Hello,
Need recommendation for the project, is there any preferences in down-converters from C & Ku bands to LBand

Please advise

To down convert from C band or Ku band to L band use a Low Noise Block Downconverter (LNB). If frequency accuracy is important use the Phase Lock Loop (PLL) variety. The most accurate results are obtained with an external 10 MHz reference from indoor reference oscillator based on GPS or Rubidium. The 10 MHz signal is sent up the cable to the LNB, along with DC power.

Regarding the design of RF over optical fibre links this is good reading:
RF-and-Microwave-Fiber-Optic-Design-Guide

Best regards, Eric.

Could you please recommend down-converter from C & Ku bands to LBand, which brand you would go with ?

Look forward to your reply

First, I would consider the least expensive solution and that is to use a conventional LNB directly connected to the antenna feed.

There are considerations of dual polarisation and, in the case of Ku band, upper and lower parts of Ku band. What do you want to receive ?
Some Ku LNBs have multiple outputs. For example low band vertical, low band horizontal, high band vertical and high band horizontal.
Some LNBs have a 'stacking' feature and merge outputs from two different input polarisations. It all depends on what you want to do. If you are sending all signals via fibre then multiplexing the signals will reduce the number of fibres.
Is frequency stability/accuracy important? If so, use PLL type LNBs.

If you intend to use wide band LNAs followed by indoor block down converters then read here:

https://www.norsat.com/solutions/microwave-products/bdc-ku-band/

https://www.norsat.com/wp-content/uploads/1000_bdc1.pdf

https://www.norsat.com/solutions/microwave-products/bdc-c-band/

https://www.norsat.com/wp-content/uploads/3000_bdc.pdf

These units have coaxial cable input, and are not low-noise devices. They are intended to be installed following a wide band LNA and, typically, a splitter.

Best regards, Eric

Thank you for your help, we will contact norsat.

Ron