Request for information regarding the matching network in the schematic of NRF9160 DevKit

[Goldie]

Hello all!!! 
I am new to the field of antennas and have lately been struggling in understanding antenna matching networks. All that I know as of now is that antenna matching networks are necessary only when there is a mismatch between source and load impedances (output and input respectively). My doubts are as follows:
1. What does the N.C. stand for below inductor L5 and capacitor C85? (in the image below)

2. What is the second terminal of the capacitor and the inductor connected (refer the highlight in the image below) (P.S.: I am assuming it is connected to ground)

3. In the image below, since the antenna A1A FR01-S4-210 and the switch QM12038 both are matched to 50 ohm impedance, what is the need for the additional circuit connected to the antenna (I am assuming it is a antenna matching circuit :) )?

The help will be sincerely appreciated. Thank you!!!!

[Kazi Afroza Sultana]

Hello,

1. What does the N.C. stand for below inductor L5 and capacitor C85? (in the image below)

N.C means non connected. 

2. What is the second terminal of the capacitor and the inductor connected (refer the highlight in the image below) (P.S.: I am assuming it is connected to ground)

Yes you are right. They are connected to second pin of the antenna.

3. In the image below, since the antenna A1A FR01-S4-210 and the switch QM12038 both are matched to 50 ohm impedance, what is the need for the additional circuit connected to the antenna (I am assuming it is a antenna matching circuit :) )?

Yes its a antenna matching circuit.

The antenna tuner is all the components between and including U2 and U3 which are required to receive signals at different  LTE band. ANT_LTE is the output from the antenna tuner and is directly connected to the antenna. C15 and L8 are the antenna matching components.

It is recommended to use radio matching network and antenna matching network for avoiding subdue harmonics and make sure that the signal in the transmission line is close to 50 ohms impedance. There is no 100% guarantee that the signal out from the ANT pin is 50ohms impedance. So, matching network is there to make sure that the signal sees as close as possible to a 50ohm impedance into the antenna. If there is a mismatch in impedance you can get a loos in power and reduced performance.

Thanks.

Best Regards,

Kazi

[Goldie]

Thank you!!!! Kazi Afroza Sultana for your apt reply and support. It did really help me fast track my work.

[Goldie]

It would a great help to me if you could help me with this question too (P.S.: I am sorry for asking this question late). Can I use the Qorvo QM12038 RF Switch (which is in the second image above) with Ethertronics P822601 LTE Antenna as this is available with me. Also is it possible to tune the antenna using the Qorvo RF Switch? If yes, do we need to change the values of the tuning circuit already present (in the second image above) and calculate the same for the Ethertronics antenna. Thank you!!!

[Kazi Afroza Sultana]

Hello,

Do you want to change antenna on Thingy:91 (as Qorvo switch is used there) or you want to use QM12038 on nRF9160 DK with your mentioned antenna? Why do you need to add a switch on DK ? Could you please explain more about this?

Any change in the design will make it obvious to re-tune the antenna. Change on Thingy:91 is very time consuming. 

Thanks.

Best Regards,

Kazi Afroza Sultana

[Goldie]

Yes Kazi Afroza Sultana I am building a product using the nrf9160 and I don't have any access to the Thingy91 or the DevKit and thus I wanted to know if I could use the Qorvo QM12038 for band switching of the antenna alongside the Ethertronics P822601 LTE.

The main question that I would like you to address is since I am new to antenna tuning, I was wondering if I could use the Qorvo switch for band switching by using the matching circuit in the Thingy91 schematic as is to connect to P822601 antenna; since the antenna that I intend to use i.e., the P822601 has the impedance as that of the actual antenna used in the Thingy91 schematic i.e. AIA FR01-S4-210 and the QM12038 switch, viz 50 ohms. 

[Kazi Afroza Sultana]

Hello,

This is very specific design idea, and many factors are involved like frequency bands, tuning, matching networks. If you want to implement only single band that could be simpler. For the combination of LTE and GPS, it will be difficult to get good results on both. The tuning will be really a complex process and Antenna manufacturer can help in this case to find a better way. 

Would you like to implement this for both LTE and GPS or for only LTE? 

LTE has many operating frequency bands and for these specific bands the matching components are different on Thingy:91. It is also complicated as the size of the board matters. The challenge with the Thingy:91 is that the PCB board is small (50mm x 50mm) for getting proper LTE performance on low band. The ground plane of the PCB should be longer than the 1/4 of the wavelength; For 700 MHz band it should be 10 cm or so. When a PCB size is that much small like 50mmx50mm, it is difficult to get the proper ground plane; therefore, to get the good performance, there are switches (QM12038) to tune the antenna between bands. 

 So, using switch is design specific for Thingy:91 according to our recommended hardware design.

In nRF9160 reference design we do not recommend any switch in tuning network. 

Any RF switch which has the same specification as Qorvo QM12038 used on Thingy:91 should work, but consulting with an antenna expert and then get the design evaluated by our hardware engineers is the best way to proceed. 

I can show you dome documentations from our infocenter for further reading: 

https://infocenter.nordicsemi.com/topic/nwp_033/WP/nwp_033/nwp_033_intro.html?cp=2_0_6_4 

https://infocenter.nordicsemi.com/topic/nwp_037/WP/nwp_037/ant_if.html?cp=2_0_6_2_3_12

 https://infocenter.nordicsemi.com/topic/nwp_037/WP/nwp_037/gps_if.html?cp=2_0_6_2_3_14  

https://infocenter.nordicsemi.com/topic/nwp_037/WP/nwp_037/antenna.html?cp=2_0_6_2_4_2 

https://infocenter.nordicsemi.com/topic/nwp_037/WP/nwp_037/design_overview.html?cp=2_0_6_2_2_2 

I hope it helps.

Thanks. 

Best Regards,

Kazi Afroza Sultana