1. Long-term stability of an SiGe HBT-
based active cold load
Emilie Leynia de la Jarrige1,
eleyniad@laas.fr
L.Escotte1, E.Gonneau2, J.M.Goutoule3
1CNRS ; LAAS ; Université de Toulouse, 7 avenue du colonel Roche, F-31077 Toulouse, France
2 LERISM, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France
3 EADS- Astrium, 31 rue des Cosmonautes, 31042 Toulouse France
1

2. Introduction
Radiometer calibration onboard a satellite
External target:
Reference - cold sky, pole…
sources
Internal reference:
- passive termination, noise diodes …
Cold source : Low value of noise temperature (< target brightness temperature)
+
Long-term stability
Alternative solution : Active Cold Load (ACL)
=> Significant improvement (> 40%) in radiometric resolution has been reported
at L-band (with traditional noise injection radiometer)
2

3. Introduction
Active cold loads
Year author Technology Frequency Noise
temperature
1981 R.H. Frater GaAs MESFET 1.4 GHz 50 K
1997 L.P. Dunleavy 0.15 μm InP HEMT 18 GHz 126 K
2-10 GHz 90 K
2000 P.M. Buhles 0.15 μm GaAs
10-26 GHZ 125K
MHEMT
2009 N. Skou GaAs PHEMT 10.69 GHz 77 K
2010 C. Bredin 70 nm GaAs MHEMT 23.8 GHz 72 K
-> several technologies, topologies and frequency ranges : FET-based ACL
SiGe HBT : low cost, low excess noise : feasibility ?
3

4. Outline
I – ACL implementation
1. Topology
2. Results
II – Radiometer for long-term stability measurements
1. Test-bed
2. Characteristics
III – Stability results
1. Short-term
2. Long-term
Conclusion
4

5. Outline
I – ACL implementation
1. Topology
2. Results
II – Radiometer for long-term stability measurements
1. Test-bed
2. Characteristics
III – Stability results
1. Short-term
2. Long-term
Conclusion
5

6. ACL implementation 1/2
Technical specifications :
• Frequency band : 1400 – 1427 MHz
• Return loss > 30 dB Typically for sea surface
• Noise temperature < 80 K salinity and soil moisture
measurements applications
• Stability : to be evaluated
80
Min. noise temperature (K)
1.8 GHz
Active device : 60
1.4 GHz
SiGe HBT from Infineon Technologies 40
(BFP640 – SOT343 package) 0.9 GHz
20
Tmin < 40 K 0
0 2 4 6 8 10 12 14
Ic (mA)
6

7. ACL implementation 1/2
Electrical circuit :
Vcc
Cdec
- 0.635 mm alumina substrate
- Surface Mount Components
Rc
Cdec
Rb
Cdec - 50 Ω microstrip lines
Stub L1 L2
Cc
output 50Ω
Cb Lc
Lb Le
Bias stability :
- Resistive collector-to-base circuit
Photograph of the ACL
- Adjustable voltage regulator
7

8. ACL implementation 2/2
2. Results
For Ic = 5.6 mA and Vcc = 1.12 V
0 70
Reflection Coefficient (dB)
Noise temperature (K)
-5
-10 65
-15
-20
60
-25
-30
-35 55
-40
-45 50
1,2 1,25 1,3 1,35 1,4 1,45 1,5 1,55 1,6 1,39 1,4 1,41 1,42 1,43 1,44
Frequency (GHz)
Frequency (GHz)
 Sensitivity test of temperature variations : ≈ 0.3 K/°C
(ex : GaAs FET-based ACL : 0.35 and 0.38 K/°C)
Reflection coefficient < -35 dB
Noise temperature < 66 K
8

9. Outline
I – ACL implementation
1. Topology
2. Results
II – Radiometer for long-term stability measurements
1. Test-bed
2. Characteristics
III – Stability results
1. Short-term
2. Long-term
Conclusion
9

10. Radiometer for long-term stability measurements
1. Test-bed
Dedicated radiometer : two-load radiometer with noise injection
 measurements errors due to power gain variations are reduced
matched
load
T0
ACL A/D
+
Tn
+
Receiver TR , G
cold
Typical measurement cycle
load T2
Directional coupler
+
Noise diode
10

11. Radiometer for long-term stability measurements
1. Test-bed
Temperature-stabilized enclosure
Matched USB
load
Output
voltage
LNA
ACL dc amplifier
Receiver
Noise DAQ
.
diode .
device
Thermistors
.
Command
circuit
- DAQ U2351A from Agilent
…
Thermistors for temperature control
t
t0 t1 t2 11

12. Radiometer for long-term stability measurements
1. Test-bed
Temperature-stabilized enclosure Coaxial switch :
Matched USB Insertion loss : 0.04 dB
load Output
voltage Isolation : > 120 dB
ACL LNA dc amplifier
Noise Receiver DAQ
.
diode . device
Thermistors .
Command
circuit
Transmission coefficient (dB) 70
60
LNA +Filter : 50
40
Gain > 60 dB 30
Noise Bandwidth : 60 MHz 20
10
Noise temperature : 40 K
0
-10
1200 1250 1300 1350 1400 1450 1500 1550 1600
Frequency (MHz) 12

13. Radiometer for long-term stability measurements
1. Test-bed
Temperature-stabilized enclosure
Matched USB
load
Output
voltage
ACL LNA DC
Noise Receiver amplifier DAQ
diode .
Thermistors . device
.
Command
circuit
Square-law detector:
- Tunnel diode
- Responsivity : 1 mV/µW à 1.4GHz
- Noise : 2 nV/√Hz à 100Hz
13

14. Radiometer for long-term stability measurements
1. Test-bed
Temperature-stabilized enclosure
Matched USB
load
Output
voltage
ACL LNA DC
Noise Receiver amplifier DAQ
diode .
Thermistors . device
.
Command
circuit
Square-law detector: Low-noise voltage amplifier:
- Tunnel diode - Gain : 80 dB
- Responsivity : 1 mV/µW à 1.4GHz - Noise floor : 1 nV/√Hz
- Noise : 2 nV/√Hz à 100Hz
14

15. Radiometer for long-term stability measurements
1. Test-bed
Temperature-stabilized enclosure
Matched USB
load
Output
voltage
ACL LNA DC
Noise Receiver amplifier DAQ
diode .
Thermistors . device
.
Command
circuit
Square-law detector: Low-noise voltage amplifier:
- Tunnel diode - Gain : 80 dB
- Responsivity : 1 mV/µW à 1.4GHz - Noise floor : 1 nV/√Hz
- Noise : 2 nV/√Hz à 100Hz Low-pass filter (RC circuit)
- Time constant : 5 ms
15

16. Radiometer for long-term stability measurements
1. Test-bed
Temperature-stabilized enclosure
Matched USB
load
Output
voltage
ACL LNA DC
Noise Receiver amplifier DAQ
diode .
Thermistors . device
.
Command
circuit
Noise injection :
- Excess noise ratio of 15.6 dB at 1.4 GHz
- Coupler : 16 dB
- Noise temperature : T2 ≈ 280K
16

17. Radiometer for long-term stability measurements
1. Test-bed
-> in order to limit the impact of temperature variations, we realize temperature stabilized
enclosure :
ACL temperature ≈ 38.4°C
ACL temperature variations :
< 0.01 °C during 1 day
< 0.03 °C during 4.5 months
10 mm- thick aluminum enclosure
+
50 mm-thick extruded polystyrene
+
Temperature control circuit 17

18. Radiometer for long-term stability measurements
2. Characteristics matched
load
T0
ACL noise temperature : ACL A/D
+
Tn

 V0  V1 
 +
Tn  T0  L
Receiver TR , G
T
V
 2  V1  2
 T2
cold
load
matched load Injected noise temperature
loss of the coupler
   

1   V0  V2   V1  V0 
Tn  LT2 V0 ²


 ²  V1 ²   ²  V2 ²  ²
 V V   (V  V )²   (V  V )² 
 2 1  2 1   2 1 
Important characteristics :
- low value for L
- V0 and V2 close
- smallest variance values ; depend on integration time 18

19. Radiometer for long-term stability measurements
2. Characteristics
1
Allan variance:  y (t )  ( yn1  yn )2 
2
2
1
Conditions :
- T°amb ≈ 38.4°C Diode
Allan deviation (mV)
- Maximum deviation < 0.01°C Matched load
- Period = 150 min
0,1
Optimum integration time: White noise ACL
τ 0 = 50 s Random
Flicker noise walk noise
τ 1 = 15 s
τ 2 = 30 s 0,01
0,1 1 10 100
Integration time (s)
- Sensitivity < 31mK
- Stability < 25mK for a total integration time of 95s
19

20. Outline
I – ACL implementation
1. Topology
2. Results
II – Radiometer for long-term stability measurements
1. Test-bed
2. Characteristics
III – Stability results
1. Short-term
2. Long-term
Conclusion
20

21. Stability results
1. Short-term
87.65
ACL Noise Temperature (K)
87.6
87.55
87.5
87.45
87.4
87.35
87.3
0 1 2 3
Time (day)
Characteristics :
- Mean : 87.48 K
- Standard deviation : 32 mK
- Maximum deviation : 0.205 K
21

22. Stability results
2. Long-term
87.7
ACL Noise Temperature (K)
87.6
87.5
87.4
87.3
87.2
0 20 40 60 80 100 120 140
Time (day)
ACL noise temperature :
- Maximum deviation < 0.35 K
- Slope : -2.1 mK/day
 extrapolated stability < 1 K/year
22

23. Outline
I – ACL implementation
1. Topology
2. Results
II – Radiometer for long-term stability measurements
1. Test-bed
2. Characteristics
III – Stability results
1. Short-term
2. Long-term
Conclusion
23

24. Conclusion
ACL
- Reflection coefficient < -35 dB
- Noise temperature < 66 K
Radiometer
- Sensitivity < 31 mK
- Stability < 25 mK for a total integration time of 95 s
- Long-term stability (4.5 months) :
• Gain : maximum deviation < 0.03 dB
• Receiver noise temperature : maximum deviation < 1 K
• Physical temperature variations < 0.03°C
Long term stability (4.5months)
- Maximum deviation < 0.35 K
- Slope : -2.1 mK/day  extrapolated stability < 1 K/year
Author Frequency Technology Noise Stability Duration
temp.
N. Skou 10.69 GHz GaAs PHEMT 77 K 2K 1 year
1K 8 months
L.P. Dunleavy 18 GHz 0.15 μm InP HEMT 126 K 2K 1 year
C. Bredin 23.8 GHz 70 nm GaAs MHEMT 72 K 0.4 K 1 month
24

25. Thank you for your attention
25