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4MHz filterbank. 

Physically, there are a total of 9 4MHz filterbanks, each with a fixed
resolution of 4MHz and bandwidth of 1024 MHz.  They should therefore
normally be used with receivers in /WIDTH WIDE mode.

The 4MHz filterbanks can be connected either with:

1 to 4 parts to 1 to 4 (different!) single-pixel receivers, e.g.:
BACKEND 4MHz 1 /RECEIVER C150
BACKEND 4MHz 2 /RECEIVER C270
BACKEND 4MHz 3 /RECEIVER D150
BACKEND 4MHz 4 /RECEIVER D270

or 1 part (times 9) to either HERA1 or HERA2, e.g.:
BACKEND 4MHz 1 /RECEIVER HERA1
or
BACKEND 4MHz 1 /RECEIVER HERA2

The last possibility connects the 9 physical parts to the 9 HERA1 or
HERA2 pixels.

NOTES: it is not possible to connect 2 parts to the same receiver!
It is also not possible to connect the 4MHz filterbanks to selected
pixels of HERA1 and HERA2

