SIS Mixers
• Most common front-end element for mm and sub-mm coherent receivers
• Based on superconducting tunnel junction in planar superconducting circuit
• Superconducting gap energy provides highly non-linear current-voltage curve –
usually in niobium, giving a energy gap at 2.8 mV ~700 GHz
• LO signal causes photon assisted tunnelling – photon steps
• Non-linearity has smaller energy scale than mm photon – quantum mixing
• Allows for very low conversion loss, and even conversion gain
• Fundamental noise limit is quantum noise – 2.4 K/100 GHz
• Additional noise from – RF losses, shot noise in SIS junction, IF amplifier
(multiplied by conversion loss of mixer)
Oxford Astrophysics
SIS Mixers
• Need good coupling from signal to very small SIS device (~1 micron square)
• Most mm-wave mixers are waveguide devices – quasi-optical coupling used at
sub-mm and >1 THz
• Coupling from waveguide to planar circuit via waveguide probes or finlines
• Planar circuit also contains tuning circuit to tune out capacitance of junction
• IF signal (typically few GHz) from mixer coupled out via bias tee to LNA
• IF low noise amplifier is usually as separate module – although now starting to
be integrated into mixer block
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State of the art - sensitivity
• Major programs in SIS mixers have been aimed at ALMA and Herschel HIFI
• SIS mixers also used on – JCMT, CSO, SMA, etc. and in atmospheric science
• Typical IF bands – 4-6 GHz (JCMT, SMA), 4-8 GHz (HIFI), 4-12 GHz (ALMA)
HIFI SIS Mixers as delivered
Oxford Astrophysics
State of the art - receivers
• Receiver architectures in use at mm frequencies:
– Waveguide and on-chip LO coupling injection (Chalmers, many others)
– Waveguide side-band separating mixers (ALMA, IRAM, many others)
– Dual polarisation mixers (Waveguide OMTs (ALMA), quasi-optical (ALMA, HIFI))
– Balanced mixers (Waveguide and single chip designs)
IRAM
Chalmers
• Imaging arrays currently in development/commissioning (not complete list!):
– 16 pixel HARP-B (350 GHz) – commissioning/operational on JCMT
– 9 pixel HERA (230 GHz) –operational on IRAM 30m – 49 pixel Super
HERA under development plus 150 GHz array with photonic LO
– 64 pixel SuperCAM (350 GHz) – in development at Arizona for SMT
Oxford Astrophysics
SIS Mixers for CMB/SZ
• SIS mixer advantages
– Coherent
– Spectral resolution
– Phase switching via LO signal (or mixer bias?)
– Correlation/pseudo-correlation of IF signal post-amplification – allows
interferometry on large numbers of baselines
– 4 K cryogenics
• SIS mixer disadvantages
– IF bandwidth – limited by LNAs, mixer design and IF processing
– LO distribution – particularly in imaging arrays
– Need magnetic field at junction to suppress Josephson currents
Oxford Astrophysics
SIS Mixers for CMB/SZ
• Possible applications:
• S-Z imaging/spectral interferometer – See GUBBINS talk later
– Can get moderate spectral resolution with analogue backends
– Using both sidebands can give differential measurement across null
frequency
• Polarimetric interferometry
– Dual polarization mixers, wide bandwidth correlators
• Mm-wave phase switched pseudo-correlation polarimeter
– Could build on current imaging array developments
Oxford Astrophysics
SIS Mixers for CMB/SZ
• Mixer developments required to overcome disadvantages:
• IF bandwidth:
– Design tuning circuits, IF outputs for wide IF – relies heavily on CAD
software (HFSS, Sonnet, CST, SuperMix)
– Tight integration with LNAs – reduce parasitics, no need to go via 50
Ohm coax, could include LNA matching on superconducting circuit
– IF LNA and backend processing bandwidth needs development
• LO distribution:
– On-chip/in-block LO coupling – cleaner optics, reduced LO power
requirements (particularly in balanced mixers)
– Photonic LOs – cryogenic module fed by optical fibre close to mixer chip
• Magnetic field control (particularly for imaging arrays):
– Smaller magnets to adjust field close to chip
– On chip magnetic field generation for tweaking local fields
Oxford Astrophysics
SIS Development at Oxford
• Wide IF band mixers (>20 GHz) at 230 GHz
• New mixer design on silicon – easier fabrication
• Step towards SOI techniques for easier integration
– Gives very thin silicon substrates (0.5->15 micron)
– Allows beamlead techniques for chip grounding and IF connection
– Suitable for use with Planar OMT designs
UVa/Arizona
• SIS mixers at 700 GHz
• On-chip LO injection and sideband-separation
• Photonic LO for 230 GHz and eventually 700 GHz
Oxford Astrophysics
GUBBINS mixers
• GUBBINS – Single baseline 220 GHz interferometer prototype for S-Z
– 0.5m baseline, 0.4 m antennas feeding SIS mixers with 20 GHz IF bandwidth
– LO tunable from 195-260 GHz
– 2-20 GHz analogue correlator – sideband separating, 16 frequency channels
per sideband
• Initial mixer chips use finline single-ended mixers with RF-bandpass filter to
prevent IF leaking into finline - LO directional coupler in mixer block
• IF transformer to convert 20 Ohm mixer output to 50 Ohm required by LNAs
• First batch of chip being tested – problem with RF bandpass filter, will be
resolved on next batch
• Next generation chip design using 50 Ohm characteristic impedance and new
unilateral finline – eventually test balanced and sideband–separating mixers
Oxford Astrophysics
Supporting technologies
• Photonic Local Oscillators - Mix two 1.5 um lasers with mm-wave difference
frequency
– Uses commercially available photodiodes – LO distribution via optical fibre
– Mm-wave signal generated close to mixer – can operate at cryogenic temps
• Oxford have joint student (Boon Kok Tan) with RAL MMT group (Peter
Huggard) developing photonic LO
• RAL LOs used on IRAM 150 GHz SIS array (Room temp LO, inside cryostat)
• Oxford SIS mixer pumped with RAL LO at 230 GHz (optimised for 150 GHz)
10
Single piece
filter/ probe
Photodiode
chip
10
D etec te d po w e r (W )
Position of
optical fibre.
10
10
10
10
-3
-4
-5
(Frequency )
-6
-7
-8
100
1000
Frequency (GHz)
Oxford Astrophysics
-4
Supporting technologies
Oxford Astrophysics
Supporting technologies
• Planar OMTs - Four probes into circular waveguide on Si or SiN membrane
– Direct coupling onto detector chip (or to coax at low frequencies)
– Compact, simple to make, trivial waveguide machining
– Extremely high performance - -50-70 dB isolation over >40% bandwidth
• Smooth wall horns – Uses number of discontinuities to generate corrugated
horn-like beam pattern
– Can be made by direct drilling up to and above 230 GHz
– Can make very cheap arrays
Oxford Astrophysics