MER Modulation Error Ratio Testing For Digital Headend Equipments

mer testing

Introduction

MER is known as Modulation Error Ratio. In simple words MER is errors detected in modulation between what we transmitted and what we received. MER is measured in dB.

We face a problem generally in cable tv industry that all the channels of all Transport Streams are working fine in headend but in the field we are getting freezing in some channels while signal level of all the Transport Streams is ok. To maintain only the signal level in field is nothing while there are some other parameters responsible for proper delivery of all channels like BER and MER etc.

Sometimes we faces this kind of problem in Digital Headend also and we gets freezing. Although there can be several factors responsible but in this article we will cover MER degradation problem.

To troubleshoot with this MER degradation we need to follow step by step process from QAM to STB. All the values which we will discuss here will be practical values only.

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MER Testing

One thing which we need to keep in mind that the maximum value of MER which we gets from QAMs output is 39 dB for QAM 64. As long as signal goes into the field and passes through different equipmetns we gets degradation of MER due to losses in cables and equipments. For proper delivery of all cable tv services in the field we need to maintain MER as much as can possible close to 39 dB. But practically it becomes very hard to get 39 MER at subscriber end so it can be in between 33 dB to 39 dB for proper delivery of CATV services. Below 33 dB MER you will start facing minute problems in services like freezing even input signal level in ok in stb.

So to do MER Testing in our network we need to follow step by step process from our Digital Headend to Subscriber end. We have divided our network in two parts one is Headend MER Testing and second one is Field MER Testing.

Headend MER Testing

digital headend mer testig

As shown above in the figure we need to follow step by step process to found the MER degradation in headend side. First of all we need to check the RF output of QAM as shown above in the figure there is 4 RF output of qam. So we need to check the MER on each RF port of QAM. In this case we are considering QAM 64. So we need to get 39dB MER on each RF output port of QAM.  It should be 39 dB. If its not there is problem in modulation of QAM.

Now check the RF output of combiner whether it is near by 39 or not. If its not means you are getting 35 dB then you should change the combiner and use other one.

Now check the RF output of Splitter whether it is near by 39 or not. If its not then you need to change it. Now the final output is going into the transmitter for field distribution. See next diagram of “Field MER Testing”.

Field MER Testing

Field MER Testing

The transmitter is giving Optical Fiber output. to the node. Keep in mind one thing here is that there should not be any kind of band in fiber from transmitter to node. If there is band in fiber then also it will degrade the MER value. Now check the RF output of the Node. It should be more than 33 dB. If there is higher loss of MER then first check your node by replacing other one.

If the problem still persist then check your headend transmitter whether it is ok or not. If your headend transmitter is once repaired then there are maximum chances of degrading the MER.

Now check amplifier output MER it should not below 33 dB. If it is then replace the amplifier and check output. So then final input of MER into the STB should be at least 33 dB for better service.

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There are many other hidden factors are responsible for degrading MER are due to high losses in coaxial cable.

Conclusion

MER is a parameter used to denote errors in the modulation for delivery of catv services. For proper delivery of services for QAM 64 its value should be in between 33db to 39 dB. Its maximum value which we gets from qam output is 39dB. We need to maintain this higher value of MER in field also for proper deliver of services. There are various handy RF meters available in the market to measure the value of MER.

Know More About  What Is MER BER

 

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