ST25R3916B Target Phase Automatic Antenna Tuning

[LishengZou]

ST25R3916B offers two types of AAT algorithms, target amplitude and target phase. I understand that the target amplitude algorithm is used to achieve certain power level or detectable range.

However, I don't understand the use cases for the target phase algorithm. What is the benefit of having a certain phase difference between the RFO and RFI? What parameter can be affected by the phase shift between the RFO and RFI. 

Thanks in advance. 

[Travis Palmer]

Hello Lisheng Zou,

The amplitude algorithm adjusts to the highest amplitude of antenna voltage (=highest amplitude on RFI).

In most cases this is at the lowest possible target matching impedance while still being on the resonance sweet spot - especially when using series and parallel cap tuning.

If you want to target a specific impedance and don't want to go below a certain impedance you can also use the phase algorithm. Here you can define the phase relation between RFO and RFI voltage. Due to the measurement via captive voltage divider, this measured phase is not the phase of the target matching impedance.

I hope this answers your question. In addition please have a look at AN5322.

BR Travis

[LishengZou]

Thanks for your reply, Travis. 

I have read AN5322 and AN4914, but still have something unclear. According to AN4914, there are three types of algorithms, maximum amplitude, target amplitude, and target phase.

1. The maximum amplitude algorithm finds the highest amplitude (or lowest matching impedance).

2. The target amplitude algorithm finds the closest value for the target amplitude (or target matching impedance).

3. The target phase algorithm finds the closest value for the target phase difference between RFI and RFO. 

You mentioned that the target algorithm should be used to target a specific matching impedance. For me, It sounds like the target phase algorithm is doing the same thing as the target amplitude algorithm. 

My two follow-up questions would be: 

1. Can you explain further what is the difference between the target phase and target amplitude algorithm regarding the use cases? Basically, when should I use the target phase instead of the target amplitude?

2. How does the phase shift affect the matching impedance? Does the matching impedance only depend on the resonance frequency? 

Thanks in advance.

[Travis Palmer]

Hello Lisheng Zou,

For now, lets assume max amplitude and target amplitude is the same. (Max is just a different target).

The target phase algorithm can be beneficial if you want to end up with the same/ good wave shapes along de-tuning.(dependent on how much your antenna will be de-tuned.

The target amplitude you can ensure a certain target matching impedance and therefore output power - if reachable over de-tuning.

The target phase should for example find a matching impedance along the orange line and the target amplitude along the green line. 

If you would combine both algorithm using a weighting function, it should be possible to target an exact target matching impedance.

br Travis