Challenging the conventional bandwidth limit, we design an extremely wide-band circular waveguide septum polarizer, covering 42% bandwidth, from 77 GHz to 118 GHz, without any high-order resonance. The performance of this polarizer has been verified in between 75 GHz and 115 GHz. The Stokes parameters constructed from the measured data show that the leakages from I to Q are below ±2% and the Q U mutual leakage below ±1%. This result removes the major weakness of the septum polarizer and opens up a new domain of astronomical instrumentation for polarization measurement. Despite this polarizer is designed to cover 77-118 GHz, it can be straightforwardly downsized to cover higher frequencies with minor change. The measurement result of a G-band (140-220 GHz) polarizer is also presented.
We propose analog-to-digital converters (ADCs) using spread spectrum technology in cryogenic receivers or at warm
room temperature for coherent receiver backend systems. As receiver signals are processed and stored digitally, ADCs
play a critical role in backend read-out systems. To minimize signal distortion, the down-converted signals should be
digitized without further transportation. However, digitizing the signals in or near receivers may cause radio frequency
interference. We suggest that spread spectrum technology can reduce the interference significantly. Moreover, cryogenic
ADCs at regulated temperature in receiver dewars may also increase the bandwidth usage and simplify the backend
digital signal process with fewer temperature-dependant components. While industrial semiconductor technology
continuously reduces transistor power consumption, low power high speed cryogenic ADCs may become a better
solution for coherent receivers. To examine the performance of cooled ADCs, first, we design 4 bit 65 nm and 40 nm
CMOS ADCs specifically at 10 K temperature, which commonly is the second stage temperature in dewars. While the
development of 65 nm and 40 nm CMOS ADCs are still on-going, we estimate the ENOB is 2.4 at 10 GSPS,
corresponding to the correlation efficiency, 0.87. The power consumption is less than 20 mW.
NTU-Array is a W-band, dual-polarization, 6-receiver interferometer telescope aiming to detect the cross-over of CMB
primary and secondary anisotropies. The telescope has 34Ghz instantaneous bandwidth for the continuum observation.
The ultra-wide bandwidth is down-converted to four base-bands of 0-8.7Ghz for the ensuing digital correlation. We have
completed the development of an FX digital correlator system for NTU-Array, which utilizes 18Ghz, 1-bit samplers for
digitization and Virtex-4 FPGAs for subsequent digital processing of Fourier transformation and cross-correlation. This
new digital correlator has 275Mhz frequency resolution and is processing in real time the 850 Gbps input data at power
consumption about 1 KW. We stress that our present setup substantially under-rates this FPGA computing machine, as it
is designed to process 2.5 Tbps input data in real time from 18Ghz, 3-bit ADCs. Verification of this new digital
correloator has been completed, and it demonstrates that the correlator can detect small signals with -40db S/N within
one second integration per frequency channel.
NTU-Array is designed for W-band (78-113Ghz) interferometric observations of Sunyaev-Zeldovich effects. The first
phase operation of the telescope with 6 receivers had its first light in 2008 with single-polarization and half the full
bandwidth. The second-phase operation of NTU-Array in Nevada will begin the dual-polarization, full-band observation
in 2010. One-bit sampling at 18Ghz and digital correlation are in use in this telescope. Due to the ultra broadband
coverage, the IF system divides the 35GHz full-band into four 8.7GHz sub-bands. The first stage of IF module
containing a 35GHz broadband amplifier with fairly flat-gain performance over 25db gain divides the first-stage IF into
two outputs. The 2nd-stage IF module further divides the two input IF signals and down-converts them to four
basebands of DC-8.7Ghz. An LO module with 8.7Ghz input is to generate outputs with x2, x3 and x9 harmonics for the
down-conversion. The Walsh function is injected into the x9 LO via an IQ mixer. Each IF baseband is transmitted
through an optical link to the 18Ghz, 1-bit sampling ADC located in the control room. The analog optical link contains a
driver and equalizer to compensate for the path loss. Considering the limited size of the telescope mount, the entire
IF/LO system of each receiver has a compact size about 20cm cubed. This physical size can be further reduced to fit the
future 19-pixel-receiver upgrade of NTU-Array
A wideband correlator system with a bandwidth of 16 GHz or more is required for Array for Microwave Background Anisotropy (AMiBA) to achieve the sensitivity of 10μK in one hour of observation. Double-balanced diode mixers were used as multipliers in 4-lag correlator modules. Several wideband modules were developed for IF signal distribution between receivers and correlators. Correlator outputs were amplified, and digitized by voltage-to-frequency converters. Data acquisition circuits were designed using field programmable gate arrays (FPGA). Subsequent data transfer and control software were based on the configuration for Australia Telescope Compact Array. Transform matrix method will be adopted during calibration to take into account the phase and amplitude variations of analog devices across the passband.