Requirement for Maximum Intermodulation-Produced RFI

The LWA is subject to strong RFI outside our desired observing band of 20-85 MHz, including FM radio at 88-108 MHz and HF radio at 3-20 MHz. FM radio signals are primarily local and nearly line-of-sight; the power level is stable. The HF radio signals are primarily distant and arrive by reflections from the ionosphere; the power level is highly variable. The out-of-band signals are suppressed by filtering in the analog signal path so that by the time the entire signal is digitized it is well within the dynamic range of the digitizer [1]. However, non-linearity anywhere in the analog signal path can produce intermodulation among the RFI signals such that the intermodulation products fall within the observing band; these cannot be suppressed by filtering. Therefore non-linearity must be small enough to keep the inband products sufficiently small. What is sufficiently small? This memo proposes a design criterion to achieve an acceptable level.