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230 lines
12 KiB
Plaintext
230 lines
12 KiB
Plaintext
=== 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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+
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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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- In most cases you will want to start by deleting any existing file
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`fmt.all` in the directory where your log files are kept.
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- To cycle automatically through your chosen list of calibration
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frequencies, check *Execute frequency calibration cycle* on the
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*Tools* menu. _WSJT-X_ will spend 30 seconds at each
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frequency. Initially no measurement data is saved to the `fmt.all`
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file although it is displayed on screen, this allows you to check your
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current calibration parameters.
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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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- To start a measurement session, check the *Measure* option and let
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the calibration cycle run for at least one complete sequence. Note
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that, while measuring, any existing calibration parameters are
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automatically disabled so you may have to increase the *FTol* range if
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your rig is off frequency by more than a few Hertz in order to capture
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valid measurements.
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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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After running *Execute frequency calibration cycle* at least once with
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good results, check and edit the file `fmt.all` in the log directory
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and delete any spurious or outlier measurements. The line-fitting
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procedure can then be carried out automatically by clicking *Solve for
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calibration parameters* on the *Tools* menu. The results will be
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displayed as in the following screen shot. Estimated uncertainties
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are included for slope and intercept; `N` is the number of averaged
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frequency measurements included in the fit, and `StdDev` is the root
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mean square deviation of averaged measurements from the fitted
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straight line. If the solution seems valid you will be offered an
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*Apply* button to push that will automatically set the calibration
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parameters in *Settings -> Frequencies -> Frequency Calibration*.
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image::FreqCal_Results.png[align="center",alt="FreqCal_Results"]
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For a quick visual check of the resulting calibration, stay in
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*FreqCal* mode with the *Measure* option cleared. _WSJT-X_ will show
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the adjusted results directly on the waterfall and the displayed
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records.
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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
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the slow modes, select *Measure reference spectrum* from the *Tools*
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menu. Wait for about a minute and then hit the *Stop* button. A file
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named `refspec.dat` will appear in your log directory. When you check
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*Ref Spec* on the *Wide Graph*, the recorded reference spectrum will
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then be used to flatten your overall effective passband.
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=== Phase Equalization
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*Measure phase response* under the *Tools* menu is for advanced MSK144
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users. Phase equalization is used to compensate for group-delay
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variation across your receiver passband. Careful application of this
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facility can reduce intersymbol interference, resulting in improved
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decoding sensitivity. If you use a software-defined receiver with
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linear-phase filters there is no need to apply phase equalization.
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After a frame of received data has been decoded, *Measure phase
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response* generates an undistorted audio waveform equal to the one
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generated by the transmitting station. Its Fourier transform is then
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used as a frequency-dependent phase reference to compare with the
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phase of the received frame's Fourier coefficients. Phase differences
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between the reference spectrum and received spectrum will include
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contributions from the originating station's transmit filter, the
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propagation channel, and filters in the receiver. If the received
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frame originates from a station known to transmit signals having
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little phase distortion (say, a station known to use a properly
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adjusted software-defined-transceiver) and if the received signal is
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relatively free from multipath distortion so that the channel phase is
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close to linear, the measured phase differences will be representative
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of the local receiver's phase response.
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Complete the following steps to generate a phase equalization curve:
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- Record a number of wav files that contain decodable signals from
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your chosen reference station. Best results will be obtained when the
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signal-to-noise ratio of the reference signals is 10 dB or greater.
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- Enter the callsign of the reference station in the DX Call box.
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- Select *Measure phase response* from the *Tools* menu, and open each
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of the wav files in turn. The mode character on decoded text lines
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will change from `&` to `^` while _WSJT-X_ is measuring the phase
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response, and it will change back to `&` after the measurement is
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completed. The program needs to average a number of high-SNR frames to
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accurately estimate the phase, so it may be necessary to process
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several wav files. The measurement can be aborted at any time by
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selecting *Measure phase response* again to toggle the phase
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measurement off.
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When the measurement is complete _WSJT-X_ will save the measured
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phase response in the *Log directory*, in a file with suffix
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".pcoeff". The filename will contain the callsign of the reference
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station and a timestamp, for example `K0TPP_170923_112027.pcoeff`.
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- Select *Equalization tools ...* under the *Tools* menu and click the
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*Phase ...* button to view the contents of the *Log directory*. Select
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the desired pcoeff file. The measured phase values will be plotted as
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filled circles along with a fitted red curve labeled "Proposed". This is
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the proposed phase equalization curve. It's a good idea to repeat the
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phase measurement several times, using different wav files for each
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measurement, to ensure that your measurements are repeatable.
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- Once you are satisfied with a fitted curve, push the *Apply* button
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to save the proposed response. The red curve will be replaced with a
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light green curve labeled "Current" to indicate that the phase
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equalization curve is now being applied to the received data. Another
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curve labeled "Group Delay" will appear. The "Group Delay" curve shows
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the group delay variation across the passband, in ms. Click the
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*Discard Measured* button to remove the captured data from the plot,
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leaving only the applied phase equalization curve and corresponding
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group delay curve.
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- To revert to no phase equalization, push the *Restore Defaults*
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button followed by the *Apply* button.
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The three numbers printed at the end of each MSK144 decode line can be
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used to assess the improvement provided by equalization. These numbers
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are: `N` = Number of frames averaged, `H` = Number of hard bit errors
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corrected, `E` = Size of MSK eye diagram opening.
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Here is a decode of K0TPP obtained while *Measure phase response* was measuring
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the phase response:
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103900 17 6.5 1493 ^ WA8CLT K0TPP +07 1 0 1.2
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The "^" symbol indicates that a phase measurement is being accumulated
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but is not yet finished. The three numbers at the end of the line
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indicate that one frame was used to obtain the decode, there were no
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hard bit errors, and the eye-opening was 1.2 on a -2 to +2
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scale. Here's how the same decode looks after phase equalization:
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103900 17 6.5 1493 & WA8CLT K0TPP +07 1 0 1.6
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In this case, equalization has increased the eye opening from 1.2 to
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1.6. Larger positive eye openings are associated with reduced
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likelihood of bit errors and higher likelihood that a frame will be
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successfully decoded. In this case, the larger eye-opening tells us
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that phase equalization was successful, but it is important to note
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that this test does not by itself tell us whether the applied phase
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equalization curve is going to improve decoding of signals other than
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those from the reference station, K0TPP.
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It's a good idea to carry out before and after comparisons using a
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large number of saved wav files with signals from many different
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stations, to help decide whether your equalization curve improves
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decoding for most signals. When doing such comparisons, keep in mind
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that equalization may cause _WSJT-X_ to successfully decode a frame
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that was not decoded before equalization was applied. For this
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reason, be sure that the time "T" of the two decodes are the same
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before comparing their end-of-line quality numbers.
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When comparing before and after decodes having the same "T", keep in
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mind that a smaller first number means that decoding has improved,
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even if the second and third numbers appear to be "worse". For
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example, suppose that the end-of-line quality numbers before
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equalization are `2 0 0.2` and after equalization `1 5 -0.5`. These
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numbers show improved decoding because the decode was obtained using
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only a single frame after equalization whereas a 2-frame average was
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needed before equalization. This implies that shorter and/or weaker
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pings could be decodable.
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NOTE: Further details on phase equalization and examples of fitted
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phase curves and eye diagrams can be found in the article on MSK144 by
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K9AN and K1JT published in {msk144}.
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