=== 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 <> 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. - In most cases you will want to start by deleting any existing file `fmt.all` in the directory where your log files are kept. - 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. Initially no measurement data is saved to the `fmt.all` file although it is displayed on screen, this allows you to check you current calibration parameters. - 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. - To start a measurement session check the *Measure* option and let the calibration cycle run for at least one complete sequence. Note that, while measuring, any existing calibration parameters are automatically disabled so you may have to increase the *FTol* range if your rig is off freqeuncy by more than a few Hertz in order to capture valid measurements. 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"] After running *Execute frequency calibration cycle* at least once with good results, check and edit the file `fmt.all` in the log directory and delete any spurious or outlier measurements. The line-fitting procedure can then be carried out automatically by clicking *Solve for calibration parameters* on the *Tools* menu. The results will be displayed as in the following screen shot. Estimated uncertainties are included for slope and intercept; `N` is the number of averaged frequency measurements included in the fit, and `StdDev` is the root mean square deviation of averaged measurements from the fitted straight line. If the solution seems valid you will be offered an *Apply* button to push that will automatically set the calibration parameters in *Settings -> Frequencies -> Frequency Calibration*. image::FreqCal_Results.png[align="center",alt="FreqCal_Results"] For a quick visual check of the resulting calibration, stay in *FreqCal* mode with the *Measure* option cleared. _WSJT-X_ will show the adjusted results directly on the waterfall and the displayed records. === 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. [ ... more to come ... ] === Phase Equalization *Measure phase response* under the *Tools* menu is for advanced MSK144 users. Phase equalization is used to compensate for group-delay variation across your receiver passband. Careful application of this facility can reduce intersymbol interference, resulting in improved decoding sensitivity. If you use a software-defined receiver with linear-phase filters there is no need to apply phase equalization. After a frame of received data has been decoded, *Measure phase response* generates an undistorted audio waveform equal to the one generated by the transmitting station. Its Fourier transform is then used as a frequency-dependent phase reference to compare with the phase of the received frame's Fourier coefficients. Phase differences between the reference spectrum and received spectrum will include contributions from the originating station's transmit filter, the propagation channel, and filters in the receiver. If the received frame originates from a station known to transmit signals having little phase distortion (say, a station known to use a properly adjusted software-defined-transceiver) and if the received signal is relatively free from multipath distortion so that the channel phase is close to linear, the measured phase differences will be representative of the local receiver's phase response. Complete the following steps to generate a phase equalization curve: - Record a number of wav files that contain decodable signals from your chosen reference station. Best results will be obtained when the signal-to-noise ratio of the reference signals is 10 dB or greater. - Enter the callsign of the reference station in the DX Call box. - Select *Measure phase response* from the *Tools* menu, and open each of the wav files in turn. The mode character on decoded text lines will change from `&` to `^` while _WSJT-X_ is measuring the phase response, and it will change back to `&` after the measurement is completed. The program needs to average a number of high-SNR frames to accurately estimate the phase, so it may be necessary to process several wav files. The measurement can be aborted at any time by selecting *Measure phase response* again to toggle the phase measurement off. + When the measurement is complete _WSJT-X_ will save the measured phase response in the *Log directory*, in a file with suffix ".pcoeff". The filename will contain the callsign of the reference station and a timestamp, for example `K0TPP_170923_112027.pcoeff`. - Select *Equalization tools ...* under the *Tools* menu and click the *Phase ...* button to view the contents of the *Log directory*. Select the desired pcoeff file. The measured phase values will be plotted as filled circles along with a fitted red curve labeled "Proposed". This is the proposed phase equalization curve. It's a good idea to repeat the phase measurement several times, using different wav files for each measurement, to ensure that your measurements are repeatable. - Once you are satisfied with a fitted curve, push the *Apply* button to save the proposed response. The red curve will be replaced with a light green curve labeled "Current" to indicate that the phase equalization curve is now being applied to the received data. Another curve labeled "Group Delay" will appear. The "Group Delay" curve shows the group delay variation across the passband, in ms. Click the *Discard Measured* button to remove the captured data from the plot, leaving only the applied phase equalization curve and corresponding group delay curve. - To revert to no phase equalization, push the *Restore Defaults* button followed by the *Apply* button. The three numbers printed at the end of each MSK144 decode line can be used to assess the improvement provided by equalization. These numbers are: `N` = Number of frames averaged, `H` = Number of hard bit errors corrected, `E` = Size of MSK eye diagram opening. Here is a decode of K0TPP obtained while *Measure phase response* was measuring the phase response: 103900 17 6.5 1493 ^ WA8CLT K0TPP +07 1 0 1.2 The "^" symbol indicates that a phase measurement is being accumulated but is not yet finished. The three numbers at the end of the line indicate that one frame was used to obtain the decode, there were no hard bit errors, and the eye-opening was 1.2 on a -2 to +2 scale. Here's how the same decode looks after phase equalization: 103900 17 6.5 1493 & WA8CLT K0TPP +07 1 0 1.6 In this case, equalization has increased the eye opening from 1.2 to 1.6. Larger positive eye openings are associated with reduced likelihood of bit errors and higher likelihood that a frame will be successfully decoded. In this case, the larger eye-opening tells us that phase equalization was successful, but it is important to note that this test does not by itself tell us whether the applied phase equalization curve is going to improve decoding of signals other than those from the reference station, K0TPP. It's a good idea to carry out before and after comparisons using a large number of saved wav files with signals from many different stations, to help decide whether your equalization curve improves decoding for most signals. When doing such comparisons, keep in mind that equalization may cause _WSJT-X_ to successfully decode a frame that was not decoded before equalization was applied. For this reason, be sure that the time "T" of the two decodes are the same before comparing their end-of-line quality numbers. When comparing before and after decodes having the same "T", keep in mind that a smaller first number means that decoding has improved, even if the second and third numbers appear to be "worse". For example, suppose that the end-of-line quality numbers before equalization are `2 0 0.2` and after equalization `1 5 -0.5`. These numbers show improved decoding because the decode was obtained using only a single frame after equalization whereas a 2-frame average was needed before equalization. This implies that shorter and/or weaker pings could be decodable. NOTE: Further details on phase equalization and examples of fitted phase curves and eye diagrams can be found in the article on MSK144 by K9AN and K1JT published in {msk144}.