Maintaining the RF impedance along the RF trace

[tennythomas]

The module (WLAN + Bluetooth) is connected to 2.4 GHz antenna (on the other board) via the connector through the 50 ohm (impedance) RF trace.

Should i add some impedance matching circuit to maintain 50 ohm impedance along the RF trace which connects the module, the connector and 2.4 GHz antenna? If so, what is the circuit?

Thanks in advance.

 

[terry andy]

Re: RF Impedance

matching circuit should be on the module board.
T or TT shape.

 

[tennythomas]

Re: RF Impedance

Can you please show me how to get the values for T or TT ckt?

 

[terry andy]

Re: RF Impedance

the values depends on the impedance of the antenna .
you can get the smith chart first,and then u can use software to compute the values..

 

[vfone]

Re: RF Impedance

Using the VNA measure the impedance of the antenna itself.
Replace the antenna with a 50 ohms load (or a small 50 ohms resistor) and using the VNA measure the impedance of the RF trace connected to the antenna.
In this way you can find if you need (and where) impedance matching.

 

[tennythomas]

Re: RF Impedance

The module (WLAN + Bluetooth) is connected to 2.4 GHz antenna (on the other board) via the connector through the 50 ohm (impedance) RF trace.

Should i add some impedance matching circuit to maintain 50 ohm impedance along the RF trace which connects the module, the connector and 2.4 GHz antenna? If so, what is the circuit?

Thanks in advance.

Should impedance matching circuit be not needed, when the impedance of 2.4GHz antenna is 50 ohms?

 

[bkd]

Re: RF Impedance

If both the impedance of RF trace and impedance of antenna is 50 ohm, you do not need impedance matching. What you need may be is a capacitor to avoid DC leak to antenna

 

[tennythomas]

Re: RF Impedance

Thanks for the reply.

The module (WLAN + Bluetooth) is connected to 50 ohm 2.4 GHz antenna (on the other board) via the connector through the 50 ohm (impedance) RF trace.

Will this connector introduce a change in impedance?

 

[volker_muehlhaus]

Re: RF Impedance

Will this connector introduce a change in impedance?

RF connectors are designed to keep the impedance. If you use a 50 ohm RF connector, there is no need for additional compensation.

 

[FvM]

Re: RF Impedance

RF connectors have a designed impedance, they intend to keep this impedance along the transmission line through the connector exactly. Of course, they can't be perfect. The error introduced by a connector is specified by a VSWR (voltage standing wave ratio) value. The largest contribution can be expected from the microstrip to (coaxial) connector transition. The transition can be improved by adjusting the microstrip shape near the connector for minimum VSWR. For a small band application, e.g. 2.4 GHz, the connector effect can be expected to be hidden by the antenna and transceiver impedance mismatch anyway. Simply follow the microstrip layout suggestions from the connector vendor.

 

[tennythomas]

Re: RF Impedance

In my project, no RF connector (50 ohm) is used. But an ordinary connector (through hole) is used for soldering both module board and other board. Please suggest on this.

Thanks in advance.

 

[volker_muehlhaus]

Re: RF Impedance

But an ordinary connector (through hole) is used for soldering both module board and other board.

Do you have more details?

 

[tennythomas]

Re: RF Impedance

Please find attached the picture.

The impedance of the antenna on the other board is 50 ohm. I think that the non RF connector would introduce a change in impedance. How to set this connector to 50 ohm?

Thanks in advance.

 

[volker_muehlhaus]

Re: RF Impedance

I think that the non RF connector would introduce a change in impedance.

Very possible. For controlled impedance, you would have a well defined ratio of series L and shunt C. If your connector is too inductive, you can compensate that by adding shunt C. This has two effects: it creates some low pass behaviour, and improves the matching within the pass band (compared to the series inductor without shunt C). But compensation requires that we know the connector properties. The easiest way would be to go to the lab, build a test fixture for the connector and measure it with a VNA (vector network analyzer).

This is how it can be done, if really needed. But most antenna stuff is robust and some small mismatch will not kill your circuit function. If your connector is electrically small compared to the wavelength, then you are lucky because the mismatch created by the connector is small.

 

[tennythomas]

Re: RF Impedance

I want to know whether i can place impedance matching ckt on the module board before the non RF connector OR on the other board before the antenna. Which one is a must?

Thanks in advance.

 

[volker_muehlhaus]

Re: RF Impedance

If needed, I would compensate the discontinuity where the discontinuity is: close to the connector.

 

[spid3rx]

Re: RF Impedance

depending on the board space, try to have T and TT together so you have more options when you are doing tuning.
I faced some problems when the TT matching circuit is not sufficient to tune the antenna. Have either one of them
close the connector and the other one near the antenna, just on the safe side