868 MHz, 2-way remote control: How to test the matching network?

[edaenrico]

Hi,

I'm designing a 2-way remote control working at 868 MHz.

The RF chip is made by Texas Instruments and the feeding is coplanar.

How can I test the matching network and the antenna design?

The PCB is 2 x 2 inches.

I got a Vector Network Analyzer.

Thanks in advance,
Enrico Migliore

 

[jiripolivka]

Hi,

I'm designing a 2-way remote control working at 868 MHz.

The RF chip is made by Texas Instruments and the feeding is coplanar.

How can I test the matching network and the antenna design?

The PCB is 2 x 2 inches.

I got a Vector Network Analyzer.

Thanks in advance,
Enrico Migliore

This is a quite typical case. You have a device generating a signal (transmitter) which, to be tested, must be matched to 50 Ohms which is the important impedance of most of test instrumentation.
Then you have an antenna and it should be tested to match devices with 50-Ohm impedance.

You have a VNA which tests impedance matching of any device to its 50-Ohm impedance.

First read carefully VNA instructions. To test your transmitter, its output power must be adjusted to VNA specifications. You can connect it as an external source, and find S22 of it to VNA matched load of 50 OHms.

To test your antenna, you will use it in place of VNA load. VNA internal signal source will be used to feed the antenna, and VNA will measure S11 of your antenna. Make sure that the antenna is located in "free-space" conditions, at least pointed out of laboratory window. No conductive objects closer than 0.5 meter. During antenna S11 test with swept frequency VNA output, you might see variations of the plot if you move your hand around the antenna under test.

If you "tune" your transmitter and antenna so their S22 and S11 are close to "1", then both devices are matched to 50 Ohms. You can connect them together with 50-Ohm connectors, or with a section of a 50-Ohm cable suitable for 868 MHz. Usually, the receiver section of your transceiver needs no separate matching circuit.

As you mentioned your device is installed in coplanar line, you will have to buy or make a transition to 50 Ohms coaxial. Such device can include tuning elements by which you can adjust the best matching to 50-Ohm connector, with the help of your VNA as above described.

 

[edaenrico]

Hi,

thank you for your clear answer.

The problem is that the transmitter (RF Integrated circuit), the matching network and the antenna are really close to each other.

I would like to design a PCB antena.

Enrico Migliore

 

[jiripolivka]

If the data sheet of your RF IC indicates the output impedance and hopefully close to 50 Ohms, then you should match your antenna to this impedance (if not exactly 50 Ohms, then the complex conjugate). Or a simpler way can be to match the antenna(s) to 50 Ohms, then try to use tuning elements around the line joining the RF IC and antenna while monitoring the received signal level at the opposite receiver, to optimize the system.
Some RF ICs have an antenna tuning section for such optimization.

 

[edaenrico]

Hi,

> Some RF ICs have an antenna tuning section for such optimization

Could you be more precise on that?

Enrico Migliore

 

[jiripolivka]

I have seen new RF ICs for GSM band which have antenna-tuning sections, allowing to optimize antenna tuning. Please check the maker of your RF IC.