# Hardware Design of a High Dynamic Range Radio Frequency (RF) Harmonic Measurement System
By consequence, many RF and microwave circuit elements exhibit unintended nonlinear properties. An example is the RF amplifier. Amplifiers are intended to operate linearly, boosting the input signal without creating extraneous frequencies at the output. In practice, creating a linear amplifier is not possible and additional frequency content is generated that distorts the desired signal. Much research has been done to linearize amplifiers [16; 17; 18; 19; 20]. The unintended frequency content generated by the nonlinear properties of the amplifier interferes with other radar and communication systems [21; 22; 23], as well as affect the sensitivity of the receiver. There are other nonlinear effects that are subtler and do not manifest as often. Among these is PIM, which is observed when high power signals interact with components that are weakly nonlinear. Such components do not exhibit measurable nonlinear distortion under normal conditions. In communication systems, the PIM produced can fall close to the fundamental band and interfere with adjacent communication channels [24; 25]. To combat this, much research has gone into linearizing communication systems [3; 24; 26]. For close-in intermodulation distortion (IMD), the frequency separation between the fundamental signal and PIM is too small to effectively filter out. Additionally, the communication channels change frequency quickly to accommodate multiple users. So, adaptable filters with large Q values would be needed; however, reconfigurable, high Q filters do not exist. For this reason, adaptive techniques are used to predict and cancel the nonlinearities. Such techniques include predistortion, feedforward linearization, channel equalization, etc. [27; 28; 29; 30; 31; 32]. Measuring weakly nonlinear RF circuit components requires specialized RF hardware [3], which itself must be highly linear and devoid of any self-generated nonlinearities. If the measurement system is not highly linear, the measurements will reflect the distortions caused by the test hardware in addition to the device under test (DUT). Commercially-available high dynamic range PIM measurement systems are accessible today [33; 34]. These systems are typically designed for specific frequencies, usually around the cell band. They use a two-tone test setup and achieve up to 170 dBc of dynamic range using high Q filters that are fixed in frequency, but lack frequency agility. A commercially-available nonlinear vector network analyzer, PNA-X, demonstrates far more flexibility than the fixed frequency PIM testing systems, and has the ability to vary tone spacings and tone amplitudes [35]. The PNA-X system also tracks intermodulation (IM) products and harmonics, keeping track of all the nonlinear terms [36]. However, it lacks the dynamic range necessary to measure nonlinear distortion from weakly nonlinear devices, as they are specified to generate harmonics lower than 60 dBc [37]. Feedforward cancellation systems have been developed that enable close-in PIM to be measured with a 160-dBc dynamic range, measured from the probe signal power to the spurious free IM frequency [16; 38; 39; 40]. The systems achieve a large dynamic range by taking great care to linearize their measurement system, which requires high levels of isolation between nonlinear circuit elements in the test setup to the DUTs. These systems also employ an adaptable cancellation system that removes the probe signals before PIM measurements are made. The linearization, isolation, and removal of the probe signal are essential to any high dynamic range measurement system. The downside to constructing such a test setup is the complexity of the system. These systems require iterative feedback from the receiver to cancel the fundamental probe signal. They also require an optimization algorithm to maximize the cancellation of the probe signal to maximize the dynamic range of the measurements. This paper describes an alternate approach to measuring low level nonlinear distortion from weakly nonlinear targets.