Improved Multi-Octave 3 dB Hybrid for Radio Astronomy Cryogenic

3dB HYBRID
CRYOGENIC
ABSTRACT
DESIGN
This poster describes the design,
construction and measurement of a
multioctave stripline hybrid for the 4 to
12 GHz band specially conceived to
survive and fulfill accurate specifications
when cooled to 15 K (-258 ºC).
The materials and mechanical
construction procedure have been
specially selected and the result is a very
compact, reliable and low thermal mass
device, able to survive extreme thermal
cycling. The coupling and reflection
show
very
little
temperature
dependence.
Finally, the comparison of the results
of a typical commercial unit with the new
design presented clearly shows its
advantages at cryogenic temperature.
Inmaculada Malo Gómez.
Juan Daniel Gallego Puyol, IEEE, Member.
Carmen Diez González.
§ Three sections of λ/4 coupled lines:
21.5 mm
Ø Maximum coupling for transmision
offset
lines of n λ/4 length (n: odd).
Ø Three sections allows coupling ripple
of 0.1dB in two octaves band.
λ/4
λ/4
λ/4
1
offset2
Schematic top view
§ The coupled lines are “offset broadside coupled striplines”:
Ø Offset of central section is used to tune the coupling factor.
Ø Perfect alignment of sustrates is critical to obtain the design
specifications but is dificult to achieve in the fabrication
process.
sustrate1
§ Stripline technology in
mylar
shielded chassis, with
chassis
offset
line1 line2
Ø J.P.Shelton model (“Impedances of offset parallel-coupled
strip transmission lines”. IEEE Trans. Microw. Theory Tech.,
vol.14, Jan.1966).
Ø Coupling is very sensitive to the dimensions of the central
λ/4 section geometry, the most critical parameter is the
separation between sustrates.
Ø First and third λ/4 sections are equal. They reduce the ripple
of the coupling and influence on input reflection.
coup
37.5 mm
sustrate2
a mylar separation layer.
§ Simulation:
16 mm
Modern ultra low noise receivers
used for radio astronomy have evolved
to provide very wide instantaneous
bandwidth. Some of the configurations
used in present cryogenic front-ends,
like sideband separating mixers and
balanced amplifiers, need 90º hybrids at
the IF, typically in the 4-12 GHz band.
There are commercially available
devices covering this band with good
ambient temperature characteristics, but
their cryogenic performance drops to
unacceptable levels.
FABRICATION
direct
One half of the coupler without the mylar film,
in the Al chassis, with the sliding pin SMA connectors.
Sustrate dimensions: 21.4 x 16 x 1,073mm.
§ Chassis in Aluminium alloy.
§ Substrates: RT/duroid ® 6002.
Ø Thermal expansion coefficient matching Aluminum.
Ø εr = 2.94. Thickness: 20mills. 17 µm of copper in one
side only.
Ø Machined with LPKF Proto Laser 200 (it engraves and
cuts the sustrate in the same operation making the
aligment between top and bottom sustrates very
accurate).
§ Dielectric separation layer: Mylar film (thickness 23µm).
Ø εr similar to Duroid.
Ø High rigidity and good thermal stability.
in
iso
Ø Carefull design of transitions between sections improves the
input reflection.
Ø Simulation tool: Momentum 2.5D tool from Agilent EESOF
ADS software.
Carlos Cortés.
César Briso Rodríguez.
CAY
Centro Astronómico de Yebes.
Observatorio Astronómico Nacional.
Apto. correos, 148. 19080 Guadalajara. Spain.
§ Input/output connector: Sliding pin SMA.
Ø Pin shifts absorbing mechanical contractions due to
thermal cycling.
§ Experimental optimization of the transition connector –
line to allow good input matching.
e-mail: i.malo@oan.es
EUIT Telecomunicación.
Departamento IAC
UPM. A-3, km 7. 28031 Madrid. Spain.
3dB HYBRID
CRYOGENIC
Centro Astronómico de Yebes. Observatorio Astronómico Nacional. Apto.
correos, 148. 19080 Guadalajara. Spain.
e-mail: i.malo@oan.es
EUIT Telecomunicación. UPM. A-3, km 7. 28031 Madrid. Spain
MEASUREMENTS
Good cryo performance, similar to room temperature
-20
-40
0
4
16
-3
-6
-9
0
4
8
Freq (GHz)
95
90
85
80
0
4
-2.5
S12, S13 (dB)
S12, S13 (dB)
0
8
12
Freq (GHz)
100
12
12
16
-3.5
0
4
Av.Eq.Insertion Loss (dB)
Average equivalent insertion loss 0.2 dB @ 15 K
calculated from measured S parameters
0,0
8
Freq (GHz)
12
16
CAY 3dB hybrid
in the test dewar
-0,6
-0,8
2
300K
2
2
2
 2
Leq = 10⋅ log  s11 + s12 + s13 + s14 
10


4
6
8
10 12
Frequency (GHz)
14
16
“Side band
separating
receivers
(2SB)”
IF balanced amplifier in “millimeter and submilimeter coherent
heterodyne receivers (SIS)”
Comparison with
best commercial unit
Measurements
@ 15 K, 4-12
GHz
15K
-0,4
The utility of the hybrid developed is demonstrated in a cryogenic
balanced amplifier where the noise temperature obtained (6 K) shows
clear advantage (about 33%) over the classical isolator-amplifier
combination normally used in radio astronomy receivers. Besides, a
superior insensitivity of the noise to the input termination mismatch is
obtained.
Radio astronomy
APPLICATIONS
Very low Insertion loss
-0,2
We have completed the design, manufacture and measurement of a
3 dB 90º hybrid for the 4-12 GHz band specially conceived to survive and
fulfill accurate specifications when cooled to 15 K. Coupling and reflection
show very little temperature dependence. The result is a very compact,
low loss, reliable, repetitive and low thermal mass device, able to survive
extreme thermal cycling.
-3.0
-4.0
16
8
Freq (GHz)
Measurements of
the hybrid at 15 K
(thick lines) and
300 K (thin lines).
Note the almost
invariant behavior
with temperature
due to the careful
selection of
materials and
technologies.
Radio astronomy receivers require the use of devices that can
operate at cryogenic temperatures to obtain extremely high sensitivity.
Return loss [dB]
Amplitude
unbalance [dB]
Phase
unbalance [º]
CAY
(3 units)
< -22
Pasternack
(best)
< -19
A way to improve the poor input
Mismatch of the IF LNAs,
replacing traditional amplifier
with input isolator configuration.
SIS
IF
LO
LNAs
3-dB HYBRID
± 0.3
±2
± 0.9
±3
40
LNA
3-dB HYBRID
Tn YXA 1025
Tn YXA 1026
Tn BALANCED
Tn ISO+YXA1026
Gain YXA 1025
Gain YXA 1026
Gain BALANCED
Gain ISO+YXA1026
16
35
14
30
12
25
10
20
8
15
6
10
4
5
2
0
3
4
5
6
7
8
f [GHz]
9
10
11
12
0
13
Noise Temperature [K]
phase unbalance (deg)
S11 (dB)
0
-60
CONCLUSIONS
Gain [dB]
CAY
Inmaculada Malo Gómez
Juan Daniel Gallego Puyol
Carmen Diez González
Carlos Cortés
César Briso Rodríguez