Synchronous averging 02 2003

Some History?„ The first Digital Signal Analyzer were Waveform Averagers HP 5480A Circa 1967 External Trigger Analog-to-Digital Converter Digital Data Operator Controls... „ When the Sig

Synchronous Averaging - The

Trigger is Everything

Gerry Priebe - Pre-B-Tek

Representing Data Physics Corporation in North and

South Carolina

Synchronous Averaging is used to detect a signal in uncorrelated noise

Synchronous Averaging – Case Study

Case Study – Paper Machine Calender Section Used for changing and/or controlling sheet caliper (thickness)

Synchronous Averaging – Case Study

Paper Machine Calender – Before Problem

Pk to Pk: 0.261507 Mean: 0.00723551

Pk to Pk: 1.64547 Mean: 0.00741009

Vibration Synchronous to Top Roll – 1.64 ips, pk-pk

Vibration Synchronous to Bottom Roll – 0.26 ips, pk-pk

Synchronous Averaging – Case Study Paper Machine Calender – After Problem

Correction

0°

180°

90° 270°

1.0 in/s

-1.0 in/s

RMS: 0.0717688 Max: 0.13472 Min: -0.131216

Pk to Pk: 0.265936 Mean: 0.00349375

0°

180°

90° 270°

1.0 in/s

-1.0 in/s

RMS: 0.00733213 Max: 0.0220629

Pk to Pk: 0.0353916 Mean: 0.00328174

Vibration Synchronous to Top Roll – 0.03 ips, pk-pk

Vibration Synchronous to Bottom Roll – 0.26 ips, pk-pk

Synchronous Averaging

„ What is it?

„ What are the benefits?

„ What can go wrong?

Synchronous Averaging

„ Synchronous averaging

involves triggering data

acquisition with respect to

external events such as

tachometer pulses

„ Triggered acquisitions are

averaged to remove the

What is Averaging?

„ A data frame of some arbitrary length (N) is gathered into an array (Block 1)

Frame 1

New Result

Average

What is Averaging?

„ Eight records with a signal buried in noise

R1 -0.12323 -1.01524 1.381033 -1.92251 0.456146 0.392856 1.874923 -1.96605 -0.25703 -1.08908 R2 0.369698 0.761429 0.920689 1.441884 -0.3041 1.598214 2.187615 -0.49151 0.514061 0.544538 R3 -0.49293 0.25381 1.841378 0.961256 -0.45615 3.205358 2.125077 0.491513 -0.77109 -0.81681 R4 -0.24647 -0.50762 -1.38103 0.480628 0.152049 2.803572 2.250154 1.966052 -1.02812 -0.54454 R5 0.492931 -0.25381 -1.84138 -0.96126 -0.15205 3.607144 1.937462 0.983026 0.257031 0.272269 R6 -0.3697 -0.76143 0.460344 -1.44188 0.608194 0.794642 1.749846 1.474539 0.771092 0.816807 R7 0.123233 1.015239 -0.92069 1.922513 0.304097 2.401786 1.812385 -1.47454 -0.51406 -0.27227 R8 0.246465 0.507619 -0.46034 -0.48063 -0.60819 1.196428 2.062538 -0.98303 1.028122 1.089076

What is Averaging?

„ Eight records with a signal buried in noise

-3 -2 -1 0 1 2 3 4

R1 R2 R3 R4 R5 R6 R7 R8

What is Averaging?

„ Eight records with a signal buried in noise

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

A2

What is Averaging?

„ Eight records with a signal buried in noise

-1 -0.5 0 0.5 1 1.5 2 2.5

A4

What is Averaging?

„ Eight records with a signal buried in noise

-0.5 0 0.5 1 1.5 2 2.5

A8

Some History?

„ The first Digital Signal Analyzer were Waveform Averagers (HP 5480A Circa 1967)

External Trigger

Analog-to-Digital Converter

Digital Data

Operator Controls

What are the benefits of

Synchronous Averaging?

„ Reduce noise

– Noise Reduction is

proportional to the square root of the number of averages 1

Records Averaged

Noise Reduction (dB)

What are the benefits of

When Synchronous Averaging

won’t work?

„ When the Signal being averaged does not appear at the same place each and every frame

When Synchronous Averaging

won’t work?

„ When the Signal being averaged does not appear at the same place each and every frame

– Trigger jitter

– Signal jitter

– Poor trigger signal shape

– Noise on the trigger signal

– Synchronous noise - can’t be removed

-500.0m 0 500.0m 1.0

Signal Jitter

Detected by examining the average of the trigger signal

Examination of the first trigger pulse in the average shows that the pulse is the correct shape Therefore, the average appears to be starting at the same place each record However, the fact that the average of the trigger deteriorates as time increases suggests

that even though the trigger is stable the time between pulses is not stable

-1.5 -1.0 -500.0m 0 500.0m 1.0 1.5 2.0

sec

What Causes Jitter?

„ Relative motion between the machine and the photo-tachometer2

Laser Tachometer

Poor Trigger Signal Shape

Noise on the Trigger Signal

Synchronous noise

-4.0 -3.0 -2.0 -1.0 0 1.0 2.0 3.0 4.0

What Causes Jitter?

„ When the signal being averaged does not appear at the same place each and every frame

– Poor trigger processing in the analyzer

What Causes Jitter?

„ Poor trigger processing in analyzer

Square waveform

Square waveform

What Causes Jitter?

„ Test One: Trigger on Input Channel

Square waveform

Square waveform

What Causes Jitter?

„ Test One: Results

750.0m

-8.0 -6.0 -4.0 -2.0 0 2.0 4.0 6.0 8.0

What Causes Jitter?

„ Test Two: External Trigger Input

Square waveform

Square waveform

What Causes Jitter?

„ Test Two: Results

750.0m

-8.0 -6.0 -4.0 -2.0 0 2.0 4.0 6.0 8.0

What Causes Jitter?

– What is different between triggering on an

What Causes Jitter?

„ The Sampling Frequency

– When the trigger is connected to the input

„ In the case of our example the Sampling Frequency Fs = ~ 2,600 Hz

„ Therefore, Delta T = 400,000 nS

„ The trigger jitter = + 200,000 nS

What Causes Jitter?

„ The Sampling Frequency

– When the trigger is connected to the

much higher rate to minimize the jitter

„ In the case of the DP Mobilyzer the external trigger is sampled at a frequency of 10 MHz

„ Therefore, Delta T = 100 nSec

„ The trigger jitter = + 50 nSec

Frequency Domain Errors

„ Consider the analysis of a Square Waveform

– If a perfect square wave is analyzed:

Frequency Domain Errors

Note that even though AE, RC & RE don’t agree with theory (T) they do agree with each other

How big is the error?

% Error of Each Harmonic

How do I know?

„ Look for evidence of jitter

– Examine the average of the tachometer

„ Is the average of the tachometer good at the beginning and bad at the end?

„ Is the shape as expected?

averaging

Order Tracking vs RPM-related

„ RPM-related for slowly changing machine speed, constant speed within a revolution

– Needs only a once per rev tach

„ Order tracking for quickly changing machine speed, or

speed that varies though the cycle

Order Tracking

How is Triggering done?

„ Check the analyzer specifications

– How does the trigger input circuit work?

– Determine the speed of the trigger sample clock

Abacus – Data Flow

DSP

32 8

INPUTS measure voltage

How is Triggering done?

How is Triggering done?

„ Check the analyzer specifications

– Is the trigger threshold adjustable?

– Is the Slope (+ or -) selectable?

– Is trigger Hysteresis available and adjustable?

– Is trigger Hold Off available and adjustable?

„ Typically these features are not available for analyzers with poor triggering

How is Triggering done?

„ DP Abacus

(Mobilyzer II) = 25

MHz (i.e + 20 nSec)

How is Triggering done?

„ Agilent E1432A = 20 MHz

(i.e + 25 nSec)

How is Triggering done?

„ DP Mobilyzer = 10 MHz (i.e + 50 nSec)

How is Triggering done?

„ DP ACE = 51.2 kHz (i.e + 9,700 nSec)

How is Triggering done?

„ DP Abacus (Mobilyzer II) = 25 MHz (i.e + 20 nSec)

„ Agilent E1432A = 20 MHz (i.e + 25 nSec)

„ DP Mobilyzer = 10 MHz (i.e + 50 nSec)

„ DP ACE = 51.2 kHz (i.e + 9,700 nSec)

„ DAT Tape (5 kHz) = 12.8 kHz (i.e + 39,000 nSec)

„ Input Channel Trigger = 2.6 kHz (i.e + 200,000 nSec)

„ YOUR ANALYZER = ?

„ Jitter effects can be subtle & hidden

„ Not all analyzers are created eQuaL

„ The same problems arise for digital

tape recorders

„ Best analysis is “live” and on the scene with a well designed DSA

End of Presentation