=== Frequency Calibration

Many _WSJT-X_ capabilities depend on signal-detection bandwidths no
more than a few Hz.  Frequency accuracy and stability are therefore
unusually important.  We provide tools to enable accurate frequency
calibration of your radio, as well as precise frequency measurement of
on-the-air signals.  The calibration procedure works by automatically
cycling your CAT-controlled radio through a series of preset
frequencies of carrier-based signals at reliably known frequencies,
measuring the error in dial frequency for each signal.

You will probably find it convenient to define and use a special
<<CONFIG-MENU,Configuration>> dedicated to frequency calibration.
Then complete the following steps, as appropriate for your system.

- Switch to FreqCal mode

- In the _Working Frequencies_ box on the *Settings -> Frequencies*
tab, delete any default frequencies for *FreqCal* mode that are not
relevant for your location.  You may want to replace some of them with
reliably known frequencies receivable at your location.

TIP: We find major-city AM broadcast stations generally serve well as
frequency calibrators at the low frequency end of the spectrum.  In
North America we also use the standard time-and-frequency broadcasts
of WWV at 2.500, 5.000, 10.000, 15.000, and 20.000 MHz, and CHU at
3.330, 7.850, and 14.670 MHz.  Similar shortwave signals are available
in other parts of the world.

- To cycle automatically through your chosen list of calibration
frequencies, check *Execute frequency calibration cycle* on the
*Tools* menu.  _WSJT-X_ will spend 30 seconds at each frequency,
writing its measurements to the file `fmt.all` in the directory
where your log files are kept.

- During the calibration procedure, the radio's USB dial frequency is
offset 1500 Hz below each *FreqCal* entry in the default frequencies
list.  As shown in the screen shot below, detected signal carriers
therefore appear at about 1500 Hz in the _WSJT-X_ waterfall.

image::FreqCal.png[align="left",alt="FreqCal"]

With modern synthesized radios, small measured offsets from 1500 Hz
will exhibit a straight-line dependence on frequency.  You can
approximate the calibration of your radio by simply dividing the
measured frequency offset (in Hz) at the highest reliable frequency by
the nominal frequency itself (in MHz).  For example, the 20 MHz
measurement for WWV shown above produced a measured tone offset of
24.6 Hz, displayed in the _WSJT-X_ decoded text window.  The resulting
calibration constant is 24.6/20=1.23 parts per million.  This number
may be entered as *Slope* on the *settings -> Frequencies* tab.

A more precise calibration can be effected by fitting the intercept
and slope of a straight line to the whole sequence of calibration
measurements, as shown for these measurements in the graph plotted
below.  Software tools for completing this task are included with the
_WSJT-X_ installation, and detailed instructions for their use are
available at https://physics.princeton.edu/pulsar/k1jt/FMT_User.pdf.
Using these tools and no specialized hardware beyond your
CAT-interfaced radio, you can calibrate the radio to better than 1 Hz
and compete very effectively in the ARRL's periodic Frequency
Measuring Tests.

image::FreqCal_Graph.png[align="left",alt="FreqCal_Graph"]

=== Reference Spectrum

WSJT-X provides a tool that can be used to determine the detailed
shape of your receiver's passband.  Disconnect your antenna or tune to
a quiet frequency with no signals.  With WSJT-X running in one of the
slow modes, select *Measure reference spectrum* from the *Tools* menu.
Wait for about a minute and then hit the *Stop* button.  A file named
`refspec.dat` will appear in your log directory.  

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=== Equalization

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