hz 109 M Hz error DC error Am bientLoad C alibration 1stD O W N CONVERTER LO ~109 M Hz 5+/-1.25 GHz BPF ISO LATO R 5+/-1.25 GHz BPF D irectD igital Synthesizer 4-W y P D 4-W y Sw D igitalPLL Assy DC error G UNN PLL & M U LTIPLIER ASSY (1 PER RECEIVER INSERT R EQ U IR ED ) TO O THER G UNN PLL & M U LTIPLIER ASSY'S TO O THER G UNN PLL & M U LTIPLIER ASSY'S FIBER -C FIBER -A AN T-8 CSO JC M T . . . DEMUX 8-B ITS SE TO DIFF 16-B ITS D IFFE R EN TIAL ECL /CHUNK 2nd D O WN CONVERTER 52 M Hz C H U N KS 1-4 C H U N KS 4-8 C H U N KS 9-12 C H U N KS 13-16 C H U N KS 17-20 C H U N KS 21-24 1 2 6 . . . FAN OUT GPS R eference x10 & 100 M Hz PLL x2 x10 DDS (~9 + 100 (M Hz) 52 M Hz PLS SUM x6,7 or8 M U LTIPLIER 52 MHz (1 PER AN TEN N A IF R EQ U IR ED ) M RG YIG -1 PLL H P 8644B fwd F(s) fine tune 0.2 to 500 M Hz coarse tune 6,7,or8 G Hz 5.5 to 8.5 G Hz IF-2 O U T (5 G H z) -30 dBm TO RCVR-B SYSTEM (NO T SHO W N) SUM from MRG YIG -2 PLL ListofAcronym s: BPF = Band Pass Filter FO R x = FiberO ptic Receiver FO Tx = FiberO ptic Transm itter M RG = M asterR eference G enerator NO M = N om inal PD = Pow erD ivider PLL = Phase Lock Loop W DM = W ave D ivision M ultiplexer LO-1 LO-2 Contact: Derek Kubo, [email protected] March 18, 2008
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IF/LO Thermal Controller for SMA Antennas D. Y. Kubo a , S.W. Chang a , B. N. Koge a , R. Wilson b a Academia Sinica, Institute for Astronomy & Astrophysics, Hilo, HI b Harvard-Smithsonian Center for Astrophysics, Cambridge, MA. 1. IF Functional Description and Impact on Phase Stability - PowerPoint PPT Presentation
Transcript of Abstract
IF/LO Thermal Controller for SMA AntennasD. Y. Kuboa, S.W. Changa, B. N. Kogea, R. Wilsonb
aAcademia Sinica, Institute for Astronomy & Astrophysics, Hilo, HIbHarvard-Smithsonian Center for Astrophysics, Cambridge, MA
AbstractAs part of an ongoing effort to refine the SMA's technical performance we investigated the YIG oscillator's phase to temperature coefficient. Measured lab data shows approximately 1 degree phase per degree C change in YIG oscillator temperature. A typical post YIG multiplier value for observation at 345 GHz is 48, thus a 1 degree YIG phase change results in a nontrivial 48 degrees at the sky frequency. The YIG oscillator temperature varies quite slowly with diurnal variations and weather, and somewhat quickly (~15 minutes) with YIG tuning voltages. The present method to mitigate this phase movement is through regular (~20 minutes) astronomical calibration on bright celestial sources. This method, through the use of interpolation between calibrations, is used to remove the phase variations from a multitude of sources, one being the antenna YIG oscillator. We have developed an active fan speed controller to maintain a constant temperature within the IF/LO electronics enclosure located in the antenna cabin. After struggling with oscillation issues we have obtained some impressive temperature stability results. We are currently in the process of replicating the design for all 8 antennas.
1. IF Functional Description and Impact on Phase StabilityThe IF path which carries the 4 to 6 GHz signal begins within the antenna cryostat just after the SIS mixer (lower left of Fig 1). This signal is leveled then optically modulated for transmission over fiber to the main control building. The received optical signal is demodulated to electrical, leveled to accommodate for optical loss variations, then passes through the 1st down conversion which subdivides the IF signal into six blocks. A 2nd down conversion further subdivides the IF into 24 chunks each centered at 153 MHz and with a bandwidth of 82 MHz (total bandwidth of 1968 MHz). Each of these 24 chunks are fine leveled to accommodate for channel slope and sky variations then digitized using 2-bit samplers operating at 208 Msps (upper right of Fig 1). Phase movements or drifts of the IF signal do not scale with LO frequency. We have shown that the IF drift is much less than 1 degree and is therefore negligible even without the aid of thermal stabilization. Thermal stabilization does, however, improve the gain stability of the amplifiers and continuum detectors.
2. LO Functional Description and Impact on Phase StabilityThe LO system begins with a 10 MHz crystal oscillator phase locked to a GPS reference. A set of ~109 MHz and 200 MHz references are generated then optically modulated for transmission over fiber to the antenna. A common tunable LO (6.0 – 8.5 GHz) denoted as MRG YIG-1 PLL in the lower right of Fig 1 is optically modulated and power divided to each of the 8 antennas. Each antenna receives these LOs on fibers A and C and are demodulated to electrical then leveled to accommodate for optical loss variations. The antenna YIG-1 PLL phase locks to the 200 MHz and LO-1 to produce an output at either LO-1 +/- 200 MHz. Following the YIG-1 output is a harmonic mixer which produces multiple harmonics (M) and mixes them with the Gunn oscillator output operating in the 80 to 120 GHz band. The Gunn output is followed by a final fixed multiplier whose value is denoted as N. Table I provides example LO frequencies for some typical tuning frequencies. It becomes obvious from the table that a small movement in the antenna YIG phase translates to a non-trivial phase movement at the final LO. For example, a 1.0 degree phase movement at the antenna YIG output translates to 32, 48, and 84 degrees at the final LO for 230, 345, and 690 GHz, respectively. Regular calibration is clearly necessary to avoid errors in the position measurement of astronomical sources and to a reduction of the image fidelity. This calibration, however, includes phase corrections for a multitude of sources and is performed only periodically at intervals of approximately 20 minutes.
Fig 2. Antenna IF/LO enclosure attached to cabin wall. Left photo – front cover removed, right photo – internal covers removed. Enclosure size is 36 x 24 x 9 inches (91.4 x 61.0 x 22.9 cm), weight is ~135 LBS (61 Kg).
Fig 1. Smithsonian Submillimeter Array RF System Block Diagram
MRG YIG ANTENNA YIG M GUNN N FINAL LO M*N
6.825 GHz 7.025 GHz 16 112.509 GHz 2 225.018 GHz 32
6.877 GHz 7.077 GHz 16 113.341 GHz 3 340.023 GHz 48
7.947 GHz 8.147 GHz 14 114.167 GHz 6 685.002 GHz 84
Table I. Typical Post Antenna YIG Multiply Ratios
3. Fan Controller Functional DescriptionA photo of the fan controller assembly is shown in Fig 3. The assembly is entirely self contained, including its own power supply, and can easily be installed onto the IF/LO enclosure by replacing the stock cover. The reference thermistor cable (not shown) extends from the controller board and attaches to the center of the enclosure base plate (just below the lower left corner of YIG-1 in Fig 2). The thermistor (negative temperature coefficient resistor) is biased to produce a voltage which is proportional to temperature. This voltage is amplified, filtered then fed to a commercial Microchip TC648 fan speed controller. The TC648 provides pulse width modulation (PWM) of the +12VDC supply to the intake and exhaust fans. PWM is preferred over linear voltage control to prevent stalling of the fan blades at low rotational speeds. The temperature set point can be adjusted from the outside via a feed-through trimmer potentiometer.
Fig 3. Fan controller assembly integrated into enclosure cover. Left and right photos are front and rear views, respectively. The intake baffle was added to prevent excessive cooling of the upper assemblies, the mixing fans were added to prevent thermal oscillation.
4. Fan Controller PerformanceFig 4 shows the performance of the fan speed controller mounted on the IF/LO enclosure for antenna 5 (acc5) over a five day interval. The other 7 antennas are unregulated. Note that in addition to a smaller diurnal variation, antenna 5 also operates several degrees lower than the other antennas because of the active cooling. A baffle was added to the upper intake fan to prevent excessive cooling of the upper two assemblies. We encountered thermal oscillation with a very long period of ~40 minutes and finally resolved it with the use of 4 internal mixing fans. These mixing fans are run at 100% speed and circulate the air in a counter-clockwise direction as viewed from the front. One difficulty that we are presently encountering is the determination of the optimum temperature set point which appears to be different for different antennas. The solution may have to be empirically derived separately for each antenna.
Fig 4. Comparison of IF/LO enclosure temperatures for all 8 antennas over a five day interval.
5. Future PlansThe next level of thermal stabilization will involve the development of a complementary heater for the YIG oscillator component itself. The YIG oscillator produces a varying amount of heat which is a function of tuning frequency. The YIG frequency is often changed in the middle of the night during the change over from one science track to the next and results in a slow change in the overall assembly temperatures.
MIRROR 1
2
3
4
5
6
POLARIZATIONSPLITTER
SIS
ISOLATOR/
SIS BIAS
HEMT
1ST STAGE(65K)
3RD STAGE(4K)
2ND STAGE(14K)
RECEIVER SET A(4 INSERTS, 1
SHOWN)
RECEIVER SET B (4INSERTS, NONE
SHOWN)
xNGUNNOSC.
xM
fwd
F(s)
finetune
Fo
FIN
AL
LO (
SK
Y +
/- 5
GH
z)
coursetune servo
CRYOSTAT
LOCATED ON OPTICS CAGE
IF-1 OUT (5 GHz)
~109
MH
z RE
FE
RE
NC
E
-30 dBm
YIG-1OSC.
fwd
F(s)
finetune
Fin
- Fo
=2
00 M
Hz
coursetune
FORx
FORx
FOTx
1550 nm
1310 nm
DIP
LE
XE
R
20
0 MH
z RE
FE
RE
NC
E
200 MHz
109 MHz
ANTENNA IF/LO Box
+5
dB
m (n
om
)+
20
dB
m(m
ax)
FOTx
10 MHzPLL 1
00 M
Hz
FOTx
1550 nm
WD
MF
OR
x
1310 nm
.
.
.
6-WA
Y P
D
4-W
AY
PD
.
.
.
SAMPLER2-BITS
x4
20
8 MH
z
LENS
2ND STAGEAMP
ISOLATOR
2ND STAGEAMP
WD
M
6.2 to 8.3 GH
z LO52 MHz CLOCK
6 BLOCKSFc = 1 GHz
(328 MHz EACH)
4 CHUNKS / BLOCKFc = 153 MHz
(82 MHz EACH)
2ND LOSYNTH.
724 MHz806
12761194
TOTAL OF 6 X 4 = 24CHUNKS PERRECEIVER IF
1ST LOSYNTH.
3180 MHz
35083836
61646492
6820
(flat)
(flat)
(oval)
(oval)
TO
CO
RR
ELA
TO
R
~109/200 MHz
IFA
NT
EN
NA
CO
NT
RO
L B
LD
G
-7 dBm(nom)
+4 dBm(nom)
1310 nm
+2 dBmnom
+11 dBm(nom)
OP
TIC
AL
SP
LIT
TE
R
10-WAY PDANT-1
PD
TO RCVR-B YIG-2
(HARDWARE SHOWNONLY FOR RCVR-A)
-3 dBm
(-10.5 dB)
+4 dBm(nom)
1310 nm
(-0.5 dB)
1310 nm
ORTEL
ORTEL
ORTEL
(-0.5 dB)
-5 dBm(nom)
1550 nm
LASERTRON
horn
harmonicmixer
fixed multiplier
fwd = LOrvrs = {Fgunn - (M x LO)} = ~109 MHz
109 MHzerrorDC
error
Ambient LoadCalibration
1st DOWNCONVERTERLO
~109 MHz
5+/-1.25GHz
BPF
ISOLATOR
5+/-1.25GHz
BPF Direct DigitalSynthesizer
4-Wy PD
4-Wy Sw
Digital PLL Assy
DCerror
GUNN PLL &MULTIPLIER ASSY
(1 PER RECEIVER INSERTREQUIRED)
TO
OT
HE
R G
UN
NP
LL &
MU
LTIP
LIE
RA
SS
Y'S
TO
OT
HE
R G
UN
NP
LL &
MU
LTIP
LIE
RA
SS
Y'S
FIBER-C
FIBER-A
ANT-8CSO
JCMT
.
.
.
DEMUX8-BITS
SE TODIFF
16-BITSDIFFERENTIALECL / CHUNK
2nd DOWNCONVERTER
52
MH
z
CHUNKS 1-4
CHUNKS 4-8
CHUNKS 9-12
CHUNKS 13-16
CHUNKS 17-20
CHUNKS 21-24
1
2
6
.
.
.
FANOUT
GPSReference
x10 &100 MHz
PLLx2
x10
DDS(~9 + 100
(MHz)
52 MHzPLS
SUM
x6, 7 or 8MULTIPLIER
52MHz
(1 PER ANTENNA IFREQUIRED)
MRG YIG-1PLL
HP 8644B
fwd
F(s)
finetune
0.2 to 500 MHz
coarsetune
6, 7, or 8 GHz
5.5 to 8.5 GHz
IF-2 OUT (5 GHz)
-30 dBmTO RCVR-B SYSTEM(NOT SHOWN)
SU
M
from MRGYIG-2 PLL
List of Acronyms:BPF = Band Pass FilterFORx = Fiber Optic ReceiverFOTx = Fiber Optic TransmitterMRG = Master Reference GeneratorNOM = NominalPD = Power DividerPLL = Phase Lock LoopWDM = Wave Division Multiplexer
LO-1
LO-2
Contact: Derek Kubo, [email protected] 18, 2008
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