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Software Defined Radio

DiRaWave 032

Functional Description


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Following a new path

The main focus of the hardware design was to digitize an anologue shortwave signal as accurately as possible without quality loss. Further signal processing and demodulation are done in software.

The radio is a dual conversion superheterodyne receiver, wherein the RF signal is converted first to a high IF frequeny of 45 MHz by using a high-IP actice mixer in conjunction with a very low-noise VFO.

An ultra steep special discrete 10-pole crystal roofing filter with 16KHz bandwidth is used for 45MHz IF filtering. Next conversion is done to the very low second IF of 12KHz. This working principle is also state-of-the-art used for professional communication receivers.

DiRaWave 032 Blockschaltung

Then a novel method is used for optimized analogue-to-digital conversion:

The second intermediate frequency is splitted into two different channels (RX & DX channel), whereby the DX channel is amplifed by an amount of 27dB to increase the resolution for weak signals just above the noise threshold. Then each channel is converted with 16dB ADC resolution and transferred to the PC via USB.

IGC replaces AGC

By splitting into two channels, a conventional automatic gain control (AGC) is superflous. A standard AGC may cause signal degradation problems:

Within the passband of the roofing filter, the strongest signal determines the setting of the AGC. If a strong and a weak signal are close to each other, the weaker signal will also be attenuated, because of the reduced gain and therefore its signal-to-noise ratio (SNR) will be also reduced.

Instead, a software-based gain control (IGC- Intelligent Gain Control) is used, whereby both channels are monitored and adjusted simultaniously to obtain always the maximum achievable SNR.

IGC - RX-Kanal & DX-Kanal

RX channel (red) and DX channel (white) spectrum signals simultaneously displayed by an AM transmitter on longwave. The lowered noise floor of the DX channel is clearly visible.

Taking all control options, level differences of 137dB could be processed.