This article intends to share knowledge about the considerations in deciding on IFL coax cable length and resistance.
In most low power BUCs, power is supplied to the BUC from the modem via the IFL cable. When selecting the proper type of cable for the IF, attention should be given not only to the impedance of the cable (75 Ohm or 50 Ohm), but also to the voltage drop along the cable due to its DC (Ohmic) resistance.
Two independent parameters limit the total resistance allowed in the IFL cable:
1. The input voltage to the BUC should be above a specified value.
2. The total current draw from the power supply or the modem should not exceed the specification of that power supply.
The voltage drop along the cable and the voltage at the output of the IFL cable can be calculated using the following formulae: