Hi, my name is Poornima, I'm referring to the z-wave data sheet and schematic for design to my module, in that C21 doesn't have value still it is connected with the C20paralell and series with L1, Kindly tell me, what is the value of C21?
Hi Poornima, That's a Pi network. It's part of the circuit upstream on the antenna. The purpose is described here
It's not populated by design, the reason being that you would;
1. Design the antenna; coiled wire, meander path on PCB, etc. Properties such as the PCB board type, case, proximity with other components, flesh (!) have an impact on the natural resonance of the antenna too. Make the environment as close to the normal operating conditions as you can.
2. Send a trusted 868.4Mhz signal through it with a sig gen (Matching balun would be good here).
3. Assess the radiated frequency using a frequency analyser and then calculate the components of the network. Ideally sticking with caps rather than inductors to keep the cost down.
Document INS14283 is a good starting point. See section 3.
If you're only at the stage of getting your PCB design complete then I would suggest bridging L1 with a 0 ohm resistor and not mounting C20 & C21. Then you can match the antenna under controlled conditions later.