Maximizing Eb/No (C/N) Accuracy (Application Note 701)

The CNG series of generators have the ability to make highly accurate Eb/N0 or C/N ratios when used correctly. The basic operation of the CNG is depicted by the RF test block diagram in Figure 1, which is on the front panel of the instrument, dynamically displayed with the internal power meter. Prior to entering a ratio, the instrument measures the test signal's amplitude with the embedded power measurement system. Once it has this value it prompts the user to enter in an Eb/N0 ratio within the instruments allowable range. Upon entering the values, the instrument's CPU instructs the attenuator in the noise path to automatically actuate to the correct value to achieve the set ratio. In theory this sounds like an exceedingly simple device. The challenge is to be able to set this ratio accurately over multi-parameter variables such as frequency, bandwidth and test signal amplitude. As is clear from the BER vs. Eb/N0 waterfall curve shown in Figure 2, any uncertainty in Eb/N0 translates dramatically to uncertainty of BER. The combination of the CNG's hardware and software algorithms, calibration routines and user input parameters all work in unison to insure accurate Eb/N0. The three detailed items to follow are the most critical in this process.