Observing Bit Mode

There are 3 available bit modes available while observing with the station. 4, 8 and 16-bit, each of which describe the size of the signed word of each beamlet sample. 8-bit is the standard mode used for observations with I-LOFAR.

Adjusting the bit mode of the station varies the number of beamlets (pointings, frequencies) that can be formed at a given time, allowing for a trade off between digitisation accuracy and observing bandwidth.

Bitmode	Beamlets	Bandwidth (MHz)
16	244	47.65625
8	488	95.3125
4	976	190.625

4-bit Mode

4-bit mode is rarely used as unlike 8-bit mode, during the reduction operation from 8-bit to 4-bit data is not fully reduced to fit in the -8,7 range, resulting in samples being clipped. This can be compensated for by increasing the RCU attenuation, but requires analysis on a source-by-source basis. Data produced in this mode needs to be extracted in a special way, as each sample takes up the upper or lower half of a single byte.

When used correctly, it can allow for an extremely large sky coverage, or observations across multiple observing modes.

8-bit Mode

8-bit mode is the standard for all HBA and most LBA observations at I-LOFAR, offering up a 95MHz bandwidth for observers to code an entire observing mode with the 200MHz clock (excluding parts of the Nyquist zones at 100 and 200MHz).

16-bit Mode

16-bit mode is sometimes used for LBA observations to slightly increase the sensitivity at lower frequencies. This is often a useful tradeoff due to the significant amount of RFI present below 30MHz from ionospheric reflections and local sources (AM radio, FM radio about 80MHz, etc).