Frontend operations

General rules

To change the frontend Local Oscillator frequency, use the following command:

> setLO=[freq1];[freq2];…;[freqN]

Notice the semicolon. Ideally, different values could be assigned to different IFs, thus tuning each section to a different sub-band. For the TPB, though, this is not possible, so a single value must be specified:

e.g. ``> setLO=5600``

Remember that the actually observed band begins at a frequency which is usually different from the LO one (see Antenna overall setup)

For the L-P band receiver, filters are available in order to set the observed sub-band and select the polarisation (circular or linear), specifying a code with the command:

> receiversMode=[code]    (see the next section for the details)

The temperature of the calibration mark in use is recorded in the logfile whenever a Tsys is measured. It is also stored in the FITS/MBFITS output files. The calibration mark can be manually switched on and off respectively with:

> calOn
> calOff

If users want to perform the setup for the frontend only (without affecting the mount, the minor servo or the backend), the command is:

> receiversSetup=[code]   (PPP, LLP, PLP, CCB, KKG)

Specific notes for the L/P receiver

The L/P receiver is actually seen by DISCOS as a group of three different receivers, each having its own setup code:

  • PPP for the usage of the P band only
  • LLP for the usage of the L band only
  • PLP for the dual-feed usage

These are the codes to be used in the setupXXX command. Then the configuration of the band filter and of the polarisation mode must be performed, by means of the receiversMode command. Here follows the list of options.

PPP

Only the P band can be used. The possible configurations are:

> receiversMode=C1XX
> receiversMode=C2XX
> receiversMode=C3XX   (default)
> receiversMode=L1XX
> receiversMode=L2XX
> receiversMode=L3XX

The first two characters in the code identify the polarisation mode C = Circular, L = linear) and the band filter index, where:

  1. all band, 305-410 MHz (no filter)
  2. 310-350 MHz
  3. 305-410 MHz (band-pass filter)

The XX chars are placeholders for the L-band configuration, which is not allowed in PPP mode.

LLP

Only the L band can be used. The possible configurations are:

> receiversMode=XXC1
> receiversMode=XXC2
> receiversMode=XXC3
> receiversMode=XXC4   (default)
> receiversMode=XXC5
> receiversMode=XXL1
> receiversMode=XXL2
> receiversMode=XXL3
> receiversMode=XXL4
> receiversMode=XXL5

The last two characters in the code identify the polarisation mode (C = Circular, L = linear) and the band filter index, where:

  1. all band, 1300-1800 MHz (no filter)
  2. 1320-1780 MHz
  3. 1350-1450 MHz (VLBI)
  4. 1300-1800 MHz (band-pass)
  5. 1625-1715 MHz (VLBI)

The XX chars are placeholders for the P-band configuration, which is not allowed in LLL mode.

Local Oscillator for L-band observations

Starting from the release of DISCOS 0.6, it is possible to use the setLO command for this receiver, in the operator input console:

> receiversSetup=LLP
> setLO=2300

This new feature is only available using the L-band feed (it is not possible to set the LO in the PPP configuration).

It is not allowed to set the LO inside the observed sky band. For instance:

> receiversMode=XXL5    (Bandwidth 1625:1715)
> setLO=1650
Error - Value out of legal ranges

It is also not possible to set a receiver mode in case the current LO value is inside the mode band:

> receiversMode=XXL5    (Bandwidth 1625:1715)
> setLO=1500            (Ok, it is outside the RF band)
> receiversMode=XXL4    (Bandwidth 1300:1800)
Error - Value out of legal ranges

Notice that, for this receiver, the LO frequency is higher than the observed frequency. For this reason the bandwidth value displayed in the Receivers monitor is positive while the IF is negative, indicating that the IF band is inverted. For instance, in the following case:

> receiversMode=XXL5    (Bandwidth 1625:1715)
> setLO=2300

the receiver panel will show startFreq=-675 and bandwidth=90. The band initial frequency can be obtained as LO+startFreq.

A 1000 MHz low-pass filter is applied to the IFs, so it is not possible to set a LO value that brings the IFs outside the range of the low-pass filter:

> receiversMode=XXL4    (Band 1300:1800)
> setLO=200             (IF band: 1100:1600)
Error - Value out of legal ranges

IF distributor

The SRT has a new device (IFDistributor) capable to perform down conversion and IF filtering when the L-band receiver is configured. The control of the local oscillator and the down conversion is transparent and can be done as already described. In particular, all the caveats regarding the limits still hold. In case a filtering of the IF bandwidth is required (e.g. when observing with the DFB3) the command ifd must be used:

> ifd=[filterid]
FILTER ID Central Freq (MHz) Bandwidth (MHz)
BW-NARROW 576 115
BW-MEDIUM 640 230
BW-WIDE 768 460
BW-UNFILTERED see L-Band  

PLP (dual band)

In this setup, both bands are used and can be configured, according to the following scheme, based on the permutation of all the previously described codes:

> receiversMode=C1C1
> receiversMode=C2C1
> receiversMode=C3C1
> receiversMode=C1C2
...
> receiversMode=C3C4  (default)
> receiversMode=C3C5
...
> receiversMode=L1C1
...
> receiversMode=C1L1
...
> receiversMode=L1L1
...