Wide-band high dynamic range variable passive intermodulation generator using fabric-over-foam gasket

Abstract

Accurate measurement of Radio Frequency (RF) component’s passive intermodulation (PIM) and systematic evaluation of PIM’s effect on mobile communication require flexible PIM generator. This work presents a wide-band high dynamic-range PIM generator using a fabric-over-foam (also known as RF Interference Shielding Gasket) as superstrate of low-PIM microstrip line (MS-line). An air gap of 1 mm introduced between the foam and the MS-line to ensure long-term repeatability by avoiding the nonlinearity that can be generated by the metal-to-metal contact. To make the PIM generator adjustable, the foam moved transversely over the MS-line. The size of the gasket selected in such a way that it slightly changes the S-parameters of the MS-line. Due to the variation of the exposure area of the foam (which is an electromagnetic magnetic material) to the electromagnetic (EM) field generated by the MS-line; the PIM level of the MS-line changes with the foam’s location. The minimum PIM level of the proposed generator determined by the PIM level of the MS-line, which is no more than $-125\mathrm{d}\mathrm{B}\mathrm{m}$ @$2\times 43\mathrm{d}\mathrm{B}\mathrm{m}$. However, the maximum PIM level can be as high as $-48\mathrm{d}\mathrm{B}\mathrm{m}$ @$2\times 43\mathrm{d}\mathrm{B}\mathrm{m}$, achieved by placing a gasket piece on the top of the MS-line. The proposed generator verified from $700\mathrm{M}\mathrm{H}\mathrm{z}$ to $2.7\mathrm{G}\mathrm{H}\mathrm{z}$ and all of the measurement results show its feasibility. Moreover, presented PIM generator can be used a single-port or two-port calibration device.

Introduction

Growing number of users, introduction of new devices, and antenna sharing schemes, increase the possibility of passive intermodulation (PIM) interference in mobile communication system. PIM has been a severe problem for service providers and telecom equipment vendors. PIM measurement helps the designers and engineers to choose the desired components, i.e. low PIM components for their communication set-up. Besides, PIM measurement can also help engineers to evaluate system performance, especially for the case that multiple potential PIM sources present their effect concurrently.

The main problem during the process of establishing of a PIM measurement setup is itself vulnerability to the intermodulation generation. This type of intermodulation commonly known as system noise floor (NF) or residual PIM. This NF may make the obtained measurement results be erroneous or has large fluctuation due to poor signal-to-noise ratio. Due to this reason, it is quite possible that a good DUT will regard as a bad one, and vice-versa. This will harm the vendor’s reputation or increase the production cost. Thus, a wideband tunable PIM generator with a high dynamic range is in high demand.

PIM-generating patch antenna for PIM generation proposed in [1], while dual notched-ring PIM generator proposed by [2]. Ref. [3] developed a dual-port tunable IM3 generator using cascading coupling network and a Schottky diode. The narrow bandwidth issue of [3] solved in [4] by improving the circuit of the generator. However, this modification results into a reduced dynamic range around $20\mathrm{d}\mathrm{B}$. A $50\mathrm{d}\mathrm{B}$ dynamic range programmable PIM3 generator has been developed in [5], using a partly Ni-coated disk. Three solutions to realize tunable PIM generator discussed in [6] and the maximum dynamic range achieved is $50\mathrm{d}\mathrm{B}$ (covering the $700\mathrm{M}\mathrm{H}\mathrm{z}-2.6$ GHz band). Most recently, a $80\mathrm{d}\mathrm{B}$ dynamic range generator is achieved by [7] with small operation frequency range.

The proposed PIM generator in this work takes advantage of the magnetic material’s nonlinearity. It covers the $700\mathrm{M}\mathrm{H}\mathrm{z}-2.7$ GHz band, i.e., 2G to 4G mobile communication. Totally speaking, its dynamic range is larger than $70\mathrm{d}\mathrm{B}.$ Moreover, the PIM levels generated are stable and repeatable. Table 1 illustrates the detailed comparison of the proposed generator with the preceding ones. Two-tones having $43\mathrm{d}\mathrm{B}\mathrm{m}$ input power (i.e.,$2\times 43\mathrm{d}\mathrm{B}\mathrm{m}$) used to obtain the measurement results presented in Table 1.