A/D Converter and Digital Correlator
Below is a description of the digital section of the receiver.
The signals from the analog amplifier chain are sent to the digital part of the receiver.
The digital processor is composed of a mixer that converts the radio frequency (RF) signal to intermediate frequency (IF), analog-to-digital converters (ADC), and a field-programmable gate array (FPGA). The FPGA performs filtering, windowing, a 256-channel Fast Fourier Transform (FFT), and complex multiplication of the two polarizations, followed by summing of the correlation products. A single clock generator (SI5332H) provides the reference clock signal.
The downconversion and analog-to-digital conversion are performed by two LMS6002 by Lime Microsystems. The combined gain of the LMS6002 (built-in LNA, VGA1, and VGA2) is about 35 dB.
The incoming digitized samples are processed on a Xilinx Artix FPGA, which is a type of field programmable gate array. The processing includes the following steps:
Detection of any outliers in the samples, which are identified as those with either too high or too low voltage. This is done by using a dynamically adjusted threshold, and if the number of outliers exceeds a specified threshold limit, the entire frame is discarded.
Application of a Blackman window function on each frame of 256 samples.
Fast-Fourier Transform is applied on each frame, converting it from the time domain into the frequency domain.
Auto and cross-correlation products are calculated in the frequency domain.
The sum of 20,000 frames is formed to create a single integration, with the RF switch connected to the antenna, and an additional 20,000 frames of the calibration signal. This process takes around 700 ms but can take significantly longer depending on how many frames are discarded in the first step.
The correlator has three complex multipliers and four adders (integrators) at its core. Two complex inputs from channels 1 and 2 (horizontal and vertical polarization) form four real correlation products, which we call HH, VV, HV, and VH.
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