Nuts and Volts - April 2014

52 April 2014

WHAT IS AN SDR?

First — as a quickie review — a software-defined radio is a digital radio where some or most of the operations of a transmitter or receiver are performed digitally with mathematical algorithms. These digital operations can take place in a digital signal processor (DSP) or in a field programmable gate array (FPGA). The most common operations are filtering, mixing, modulation, and demodulation. Amplification is still an analog operation, however.

An SDR transmitter would first put any information (voice, video, etc.) to be sent into digital form with an ADC (analog-to-digital converter) and send it to the DSP where the modulation is applied. Refer to Figure 1. The DSP generates two digital outputs called the in-phase (I) signal and the are called the baseband (BB) signals.

The signals are next sent to DACs (digital-to-analog converters) where the analog radio signals are developed then filtered by low pass filters (LPF). The signals are then upconverted by a pair of mixers. The mixers are fed with a local oscillator (LO) signal set to the final transmit frequency. One LO signal is shifted 90 degrees from the other. The mixer outputs are added together then sent to a power amplifier (PA) and the antenna.

Note: The reason for the I and Q signals is that both are needed at the receiver to recover the original modulating data. The I and Q signals provide the amplitude and phase information needed by the mathematical algorithms for demodulation.

The ideal SDR receiver would be an antenna connected to a low noise amplifier (LNA) and then to an ADC. Check out Figure 2. An input filter is

n case you haven’t noticed, virtually all radios today — cell phones and Wi-Fi WLAN, for example — are software-defined radios (SDRs). More and more, the functions of a radio are gradually moving from hardware to software. Furthermore, with ever decreasing semiconductor sizes, chip companies can put more circuits and functions on a chip that operates at higher and higher frequencies.

Today, it is no problem to put most of a radio on a single chip. Two recently announced integrated circuits really point this out. Here is a look at the latest in SDR RF chips.