U14 is a straight 300 MHz unity gain follower that buffers the output of the multiplexer. The low impedance output of this buffer drives the attenuator section. 3 Resistor networks give attenuation of 1, 2 and 5.273. The other option is a x 4.583 gain stage to boost low level signals.
The 4 range options are switched by MUX U18 (4052). This MUX is addressed by RNG0, RNG1 outputs from the PIC.
Depending on the 4052 (MAXIM is best) you may need to add a small speedup cap C69 to the 5:1 range for ideal frequency response. A poor mans trimmer can be made from some adhesive copper foil (stained glass supplier) and a bit of paper.
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