Antenna tuning with chipset antenna

RE: Antenna tuning with chipset antenna

vidal — Thu, 03 Aug 2017 10:10:16 GMT

Hello Kristin, Right now I can not do it, so I will try to measure the impedance with VNA. To do that I have to cut off the track in the same way of tuning antenna process????.

Also I was doing some math. According to “nRF52 matching network.pdf” document shown that trace has 10nH/cm and I did some simulation and that trace has around 4nH/cm there are a lot of difference… do you know why???. Conductor Width is 9 mils. Also I was checking the distance. In Solar Beacon board, the distance is 1.694mm and in “nRF52 matching network.pdf” document is 0.9mm, do you know why???. I suppose that you are taking other reference. You can see in the picture below my reference of 1.694mm because I think that GND is the thermal/ground PAD of the nRF52.

Thanks.

RE: Antenna tuning with chipset antenna

FormerMember — Tue, 25 Jul 2017 06:53:43 GMT

Measuring the impedance of the ANT pin on the chip is an alternative way to verify that the output power from the chip is what it should be: When the impedance is (approximately) 50 Ohm across the band, the output power in the 50 Ohm reference point will be the same as what it was set to be. However, measuring the impedance does not, unfortunately, replace the measurement of harmonics. The only way to measure the harmonics is to use a spectrum analyzer. This .pdf explains an important aspect of C3/L1 in terms of suppressing harmonics.

When using the radio_test example and you want to measure the output power, the signal should be unmodulated. Unmodulated signal is the default. In addition, you would need to set the following settings:

Constant channel (TX): a, p, c

Sweep (TX): a, b, p, t

RE: Antenna tuning with chipset antenna

vidal — Mon, 24 Jul 2017 15:16:23 GMT

Hello Kristin,

Thanks for your reply. I only have two questions more and I will do some test.

As you say I do not have tools to measure the second harmonic so I have to measure the impedance of the chip instead of output power. To do that I have to cut off the track in the same way of tuning antenna process?. I have to program nRF52 with radio_test" example (which configuration of the app)?.

Best regards.

Jose.

RE: Antenna tuning with chipset antenna

FormerMember — Mon, 24 Jul 2017 12:05:56 GMT

Measuring the harmonics: Yes, to measure the harmonics a spectrum analyzer for higher frequencies are needed. If you have copied our reference layout for L1 and C3, with the same component values and distances as in the reference layout, the harmonics will normally not create any problems. The reason why it is necessary to measure the harmonics is to pass the various regulatory standards. All regulatory standards have a maximum limit for signals that are out-of-band.

Output power: You can use the "radio_test" example in the SDK to easily set the radio in continuous transmission with the desired output power.

RE: Antenna tuning with chipset antenna

vidal — Thu, 20 Jul 2017 14:09:01 GMT

Please,

Can you answer my doubts?.

Thanks in advance.

Jose.

RE: Antenna tuning with chipset antenna

vidal — Tue, 04 Jul 2017 08:39:20 GMT

Dear Kristin,

Thanks for your swift reply. Now I understand a little bit more. I am only have some questions:

First at all, I have a VNA from 1MHz to 3GHz and a spectrum analyzer (MDO4034C + SA3) of 3GHz, so it is impossible to me measure at the harmonic at 4.88 GHz...Is this a great problem?. Why is not possible the measure the principal frequency (2.44GHz)?.

Second, when you are referring to measure the output power, I suppose that is about to validate if I set TXPower register (0x50C) of nRF52 to 0x00 (0dBm) I see in the spectrum analyzer 0dBm. To do that I suppose that nRF52 have an special configuration in order to test it...similar to TX in continuous mode?.

Thanks in advance.

Jose.

RE: Antenna tuning with chipset antenna

FormerMember — Wed, 28 Jun 2017 10:56:50 GMT

I have updated my answer.

The output power can be measured using a spectrum analyzer. It is also possible to use a VNA to measure the impedance of the chip, see the updated answer.

Tuning of antenna: When connecting the VNA, the center "cable" should be connected to the C13, and the outer "cable" should be connected to ground (ground plane) close to where the center cable is connected.

What do you mean by frequency resonance?

RE: Antenna tuning with chipset antenna

vidal — Tue, 27 Jun 2017 19:30:14 GMT

Hello Kristin,

Thanks a lot for your swift reply. I have a new questions...

How can I measure the output power?.

About VNA I suppose that the signal of VNA is in C13 and the GND of VNA in L4 (GND), that is right?. About this process...that means that doing that we will be sure about the impedance adaptation between all parts but...what about the frequency resonance?. Imagin that impedance is right but frequency resonance is wrong... I suppose that you make some calculations to do that. Can you shared with us this calculations.

Thanks in advance.

Jose.

RE: Antenna tuning with chipset antenna

FormerMember — Tue, 27 Jun 2017 08:11:11 GMT

Impedance/output power of the chip: There are two ways to verify that the impedance is approximately 50 Ohm:

Measure output power: If you have followed (copied) the reference layout, it should normally not be necessary to modify the values of C3 and L1. You can verify that the impedance is correct (50 Ohm) by measuring the output power using a spectrum analyzer. If the measured output power is the same as what it was set to be, the impedance is 50 Ohm.

NB: Remember to measure the signal level of the harmonics and the frequency of 2xRX LO (center frequency 4.88 GHz) as well.

Measure the impedance: A vector network analyzer (VNA) can be used to measure the impedance of the chip. Connect the VNA "to the chip" after C3 and L1, set the radio in nRF52832 in RX or TX mode, and measure the impedance. The impedance should be approximately 50 Ohm.

Tuning of the antenna: To tune the antenna, a VNA should be connected to the antenna, by connecting it right before the matching network, the order will be the following: VNA connected - matching network - antenna. The connection towards C3/L1 should be cut of, see picture below.

When the VNA is connected, you should change the values in the matching network until the measured impedance is as close to 50 Ohm as possible across the band. Use the smith chart to get an approximation of the values for the matching network. Start with the components closest to the antenna.

Typically, chip antennas are quite narrow banded. We usually say that if the measured SWR is below 2 across the band, the antenna is well tuned.

As initial values, when doing the tuning, I would recommend the following: C13 and L5 is 0 Ohm, and L4 is not mounted.