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.
KEYWORDS: Field programmable gate arrays, Optical correlators, Signal processing, Clocks, Digital signal processing, Information operations, Telescopes, Signal detection, Logic, Receivers
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.
KEYWORDS: Receivers, Optical amplifiers, Mirrors, Telescopes, Transmitters, Analog electronics, Interferometers, Interferometry, Linear filtering, Digital signal processing
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
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