From eb4a98e5b44a916e1fbd67cb4adf3d723953bade Mon Sep 17 00:00:00 2001 From: Steven Franke Date: Sat, 23 Sep 2017 19:56:45 +0000 Subject: [PATCH] Add some text for section 13.3 of the User Guide. git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@8115 ab8295b8-cf94-4d9e-aec4-7959e3be5d79 --- doc/user_guide/en/measurement_tools.adoc | 108 ++++++++++++++++++++++- 1 file changed, 105 insertions(+), 3 deletions(-) diff --git a/doc/user_guide/en/measurement_tools.adoc b/doc/user_guide/en/measurement_tools.adoc index 12348febe..1deb5f8f2 100644 --- a/doc/user_guide/en/measurement_tools.adoc +++ b/doc/user_guide/en/measurement_tools.adoc @@ -65,7 +65,7 @@ image::FreqCal_Graph.png[align="left",alt="FreqCal_Graph"] === Reference Spectrum -WSJT-X provides a tool that can be used to determine the detailed +_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. @@ -74,7 +74,109 @@ Wait for about a minute and then hit the *Stop* button. A file named [ ... TBD ... ] -=== Equalization +=== Phase Response and Equalization - [ ... TBD ... ] +*Measure phase response* under the *Tools* menu is for advanced +MSK144 users. Phase equalization is used to compensate for group-delay +variation across the passband of receiver filters. 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 received frame is decoded *Measure phase response* generates +an undistorted waveform whose Fourier transform is used as a +frequency-dependent phase reference to compare with the phase of the +received frame's Fourier coefficients. +Phase differences between the reference +and the received waveform 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 SNR of the +reference signals is at least 9 dB. + +- Enter the callsign of the reference station in the DX Call box. + +- Select *Measure phase response* from the *Tools* menu, and process +the wav files. The mode character 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 discrete +circles along with a fitted 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. + +The three numbers that are printed at the end of each MSK144 decode line +can be used to assess the improvement provided by equalization. These numbers +`N` `H` `E` are: + `N` - Number of frames averaged, + `H` - Number of 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. The +three numbers at the end of the line indicate that one frame was +used to obtain the decode, there were no bit errors, and the +eye-opening was 1.2. 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 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 tell us whether the applied phase equalization curve +is going to improve decoding of signals other than those from the reference +station, K0TPP! + +We strongly advise you to carry out before and after comparisons +using a large number of saved wav files with signals from many different +stations to decide whether or not the equalization curve improves decoding for most +signals. When doing before and after 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 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.