Joshua M. Wetherington’s research while affiliated with North Carolina State University and other places

The paper presents a conceptual discussion of the characterisation and phenomenology of passive intermodulation (PIM) by the localised and distributed nonlinearities in passive devices and antennas. The PIM distinctive nature and its impact on signal distortions are examined in comparison with similar effects in power amplifiers. The main features of PIM generation are discussed and illustrated by the example of PIM due to electro-thermal nonlinearity. The issues of measurement, discrimination and modelling of PIM generated by nonlinearities in passive RF components and antennas are addressed.

A high-dynamic-range vibration detection system is presented based on analog cancellation. The prototype system operates at 900 MHz and is shown to detect vibrational displacement as low as 11 nm for a rectangular aluminum plate at a standoff distance of 2 m. By operating at a lower frequency than previous radar vibrometers, the system gains ground-penetrating capability and is shown to detect vibrations through 13 cm of sand. A fundamental relationship is introduced relating the minimum detectable vibration displacement to the standoff distance and system dynamic range.

Passive intermodulation distortion (PIM) generated on antennas is experimentally investigated and shown to agree with calculations based on an electro-thermal origin. Two-tone testing is used in the investigations with tone spacings ranging from 3 Hz to 100 kHz. The two-tone measurement system has a minimum spurious-free dynamic range of 125 dB at 3 Hz tone separation. Results show that at least some of the PIM generated by an antenna is due to electro-thermal effects but it is apparent that there are sources of PIM that cannot be described by electro-thermal effects alone.

An active analog canceller system is implemented to maximize the dynamic range of a two-tone nonlinear measurement, a common test for nonlinear radio frequency behavior. The canceller uses active feedforward cancellation, controlled by a fully automated iterative algorithm. The system enables testing of a variety of nonlinear channels and devices, either by direct connection or wireless probing. In this report, the advantages of feedforward cancellation are summarized, a two-tone architecture using this cancellation is presented, the software algorithm for automating measurements using this architecture is explained, and the performance of the cancellation system recently constructed at the U.S. Army Research Laboratory (ARL) is evaluated using measured data.

A resonant antenna displays passive intermodulation related to antenna vibrations at levels that impact communications systems. A non-resonant antenna was immune to structure vibrations. Both antennas were of solid construction with thick metalization and did not exhibit electro-thermal passive intermodulation distortion previously observed with microstrip structures. The ultimate dynamic ranges of two antennas are explored using a measurement system with a dynamic range of 98 dB at one hertz offset from a 30 dBm, 500 MHz transmit signal. This increased to 140 dB when the separation increased to 100 Hz.

The essence of radar, radio and wireless sensor engineering is extracting small information-bearing signals. This is notoriously difficult and engineers compensate by transmitting high power signals, reducing range, and spacing wireless systems in frequency and time. New understandings of passive intermodulation distortion, thermal effects, time-frequency effects, and noise are presented. It is seen that the familiar frequency-domain-based abstractions have missed important underlying physics. Through greater understanding, RF engineers can develop microwave systems with far lower levels of distortion and noise.

An automated analog canceller is presented that uses feedforward cancellation in a bridge configuration. A minimum of 70 dB of analog cancellation is obtained. The canceller is used to construct an intermodulation distortion measurement system achieving up to 120 dBc of intermodulation dynamic range in two-port transmission testing and 140 dBc of intermodulation dynamic range in one-port reflection testing for frequency separations of at least 1 kHz. At a two-tone frequency separation of 1 Hz, the respective dynamic ranges are at least 94 and 111 dBc. The relationship of cancellation performance and dynamic range is examined in terms of application-specific definitions of dynamic range. The system is then used to measure passive intermodulation distortion using a two-tone test with a tone frequency separation from 1 Hz to 100 MHz.

An acoustic tone isonifying an antenna is shown to produce radio frequency (RF) distortion in a log-periodic dipole array. This distortion is capable of interfering with a sensitive receiver. The passive distortion produces two sidebands, the powers of which have a 1:1 relationship with both the RF and acoustic tone, but have nonmonotonic behavior with respect to RF and acoustic frequency.