LAUNCHXL-CC1352R1: Antenna Matching Impedance

The diplexer will filter the signal to some extent meaning that when not using a diplexer some extra filtering has to be added to get the same total filter effect. To be compliant with regulations, filtering of the signal is required and in this design the filtering is done mainly in the diplexer + L34/C35 or with C34/L34/C35.

Are you only going to use sub 1 GHz? In that case, why are you using CC1352 vs CC1312? 

Thanks for the answer, TER.

We are still evaluating our solution and choosing between Sub1GHz and BLE. Once we can change for another version I thought that using the same chip would be a good practice.
Is there another reason instead "you do not need BLE in the uC" that I am not considering?

Does that mean you intend to build boards with both sub 1 GHz and BLE and evaluate the performance of both and then remove the one of the RF paths afterward for the final solution? If that is the case, use CC1352R. I was just trying to understand the project. 

Hello, TER.

Yes, our goal is to compare both technologies in the future. So, I was concerning also about the antenna matching.

Once the antenna dimension really matters, we chose for the miniature helical presented in the DN038 and an antenna matching topology is required for future matching impedance in the case.

So, I have two questions. In the image attached, (1) is from LAUNCHXL-CC1350EU_1_3_0_Schematics which use this antenna and (2) from WCS038B_Schematics, the reference design for CC1352R.
1) Is correct the assumption of using these two branches of the circuit marked in green connected in series is a correct impedance matching?
2) Is the impedance of the antenna (as described in DN038): 10 - 88j @ 868 MHz? The topology and values of L32 and C71 don't seem right for impedance matching of 50 Ohms when simulating in the Smith Chart.

For 1), not sure what the question is. First, in (1) only the antenna matching is included, in (2) both balun, filer and antenna tuning are included. Please rephrase the question. 

2) The impedance given is correct for the board it's measured on. 

Note that the antenna tuning components should not be copied from one design to another since the required tuning will mostly change from device to device mostly due to two factors:

- Size of ground plane, if the size change, the needed matching change.

- Casing: The plastic can change the resonance frequency. In addition display, battery holder etc can impact the result.  

In Figure (1) is marked only the matching impedance filter for the miniature helical from DN035. In Figure (2), the circuitry between the output from the CC1352R and the filter for the frequency, before the antenna matching.

I'm planning to manufacture an expressive number of these boards and it should at least work in a non-ideal condition, i.e. without antenna matching (suppose that our first tests will not be performed in a case).

Can you review in another document that isn't DN038 what is the miniature helical impedance? If the value at 868 MHz is 10-j88, the matching circuit from the Launchpad CC1350 and Antenna DK2 (antenna number 6) are wrong, once the shunt inductance doesn't allow the impedance match. Could you give me a direction about what am I missing to understand this difference?

Considering the circuit "serial capacitor and shunt inductor" is correct, could you give me the correct standard values to match theoretically the miniature helical antenna (DN038) with an input impedance of 50 Ohms?

As I wrote in my last post: The matching is a function of the size of the ground plane. DN038 uses a rather small PCB, the launchpad is a larger PCB. Note that the length of the antenna used in DN038 and on the launchpad is different (to adjust the resonant frequency) Board #6 from the antenna DK 2 has a size that is somewhere between DN038 and the launchpad.

As you can see from the measurements from DN038 and on #6 in http://www.ti.com/lit/an/swra496a/swra496a.pdf the impedance of the antenna is  different dependent on the board. Hence we don't have any theoretical matching values. The antenna impedance has to be measured on the board in question and the matching values found according to this. That is why you see placeholders for a pi filter before the antenna in our reference designs since it allows us to match to any impedance. 

Typically a small number of boards are produced first used for functional testing and for antenna tuning. That the production batch is built with the match found. 

Thanks for the answers, but I did not realize yet which design I should follow to a preliminary version of the RF path.

Should be the one presented in the swra496a.pdf or in the LAUNCHXL-CC1352R1/WCS038?

note: the filter required when is not using the diplexer in WCS038 should be another pi-filter connected in series or just replace the values of C34/C35/L34?

I see that I answered slightly wrong here:

"For 1), not sure what the question is. First, in (1) only the antenna matching is included, in (2) both balun, filer and antenna tuning are included. Please rephrase the question. "

Figure 2) which you have repeated in your last post does not include antenna matching.

So if you want to follow LAUNCHXL-CC1352R1/WCS038 but not include the diplexer, you have to include the pi filter consisting of C34/ L34/ C35 with the values given as text in the schematic. In addition you have to add a pi filter (as shown in swra496a)  after C36 to be able to tone the antenna. As a start you can have the shunts in this additional pi filter as DNM and the series component as 0 ohm until you measure the antenna.

Draw a suggestion for the schematic and I can take a quick look at it.

Hi, TER. Thanks for you attention.

The base documents is written in the Figure, below the schematic.

Thanks for you attention, TER.

If you have more theoretical information about the miniature helical antenna I would really appreciate.
I'm going to copy the schematic from the reference design, but I am not sure that the configuration of the pi-network fit observing the impedance of 10 - 88j in the Smith Chart.