Tagged: srm-3000

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Audio output from the SRM SDR receiver

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Audio output

The SRM-3000 SDR receiver can handle up to 16 independent receiver channels, but usually, we only have one sound card output in the PC. The radio receiver software defines one channel as monitored, and sends its audio output to the default audio out device. This way, you can listen to an active channel with your speakers or headphone.

Processing the audio output

Usually, after the filtering and demodulation, we want some additional post-processing of the audio output. The post-processing software can run on the same or on a separate computer. In the latter case, we can directly connect the audio out to the microphone input of the other PC using a simple jumper cable with two standard 3.5 mm jack plugs. In case we use the same computer for post-processing, we can use a virtual audio cable. This is a software that defines virtual output and input devices. It can be set as the default audio device, and thereby act as a gateway: the SRM SDR receiver can send its audio output to it, which the post-processing software can directly receive.

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Testing with different post processors and decoders

First, I used an external recording software to save audio output for later processing. I’ve employed Audacity, which is a free, open-source audio editor and recorder software for a variety of platforms.

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After that, I’ve successfully tested the connection with the well-known Spectrum Lab post-processor developed by DL4YHF.

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Finally, I’ve tested the demodulation possibility for the audio output by utilizing the Code300 software package from Hoka Electronic. The audio signals contained a real FSK transmission, and it was successfully demodulated and processed.

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FM_broadcast_if_spectrum200

Wideband SDR reception in the SRM SDR software receiver

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[Wideband SDR available bandwidth settings]

The SRM-3000 radio receiver SDR software supports the following six built-in bandwidths: +/- 6.25, 12.5, 25, 50, 100, 200 kHz. These are Single Side Band (SSB) specifications. So, for example, the widest (200 kHz) setup actually enables the reception of 400 kHz wide transmissions, as all the signals are complex. This is seen on the spectrum/waterfall display, which has a frequency range going from -200 kHz to +200 kHz in this case. The wideband SDR software radio receiver SRM automatically sets the Digital Down Converts (DDCs) on the DRU digitizer to the desired Nyquist frequency. But this DDC output sample rate is NOT equal to the bandwidth specification above! In fact, the sample rate is much higher, thus the system can deliver unattended performance within the useful bandwidth. So, for example, in case of the 200 kHz bandwidth setup, the actual DDC output sample rate is 398 kSPS – almost double. This overhead allows to have a magnitude drop of less than 1 dB within the useful ±200 kHz bandwidth.

200 kHz is wide enough to display and receive WB FM broadcast transmissions. As the input frequency range of the DRU-244A digitizer hardware can go up to 320 MHz, we just need to tune the radio receiver to the desired station.

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Thanks to the high SNR the large amplitude 19 kHz stereo pilot is easily identifiable in the audio spectrum. The audio processing runs at 48 kSPS, thus the difference channel is not processed for the time being. Full stereo decoding is an upcoming feature.

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Recall that the display only contains the 1 dB band of the DDC output, which runs in the example at 398 kSPS if IQ pairs are counted as one sample, or at 796 kSPS if counted as two. This is a significant burden on the subsequent DSP processing, which is entirely performed by the x86 Intel GPP in the PC. The processor workload is very high, so you need a powerful machine (i5 or preferably i7) to process one or more wideband radio channels. On the other hand, WB FM broadcasts may be processed in the 100 or 50 kHz bandwidth mode as well due to the robust nature of FM modulation.

FM_broadcast_if_spectrum200 FM_broadcast_if_spectrum100a

The wideband radio receiver is not only for demodulating WB FM broadcasts. The receiver may be tuned to any band of interest. E.g., the following images show the 20 m HAM radio band; the center of the ±50KHz window is at 14.050MHz. In this case, the demodulator offset functionality may be used to select and demodulate the narrowband signal of interest within the available bandwidth. This involves mixing, band filtering, and CW/USB demodulation – all performed on the PC.

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Finally, a note on safe device handling. Direct digital radio reception at wide input bandwidths (i.e., 320 MHz for the DRU-244A digitizer board) necessitates the use of input preselection filters, in order to avoid overload conditions for the sensitive receiver input circuitry. To design and simulate such filters, you may use the excellent free filter design program DIONYSUS by ComNav Engineering.

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