Question about antenna matching using a Balun

[leszcz]

Hi,
could someone pls explain me the way this balun is working ?
Can a balun be a matching circuit for the antenna at all or do I need additional 50ohm matching circuit ?

Thank u

 

[Mazz]

Balun question

Without the passives values is more uncertain but...
C61&L62 = C81&L81 makes the +/- 90 deg shift of the balun.
C71 is DC block vs GND, its value is large.
L61 makes a matching on impedance vs IC
C62 makes a matching on impedance vs antenna

I hope it can help
Mazz

 

[leszcz]

Re: Balun question

Here are the values , sry for that. U helped me a lot but im still not certain about antenna matching. If u had to match the antenna would u only change the C62 value ? Normally there is an inductance also needed for proper antenna match.
Why is actually L61 needed ? How is it working ?
Apart from that L62 is not equal L81

Thank u

 

[Mazz]

Balun question

Looking at values it seems that my assumptions (or sime of them) are wrong.
C71 is small, and I didn't noticed that the middle point is in TX/RX switch.
The circuit is more complex than I assumed to be.
Do you have other infos? Operating band?
IC data?

Mazz


May I argue 2.4~2.5 GHz?

 

[leszcz]

Re: Balun question

yes its 2.4-2.5Ghz zigbee tranceiver, here is the manual, have a look at page 19 and 55, there is some description but very general.

**broken link removed**

 

[Mazz]

Re: Balun question

Sorry for delay

I've done some simulations and I've got some results:
assuming that RF impedance is 115-J180 (complex cnj of optimum load impedance indicated in the datasheet), the balun shows a good in/out match but some unbalance (see difference between pos and neg phase that is less tha 180 deg), a couple of dB of insertion loss, a ratio of 1:4 (magnitude of input impedance is roughly 200 Ohm - in fact being 0 dBm the signal of the single ended port, we see roughly 6 dBm (voltage expressed in dBm) on differential voltage.

It is not clear to me why there is the TX/RX point connected inside the balun, maybe is for DC purpose, infact the connection point is unsensitive to high/low impedance (it is the C71 effect).