Eddy Veltman

May 28, 2020

Demodulation of distorted ASK signals

May 28, 2020 PUBLIC2

Recap WPC1605 Seattle

• Philips presented the Buck-Royer converter topology in the Resonance Task Force• Technology was demonstrated with medium-z demonstrator• Advantages:

- improves EMC performance- tunable effective resonance frequency- inherent stability for load steps even at low coupling

Nexus 5 charging on Royer demonstrator15 mm coil to coil distance

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Distorted ASK is side effect of inherent stability for load steps

Effect of a load step at Rx ( 1 kOhm => 5 Ohm )

• Coil voltage is automatically kept constant during load steps

Load is connected to Rx output

PUBLICMay 28, 2020

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Examples of distorted ASK bits as seen by the transmitter MCU

• Signal samples are directly from the MCU ADC.

• Each bit stream example is made with different z distance, receiver or load.

• The scale of the graph is equal for all signals

PUBLICMay 28, 2020

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Standard steady state based demodulation method

• Uses steady state signal levels in bits for digitizing• Very simple. Requires minimum resources from MCU• Sensitive for bit shape deformation and low frequency variation on top of ASK modulation• Not suitable for Royer topology

PUBLICMay 28, 2020

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Improved transient response based demodulation method

• This digitizer only triggers on the steepest edges in the demodulated bi-phase signal.• An edge is only considered “valid” when no steeper edge occurs within the validation period after it.• This method is non-causal. It uses information from the near future to determine whether or not an

edge is the steepest. To enable this non-causality the output signal is delayed w.r.t. the original signal.• Signal delay only requires two counters. No buffer memory is needed.

PUBLIC

ADC sampling synchronous with Tx drive

signal

Low Pass Filter

Moving window

differentiator

Low Pass Filter

Peak tracker

deltasensitivity

tuning factor (STF)

gainLPF1 gainLPF2sample phase

Validation period (Tval)

Modulated bit stream

Demodulated bit stream

x y z u

wTpos

Tneg

s

Demodulator

Non-causal peak detector

S

R

QA

A==BB

A A==B

B

Digitizer

Differentiator

May 28, 2020

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Example of non-causal transient response digitizing

Demodulated signal (y)

Digitized bit stream (w)

Validation period

Over here this rising edge is considered “valid” because no steeper rising edge occurred within the validation period after it.

Validation period

Over here this falling edge is considered “valid” because no steeper falling edge occurred within the validation period after it.

time

PUBLICMay 28, 2020

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• A method for good demodulation of distorted ASK signals has been explained• It is very robust due to its transient based and non-causal approach• It requires a minimum increase in MCU resource use w.r.t. the standard method

Conclusions

PUBLICMay 28, 2020