RF PCB Design Difference Between Controlled Impedance Track and Impedance Matching Circuit

Question

Im new on RF circuit designing. Im designing an circuit with GSM and GPS capabilities. I know when using high frequency signals the RF traces must be impedance matched. Now what I understand is, I can make it by impedance mathing circuits like pi-network but this will require antenna tuning and I dont have the tools for that, or I can use impedance controlled traces(microstrip line) on my PCB. What I don't understand is: which one is better or must I use impedance controlled lines and impedance matching circuit?

Answer 1

It is not an either/or situation.

Impedance controlled traces make feedlines of a known characteristic impedance, just like most coax is 50 or 75 ohms, and ladder-line is often 300 or 450 ohms. As a rule of thumb, any trace or wire longer than 1/10th the wavelength has to be considered a transmission line, in which case you must know the characteristic impedance. Trying to use a transmission line in your circuit without knowing the characteristic impedance is like trying to use a resistor without knowing the resistance.

Impedance matching circuits are circuits which transform one impedance to another. Designers usually try to keep them to a minimum since they cost something both in money and signal power. The matching circuit might consist of short (usually 1/4 wavelength or less) sections of transmission lines, but also inductors, capacitors, transformers, and (less commonly) resistors.

Answer 2

To add to Phil's answer: if your antenna, source and transmission line (the impedance controlled traces) are all of the same impedance, you don't need an impedance matching circuit. Most common impedance for RF applications is 50 ohms.

The impedance matching circuits are only there to connect two systems with a different impedance to each other.

Creating impedance matched traces is not necessarily hard to do. Take a look at a page like https://www.eeweb.com/tools/symmetric-stripline-impedance to calculate which dimensions to use. However, the frequencies you are talking about are rather high so things get harder to do "right".

Then again, you might not require the "perfect" board to get your "product" to work, it depends on the application... So try it out and see where you end up.