Balun simulation for CC2540 BLE chip
I wanna simulate the S parameters of the balun in ADS for CC2540 chip which has differential RF output.
Datasheet of CC2540 says that the optimized load impedance(Differential impedance as seen from the RF port (RF_P and RF_N) toward the antenna) of CC2540 is 70+j*30.
So I have made the port of the chip as 70+j*30 and trying to simulate the Balun as per the recommended components given by the manufacturer in the datasheet of CC2540.
My ADS model looks like this:
But the problem is that I am not getting 50 ohm at the output.
Can anyone suggest what's the problem.
Thanks
A sufficient reason to get different results is ignorance of parasitic circuit impedances, particular trace and component series inductance.
I think there are two possible approaches:
- use the suggested discrete balun as is, copying the layout precisely
- make your own optimized design in combination of circuit analysis (including parasitic parameters), measurement of actual circuit impedances and empirical component variation (load pulling).
A specific issue is that differential impedance matching and maximum power output doesn't automatically imply symmetrical load. The latter is a goal on it's own and responsible for suppression of even harmonics.
Hey, Thanks for your reply.
The datasheet is not giving any recommended layout for the balun so I was assuming that may be the secondary effect.
Do you think that putting manufacturers' s2p files for each component may help?
Also do you think that parasitic PCB effects can vary the impedance this much?
Thanks
TI has published evaluation board design with layout data.
Layout and component parasitics definitely affect the impedances. It's already the case for sub-GHz transceivers and even more for 2.4 GHz designs.