What is direct digital receiver?
Direct digital receiver or software-defined receiver (SDR) is a type of radio, where the analog signal is converted into the digital domain, and functionality is implemented in the digital domain employing signal processing algorithms. Conversion technology is limited in terms of bandwidth and frequency range, thus the right point for conversion has to be carefully chosen. Conversion can take place at the baseband, Intermediate Frequency (IF), or directly at the Radio Frequency (RF). In case conversion happens at the operating RF (likely after the pre-selector), we can talk about direct digital receiver. Feel free to read more in the award winning presentation by Dr. Bertalan Eged here: GET THE AWARD WINNING TECHNOLOGY INTRODUCTION CONFERENCE PAPER
Domain converter frequency parameters
Analog-to-Digital Converters (ADCs) and Digital-to-Analog Converters (DACs) are employed to bridge the analog and digital domains on the radio hardware platform. Converter parameters determine how we can use them in the radio implementations.
One of the most important parameters is the real-time bandwidth or instantaneous bandwidth. It is determined by the sampling frequency of the converter, and according to the Nyquist law, it is equal to the half of the sampling frequency.
The other very important parameter is bandwidth or frequency range of the converter itself. Usually, this is determined by the circuits involved: it starts with the analog components, and includes circuitry within the converter, like the sample-and-hold stage. The Nyquist criteria states that the bandwidth should be equal to the half of the sampling rate in order for a perfect reconstruction in both time and frequency domains. Hence, there is a possibility to generate and sample higher frequency signals too, if we keep the bandwidth inside half of the sampling rate. In other words, we can use upper half bands, called Nyquist bands. If we have a wider spectrum, we have to be sure not to alias or fold from higher Nyquist bands to the baseband. The anti-aliasing filter or SDR pre-selector is used for that propose. If we are talking about ADCs and receivers, the latter terminology is employed.
Typical ADC input frequency response for direct digital receiver
In the SRM-3000 direct digital receiver we used 80 MHz as sampling frequency for our hardware platform, so the instantaneous bandwidth is 40 MHz. We can tune to radio channels within this band using on-board hardware digital down converters (DDCs). The input bandwidth of the ADC itself is 650 MHz. This is the -3 dB point of the input stage, and it has no brick wall slope.
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