Abstract
W-Band radiometers using intermediate frequency down-conversion (super-heterodyne) and direct detection are compared. Both receivers consist of two W-band low noise amplifiers and an 80-to-101 GHz filter, which conforms to the reception frequency band, in the front-end module. The back-end module of the first receiver comprises a subharmonic mixer, intermediate frequency (IF) amplification and a square-law detector. For direct detection, a W-Band detector replaces the mixer and the intermediate frequency detection stages. The performance of the whole receivers has been simulated requiring special techniques, based on data from the experimental characterization of each subsystem. In the super-heterodyne implementation a local oscillator at 27.1 GHz (with 8 dBm) with a x3 frequency multiplier is used, exhibiting an overall conversion gain around 48 dB, a noise figure around 4 dB, and an effective bandwidth over 10 GHz. In the direct detection scheme, slightly better noise performance is obtained, with a wider bandwidth, around 20 GHz, since there is no IF bandwidth limitation (~15 GHz), and even using the same 80-to-101 GHz filter, the detector can operate through the whole W-band. Moreover, W-band detector has higher sensitivity than the IF detector, increasing slightly the gain. In both cases, the receiver performance is characterized when a broadband noise input signal is applied. The radiometer characteristics have been obtained working as a total power radiometer and as a Dicke radiometer when an optical chopper is used to modulate the incoming signal. Combining this particular super-heterodyne or direct detection topologies and total power or Dicke modes of operation, four different cases are compared and discussed, achieving similar sensitivities, but better performances in terms of equivalent bandwidth and noise for the direct detection radiometer. It should be noted that this conclusion comes from a particular set of components, which we could consider as typical, but we cannot exclude other conclusions for different components, particularly for different mixers and detectors.
Highlights
In W-band, there are two main areas of interest for radiometric systems: one of them features scientific applications that include observations of the Earth, radio astronomy [1,2]and spectroscopy [3]; the other one includes the security control applications and low visibility situations through the acquisition of millimeter wave images [4,5]
The sensitivity of the radiometer is established by the temperature of the lowest detectable source, which is generally bounded by the noise fluctuations that appear at the output of the receiver for a null input
The total power radiometer developed in the super-heterodyne version is divided into three main parts: a front-end module with low noise amplification and band-pass filtering in W-band (RF), an intermediate frequency section (IF) with amplification, and, quadratic-law diode detection
Summary
In W-band, there are two main areas of interest for radiometric systems: one of them features scientific applications that include observations of the Earth, radio astronomy [1,2]. The very low noise receivers used in radio astronomy are radiometers that require high stability and sensitivity and allow to measure the electromagnetic emission of a body, distinguishing small levels of signal power [7,8,9]. The radiometric sensitivity, defined as the minimum detectable temperature change of the antenna expressed in Kelvin, at the radiometer output depends on the system thermal noise Tsys , the Equivalent Noise Bandwidth of the High Frequency section, BHF (Hz), and the equivalent bandwidth of the Low Pass filter or integrating circuit (BLF ). Direct detection radiometers may provide a simple solution as long as high-frequency, low-noise amplifiers, filters, and detectors are available.
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