For analysis of digital signals the instrument must capture an IQ array with the specified bandwidth (sampling rate. Modern instruments’ sampling rate is about 125% of the bandwidth) and the minimum recording time of the IQ. The required bandwidth and the required recording time of IQ are determined by the standard of the analyzed digital signal.

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For example, the DECT signals have minimum sampling rate of 2000 kHz and the minimum IQ recording time of 200 ms. If the instrument does not support sampling rate of 2000 kHz, than the program selects the nearest available frequency higher than 2000 kHz and performs resampling of IQ to the required frequency for DECT demodulation. If the instrument does not allow capturing IQ with frequency of 2000 kHz or higher, it is impossible to demodulate DECT standard (error message “insufficient sampling rate” will be displayed).

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The majority of the instruments has built-in memory for recording IQ. (Note: some instruments does not have built-in memory, but can produce IQ as continuous flow. For such instruments as Rohde & Schwarz PR100, SignalHound BB60memory size limitation is not applied.). The required minimum sample rate and minimum IQ recording duration are define a minimum memory size requirements:

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For example, when analyzing the DECT signal, minimum size of IQ memory must be 0.2 (seconds) * 2000 kHz = 400 kWords of IQ.

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If an unknown signal is analyzed for several digital data transmission standards at the same time DTEST, the IQ saved only once with a maximum sampling rate and a maximum recording duration from all selected standards to reduce the time of testing. (The program converts the IQ for each standard, the digital demodulation for all the selected standard is performed simultaneously in different threads of multi-core CPU).

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Algorithm of program’s operation with simultaneous demodulation of the different standards is following:

1. The program selects the maximum value of the IQ sampling rate from all digital standards that need to be analyzed (let’s name it “Fd”). If the maximum sample rate of the instrument is less than the required sample rate for any digital standard, this standard can not be analyzed (the error message “insufficient IQ sampling rate” will be displayed).

2. The program selects the maximum recording duration of the IQ for all digital standards that need be analyzed (let’s name it “L”).

3. The program checks the ability to record required length of IQ based on the available memory of the instrument. The required memory size for IQ recording must be equal to M = Fd * L.

4. If the IQ memory size of the instrument is less than the “M” value, then new value is calculated as “L1” = Mavailable / Fd where “L1” is maximum recording time of IQ for selected instrument.

 

If “L1” value is less then minimum required recording time of IQ of any digital standards, demodulation of these standards is not performed (error message “insufficient size of the IQ memory” will be displayed).

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For example, we have chosen the spectrum analyzer Rohde & Schwarz FSU. Digital standards TETRA, APCO25, DMR, GSM, Bluetooth, DECT are selected for analysis.

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The minimum required sample rate for the simultaneous analysis of all selected standard is Fd = 2000 kHz (determined by DECT standard). The instrument is able to support this sample rate. The required recording time of the IQ is 1 second (L = 1 second defined by GSM standard). Required IQ memory size is M = 2000000 IQ words.

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Rohde & Schwarz FSU spectrum analyzer has only memory size of 523,776 IQ words. This memory is enough to record IQ with length L1 = 523776/2000000 = 0.26 seconds with a 2000 kHz sample rate.

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For the analysis of TETRA standard, IQ record length of 1,076 seconds is required. TETRA analysis test will not be performed, the error message “insufficient size of the IQ memory” will be displayed.

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For the analysis of APCO25 standard, IQ record length of 0,8 seconds is required. APCO25 analysis test will not be performed, the error message “insufficient size of the IQ memory” will be displayed.

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For the analysis of DMR standard, IQ record length of 0,8 seconds is required. DMR analysis test will not be performed, the error message “insufficient size of the IQ memory” will be displayed.

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For the analysis of DECT standard, IQ record length of 0,2 seconds is required. DECT analysis test will be performed.

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For the analysis of Analogue TV standard, IQ record length of 0,1 seconds is required. Analogue TV analysis test will be performed.

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For the analysis of GSM standard, IQ record length of 0,98 seconds is required. GSM analysis test will not be performed, the error message ” insufficient size of the IQ memory ” will be displayed.

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To determine which standards can be tested at the same time for the selected instrument is possible to achieve by empirical methods.

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AM and FM analogue signal demodulation

AM and FM analogue signals demodulation is possible for instruments that send a real-time flow of IQ data (currently available only from software defined radios or spectrum analyzers). Demodulation is possible over any frequency range provided by the instrument– from the maximum down to 200 Hz. For AM signals phase-locked loop techniques may be possible.

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DECT

Demodulation of DECT signals provides detection of base station addresses (RFPI addresses) and connected handsets which are in active mode (talk mode). For each base station and active handsets, signal level is determined in order search for them using amplitude direction finding techniques. Determining the list of legal addresses of a DECT base station allows the operator to discover any new DECT voice data channels present in a controlled premise, which might be used as radio microphones. RadioInspector does not perform voice demodulation as DECT uses data encryption.

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