Specification and Performance

In Tables 1 and 2 we compile the specifications and the actually achieved performance of the APEX FFTS, its ADC/FPGA board and the FPGA/FFT-processing pipeline

ADC input sample rate max 2 GS/s
ADC input bandwidth (-3\,dB) 0-1 GHz
ADC input resolution (quantisation) 8 Bit
Maximum dynamic range (linear) 48 dB
Input full scale range, selectable -22 dB – 16 dB
Ghost free dynamic range > 30 dB
FPGA Data Processing Unit XILINX Virtex 2 Pro 70
PCIbus interface 32-bit, 33 MHz

Table 1: Hardware specifications of the ADC/FPGA board AC240

Fast Fourier Transform (FFT) RFEL, HyperSpeed FFT
FFT - number of frequency channels 16384
FFT - channel separation 61.035 kHz
FFT - Blackman-Harris window 3 term, -61 dB side-lobe
FFT - frequency resolution 98.267 kHz
FFT - arithmetic 2's complement, 18-bit
Digital Half Band Filter (DHBF) 50 – 950 MHz
DHBF - Stopband rejection +48 dB
DHBF - Passband ripple max ±0.1 dB
Power spectra builder (sum of squares) full precision, 33-bit
On-board accumulation (integration) 54-bit precision
Integration - max time (full precision) ~35 s
Integration - min time 32.768 µs
Stability/Allan-Variance: only FFTS ~4000 s
Stability/Allan-Variance: IF + FFTS ~500 s (Fig. 4)
Table 2: Specifications of the FPGA/FFT-processing pipeline

To illustrate the performance we plot in Figures 1 and 2 the results of stability tests, performed pre-shipment at MPIfR and after installation at the telescope.

Figure 1 presents the continuous decrease of the accumulated noise vs. integration time, which followed the radiometer formula, i.e. 1/√t was, for nearly two days of operation.

Figure 2 displays an Allan-Variance plot of the spectrometer (incl. now the IF unit) as operated at the telescope, emphasizing the remarkable stability of both, the APEX IF and FFTS, on a timescale of ~500 s.

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