SA update to version v 6.2.3.3 : OFDM Module.

SA update to version v 6.2.3.3

Long enough time ago SA users SA had paid attention that the copy of the selected on the sonogram fragment visually looks shorter than the original. This interesting phenomenon, entirely linked with property of uncertainty of FFT transformation.

The matter is that transition from the time area into the frequency area, which provides FFT transformation, excludes possibility of exact definition of position of concrete frequency component by the time. FFT only "says" that the component is entering into the block, but where is the component located in the block is unknown.

It is the very old and standard problem. It has no exact decision, but we counted possible to lower a sharpness of this phenomenon. As the basic problems arise within the small blocks, when the resolution on time on

the sonograms is rather good.

This is realized in the current version.

We just make replacement of one problem by another. If in the old versions, copied by the sonogram fragment, physically had such duration which had been displayed at the moment of copying. That in the current version, the physical duration is longer than it is specified at copying.

We have considered possible to go on such exchange, for two reasons.

In the first, users confuses that the copied fragment visually looks shorter, and this situation demanded actions to regulate it.

In the second, the work on the sonogram, assumes visual perception, and it is better to see the copied fragment the way it has been selected, neglecting real accuracy. Even more so further it is necessary to work with the fragments in the area of the sonograms displaying.

The mode of a signal review has been added by the users offer.

The review is provided by FFT window with the size in 1024 counts. The primary goal of the mode it is to pass part of the signal with preamble or another problem segment (by having placed the start point in the necessary position), before the correlation triangle search will begin.

In the review mode, FFT window moves on the signal loaded in OFDM module, by the slider which is shown on the picture.

After key parameters of the signal are gotten, it is possible to define the detailed parameters of the preamble or other fragmen, by moving symbol-by-symbol backwards on the part passed earlier.

The detailed example on the real signal is described in the article OFDM analysis in SA version 6.2.3.3

Also the problems concerned users bug reports are removed.

Using SA to measure and correct sound card digitizer errors.

Using SA to measure and correct sound card digitizer errors.

This article had been written completely on the basis of letters sent by SA users.


All sound cards and A/D converters have some clock error. This is especially true if converters are commercial and cheap ones, like PC sound cards and similar. Professional and expensive converters exhibit a much better clock stability and jitter.


To correct this error, one must know nominal parameters of the signal under test. If these parameters are known, it is quite easy to correct digitizing clock error using SA.


The SA method of “Correction of BR” is quite good for this job, but perhaps using resampler as data input is a better procedure.


The correction factor will have to be measured for any digitizing speed and mode.
Bear in mind that for cheap cards, the speed can vary due to various factors, so if high precission measurements are required, a new calculus should be carried out.


For a good measurement, a quite big signal is needed.

This method is useful for PSK,FSK,MFSK and other modulations. To use it with OFDM, some more operations should be carried out using SA.


Ideally, an external signal like GPS 1000 Hz or a signal from a high end signal generator will provide the best results. Also, a radio timing signal should be good enough.


The best way to understand the subject is an example.
A well known signal as Stanag-4285 (sampled at 8000 sps) will be used to show the method.




We know 4285 has a nominal speed of 2400 sps. Since a frame is 256 symbols, the frame time must be 106,666666 mS. This is the value that should be obtained if using the VMW feature of SA when signal structure is perfectly vertical.





As we can see, the mesured value is 106,69241.


Correction factor =measured value/nominal value=106,69241/106,66666= 1,000241
Error PPM=(correction factor-1)* 1000000= (1,000241-1)*1000000= 241 PPM.


Real Digitizing speed= 8000*1,000241 = 8001,931
Real modulation speed (Br) = 2400/1,000241 = 2399,421739


Measured SA Br= 2399,41


Now, lets go to correct the signal using calculated BR in SA.

The signal parameters are almost perfect, so we can save it and after this procedure, we can be quite sure the new signal will be demodulated using any comercial demodulator.

Also, we know the correction factor for the used card in that speed.

As soon as parameters of a sound card in the record channel are received, then it is possible to realize precise measurements of replay( reproduction) channel for sound cards, which have separate record/reproduction channeles.

You can perform these measuremens simply by having connected an output of such card with an input, playingback a known/synthesized signal/file. Measurements should be realized by the technique described above. As the error of the channel of record is already known, it can be easily considered, and there will be only an error of the channel of reproduction/replay/playback.

PSK: CIS-3000

CIS-3000

PSK-8, Br-3000.

PSK

The record of this signal is here. Send by RadioKoteg.

Author: SergUA6

Band Width ~3400 Hz

Low Range ~600 Hz, in this example only!

Baud Rate 3000 Hz

n-Ary (PSK/MPSK) PSK-8

Carrier frequency ~2313 Hz, in this example only!

ACF Preamble ~ 85 ms

RX mode SSB

Sonograms

pic.1 Generall view

pic.2 General structure of the signal

Diagrams:

pic.3 8th harmonic

Pictures:

pic. 4 Phase constellation

pic.5 ACF preambles

CIS-3000 - With very high probability this signal has the Russian origin roots. More and more often there are signals which are leaving the limits of standard phone channel. This signal represents that kind of signals. The spectrum of this signal occupies 3400 Hertz. The maximum technical speed 3*3000 = 9000 bps.

SA update to version 6.2.3.0

SA Update to version 6.2.3.0.

Changes has been added in OFDM module operation.

Algorithm of search of a correlative triangle has been also changed. Current implementation of this algorithm successfully enough copes with the short signals.

Function of obtaining of exact value of clock frequency is updated. Now besides obtaining of the clock frequency, the task of obtaining of an optimal position of the signal by frequency is solving. It does automatically translates the triangle in the necessary positive polarity. All that demands some time, so no need to be nervous:-)

Some pauses (till 3-5 seconds) in operation of "Get Br" function are possible, The pauses depend on many factors:

from sampling rate of a signal

from its position on frequency

from the sizes LS and LG etc.

In generall, the program solves wide enough range of tasks in an automatic mode. From the user it is required only to situate the triangle in "a good" position and to call function "Get Br", and it is not important if there will be the positive triangle or negative, the program itself will translate the triangle into the necessary type, through search of optimal shift by frequency.

It is necessary to note that shift by frequency isn't precise because precision isn't necessary, while it is very close to optimal value, from the point of view of the program certainly.

All these changes and additions are preliminary preparation of the module for the decision of the primary task: obtaining LS and LG values of concrete signal, within the limits of our ideas, which were described in the article Parameters of OFDM signals.

Some minor problems had been found out by users and solved by us.

The more detailed information of operation with OFDM signals in version 6.2.3.0 will be described in separate article.

Good luck!