Antenna tuning with chipset antenna

[vidal]

Hello,

I developed a Nordic board with 2450AT18B100 chipset antenna. I follow the design of Solar Powered Sensor Beacon.

infocenter.nordicsemi.com/index.jsp I test it and it works fine, but I want to tuning the antenna in order to get minimum return lose in signal.

So, I implement this antenna circuit in my board.

From johanson technology values are:

I suppose that from C3 and L1 you have to modify some references values of johanson chip antenna.

In general, my questions are:

Can you explain me each part of this circuit?.

Can you explain me why do you change this values and how do you calculated?.

Can you explain me how tuning the antenna?. Because your reference document for that do not explain how to do it in this case.

infocenter.nordicsemi.com/.../nwp_017.pdf

Thanks in advance.

Jose.

Others attached files. Conductor Impedance

Trace Lenght Solar Beacon

[vidal]

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.

[FormerMember]

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?

[vidal]

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.

[FormerMember]

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.

[vidal]

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.

[FormerMember]

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