We present a detailed description and users manual for a new tool to evaluate seismic station performance and characteristics by providing quick and easy transitions between visualizations of the frequency and time domains. The software is based on the probability density functions (PDF) of power spectral densities (PSD) (McNamara and Buland, 2004) and builds on the original development of the PDF stand-alone software system (McNamara and Boaz, 2005) and the seismological data viewer application PQL (IRIS-PASSCAL Quick Look) and PQLII (available through the IRIS PASSCAL program: http://www.passcal.nmt.edu/content/pql-ii-program-viewing-data). With PQLX (PQL eXtended), computed PSDs are stored in a MySQL database, allowing a user to access specific time periods of PSDs (PDF subsets) and time series segments through a GUI-driven interface. The power of the method and software lies in the fact that there is no need to screen the data for system transients, earthquakes, or general data artifacts, because they map into a background probability level. In fact, examination of artifacts related to station operation and episodic cultural noise allow us to estimate both the overall station quality and a baseline level of Earth noise at each site. The output of this analysis tool is useful for both operational and scientific applications. Operationally, it is useful for characterizing the current and past performance of existing broadband stations, for conducting tests on potential new seismic station locations, for evaluating station baseline noise levels (McNamara and others, 2009), for detecting problems with the recording system or sensors, and for evaluating the overall quality of data and metadata. Scientifically, the tool allows for mining of PSDs for investigations on the evolution of seismic noise (for example, Aster and others, 2008; and Aster and others, 2010) and other phenomena. Currently, PQLX is operational at several organizations including the USGS National Earthquake Information Center (NEIC), the USGS Albuquerque Seismological Laboratory (ASL), and the Incorporated Research Institutions in Seismology (IRIS) Data Management Center (DMC) for station monitoring and instrument response quality control. The PQLX system is available to the community at large through the U.S. Geological Survey (USGS) (http://ehpm-earthquake.wr.usgs.gov/research/software/pqlx.php) and IRIS (http://www.iris.edu/software/pqlx). Also provided is a fully searchable website for bug reporting and enhancement requests (http://wush.net/bugzilla/PQLX). The first part of this document aims to describe and illustrate some of the features and capabilities of the software. The second part of this document is a detailed users manual that covers installation procedures, system requirements, operations, bug reporting, and software components (Appendix).