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# Why You Need to Measure Both BER and MER on QAM Digital Signals

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…a step
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2-14-2002

## 2. Introduction

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Most Digital Analyzers measure Modulation Error Ratio
(MER) and Bit Error Rate (BER)
MER and BER each have their limitations
This seminar explains why it is important to measure both
MER and BER and what types of impairments that will be
missed if you only measure one or the other.
Viewing of the Constellations Demystified Seminar and
the Modulation Error Ratio Demystified Seminar is
recommended prior to viewing this seminar.
2-14-2002

## 3. Modulation Error Ratio

RMS error magnitude
MER is defined as follows:
10 log
MER is expressed in dB.
average symbol
magnitude
RMS Error
Magnitude
Ideal
Symbol
Average
Symbol
Magnitude
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2-14-2002

## 4. MER

MER effectively assigns a value to the fuzziness of the
symbol cluster.
The larger or fuzzier the cluster, the poorer the MER.
The further from the ideal locations, the poorer the MER.
Constellation With
“Good” MER
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Constellation With
“Poor” MER
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## 5. How Errors Occur

7
Each symbol on the constellation is framed by decision
boundaries
5
When the carrier falls inside the decision boundaries the information is
3
transmitted error free.
1
-7
-5
-3
-1
1
-1
3
5
7
-3
Correct Locations Fall
Within Decision
Boundaries
-5
-7
Locations in Error Fall Outside
Decision Boundaries
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2-14-2002

## 6. Comparison Between Three Error Free Constellations

All constellations below have a perfect BER with no errors, because
the carrier
always falls within the decision
boundaries.
7
7
7
The constellations to the right have significantly better MER with less
5
5
noise. 5
When the
boundaries, BER is not 3an
3
3 cluster falls within the decision
effective measurement of quality because the BER is perfect.
1
1
-7
-5
-3
-1
1
-1
5
7 -7
-5
-3
-1
1
-1
3
5
-77
-5
-3
-1
1
-1
-3
-3
-3
-5
-5
-5
-7
-7
-7
Poor MER
Perfect BER
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3
1
Good MER
Perfect BER
3
5
Best MER
Perfect BER
2-14-2002
7

## 7. MER vs BER With Only Gaussian Noise Impairing the 64 QAM Signal

Note there are no errors in
this range of MER.
1.00E-09
No
Errors
1.00E-08
BER
1.00E-07
1.00E-06
1.00E-05
1.00E-04
1.00E-03
35 34 33 32 31 30 29 28 27 26 25 24 23 22 21
1.00E-02
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MER
In practice errors will tend to occur at higher MERs due to
other forms of impairments besides Gaussian Noise.
2-14-2002

## 8. Why Measure BER?

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Since MER can quantify signal quality when no errors
exist the question can be raised, why measure BER at all if
MER will do?
The major limitation of MER is the inability of the
measurement to capture fast intermittent transients.
A signal can have a very good MER, but poor BER due to
intermittent interference.
2-14-2002

## 9. Examples of Intermittent Interference That Cause Poor BER But Good MER

Laser Clipping
- Occasional overload of the laser due to analog sync pulses lining
up.
Loose Connections
- Corroded or loose connections.
Sweep System Interference
- Sweep pulses from a sweep system set up to sweep empty
spectrum.
Microphonics
- Vibration of digital origination equipment can cause intermittent
errors.
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2-14-2002

## 10. Troubleshooting By Measuring Both MER and BER

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One way to determine if you have
intermittent problems is to measure both
MER and BER.
If the MER is high, but you still see errors,
then the errors are probably caused by a
intermittent problem.
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## 11. Intermittent Errors on a Constellation Display

Intermittent errors will show
up on a constellation display
as lone dots away from the
main cluster.
Zoomed Constellation
with Intermittent
Interference
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2-14-2002

## 12. Conclusions

In order to see the effects of all
types of impairments on a QAM
signal you need to measure both
MER and BER.
MER can quantify the quality of
a digital signal that doesn’t have
any errors.
MER has the limitation of not
being able to see intermittent
errors so a signal can have a
good MER but a poor bit error
rate.
The most common type of
intermittent errors are caused by
laser clipping.
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CM1000 Cable
Modem System
Analyzer
AT2000
Spectrum
Analyzer
2-14-2002