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
This commit is contained in:
Steven Franke 2017-09-23 19:56:45 +00:00
parent ddbfc0da74
commit eb4a98e5b4

View File

@ -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.