The Spectral Processor is located in the electronics equipment room (206). It consists of numerous components of varying complexity. A depiction of the spectral processor racks is shown in Figure 4. Of the spectral processor's numerous components, the more significant ones are described below. Please see the paragraph on technical references for further details.
Figure 3
Seven of these units are included. Their purpose is to accept intermediate frequency (IF) (i.e., the difference between the received astronomical radio source signal and that generated by the Local Oscillator) inputs in the range of 70-500 MHz and convert them down to baseband in two stages. Since the system is vulnerable to impulsive RFI, detectors are employed in the two spectral processor racks.
Four of these units are included. The radio signals collected by the telescope's front end receivers arrive in an analog form. The A/D's convert these signals to a digital format before being transferred to the Window Multipliers for further processing.
Before entering the FFT section of the spectral processor, the data is weighted by a window or "taper" function. In this stage of the device, a weighting factor is applied to data sampling before it's sent to the FFT's. This is done in order to optimize the spectrometer's frequency resolution and immunity to strong RFI.
Fourier Transform refers to a mathematical tool used by scientists and engineers to alter a problem into one that can be more easily solved. In our case looking at astronomical data, this means converting time domain data to frequency domain data to produce a voltage spectrum. The FFT's in the spectral processor are the actual spectrometers that make up this complex system. Two are incorporated. Each is designed to accept complex data samples and turn them into 1024 frequency channels. The mathematics involved in FFT processes is very complex. If needed, please refer to published literature on this subject for a deeper understanding.
This component is responsible for computing the intensity of complex vectors for each frequency channel before they're sent to the accumulator. In simple terms, in the spectrometer mode of operation, complex voltages are turned into power values. In a polarimetry mode, things called cross products are produced which are in turn used to generate Stokes parameters. (Note: . Stokes parameters relate the amplitudes of components of an electric field and are a set of values used to specify the phase and polarization of radiation. These parameters provide an astronomer a very useful description of the polarization state of an electromagnetic wave).
This is the most complex part of the digital hardware as it's responsible for "accumulating" and managing an incoming high-speed data stream. The accumulator serves an additional purpose in that it may delete data that contains pulsed wideband RFI. Two accumulators are installed to handle the spectral processor's 1024 output channels in the following ways.:
By accumulating data in one 1024-word section of memory for a simple total power spectrum.
By switching between several 1024-word address ranges (corresponding to signal, reference, cal on, and cal off phases) in synchronous.
By accumulating data using its full memory range with address switching synchronous with a pulsar.
By mapping data into memory using a technique of time/frequency dedispersion to produce a time sample series for the spectrometer's full passband.
A Motorola 167 single-board computer controls the spectral processor's hardware. This computer takes scan setup information (entered via a Sun workstation using Observer-Operator interface software) reformats it and loads it into the hardware, starts and stops data flow, and informs the workstation of the current scan status.
General specifications concerning the Spectral Processor are described in the folowing Table.
| Number of spectrometers (FFT's) | 2 |
| Number of IF's 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, 32bit |
| 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 |
| Accumulation modes | Synchronous
with front-end switches
Synchronous with pulsar Doppler tracking Dedispersed time series |