Feedback on 2.4 GHz circuit design

[alexxs88]

Hi,

So this is a project that's on its second iteration, having trusted the previous PCB designer and ending up with some expensive PCB-shaped coasters, have learned quite a bit myself and also asked for help from another more experienced engineer. Looking for some feedback on this RF feedline and antenna area:

• Murata RF connector with integrated switch (MM81302600) - for inserting the VNA and tuning the PI network
• PI network
• Extra in-series 0402 footprint (C14/C15) to route the signal to J3 (UFL connector), in case I want to use an external antenna instead of the one on the PCB.
• Chip antenna: 2450AT42A100

Thanks!

[BigBoss]

Hi,
So this is a project that's on its second iteration, having trusted the previous PCB designer and ending up with some expensive PCB-shaped coasters, have learned quite a bit myself and also asked for help from another more experienced engineer. Looking for some feedback on this RF feedline and antenna area:

• Murata RF connector with integrated switch (MM81302600) - for inserting the VNA and tuning the PI network
• PI network
• Extra in-series 0402 footprint (C14/C15) to route the signal to J3 (UFL connector), in case I want to use an external antenna instead of the one on the PCB.
• Chip antenna: 2450AT42A100

Thanks!

You make some serious mistakes in this circuit.
-Antennas are never connected in parallel because RF cannot be considered low frequency.Instead, you should use a Antenna Switch to change over the antennas
-Similarly using a Connector on the Antenna Path is not a good idea because it will disturb the Characteristic Impedance of the Transmission Line.In additional to, VNA Measurements will also be very erroneous due to Loading Effect of the IC. You have to separate the Path by using-for instance- directional coupler.Or another switch..
-Layout does not conform the RF specifications. You designed it as usual Low Frequency circuit.I don't know which IC you intend to use but if there are some TL specifications about Output Circuit, you should track that because TL might be a part of Matching Circuit.
 

[ejeffrey]

You make some serious mistakes in this circuit.
-Antennas are never connected in parallel because RF cannot be considered low frequency.Instead, you should use a Antenna Switch to change over the antennas

See the note: only one anetnna is present and the coupling capacitors (C14 and C15) are populated to select it.  This should be no problem.

-Similarly using a Connector on the Antenna Path is not a good idea because it will disturb the Characteristic Impedance of the Transmission Line.In additional to, VNA Measurements will also be very erroneous due to Loading Effect of the IC. You have to separate the Path by using-for instance- directional coupler.Or another switch..

Connectors on antennas are incredibly common.  A suitable connector will not present a meaningful impedance discontinuity in the signal line and the U.FL connector chosen is very commonly used for 2.4 GHz radios in exactly this sort of application.

[alexxs88]

Thanks a lot! As ejeffrey said, I'm hoping that using a capacitor to select which antenna path to take and a dedicated RF connector with an integrated switch should work ok.

[virtualparticles]

The only comment I would add is that the connecting lines from the shunt caps to the series matching inductor, L1, should be skinny. They're so short that it probably won't make any difference but it's just good practice. The middle area of a matching circuit (or a filter) is not 50 ohms. Using fat traces just adds unnecessary capacitance to ground where the line inductance is practically the same whether skinny or fat.

[48X24X48X]

Murata RF connector with integrated switch (MM81302600)

I was looking to use this as well for a board with a chip antenna but also with the possibility to use an external antenna.
But, looking at the description on the Murata website, it does seems like they emphasized the use of the connector to be more for ad-hoc purpose like during testing.
I have used in the past an MCX switch based but that is too huge for this new board, so if anyone have used the above connector for the purpose of switching to an external antenna, please share your thoughts on this.