Design strategies aid ELF/VLF receivers

The specifications contained in MIL-STD-462 and NACSEM 5100 for electromagnetic compatibility testing require high-performance, extremely-low-frequency receivers. The considerations that govern the design of ELF reveivers are discussed. Among them are minimizing phase noise, optimizing the preamplifer noise figure and gain, setting the mixer balance, establishing power line immunity, and implementing narrow-band IF filters. If the phase noise is too high, the local oscillator is not sufficiently nulled out or the preamplifier gain is not set properly, the system's noise figure will deteriorate. It is noted that the local oscillator phase noise limits the ability of a receiver to recover modulation from the incoming signal. In the ELF receiver, the problem is especially difficult because the local oscillator must tune within 20 Hz of the IF frequency. Since the ELF receiver allows for IF bandwidths of 100 percent of the tuned frequency, this can further limit the amount of phase noise that the local oscillator can have before degrading the performance of the system. These considerations are applied to a new design of a receiver operating from 20 Hz to 10 kHz.