A DESIGN OF A DIGITALLY CONTROLLABLE WIDEBAND MICROWAVE RECEIVER

Radar echo sounders provide a safe, inexpensive and effective means of obtaining ice sheet thickness. As the roughness of ice surface/subsurface depends on the radio wavelength, wideband radar sensors can provide flexibility for ice thickness measurement under areas with various surface conditions. This paper presents the design of a digitally controllable wideband microwave receiver for a potential radar sounding system. Its radio frequency (RF) frequency ranges from 50 to 500 MHz, while the intermediate frequency (IF) bandwidth is 20 MHz. The receiver provides eight channels for different RF band choices, as well as a number of convenient gain settings. Testing measurements have also been conducted to verify the design requirements. INTRODUCTION Recent scientific researches have reported the global sea level rise over the last century. Mass balance of polar ice sheets is actively studied to assess their contributions to global sea level rise. Radar echo sounders provide a safe, inexpensive, effective means of obtaining ice thickness [1]. Over the past several decades, extensive ice thickness data sets have been collected by NASA, USGS and other institutions [2] [3]. However, current radar systems often fail to measure ice thickness on rough surface areas that typically are areas of change and interest [4]. Heavy crevassing at the ice sheet surface can increase scattering loss and absorbing attenuation of the radio signals transmitted. Basically, the roughness of ice surface/subsurface depends on the wavelength of radar sensors. Wideband microwave sensors provide flexibility for ice thickness measurement under areas with different surface/subsurface conditions. Therefore, polar research scientists may take advantage of wideband microwave receivers with convenient control of different RF band in ice sheet study. The objective of this work is to design and prototype a wideband receiver for a potential radar sounding system. Testing measurements have also been conducted to verify the design requirements. This paper describes these efforts including receiver specification, design simulation, prototyping, and testing measurements in detail. DESIGN REEQUIREMENTS The receiver is a critical component of a wireless system, having the overall purpose of recovering the desired signal from a wide spectrum of interference and noise. Figure 1 presents the basic operations of a typical radar sounding system. The radar repeatedly transmits pulses and receives their delayed copies modified by the ice surface/subsurface. After sampling and quantization, data are stored for further signal processing. Figure 1. Operations of a typical radar sounding system Our study focuses on the receiver design which includes major requirements such as high gain, wide RF range, down-conversion, digital gain control and band switch, low distortion and noise level. • High Gain In order to increase the low power of the antenna-received signal to some level suitable for the analog-digital converter, this receiver design requires a high gain up to 80 dB in some modes. Since the antenna received signal strength may change according to the transmitted signal power and different backscattering conditions [5], a certain range of gain settings is also needed to handle signal strength variation. Thus, a system gain range is desired to be digitally controllable by 40 dB (i.e., 40 ~ 80 dB). • Wide RF Frequency Range Since wideband sensors can provide measurement flexibility under different surface conditions, a wide RF frequency range from 50 to 500 MHz is specified in this design. These frequencies represent a subset of airborne ice sounding radar frequencies [5]. • Down-Conversion The antenna received RF signal is down-converted to baseband for sampling and quantization. This design specifies a baseband IF frequency of 20 MHz, which enables a theoretical range resolution of 7.5 meters (Equation 1) in air using a pulse system [5]. This resolution is adequate since the ice sheet thickness may be up to several Kilometers. Range Resolution ) ( 5 . 7 ) 10 20 ( 2 10 3 2 6 8 m B c = × × × = = (1) where c is the velocity of light in the air, B is the IF bandwidth. • Digital Gain Control and Band Switch Different gain settings and band frequencies need to be digitally controllable. The gain resolution is required as fine as 1 dB. Eight different channels are available for digital RF band switch, because of potential multi-antenna applications. • Low Distortion A low signal distortion is desired. The maximum spurious signal is required to be 35 dB down from the expected signal. • Low Noise Level Output noise level is required to be far below the maximum receiver output signal. A noise figure of 7 dB is required at the highest gain setting. • Compact Dimension The size requirement is 10 10 10 × × inches for the receiver prototype. The weight of the receiver shall be less than 10 pounds. RECEIVER DESIGN AND PROTETYPE A block diagram of this receiver design is given in Figure 2. ‘RFIN’ is the RF input port that the RX antenna signals connect. Digital on/off control is determined by “Blank” signal. Various gain setting can be achieved by the combination of “Bypass1”, “By pass2” and “A1” through “A16” signals. Band switch is controlled by “SWA1” to “SWA3” and “SWB1” to “SWB3” signals. “LO” frequency from outside oscillator provides the down conversion function via the mixer. Low noise amplifiers are used in the front end RF band to lower the overall noise figure. Seven operational amplifiers are cascaded in IF band to ensure both high gain and small distortion. Figure 2. Functional block diagram of receiver design Appropriate chip selection and printed circuit board (PCB) layout design have been carefully conducted. Micro-strip transmission line with Ω 50 characteristic impedance is used in PCB design [6]. Two printed circuit boards are prototyped. One board mainly works in RF band, while the other works in IF band. Two boards are mounted into aluminum chests and attached with I/O connectors. Figure 3 and 4 shows the final prototype with dimension of 4 5 6 × × inches and 5 pounds weight for each board. Figure 3. RF board in the chest with connectors Figure 4. IF board in the chest with connectors The receiver performance is also simulated theoretically according to device specifications. Specifically, the simulations include the receiver specifications such as gain settings, minimum detectable signal, noise figure, third order intercept point, spurious dynamic range and receiver dynamic range. TESTING MEASUREMENTS AND CONCLUSION A variety of testing measurements have been carried out to verify the design requirements. These measurements can be classified to three main categories: (1) receiver characteristics, (2) distortions, and (3) noise. Receiver characteristics include gain, mixer characteristics, filter response, switch characteristics and power consumption. Distortion measurements contain single-tone response and two-tone response. Noise test measures the system noise figure. Figure 5 and 6 displays the gain testing results in maximum gain (i.e., 80 dB) and minimum gain (i.e., 40 dB) modes. Due to the devices’ gain ripples and the transmission line loss, the system gain decreases about 3 dB between 50 MHz and 500 MHz. A compensation filter can be applied to the RF board to achieve a flat gain. The compensation filter is designed to have the opposite gain slope as the RF board. Therefore, the combination of them gives a flat gain over the function bandwidth.