RF input level calculation

The formula for calculating the input levels of optical transmitters for use in cable television has been straightforward even when adding digital carriers to the spectrum. Using all analog channels, you could apply a simple formula to calculate the difference in power between the manufacturers listed channel loading and the channel loading of your system.  10 *Log (manufacturers channel load/system channel load). For example if the manufacturer specified their transmitter for 112 analog channels and your system was going to use 80 channels the equation would be 10*Log (112/80) = 10*Log (1.4) = 10*.14612 = 1.4.  The difference in the channel loading power is 1.4 dB. 80 analog CW (Continuous Wave) carriers have 1.4 dB less power than 112 analog CW carriers do.  To match the OMI or Optical Modulation Index of the manufacturers 112 channel loading you would add 1.4 dB to the recommended input level on the 80-channel load. 

When adding digital carriers to the transmitter you probably did not think too much about it.  The digital carriers are -10 dBc to -6 dBc relative to the peak power of the analog video carrier level.(Society of Telecommunications Engineers, 2007, p. 9)  Even with the digital carriers at – 6 dBc from analog the addition of power is minimal as long as the number of digital carriers is equal to or less than the number of analog carriers. For example if we had originally installed a transmitter that was specified for 112 carriers at +15 dBmV/carrier and we input 80 carriers the input drive level would have been set for 16.4 dBmV/carrier. We just added the 1.4 dB that we calculated as the power difference between 112 and 80 channels. When we added 32 digital carriers at -6 dB from the analog peak level the additional power added would have only been +.5 dB. 

As the number of digital carriers increases above the number of analog carriers the  total power that is attributed to the digital carriers cannot be ignored. Digital carriers now account for the majority of carriers on systems and the formulas that use analog carriers to determine input level without taking the power of the digital carriers into account may not be accurate. 

As cable television systems start to reduce the number of analog carriers and increase the number of digital carriers, the power added from these carriers requires more attention. A more accurate method of determining the channel input power is to calculate the total power for the analog and digital carriers individually and then add them together.

 Reference

Society ofTelecommunications Engineers. (2007). ANSI/SCTE 22-1 2002R2007

Data-Over-Cable Service Interface Specification DOCSIS

1.0 Radio Frequency Interface (RFI) [Standard]. Exton, PA

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