===== Signal paths to the spectroscopic, VLBI, and pulsar backends =====

There are several IF chains going to the Faraday room (where the backends live), most importantly the VLBA and narrow-band (SB) IF. Furthermore, the 21-cm 7-feed system has its own IF; and some receivers provide a 2+ GHz IF over optical fibers. All IFs are usually used with different bandwidth setups and need different attenuation to bring the power to a level which is appropriate for the backend ADCs. This is done using the so-called **MultiFiBa**. 

The various IFs coming from the receivers are fed into the 16-channel MultiFiBa, which allows to apply several internal bandpass filters and attenuation. Channels 1-7 and 9-15 is almost exclusively used for the 21-cm 7-Beam system. All other receivers are attached to channels 8+16. For each channel the **mode** can be changed with the command

<code>SXXNNNZ</code>

where XX stands for the channel which one would like to change and NNN is a three-byte code for the mode. XX can be 01 to 16, or 99 (for all channels. **Note, that the MultiFiBa got a very slow processing unit, so sometimes it may take a while until the requested mode is set.** Typical modes are:

^Mode^Channels^IF name^IF range^Bandwidth^Attenuation^Comments^
^ ^ ^ ^[MHz]^[MHz]^enabled^ ^
|216|8+16|VLBA IF|700-800|100|yes| |
|185|8+16|VLBA IF|500-1000|500|yes| |
|000|8+16|Narrowband (SB)|100-200|100|yes| |
|006|8+16|Narrowband (SB)|150-200|50|yes|needs change in .front-files (ULO)|
|128|1-7,9-15|7-Beam IF|100-200|100|yes|exclusively for 21-cm 7-beam|
|161|1-7,9-15|7-Beam IF|0-300|300(*)|yes|exclusively for 21-cm 7-beam|
|033|8+16|Optical|0-2500|2500|no|used for UBB/new K-band|
|161|8+16|Baseband|0-1000|1000(+)|yes|mainly for VLBI and pulsars|
(*) 300 MHz from 7-Beam Rx/IF, MultiFiBa only applies 2.5 GHz lowpass\\
(+) 1GHz from IF, MultiFiBa only applies 2.5 GHz lowpass

One can change the attenuation using the commands
<code>AXX@NNN
AXX+NNN
AXX-NNN
</code>
where XX is again the channel number (01-16), NNN is the attenuation in 0.1dB steps, i.e., 010 is 1dB, while 100 means 10dB. With the first command one directly sets the attenuation to a value, while the plus and minus version increase/decrease by a certain amount. For example
<code>A99@100</code>
sets the attenuation in all channels to 10dB.
<code>A03-030</code>
decreases attenuation in channel 3 by 3dB. **However, not all signal paths are internally going through the attenuation unit of the MultiFiBa (as indicated in the table above).**

The commands can be sent in the "MultiFiBa" tab within ObsInp.

Note 1: For standard spectroscopy observations with the XFFTS (using MultiFiBa channels 8+16), there is the software [[information_for_astronomers:user_guide:xffts|XFFTSGui]] which can be used to set the MultiFiBa modes and levels.

Note 2: The XFFTSGui will only work correctly, when a spectroscopy core is running on the XFFTS.

Note 3: If, for some reason, you want to set the MultiFiBa manually, you should also close the XFFTSGui, as there may occur problems, when multiple clients connect to the former.


===== MultiFiBa technical scheme =====
{{:information_for_astronomers:user_guide:bsb_multifiba_rev.4_20141120.png?1000|}}\\
{{:information_for_astronomers:user_guide:bsb_multifiba_rev.4_20141120.pdf|}}