Impedance Matching

[icantthinkofaname]

Hi all,

I'm creating an RF device using the CC2420 and have started laying out the RF section. However, I became a little confused when I saw the output impedance for the differential RF signal quoted as a complex number (95+ j187):S.

I have Googled but can not find/understand how to use this; so I am asking here.

Could anyone shed some light?

Best regards,

 

[unclejed613]

look up "smith chart". the numbers given are 95 ohms plus an imaginary number (the square root of a negative number), and my guess without actually plotting it would be that it's somewhere near 50 ohms with some capacitive or inductive reactance mixed in.

 

[RadioRon]

To understand this number, you need to understand the definition of impedance when dealing with AC voltage and current. There are many good text books that cover this, here is a short version: https://en.wikipedia.org/wiki/Electrical_impedance .

What you should learn is that impedance is stated as a complex number because it can be a combination of resistance, inductive reactance and capacitive reactance. In the mathematics of electronics, the real part of the impedance is the resistance, while the imaginary part is the reactance. A positive reactance is the result of inductance while negative reactance is the result of capacitance.

Impedance matching is done between high frequency ac circuits in order to maximize power transfer. If you are willing to live with something less than absolute maximum power transfer, you can compromise the impedance match. In other words, it usually works ok even if you don't try too hard. In your case, the output is a relatively high impedance, and is differential, so when designing an antenna for this, you would be best off to choose a type that can work directly with a differential feed. A half wave dipole could do this, but they are not the best match for this higher impedance. However, they are simple to make, so you could start with a dipole and improve once you have it working. Oftentimes, a loop antenna can be used with a differential feed. You may be able to find details on the design of a loop antenna for 2450 Mhz.