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.