Frequency indicator using back-to-back rectifiers

The characteristics of parallel-tee networks are examined for sensitivity and linearity over a limited `test range¿ of frequency and are compared with those from CR and from LCR networks. Parallel-tee networks have some advantages for frequency indicators. Two such networks are used; one tuned to have its zero current at a low frequency and the other at a high frequency. The output currents are compared after rectification, their difference activating a sensitive moving-coil meter. For a 45¿55 Hz frequency range, the networks have zero-current frequencies at around 40 and 60Hz; at midrange (50Hz), the indicator is practically independent of voltage changes. It meets the specification of BS 89 for permissible errors arising from voltage and temperature effects. Its scale is practically linear. However, like most rectifier instruments, it is prone to errors on supplies with nonsinusoidal waveforms; these errors fall outside specification limits when the harmonic content is greater than 3%. For such supplies, special calibration or filters are needed. The use of the back-to-back rectifiers requires the frequency-sensitive networks to be decoupled; also, the movement must be designed to avoid mechanical resonances arising from circulating even harmonics. The performance of the instrument has been considered in the light of the recently revised standard IEC Publication 51.