Frequency and Subband selection

Major Observing modes

Signal Path

Antennas Description

Station Description and Configuration

Array Configuration

Imaging Capability and Sensitivity

Frequency and Subband Selection

Beam Definition 

Transient Buffer Boards

Data Products and Management

CEP facilities

System notes

 


 

Frequency Definition 

The combination of analogue and digital signal processing both at station level as well as at the Central Processing facility allows for the users a flexible definiton of the total frequency range, subband selection and spectral channel width.

The design of the A/D system allows for RFI detection of both broad- and narrow band features.

 

Setting the Observing Frequency

The Receiver Units (RCUs) which convert the antenna voltages to base-band frequencies are desinged to be sky noise limited.  For this, a 12 bit Analogue to Digital converter along with multiple bandpass filters is used to select the frequency bands of LOFAR.

Two sampling clock frequencies, at 200 MHz and at 160 MHz,  have been selected in the design to determine the standard obseving frequency bands shown below:

Freq. range (MHz)Clock (MHz)ArrayNyquist zoneRemarks

10—90

200

LBA*

1

Massive interference below 20 MHz during daytime.

10—70

160

LBA*

1

Aliasing of 80—90 MHz range into 80—70 MHz

[This setup is not normally used in operations]

30—90

200

LBA*

1

Short wave radio suppressed. There may however still be some intermodulation products from shortwave radio during certain ionospheric conditions.

30—70

160

LBA*

1

Aliasing of 80—90 MHz range into 80—70 MHz

[This setup is not normally used in operations] 

110—190

200

HBA

2

HBA band with the least interference

170—230

160

HBA

3

 

210—250

200

HBA

3

This band contains many digital audio stations. Final frequency selection should be done in close collaboration with observers and science support staff.

Table 1: Frequency Ranges

LOFAR Frequency Bands

 

Selecting the frequency/subband range

After digitisation from the RCU boards, the signal is further processed in the Remote Station Processing (RSP) boards, which use Field Programmable Gate Arrays (FPGAs).

Initially, the total bandwidth of the digitized signal (100 MHz for the 200 MHz clock and 80 MHz for the 160MHz clock) is split in 512 sub-bands via a poly-phase filter (PPF), followed by a 1024-point Fast Fourier Transform. 

These subbands have bandwidths of 195.3 kHz and 156.2 KHz, respectively for the 200 MHz and 160 MHz clock. This is shown in Table 2.

 

Up to 488 subbands of 0.195 MHz (195.312 kHz at 200 MHz clock) and 0.156 MHz  (156.250 kHz at 160MHz clock) can be used

This gives a total bandwidth of 95.16 MHz (200 MHz clock) or  76.13 MHz (160 MHz clock).  

 

Clock Total BandwidthSub-band width Channel width (256 channels)

(MHz)

(MHz)

(Hz)

(Hz)

200

95.16

195312.5

767.939

160

76.13

156250.0

610.352

Table 2: Subband and Spectral Line Channel Bandwidths

Following subsequent processing in the Central Processing Faciity, to realign the data streams in time, a poly-phase filter is applied to re-sample the data to the kHz level, splitting each sub-band into a fixed number of frequency channels.

 

Typically, 64 channels per subband have been routinely used. RFI excision performs well with this setup and, unless users can convincingly argue that more channels are needed for their work, observations will be perfomed with this setup. For spectral line work, up to 512 channels have been tested and proven to work in interferometric mode, therefore such a setup can be accommodated. However, note that such configuration has not been characterized. The user must take into account that the number of spectral channels proportionally increases the raw data size and the subsequent processing speed, therefore these considerations should be made when applying for processing time for the next round of proposals. 

 

 

Subband Naming Convention

Users will have to select the subbands they wish to observe when proposing and at the time of definition of the proposed observations.

In the convention of the observing system, each subband is identified by a number. This number is related to the sky frequency through the following formula:

The number of the sub-band S containing a certain frequency ν is given by

where n is the Nyquist zone (see Table above), νclk is the clock frequency (200 or 160 MHz), and ⌊ and ⌋ denotes the "floor" function.

Conversely, the central frequency of sub-band S is given by:

Because the sub-band separation is done using fixed poly-phase filters on a time series that is recorded at a fixed clock rate, LOFAR does not perform any Doppler tracking. That is, the frequencies are observed in a telescope based reference frame, not a geocentric, solar system barycentric, or sky frame. Such conversions should be performed offline by the PI, if necessary. 

Some frequently used examples of subband selections are listed below:

 
LBA frequency

subband name

clock
 30-78 154-397
 -
 HBA frequency
subband name clock
115-163
77-320
200
148-196 245-488
200
180-210
128-371
160
210-250
52-255
200

 Table 3. Naming convention of Recommended subbands

Interference in the LOFAR frequency bands

Following are typical plots of the RFI situation in the LBA and HBA bands. The HBA has been split into three parts (HBA low, mid and high), which were observed independently. Parts of the HBA high range (around 225 MHz) are contaminated by several broadband digital broadcast transmitters. Towards lower frequencies in the LBA band (below 30 MHz), the data are more affected by interferences.

 

 

 

Station Clock

The second clock system consists of a rubidium maser controlled by a GPS clock. Each station has its own independent GPS-controlled rubidium clock. In that sense, LOFAR operates by default in "VLBI mode", even on short baselines.

Since May 2010, the stations of the "superterp" CS002, CS003, CS004, CS005, CS006 and CS007, share a common clock. Since September 2012, the single clock has been extended to all Core stations. Such array is suitable for tied-array observations because no clock corrections are needed at the correlator.

Combining other stations into a tied-array will lead to decorrelation of the signal because the tied-array software currently does not correct for clock offsets and the offsets are comparable to the wavelength in the high band.

 

 

Time offsets between two independent GPS controlled rubidium masers during the course of 60 hours.

During a 2.5 day long test run, the time offsets between two stations using two GPS-stabilized rubidium masers, had an RMS variation of 3.5 ns with peaks of the order of 10 ns. These time differences correspond to path length differences of 1 to 3 m respectively.

More information on the clock system can be found in LOFAR Report 057 (pdf) by Andre van Houwelingen and Gijs Schoonderbeek.

 

Design: Kuenst.    Development: Dripl.    © 2014 ASTRON