How to choose an Application specific Signal Detection Threshold for TDA523x based ASK Mode Applications

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Signal Detector Threshold in ASK-Mode

How to choose an Application specific

Signal Detection Threshold

for TDA523x based ASK Mode Applications

The TDA523x has an integrated Signal Detector, a mechanism to distinguish in between useful data and noise.

The most relevant part of the Signal Detector is the so-called Matched Data Filter. This digital filter compares the incoming signal stream with the expected waveform of Manchester coded bits (0, 1, space, mark) in the defined data -rate. The Matched Data Filter has a Signal Power Output, which is influenced by RF input power, and the shape and accuracy of the incoming bit-stream.

In ASK-Mode, the decision between noise and data is done by comparing the signal-power of the filtered data signal against a user adjustable threshold. The chosen threshold of the Signal Detector influences Sensitivity, Dynamic Range and False Alarm Rate of the receiver system.

This document describes how to determine optimal threshold setting of this unit in customer specific ASK-Mode Applications.

At the end of this document there is also a Quick Guide, containing a short summary of required steps, and a simplified procedure.

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Equipment Required

• Arbitrary Signal Generator

• RF Waveform Generator

• Setup for reading and writing TDA523x registers

• Test environment similar to final use

• Anechoic or shielded chamber recommended

Note: For the quick procedure Signal and Waveform Generator are not required

Preparation

Make sure that all SFR’s of the TDA523x are set correctly. Use the available SW application tool for setting generation.

TDA523x set to Run Mode Slave (SFR CMC0, 0x02 set to 0x22)

System Properties and Definitions

• Message Error Rate (MER) indicates how many messages are received correctly.

messages

d

transmitte

of

number

messages

received

correctly

of

number

MER

=

1 −

• Sensitivity is the range of the signal strength at the antenna of the system, where the Message Error Rate is less or equal to 10%.

1 10− ≤

=

MER

th

streng

signal

y

Sensitivit

• Dynamic Range is the range of the signal strength at the antenna of the system, where the Message Error Rate is lower than 0.1%.

3 10− ≤

=

MER

th

streng

signal

ge

DynamicRan

• False Alarm Rate (FAR) is a quantum for how often the receiver mistakenly interprets noise present

channel

the

on

data

for

searching

periods

of

number

ups

wake

mistakenly

of

number

FAR

=

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Interdependency between False Alarm Rate and different Wake Up

Criteria

In Self-Polling Mode, while the TDA523x autonomously switches the receiver section between a low power sleep mode and active modes by following one of several available polling schemes, the TDA523x performs checks whether activity of interest was detected on one of the checked receive channels. If an activity of interest was detected, a so called Wake Up takes place and keeps the receiver in an active mode to allow the reception of the complete data.

There are four different so called Wake-Up Criteria available for the decision. The criteria, which may be applied, possess different strength in supporting the Signal Detector in providing a low False Alarm Rate of the receiver system. A stronger Wake-Up Criteria improves the False Alarm Rate behavior.

Four available Wake-Up Criteria are listed below, ordered from the strongest to the weakest:

• Pattern detection

The data pattern received during the Wake-Up check has to match a predefined Manchester coded pattern.

• Equal Bits Detection

The data pattern received during the Wake-Up check has to be either a

Manchester coded sequence of ones or a Manchester coded sequence of zeros.

• Random Bit Detection

The data pattern received during the Wake-Up check has to consist of a sequence of correctly Manchester coded bits. The length of the sequence is customizable.

• Valid Data Rate Detection

The Wake-Up check is limited to test, if the data pattern received during the test allows the data clock recovery PLL to synchronize on it.

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Special Function Registers Involved

Several Special Function Registers (SFR’s) have to be either read out or written. These SFR’s are:

Note: Most of the registers below are twice available. Once for channel A, starting with character A, once for channel B, starting with character B.

Bit Name Register Addr. Bits R/W Description

SLRXEN CMC0 0x02 1 W Slave Receiver Enable, 0…receiver is in Sleep

Mode, 1…receiver is in Run Mode

MSEL CMC0 0x02 0 W Operating Mode Select, 0…Slave Mode, 1…Self

Polling Mode

Chip Mode Control Register should be set to 0x22 to enable Run Mode Slave and allow required measurements.

ASKNP ASKNP 0xB0 5:0 R ASK Noise Power. Signal Power after Matched

Data filter. Read out to get signal and noise power for threshold calculation

The lower 5 bit of this register contains the actual signal power figure required for threshold calculation. The register is updated 16 times per data-bit-clock.

SDCNT ASIGDET0

BSIGDET0

0x71 0x91

7:6 W Signal Detector Threshold Counter. Allows to

ignore eventually occurring bit failures.

00b:disabled (default), 01b: ½ bit, 10b:1 bit, 11b:2 bit

SDTHR ASIGDET0

BSIGDET0

0x71 0x91

5:0 W Signal Detector Threshold Level. Used to set the

Signal Detector Threshold as calculated in this application note.

This is the Signal Detection Threshold Register for the Run Mode. Use ASIGDET0 if used for Channel A, use BSIGDET0 if used for Channel B.

Use of Signal Detector Threshold Counter is not recommended in normal applications; therefore SDCNT should be set to 00b to disable the counter.

Use the result of the calculation described in this application note for SDTHR.

SDLORE ASIGDETLO

BSIGDETLO

0xB6 0xB7

7 W Source selection of ASK Noise Power status

register. Use 0 for ASK

SDSEL ASIGDETLO

BSIGDETLO

0xB6 0xB7

6 W Manual selection of SIGDET0/1 range

This SFR is mainly used for FSK. But two bits are also relevant for ASK. SDLORE should bet set to 0 for ASK, and SDSEL should be also set to 0 in the majority of applications. Only if the calculated SIGDET values exceed the range of 63 decimal, set SDSEL to 1 and select a range value as below.

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SDSELLO ASIGDETSEL

BSIGDETSEL

0xB8 0xB9

3:2 W Used for FSK only

SDSEL ASIGDETSEL

BSIGDETSEL

0xB8 0xB9

1:0 W SIGDET0/1 range selection factor

00b:4, 01b:6, 10b:8(default), 11b:10

Set SDSEL to fit the SIGDET range. Usually this setting is not required, and should not be touched.

In case the later described calculation result is a value higher 63 decimal, manual

SIGDET range should be selected and SDSEL values should be increased step by step.

SDCNT ASIGDET1

BSIGDET1

0x72 0x92

7:6 W Signal Detector Threshold Counter. Allows to

ignore eventually occurring bit failures.

00b:disabled (default), 01b: ½ bit, 10b:1 bit, 11b:2 bit

SDTHR ASIGDET1

BSIGDET1

0x72 0x92

5:0 W Signal Detector Threshold Level. Used to set the

Signal Detector Threshold as calculated in this application note.

This is the Signal Detection Threshold Register for Wake Up. Use ASIGDET1 if used for Channel A, use BSIGDET1 if used for Channel B.

Usually SIGDET0 and SIGDET1 should be equal if ASK is used as well for wakeup and run mode.

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Signal Detector Threshold

The diagram below shows the signal power output of the Matched Data Filter, and the influence of the chosen threshold setting.

The black curve in the uppermost diagram shows the transmission of data at the transmitter side of the system. The high level represents valid Manchester coded data transmission in the correct bit-rate . During the low level time there is no data transmission from the transmitter.

All other diagrams symbolize the signal power present at the output of the matched data -filter of the receiver system available from the SFR ASKNP (ASK Noise Power, Addr. 0xB0, bit 5:0). The shown values are influenced by the signal power at the antenna, but also by the waveform accuracy of the bit-stream. Low levels mean noise, distorted or not fitting bits, high levels represent good data and reliable transmission.

This signal power is compared against a given threshold in the Signal Detector. The result of this comparison is used by the Signal Detector to decide between noise and valid data bits.

In the drawing above, the three lower diagrams describe the influence of the selected Signal Detector’s threshold on the receiver’s system properties. A too low threshold causes increased False Alarm rate, and may also increase MER. A too high thresho ld decreases sensitivity and dynamic range. The middle diagram symbolizes an optimal threshold setting with minimal false alarms and message errors.

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Procedure to determine a practical

Signal Detector threshold

in customized receiver systems

Generally the optimal threshold setting is as low as possible to have optimal sensitivity, but well above noise level to prevent incorrect wake ups.

Step One – Characterization

The signal power is updated with a rate of 16*fdata and can be read by the user via special

function register ASKNP (the name ASK Noise Power was chosen, to illustrate the major usage of this register).

The characterization requires a continuous ASK-modulated, Manchester coded pattern containing zeros and ones (PRBS9 recommended) to be applied to the antenna input of the receiver system and a power-level sweep for this input signal to be performed. The level sweep starts with a level below the system’s noise floor and ends when a saturation of ASKNP register readouts can be observed. Typically such a sweep starts with a power-level of -130 dBm, extends to a level between -100..-80 dBm and applies a step size of 1 dB. Typically 500 single measurements should be done for every power-level. An exemplary output of a characterization as described is shown in the diagram below:

Example:

Center frequency 433.92MHz Bit-rate 9600bps

Results of a measurement as above are shown in the following graphic:

ASKNP over Input Signal Power PRBS9 Pattern 0 10 20 30 40 50 60 70 -130 -120 -110 -100 -90 -80 Input Power [dBm] ASKNP average standard-deviation

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.

Step Two – Calculation

Two numbers reflecting two major system parameters, signal-power and noise-power, are calculated from these measurements:

Signal-power is the average of all measurements performed at a distinct power-level. Noise-power is the so-called standard deviation (sigma) of all measurements performed at a distinct power level.

For our calculation the measurements at -130dBm to -125dBm are used (the measurements with higher levels are used to verify functionality of the receiver system).

)

(

Noise

Power

Signal

Threshold

=

+

λ

∗

Signal Power…average of measurements

Noise Power…standard-deviation of measurements (=sigma)

Note: standard-deviation is calculated by (( ) ( )2 ... ( )2) 2 2 1 1 1 avg n avg avg n− x −x + x −x + + x −x

The factor λ in the rule above reflects the interdependence between the Signal-Detector’s threshold and the strength of the additional Wake-Up criteria in place. To achieve a FAR of 10-5, the initial estimation for λ lies in the range of 2..3, depending on the Wake-Up criteria.

Use λ =2.5 for Pattern Detection, λ =2.7 for Equal Bits Detection, λ =3 for Random Bit Detection and Valid Data Rate Detection. Use λ =2 only when FAR has low priority.

Example:

Using our example data, we calculated:

Signal power = 11.8, noise power = 5.8, λ =2.7 (weak Wake Up criteria) Threshold = 27.46 >> 27 decimal >> 0x1b

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ASKNP over Input Signal Power PRBS9 Pattern 0 10 20 30 40 50 60 70 80 90 -130 -120 -110 -100 -90 -80 Input Power[dBm] ASKNP

average standard-deviation average +3xsigma Threshold

Step Three – Verification

The previous step yielded in an initial estimation for the Signal Detector threshold. The calculated value has now to be used for verified for practical use.

Put special focus on:

• Sensitivity at center receive frequency

• Dynamic Range at center frequency

• False Alarm Rate

• Sensitivity over the complete receive frequency range

• Any system parameter, which is critical in a distinct application

Example:

The verification of our example showed a sensitivity of 106.5dBm, a dynamic range of -103.5dBm, and FAR was < 10-5.

The graphics below show sensitivity and dynamic range, and sensitivity over frequency offset.

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MER over Input Power

1,E-03 1,E-02 1,E-01 1,E+00

-109 -107 -105 -103 -101 -99

Input Signal Power [dBm]

Message Error Rate

Sensitivity over Frequency Range

-107 -106 -105 -104 -103 -102 -101 -100 433,72 433,77 433,82 433,87 433,92 433,97 434,02 434,07 434,12 Frequency [MHz] Sensitivity [dBm]

Step Four – Optimization

If all critical system properties have been verified and all targeted parameters were met, step four is obsolete and the determined Signal Detector’s threshold can act as the basis for further system evaluation.

The initial estimation for the Signal Detector’s threshold may not always yield the fulfillment of all target parameters and thus an optimization of the threshold may be required. In this case, the current value of the threshold should be increased or decreased by one and the procedure should be continued with the previous step.

The threshold value should be increased by one, if

• The assessment of the Dynamic Range shows a large number of Message Error Rate peaks of 10-3 or larger, although the power at the antenna of the system was substantially above the Sensitivity of the receiver system, when these Message Error peaks occurred.

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• The False Alarm Rate target was not met.

A loss of Sensitivity of the receiver system has to be expected in the case, that the Signal Detector’s threshold is increased.

The threshold value could be decreased by one, if

• All targeted system parameters where met in the preceding assessments.

A gain in Sensitivity of the receiver system could possibly be achieved in the case, that the Signal Detector’s threshold is decreased.

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Quick Guide

• Download TDA523x Config File via “TDA523x Explorer”, or configuration data via your application.

• Set TDA523x to Run Mode Slave by writing 0x22 to SFR CMC0 (address 0x02)

• Set RF Signal Generator to OFF.

• Start 500 readouts of SFR ASKNP (address 0xb0).

• Calculate average of readouts à Signal Power, and standard deviation à Noise Power

• Select a value for the factor λ between 1 and 3, depending on your wake up criteria and priority of False Alarm Rate. Typically use 2.5 to 3. For further information refer to the detailed part of this application note.

• Calculate Threshold = Signal Power + λ * Noise Power

• Load Threshold to SFR SIGDET.SDTHR (address 0x71, 0x91, 0x72, 0x92.5-0)

• Optimize Threshold. Increase by 1 if

o Dynamic range shows Message Error peaks of 1E-3 or higher at high input signal levels.

o False Alarm Rate target is not reached.

• Decrease by 1, if

o All system parameters are met. A lower Threshold can mean higher Sensitivity and Dynamic Range.