Impedance Matching after Balun

Kevin Lockwood

Other Parts Discussed in Thread: CC2540, CC2530

So the according to the data sheet, the optimum differential load at RF_N and RF_P is 70 +j30. However since I am using a Johansen Balun (which is said to be a conjugate match), the single-ended source impedance should then become 50ohms (or close to). Does it follow then that my pi matching circuit should match to a Smith Chart centre rather than some unspecified complex conjugate?

I have characterized my PCB and RF trace (using only a single 0R for matching). This placed the 2.45GHz impedance according a VNA at 23.9 - j12.99. Just want to verify I should match to the Smith chart centre and not the 70 + j30 point or 70 - j30 point. Thanks

over 9 years ago

0 Eirik V over 9 years ago

TI__Guru 56590 points

Hello Kevin,

Correct. Between the BalUn and antenna you should match to 50 Ohm as reference as it will be close to this.

0 Kevin Lockwood over 9 years ago in reply to Eirik V

Expert 1035 points

Thank you for your reply.

Has anyone done any testing using the ST balun BAL-CC25-01D3? It seems to have a better Insertion Loss specs than the Johansen Part. If it matches well with the CC2540 I might have switch over as this isn't an insignificant amount of loss.

0 Eirik V over 9 years ago in reply to Kevin Lockwood

TI__Guru 56590 points

Hello Kevin,

We have not tested it and will not test it. But it might work well as they state that it is matched to our devices.

0 Timi over 9 years ago in reply to Eirik V

Prodigy 180 points

Hi Eirik V

For CC2530,is it right that we match 50 ohm between balun and antenna ? And if it is correct ,how to comprehend the optimum load impedance :69+j29 in CC2530 datasheet.

I am looking forward to receiving your prompt reply ,thanks!

0 Eirik V over 9 years ago in reply to Timi

TI__Guru 56590 points

Hello Timi,

Yes the output of the BalUn should be close to 50 Ohm.

The optimum load specifies the wanted load that the CC2530 device sees from the pins towards the BalUn. It is a trade off between the best performance for RX and TX.

0 Timi over 9 years ago in reply to Eirik V

Prodigy 180 points

Hi Eirik V

You said The CC2530 device sees from the pins towards the BalUn . Is it like below ?right?

0 Eirik V over 9 years ago in reply to Timi

TI__Guru 56590 points

Yes, Approximately. The antenna must be connected to see this load.

0 Timi over 9 years ago in reply to Eirik V

Prodigy 180 points

Ok . Thank you so much !

0 Timi over 9 years ago

Prodigy 180 points

Hi TI

I have a confusion . we desingn balun circuit by referencing cc2530 datasheet , but the output power of CC2530 is lossed so much , does it need to match balun circuit or consider the Q value of inductor ?

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Impedance Matching after Balun