16 January 2017

PM-SA, the Poor Man Spectrum Analyzer

This is a workaround to get a spectrum-analyzer tool for people like me who run 32-bit PC (real spectrum-analyzer need 64-bit systems).

The idea is to capture and store the SDR spectrograms (the waterfall) at fixed time intervals while the SDR is recording. When the recording time is elapsed, you may browse the saved screenshots and once you detect a certain signal you can access directly to it through the I/Q recording using the timestamp of that signal (which is printed in the waterfall). My test are with SDR-Console v2.3 and 20/20 v2.2 software, the latter is the programmable screen-capture tool.

setting the 20/20 v2.2 software

First of all, 20/20 v2.2 need to be executed as windows-95 compatible (Fig. 1)

Fig. 1
 
Run 20/20 v2.2 and hit files-> preferences to set the output directory and file, auto save, image format, and auto-increment as in Fig. 2 then set the capture-parameters paying attention to the Timed Options, Capture Target and Capture To (Fig. 3).

The most important value is the Timed Options: the Time Capure shall match the time needed to fill the SDR waterfall (better few second less so to get a little overlapping between two consecutive captures). In my case I preferred 60 seconds and then I will have 60 screen-shots/hour. Hit Ok and 20/20 starts.

Fig. 2
Fig. 3
setting the SDR software (SDR-Console v2.3)

The most important settings are intended to adjust the waterfall speed and height. In my opinion, 10 lines/second is the better resolution value, allowing to fit the waterfall in about 60 seconds (Fig. 4). Remeber that the time needed to fit the waterfall shall match the Time Capture value in Fig. 3! Note also that you have to set Add timestamp option in SDR-Console (Figs 5).

Fig. 4
Fig. 5
Once found the best values and window heigth, you may start the recorder after setting the duration of the recording (Fig. 6).

Fig. 6
Now you may go for a walk or at your job, stay with your partner, have a fresh beer or simply go to sleep: 20/20 & SDR recorder will do the job for you, once recording finished you will have the chance to analyze the stored waterfall screen-shots.



Analyzing the spectrum

Remember to stop 20/20: it run in background, saving screen-shots at the scheduled time. Browse the directory where 20/20 stores the screen-shots, as indicated in Auto Save (Fig. 1), you will see something like Fig. 7

Fig. 7
Images can be seen one at time, back or foward, using the Windows images viewer or some other similar tool. This way you may carefully examine every single “capture” using also the zoomer. At the same time, run SDR-Console and playback the recordered I/Q file (Fig. 8) keeping it paused. Minimize the SDR-Console window.

Fig. 8
When you see an interesting signal (unknown or maybe known by its shape) in a certain stored screen-shot, you have to write down the related timestamp and go to the SDR-Console window. Now, working with the playback navigator you have simply to access to the time slot which is related to the seen signal and play it, i.e. as shown in Figs. 9,11 (the being analyzed screen-shots) and 10,12 (playing the needed time slots from the I/Q file). Pay attention to the different time-format in the captured screen-shots and I/Q file!

Fig. 9
Fig. 10
Fig. 11
Fig. 12
20/20 v2.2 can be downloaded from:

(googling for it ...you may turn up an XXX site).


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