Basics of Direct Sampling Software Defined Radios
The traditional radio uses analog processing, i.e. analog mixers, filters and amplifiers to convert the incoming analog signal from the antenna to a baseband signal, such as audio. In contrast, software defined radios (for simplicity I use this term for any kind of digital radio) rely on digital signal processing (DSP) techniques for reception. The DSP functionality includes the application of software running on a processor or algorithms implemented as digital circuits in microchips (FPGA/ASIC), that implement the receiver functions, like mixing, filtering, etc. in the digital domain.
SDR Receiver Architectures
In a SDR the interface between the analog world (originating in the analog signal coming from the antenna) and the digital world is realized by an AD converter. Today most receivers are a mixture of analog down conversion followed by the AD converter and digital processing. The AD converter can be placed at different stages of a receiver, thus determining which parts of the reception are executed in the analog or digital domain.
Historically the AD converter moved from the end of the reception path (where bandwidth is low) further towards the front (where bandwidth is higher). This is desirable, because digital signal processing is largely superior to analog processing (see Advantages of DSP).
For this reason it is only logical to put the AD converter as close as possible to the antenna. The ultimate concept is called direct sampling (or sometimes “all-digital receiver” or “digital radio”). Then the antenna is more or less directly connected to the AD converter with no analog down conversion. Of course connecting the antenna to the AD converter directly may not be a very good idea, because aliasing will likely spoil the signals of interest (see AD conversion basics). Therefore even a direct conversion receiver typically includes an analog (anti-aliasing) filter and sometimes a preamplifier.
Direct sampling software defined radios exhibit some special properties and challenges for the Implementer:
Advantages of Direct Sampling SDRs
Direct sampling receivers perform as much operations as possible using DSP instead of analog circuits. DSP can be implemented with almost arbitrary accuracy. Once the signal has been digitized, it is not prone to further degradation anymore. The only performance bottleneck in a direct sampling radio is the AD converter. It is important to carefully select the AD converter, since it determines the overall performance in of the radio in terms of linearity, sensitivity and bandwidth.
Challenges of Direct Sampling SDRs
Clock Jitter and SNR: For direct sampling the phase noise or jitter of the sampling clock becomes important. Clock jitter introduces extra noise and may impair the ADC’s SNR. This is especially the case for input signals with high frequency as they often occur in direct sampling with no prior analog down conversion (see AD conversion basics). It is important to use high-performance clock generators with low jitter.
Data Rates: Direct sampling receivers have to deal with high bandwidths and thus high sampling rates, that create enormous amount of data rates for DSP. Therefore direct sampling is applied more widely to lower frequencies and may be prohibitive for high frequencies. The Panoradio e.g. creates a input data rate of 16 bit x 250 MHz = 4 GBit/s and internally up to 2 x 22 bit (IQ) x 250 MHz = 11 GBit/s. Usually these data rates cannot be handled by a PC or other processor based system. Instead they require the implementation of highly-parallel digital circuits in FPGAs.