Pulsar Observing with the Spectral Processor

The Spectral Processor is an FFT spectrometer primarily designed for high time resolution pulsar observations. Because of its wide dynamic range it is also useful for spectral line observations at low frequencies where strong interference is a problem. It contains two FFT engines, each with 1024 channels over a maximum bandwidth of 40 MHz which may be divided into 1, 2, or 4 separate passbands. The two FFT engines are synchronous and their outputs may be cross-multiplied to measure polarization. The most commonly used types of observing with the spectral processor are pulsar timing and either total power or frequency switched spectral line.

While the spectral processor can support many IF configurations, the most popular IF configuration for it is 4 IF inputs of 20 MHz bandwidth each with 2 IFs of opposite polarization processed in a single FFT engine. With this configuration, one could still observe at a reduced total bandwith of 20 MHz if a hardware failure disabled one of the FFT engines. The time-frequency data matrix for this configuration is typically 256 frequency channels across an IF input with 128 time bins spread over a pulsar period. However, the machine's memory is capable of accommodating up to 512 time bins. Single pulse studies can also be made with the spectral processor using its dedispersed time series mode of data accumulation.

Table 1 gives the general specifications for the spectral processor, and Table 2 shows the possible combination of number of IF's, frequency channels per IF, and bandwidth per IF.

Table 1 :Spectral Processor General Specifications
Number of spectrometers (FFTs) 2
Number of IFs per spectrometer 1, 2 or 4
Number of frequency channels per spectrometer 1024
Total bandwidth per spectrometer 40 MHz
Shortest time resolution 12.8 us
Accumulation memory per spectrometer 256K, 32 bit
IF unwanted sideband rejection >30 dB
Spectrum dynamic range to narrowband signal >45 dB with taper
Input A/D dynamic range 10 dB over system noise power
Sensitivity : Average over 1 frequency channel 0.77
Wideband 1.0
Input IF range 70 MHz - bandwidth to 500 MHz + bandwidth

Table 2 : Spectral Processor Available IF's/Channels/Bandwidth Combinations
Single IF Banwidth IFs per FFTS/frequency channels per IF
  1/1024 2/512 2/256 4/256
40 MHz X      
20 X X X  
10 X X X X
5 X X X X
2.5 X X X X
1.25 X X X X
0.625 X X X X
0.3125 X X X X
0.15625 X X X X
0.078125 X X X X