Use of the Q-factor meter in Telecom Network

Use of the Q-factor meter: The main feature of Q-factor analysis is the short measurement time. Since the BER result is obtained purely by calculation from the Q-factor, results are obtained that may be irritating at first glance: BERs of 10 power -20 and less are determined. However, a direct comparison with a BER measurement performed using a conventional SDH/SONET analyzer cannot be made for the very reason that a single result would require a measurement time of days or even weeks. However, as long as the Q-factor is not used as a standard characteristic of the transmission signal, conversion to the more usual BER quantity will be necessary.

The Q-factor measurement will not replace an end to end measurement with an SDH/SONET analyzer, since this is used to demonstrate and record the quality of service of a transmission path for the customers of network providers. The Q-factor measurement determines the signal quality at a particular point in the transmission range.

Discrete interference

Graphical and numerical display of the results of a Q-factormeasurement

Use of the Q-factor meter in Telecom Network

The Q-factor measurement promises a significant contribution towards reducing the measurement time required. This is particularly so during the installation of DWDM systems with large numbers of channels operating at 2.5 Gbps and 10 Gbps, where a conventional BER measurement is of only limited use.

The Q-factor measurement method is the only way that all the setup parameters of a DWDM system using 80 wavelengths can be checked within an acceptable time. If the appropriate monitor points are present, the analysis can even be performed in-service.

Another important aspect regarding the use of the Q-factor measurement method is given by the system technology itself. The so-called forward error correction (FEC) decoder will be used in future to correct any residual bit error ratio. Not only are individual bit errors detected by the line equipment receiver, they are also corrected.

For this reason, the causes of single bit errors and residual error rates in a transmission system can no longer be localized by means of an SDH/SONET measurement. The number of errors corrected is indicated by the FEC rate, but troubleshooting is much more difficult.

If the bit rates at STM-16/ OC-48 and STM-64/OC-192 could be analyzed including the FEC using the Q-factor test set, thiswould be a further important step towards localizing errors when eliminating faults.

Effects of Q factor

With increasing Q or quality factor, so the bandwidth of the tuned circuit filter is reduced. As losses decrease so the tuned circuit becomes sharper as energy is stored better in the circuit. It can be seen that as the Q increases, so the 3 dB bandwidth decreases and the overall response of the tuned circuit increases.

Q factor of a resonant circuit increases so the losses decrease. This means that any oscillation set up within the circuit will take longer to die away. Actually ideal for use within an oscillator circuit because it is easier to set up and maintain an oscillation as less energy is lost in the tuned circuit.

In many RF applications there is a requirement for wide bandwidth operation. Some forms of modulation require a wide bandwidth, and other applications require fixed filters to provide wide band coverage.