Patent Application
20130317769
The present invention relates to test and measurement instruments, and more particularly to test and measurement instruments which display frequency-domain information.
Some test and measurement instruments, such as RSA6100 Series Real-Time Spectrum Analyzers and MDO4000 Series Mixed Domain Oscilloscopes, available from Tektronix, Inc. of Beaverton, Oregon, are capable of displaying both time-domain and frequency-domain views of an input signal.
When using such a test and measurement instrument, a user can sometimes encounter signals which are visible in the time-domain display but not in the frequency-domain display. This typically occurs because the time-domain display shows data from the entire acquisition bandwidth of the instrument, whereas the frequency-domain display only shows a subset of that data. That is because the frequency-domain display does not show the very ends of the acquisition bandwidth, which are distorted by attenuation and phase nonlinearity due to various analog and digital filters within the acquisition path of the instrument. As a result, a signal that is located within the acquisition bandwidth of the instrument but outside of the displayed span, i.e., a signal that is “in-band but out-of-span,” or simply, “out-of-span,” will be visible in the time-domain display but not in the frequency-domain display.
This discrepancy can result in a variety of confusing scenarios for a user.
One scenario is where time-domain and frequency-domain measurements simply do not agree. The user is left trying to figure out why these two supposedly equivalent measurements yield different results.
Another scenario is when excessive in-band but out-of-span power causes an overflow condition. A user looking in the frequency-domain display will not see the excessive power, and is left wondering why the instrument is showing an overflow condition.
Some spectrum analyzers such as the FSW Signal and Spectrum Analyzer available from Rohde & Schwarz of Munich, Germany allow the user to view an enlarged spectral display which shows the entire acquisition bandwidth of the instrument, including the band-edge roll-off However, the problem with this implementation is that the user must be knowledgeable enough to know when to activate the enlarged spectral display. For users who are not experienced with spectrum analyzers, it can be very confusing to get either differing measurement results or unexplained warning messages. Without a significant amount of experience, the user may not know he or she should make the span setting larger to see the real problem.
A test and measurement instrument according to an embodiment of the present invention automatically detects excessive in-band but out-of-span energy and notifies the user of the condition.
The objects, advantages, and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawings.
Excessive in-band but out-of-span energy may be detected in any one of various ways. For example, in various embodiments, it may be detected by determining whether the total out-of-span power exceeds any one of various quantities, such as an absolute power level, a percentage of the total in-span power, a percentage of the largest in-span signal, or a display value (e.g., the top-of-screen, reference level). These quantities may be defined by the user or defined by the instrument. Alternatively, excessive out-of-span power may be detected using analog circuitry, for example, by filtering the in-span power with a first analog filter, filtering the out-of-span power with a second filter, and then comparing the amplitudes of the resulting filtered signals.
The instrument may notify the user about the excessive in-band but out-of-span energy in various ways. For example, in some embodiments, the instrument displays a message on the time-domain display 115 or the frequency-domain display 130. In some embodiments, the instrument produces an audible tone. In some embodiments, the instrument directs the user to view the full span so that he or she can see the excessive out-of-span signal, or alternatively, automatically displays the full span for the user. In cases where the instrument is operated under remote control, the instrument may generate an interrupt. The instrument may also return any measurement results with an indication of an invalid measurement.
To aid in understanding,
It will be appreciated that, although the present invention is described in terms of test and measurement instruments that have both time-domain and frequency-domain displays, the present invention is also useful in test and measurement instruments that have only frequency-domain displays, for example, to detect when in-band but out-of-span signals result in overload conditions.
In various embodiments, elements shown and described above such as the spectrum calculation, the amplitude versus time calculation, the out-of-span energy calculation, and so on, may be implemented in hardware, software, or a combination of the two, and may comprise or be implemented on a general purpose microprocessor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or the like.
It will be appreciated from the foregoing discussion that the present invention represents a significant advance in the field of test and measurement instruments. Although specific embodiments of the invention have been illustrated and described for purposes of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.
