Hi EEVblog community,
I am a newbie Engineer who is doing his first RF project. I have learned to design RF circuits mostly by myself by following various tutorials and guides available on the Internet. however, I am stuck on the RF PCB layout part.
What is my project:I am designing a Radio communication device ( much similar to a walki-talki) that works on both VHF and UHF (137MHz to 174MHz and 400MHz to 470MHz) and has 25W of RF output power
I am using two separate RF chains for VHF and UHF. (I tried designing with a single chain but realized that it is almost impossible to match the impedances for such a broad frequency range) I am connecting the VHF and UHF RF chains near the antenna by a PIN diode SPDT switch.
Both RF chains are pretty much identical in structure and they look like this:
I have used Keysight ADS to design the circuit and simulate it. ( I have the non-linear model for the PA) I designed all the subcircuits as modules and simulated them separately and all connected together. I am very happy with the simulation results.
all the sub circuits of the RF chain ( Driver amp, PA, all the filters ) are all 50 ohms matched. (both inputs and outputs). I have only considered microstrips in the impedance matching of the PA. here is a picture of the PA and a harmonic filter.
Where I am stuck at:According to the simulations I have created the schematics in Altium but I am now stuck with The layout. I was not able to find a good tutorial on RF Layout which would answer the questions I have. Hopefully, with your guys' help, I will be able to find answers.
What are my questions:- Can any 50ohm track handle 25W of RF power? If we consider JLCPCB, with their two available PCB stack-up options we need to draw either an 11.6 mil track or 5.8 mil track for a 50ohm trace. Can a 5.8 mil track handle 25W of RF power?
- How should I place the components and do the layout ?
I am currently thinking of placing components of each subcircuit as close as possible and connecting them using wider but shorter PCB tracks. After that connect the input and outputs of each subcircuit with 50ohm tracks (as short as possible). Is this the right way to do this? - I have seen in many places that if your track length is shorter than 1/10 of the wavelength of the highest frequency (in my case for 470HMz it's 63.8 mm) you do not need to really care about the track impedance. Does this valid for only individual tracks on the PCB or for the whole RF chain. (In my case I am pretty sure that none of the tracks will exceed 63mm but for sure the whole RF chain would be longer than 63mm)
hopefully, with all your responses I would be able to learn how to do the layout properly.
Thanks in advance.
Cheers