NTM88 434MHz impedance at RF pin

[tpineau]

Hi,

I would like to design a matching network to have maximum output power at 434MHz. The NTM88 is set to maximum power step and I have matched my network to have an impedance close to 154+j*241 ohms as indicated in the doc "TPMS_Impedances_for_matching_network".

I measured the impedance with a vna and de-embedded my RF probe (coax) to measure the exact impedance at the RF PIN. I found an impedance of 109+j276 ohms at my RF PIN using the circuit below:

I'm very close to the match, but when I measure the RF power conducted, I don't have the maximum power of 8dBm, I have 4dBm. And when I try to match my network to other impedance like 85+j*152 ohms, I have a better result around 7 dBm.

I wonder if the impedance of 154+j*241 Ohm is the good target. Can you help me?

Thanks,

Titouan

[ErikaC]

Hello,

What is the procedure that you are following? (slide 4 or slide 5)

Please note that "Customers not using the same setup as NXP have to go through the de embedding process and target the recommended
impedance at the RF pin plane. In other words, the impedance recommended at the RF pin is the impedance that needs to
be targeted in case customers use any other access to the RF pin than a simple SMA connector. In that case customers
must have a reliable model of that access to do a reliable de embedding."

[tpineau]

Hi ErikaC,

Thanks for the reply. I am following the procedure of slide 4, but I also tried slide 5.
I think I have noted this well.
To give more information, I am targeting the impedance at the RF pin plane with my VNA. My VNA has a termination impedance of the conjugate of the recommended impedance, it allows me to match my network with the reflection coefficient. I have matched my network to 201+j*238 (slide 4) and 154+j241 (slide 5), but both give me bad results in power measurements. While 85+j*152 impedance, which is far from both recommended impedances, gives me better results.


Have a good day,

Titouan

[ErikaC · Accepted solution]

Regarding the impedance on slide 4: the setup should be exactly like on the slide i.e. connect a 50ohm SMA connector to the piece of PCB including the matching network and antenna. Then just measure the impedance with a VNA. This is the most precise method as the SMA connector has a standard impedance (50ohm). No calculations are needed, just measure the impedance with the VNA connected to the SMA connector. In this configuration, the target impedance measured by the VNA is 201 + j*238, and it includes the SMA connector impedance.

• In your procedure, make sure that you are measuring the impedance via a SMA connector soldered to the PCB, and not via an RF probe.

For the impedance on slide 5: the impedance of 154 + j*241 has been calculated following a de-embedding process. The de-embedding process includes modelling of the RF probe, so per definition it is not 100% accurate. And an additional error also comes from the customer’s de-embedding calculations.

If you cannot find a suitable matching network using this method, it may be because the de-embedding calculations are not accurate enough (it is not easy to have a good model of the RF probe). In such case, I would recommend going with the procedure on slide 4, as it has no approximation.

As side information, the values of the impedances presented in this document have been used by customers for years, so there should be no typos. The ppt is dated from 2019, but this is just the date of the document. The values of the impedances, especially on slide 4, have been calculated and shared before that.