2.5. HIFI Spectrometers

The HIFI instrument provides an IF bandwidth of 4GHz in all bands except for band 6 and band 7 (1408-1908GHz) where only 2.4GHz bandwidth is available. To sample this bandwidth, HIFI has 4 spectrometers. A Wide Band Spectrometer (WBS) and High Resolution Spectrometer (HRS) are available for each of the polarizations. All spectrometers can be used in parallel, although at fast data rates it is necessary to reduce how much is readout and stored since, at the highest data rates, the spectrometers provide data at a rate that is higher than the bandwidth available to HIFI on board the spacecraft.

The WBS is an Acousto-Optical Spectrometer (AOS) able to cover the full IF range available (4GHz) at a single resolution (1.1MHz). The HRS is an Auto-Correlator System (ACS) with several possible resolutions from 0.125 to 1.00MHz but with a variable bandwidth that can cover only portions of the available IF range. The HRS can be split up to allow the sampling of more than one part of the available IF range.

In the following two subsections, we describe the main workings of the two spectrometer types available to HIFI.

2.5.1. The Wide Band Spectrometer (WBS)

The WBS is based on two (vertical and horizontal polarization) four channel Acousto-Optical Spectrometers (AOS; see [8]) and includes IF processing and data acquisition. To cover the 2 x 4-8 GHz (2 x 2.4-4.8GHz for bands 6 and 7) input signals from the FPU, two complete spectrometers (horizontal + vertical polarization) are used. For redundancy reasons both spectrometers are fully independent.

Each spectrometer receives a pre-amplified and filtered IF-signal (4-8 GHz). After further amplification in the WBS electronics, the signal is split into four channels which provide the input frequency bands for the WBS optics (4 x 1.55-2.65 GHz; IF1 to IF4). The signal is further amplified and equalised (using variable attenuators), to compensate for non-uniform gain of the system, before being sent to two Bragg cells in the optics module of the WBS.

The other necessary input is to provide a frequency reference signal for the frequency calibration of WBS spectra. This is done using a 10 MHz reference signal from the Local Oscillator Source Unit (LSU), is fed into the WBS to provide a "comb" signal. The comb signal in the WBS, with regular stable 100 MHz line spacing, can be connected for frequency calibration purposes or it can be disabled to provide a zero level measurement of the AOSs. The zero allows allows more precise system temperature measurements to be made.

In the optics section of the WBS, the pre-processed IF-signal from the mixers is analysed using the acousto-optic technique. The IF-signal is fed into a Bragg cell via a transducer. The IF-signal then generates an acoustic wave pattern in the Bragg cell crystal. A laser beam which enters the Bragg cell is diffracted according to the acoustic wave pattern in the Bragg cell crystal. The diffracted laser light is afterwards detected by four linear CCDs with 2048 pixels each and each covering approximately 1GHz bandwidth. Four vertically aligned Bragg cells and CCD chains are necessary to cover the full 4 GHz IF bandwidth of HIFI.

The WBE electronic section has 4 analogue line receivers for the 4 CCD video signals. These signals are fed to 14 Bit analogue to digital converter with a conversion speed corresponding to less than 3 ms. The relatively high number of ADC-Bit is meant to keep differential non-linearity effects to a very low level. Overall non-linearity in the WBS is very low, less than 1%..

Continuous data taking is possible without dead time during data transfer, as long as the integration time is above 1 sec -- which is true for all standard operating modes of HIFI.

Every 10 ms the collected photoelectrons in the CCD photodiodes are shifted into a register and clocked out serially. After integration completion, the data can be transferred while a new integration is started. Data is transmitted to the Instrument Control Unit (ICU) with 16 or 24 Bits through a serial interface with 250 kHz clock rate which is synchronous with the CCD read-out clock. Housekeeping data is provided through the same interface. A second serial interface is used for the command interface.

2.5.2. The High Resolution Spectrometer (HRS)

With the HRS, high resolution spectra are available from any part of the input IF bandwidth (4GHz, or 2.4GHz in band 6 or 7). The HRS is an Auto-Correlator Spectrometer (ACS) that can process simultaneously the 2 signals coming from each polarization of the FPU. It is composed of two identical units: HRS-H and HRS-V. Each of which includes an IF processor, a Digital Autocorrelator Spectrometer (ACS) and associated digital electronics, plus a DC/DC converter (not discussed here). The HRS provides capability to analyse 4 subbands per polarization, placed anywhere in the 2.4 or 4 GHz input bands coming from the Focal Plane Unit (FPU). The two units of the HRS can be used to process the same 4 sub-band frequency ranges in each of the two polarizations provided by the FPU, thereby reducing the integration time and providing redundancy. Both units of the HRS operate at the same time and it is possible to look at either IF with each of the HRS spectrometers.

2.5.2.1. Overview of the HRS Subsystem

In each HRS unit the ACS processes the signals coming from its associated IF (see [9]). Each 230 MHz band width input is digitized by a 2 bit / 3 level analogue to digital converter clocked at 490 MHz. The digital signals are analysed with a total of 4080 autocorrelation channels. It is possible to configure the HRS to provide 4 standard modes of operation as given in Table 3.2, “List of HRS configurations available in each polarization ”. For example, in its nominal resolution the HRS proves two sub-band spectra each of which have a bandwidth of 230 MHz, each of which is covered by 2040 channels and has a spectral resolution of 250kHz.

It is possible to set each sub-band frequency independently anywhere in the 4 GHz IF band range.

Two buffers are used, with selection synchronised with the chopper position by the ICU. The HRS has a maximum chopping frequency of 5 Hz. The data can be accumulated in each buffer up to a maximum of 1.95 seconds. The data readout duration is about 42 ms. Data can be read out from one buffer while data accumulation occurs on the other.

2.5.2.2. Modes of HRS Operation -- Wide Band Mode

In the wide band mode all 4080 correlation channels of the ACS are used to analyse the 8 input signals. As the input signals are adjacent two by two, 4 sub-bands of each of 460 MHz bandwidth can be analysed in this mode. The four sub-bands can be independently placed anywhere in the IF bandwidth range. It is possible to analyse almost the whole 4 GHz input IF bandwidth by selecting the same polarization in the two HRS units and by setting the lose to have adjacent sub-bands.

In this mode, with a Hanning windowing of the correlation function, the spectral resolution is 1000 kHz. The total band-width per HRS unit is 2 GHz.

In each correlator ASIC one channel is dedicated to compute the analogue signal offset.

2.5.2.3. Modes of HRS Operation -- Low Resolution Mode

In the low resolution mode the 4080 correlation channels are used to analyse 4 of the 8 input signals of 230 MHz band width each. The four sub-bands can be independently placed anywhere in the IF bandwidth.

In this mode, with a Hanning windowing of the correlation function, the spectral resolution is 500 kHz. The total band-width per HRS unit is 1 GHz.

In each correlator ASIC one channel is dedicated to compute the analogue signal offset.

2.5.2.4. Modes of HRS Operation -- Nominal Resolution Mode

In the nominal resolution mode the 4080 correlation channels are used to analyse 2 of the 8 input signals of 230 MHz band width each. The two sub-bands can be independently placed anywhere in the IF bandwidth range.

In this mode, with a Hanning windowing of the correlation function, the spectral resolution is 250 kHz. The total band-width per HRS unit is 460 MHz.

In each correlator ASIC one channel is dedicated to compute the analogue signal offset.

2.5.2.5. Modes of HRS Operation -- High Resolution Mode

In the high resolution mode the 4080 correlation channels are used to analyse 1 of the 8 input signals of 230 MHz band width each. The sub-band can be placed anywhere in the IF bandwidth range.

In this mode, with a Hanning windowing of the correlation function, the spectral resolution is 125 kHz. The total band-width per HRS unit is 230 MHz.

In each correlator ASIC one channel is dedicated to compute the analogue signal offset.