LC Network in Microchip's LoRa Gateway

I am reading the microchip's LoRaWan Gateway manual. On Page 48, I got stuck at receiving side on the LoRaWan RF circuit. It looks like they have some sort of LC Low Pass Filter. I have used this online tool to calculate it's cutoff frequency. It gave me following reading:

Results:

F = 3.3553e+9 [Hz]

Z = 31.623 [Ohms]


I am not sure about the Resistor in between them. Please help me in verifying my interpretation of the circuit. The part of schematics that I am referring has been posted below:

Can someone help me in understanding the part of the circuit that have been encircled. 1257_RFINA and 1257_RFINB goes to RF inputs (Single-ended) of two different RF transceiver chips (SX1257). TA1567A is SAW Filter and SPF5043Z is LNA.

1 Answer

That is a narrow band power divider, basically two identical L matching networks with the inputs in parallel to match from the output of the filter (U9) to the inputs of the RF sand.

The resistor is there to improve the balance between the two halves of the thing, given that the loads are probably not really identical.

the SX1257 datasheet shows a remarkable lack of RF detail, where are the numbers for things like the RF S parameters (Nothing is really a resistive input at 1GHz), which you rather need to properly design matching networks? In fact even such useful things as IMD3 numbers are missing from the data.

• Can you provide some stuff or reference to learn more the design? Aug 22, 2018 at 20:10
• "Microwave Engineering" by Pozar is probably the book you want. Also the "ARRL handbook", "Experimental methods in RF design" and the handbook for your favorite manufacturers vector network analyzer. Aug 22, 2018 at 20:18
• Can you provide me formulas for calculating LC and R values? Aug 22, 2018 at 20:21
• it is just the standard L network matching equations, but you need to know the complex impedance if the input you are matching to, and the 1257 datasheet omits almost all such information. There is some information here data.eefocus.com/myspace/0/2194/bbs/1190602131/84dbcb80.pdf but it assumes a resistive termination at Zo which I don't think you have. Aug 22, 2018 at 21:45