For Digital Systems, MER and BER are the primary measurement metrics for evaluating the health of a system. Once the QAM signals leave the Headend, MER (approx. 40 dB) is subject to degradation due to distribution related problems.
BER (Bit Error Rate) is the ratio of errored bits to the total number of bits transmitted, received, or processed over a defined length of time. For example:
3 errored bits in a total of 1,000,000 transmitted bits will result in a BER of 3/1,000,000 = 0.000003 = 3 x 10-6
MER (Modulation Error Ratio) is the ratio, in decibels, of average symbol power to average error power.
MER (dB) = 10 x log (avg. symbol power / avg. error power)
MER is influenced by everything present in the signal’s transmission, such as Phase Noise, CNR, CTB Distortion, CSO Distortion, Cross Modulation, Micro-reflections (ghosting), Amplitude Tilt/Ripple, Group Delay, or Ingress.
MER can be reduced by all of the following:
- Noise: Unwanted or erroneous signals. To avoid poor MER, the desired signal must be strong enough to override the noise. Noise is added to the signal when input levels are too low. CNR (Carrier-to-Noise Ratio)
- Non-Linear Distortion from Amplifiers: Maintain Amp input/output levels according to specifications and channel loading. (CTB, CSO, XMOD)
- CTB (Composite Triple Beat) Summation of triple order distortions in decibels
- CSO (Composite Second Order) Summation of second order distortions (1.25 MHz above visual carriers) in decibels
- XMOD (Cross Modulation) is a buzzing in sound, sound lines in picture. Modulation from one or more television channels imposed on another channel or channels. Measured in decibels.
- Reflections: Caused by impedance mismatches and improper or missing termination on splitters, combiners and taps. It can also be brought upon by damaged coaxial cable or improperly installed “F” connectors (Micro-Reflections, Amplitude, Ripple/Tilt, Group Delay)
- Spurious Signals: Caused by ingress, local pickup or malfunctioning active equipment (In-channel Ingress, Laser Clipping, Data Collisions)
|Excellent||> 30 dB||> 38 dB||> 38 dB|
|Good||25 to 30 dB||30 to 38 dB||25 to 28 dB|
|Marginal||18 to 25 dB||23 to 30 dB||30 to 35 dB|
|Non-Functional||< 18 dB||< 23 dB||< 30 dB|
We recommend using a digital field strength meter, QAM analyzer, or a spectrum analyzer and constellation meter to gauge proper measurements.
Watch our RF Measurements in a CATV System webinar for further explanation on this topic.
- What level should be measured for Analog and Digital CATV systems?
- What performance metrics should be measured for the health of a signal?