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.
The Combined Array for Research in Millimeter-wave Astronomy (CARMA) have carried out a water vapor
radiometer (WVR) project to test the WVR phase correction technique for better observational effciency. We
have built two uncooled, but temperature-regulated, 22 GHz WVR prototypes to explore the feasibility of the
technique. To better isolate the effects of instrumental and atmospheric instabilities, we have optimized theWVR
design for simplicity with less high frequency components. The calibration system is Dicke switch with a single
ambient load. The thermal regulation system consists of heaters and multi-stage insulation. We have completed
testing of the WVR prototypes in a laboratory and at the CARMA site. The gain stability is about 20-100 mK
and the front-end temperature rms is about a few mK to hundreds, depending on weather conditions. Based on
the site tests, the sky temperature at 22 GHz usually varies a few K in 15 minutes, which is not necessary due
to the atmospheric water vapor. Such short time-scale background temperature variation overwhelms the limit
of the WVR dynamic range. Moreover, we have compared the WVR data rms with the phase monitor at the
site and obtain a scale factor of the 22 GHz water vapor line, 6-12, which is consistent with the results of other
WVR projects. We suggest that expanding the WVR dynamic range with diode detector models and a better
thermal regulation system are keys to the success of the CARMA WVR phase correction.
We report on the current progress of the water vapor radiometer (WVR) phase correction project for the Combined Array for Research in Millimeter-wave Astronomy (CARMA). CARMA is a new millimeter array that merges the Owens Valley Radio Observatory (OVRO) array, the Berkeley-Illinois-Maryland Association (BIMA) array and eventually the Sunyaev-Zel'dovich Array (SZA). WVRs are designed for phase correction by monitoring the water vapor in the atmosphere along the line of sight toward astronomical sources. In addition, we discuss the stability of the current OVRO water vapor radiometers in preparation for testing at the CARMA site. We will systematically analyze the receivers with atmospheric correlations to decouple the effects of instrumentation and atmospheric noise. Finally, we report on the status of the correlation receivers in development.