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What is SDR (Software Defined Radio)?

This article on SDR (Software Defined Radio) covers SDR transmitter basics, SDR architecture, difference between analog receiver and SDR receiver, benefits and more.

As the name suggests Softwave Defined Radio is the wireless communication system operating at radio frequency and configurable using software. In SDR, hardware components commonly available in analog counterpart are configured in software which is ported either on FPGA or DSP to be used in wireless chain. Following components are configured in SDR typically:
• mixers used in upconversion and downconversion
• Amplifiers • Modulator and demodulator modules
• RF detector
• Filters (BPF, LPF) using FIR filter concept

Now-a-days using SDR concept, entire PHY layer and RF chain is implemented in modem of current and next generation wireless technologies or standards such as WLAN, Mobile-WiMAX, LTE, LTE-Advanced etc.

SDR architecture-Transmitter Part

SDR transmitter

The figure-1 depicts SDR architecture with example of SDR transmitter part. As shown it consits of DSP, DUC (Digital Up Conversion), D/A converter, analog RF Up converter and power amplifier. The digital baseband part is coded in DSP which provides I/Q data as per different transmitter need. This is digitally up converted using DUC (Digital Up Conversion) with the use of digital LO and digital mixer. The digital IF samples are converted to analog IF. This analog IF is converted to analog RF using RF upconverter. The RF signal is amplified before being transmitted over the air using respective antenna.

Difference between analog receiver and SDR receiver

analog radio receiver

The figure-2 and figure-3 depicts analog radio receiver and SDR receiver block diagrams. Let us understand difference between these two architectures.

As shown in figure-2, RF signal is amplified using RF Amplifier.

The amplified signal is later fed to the RF mixer for RF down conversion. This is done by beating input amplified RF signal with locally generated LO signal.

IF signal is extracted and amplified at the output of RF mixer. Typically RF center frequencies of 455KHz and 10.7 MHz are used for AM and FM broadcasting.

This amplified IF signal is demodulated and passed to audio amplifier as shown.

In Amplitude demodulator envelope detector is used while in frequency demodulator frequency disciminator is used. Here RF mixer does the core job of converting RF signal into IF signal.

SDR Receiver

As shown in figure-3, the first module is RF tuner. This RF tuner converts RF signal to amplified IF signal. It replaces first three modules (RF amplifier, mixer, IF amplifier) as mentioned in analog receiver in figure-2.

Following modules are followed after RF tuner module.

➨A/D converter: It converts analog IF into digital IF samples.

➨The digital samples are passed to the DDC (Digital Down Conversion) which converts digital IF samples into digital baseband samples(Referred as I/Q data). DDC consists of digital mixer, digital Local Oscillator(LO) and low pass FIR filter.

➨The digital baseband samples are passed to the DSP chip where algorithms have been ported which does the functions such as demodulation, decoding and any other tasks if required.

➨ This digital implementation based architecture is referred as SDR or Software Defined Radio. Often FPGA is also used in place DSP in this software defined architecture.

The beauty to have software baseband processing chain on DSP/FPGA will help in correcting realtime baseband and RF related impairments present in I/Q data with the use of advanced algorithms. Typically algorithms such as DC offset correction, I/Q gain and phase imbalance correction, time, frequency and channel impairment correction are implemeted in SDR receiver.

Benefits of SDR-Software Defined Radio

Following are the benefits or advantages of SDR (Software Defined Radio) architecture based system:

➨Easy to upgrade different versions of the software. For example, it is easy to change LTE modem having PHY and RF compliant to LTE standard into LTE Advanced modem compliant to LTE-Advanced standard. This easy functional changes also help in upgrading and enhancing features as per client or customer need.

➨Major savings into cost due to less time required compare to analog conventional system. This is because any changes required during testing can be easily done in software unlike complex hardware counterpart.
➨It is also easy to experiment new ideas even while the system is operational.

What is Difference between terms related to SDR

RF Versus IF
Heterodyne Receiver vs Homodyne Receiver
Design of RF Frequency Up Converter
RF Transceiver Design and Development
FIR Filter VS IIR Filter
Wireless Physical Layer Implementation
Physical Layer Measurements
RF Measurements

RF and Wireless Terminologies

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