Analog Signal Path Analysis for Fiber-Connected Antennas of LWA352

Abstract

The OVRO LWA is subject to strong RFI at frequencies outside the nominal observing band, 20-85 MHz. This includes FM radio (88-108 MHz) and HF communication (<20 MHz). Intermodulation among these RFI signals can produce undesired signals in the observing band. The current design of the signal path includes strong filtering of the out-of-band signals, but that filtering occurs after the front end electronics (FEE, 36 dB gain) and after the first amplifier stage of the analog receiver (ARX, 23 dB). For the fiber-coupled antennas, it is also after the optical link. These amplifiers and the optical link are not perfectly linear, so they can produce intermodulation. They are designed to tolerate the known RFI, but some intermodulation still occurs. To minimize this residual intermodulation, signal levels at these components should be small. On the other hand, the signal path must have sufficient gain, properly distributed, to avoid having downstream components add significantly to the system noise temperature. This memo provides a quantitative analysis of the trade-off between intermodulation and noise temperature. It concentrates on the fiber-connected antennas. The coax-connected antennas have a similar issue, but whereas the coax cable can be considered completely linear there is less intermodulation and the ARX has sufficient gain to keep the noise impact small, even for the longest cables.