75 lines
3.5 KiB
Plaintext
75 lines
3.5 KiB
Plaintext
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=== Frequency Calibration
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Many _WSJT-X_ capabilities depend on signal-detection bandwidths no
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more than a few Hz. Frequency accuracy and stability are therefore
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unusually important. We provide tools to enable accurate frequency
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calibration of your radio, as well as precise frequency measurement of
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on-the-air signals. The calibration procedure works by automatically
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cycling your CAT-controlled radio through a series of preset
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frequencies of carrier-based signals at reliably known frequencies,
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measuring the error in dial frequency for each signal.
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You will probably find it convenient to define and use a special
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<<CONFIG-MENU,Configuration>> dedicated to frequency calibration.
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Then complete the following steps, as appropriate for your system.
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- Switch to FreqCal mode
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- In the _Working Frequencies_ box on the *Settings -> Frequencies*
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tab, delete any default frequencies for *FreqCal* mode that are not
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relevant for your location. You may want to replace some of them with
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reliably known frequencies receivable at your location.
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TIP: We find major-city AM broadcast stations generally serve well as
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frequency calibrators at the low frequency end of the spectrum. In
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North America we also use the standard time-and-frequency broadcasts
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of WWV at 2.500, 5.000, 10.000, 15.000, and 20.000 MHz, and CHU at
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3.330, 7.850, and 14.670 MHz. Similar shortwave signals are available
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in other parts of the world.
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- During the calibration procedure, the radio's USB dial frequency is
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offset 1500 Hz below each *FreqCal* entry in the default frequencies
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list. As shown in the screen shot below, detected signal carriers
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therefore appear at about 1500 Hz in the WSJT-X waterfall.
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image::FreqCal.png[align="left",alt="FreqCal"]
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With modern synthesized radios, small measured offsets from 1500 Hz
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will exhibit a straight-line dependence on frequency. You can
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approximate the calibration of your radio by simply dividing the
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measured frequency offset (in Hz) at the highest reliable frequency by
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the nominal frequency itself (in MHz). For example, the 20 MHz
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measurement for WWV shown above produced a measured tone offset of
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24.6 Hz, displayed in the _WSJT-X_ decoded text window. The resulting
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calibration constant is 24.6/20=1.23 parts per million. This number
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may be entered as *Slope* on the *settings -> Frequencies* tab.
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A more precise calibration can be effected by fitting the intercept
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and slope of a straight line to the whole sequence of calibration
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measurements, as shown for these measurements in the graph plotted
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below. Software tools for completing this task are included with the
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_WSJT-X_ installation, and detailed instructions for their use are
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available at https://physics.princeton.edu/pulsar/k1jt/FMT_User.pdf.
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Using these tools and no specialized hardware beyond your
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CAT-interfaced radio, you can calibrate the radio to better than 1 Hz
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and compete very effectively in the ARRL's periodic Frequency
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Measuring Tests.
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image::FreqCal_Graph.png[align="left",alt="FreqCal_Graph"]
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=== Reference Spectrum
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WSJT-X provides a tool that can be used to determine the detailed
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shape of your receiver's passband. Disconnect your antenna or tune to
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a quiet frequency with no signals. With WSJT-X running in one of the
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slow modes, select *Measure reference spectrum* from the *Tools* menu.
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Wait for about a minute and then hit the *Stop* button. A file named
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`refspec.dat` will appear in your log directory.
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[ ... TBD ... ]
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=== Equalization
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[ ... TBD ... ]
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