Software Defined Radio (SDR)

Jan 29, 2019 · 3 min read
The Secret Files of the SpyDogs

SDR is a radio communication system that can be modified by the software to operate at any frequency range via programmable hardware. SDR system consists of an antenna, a transmitter, a receiver, and a host computer.

Basic specifications of SDR are bandwidth, resolution and tuning range. Multiband (ability to operate on two or more bands either sequentially or simultaneously), multicarrier (ability to simultaneously operate on more than one frequency at a time), multimode (ability to process several different kinds of standards like FM, AM, CDMA) and variable bandwidth are some characteristics of SDR. The bandwidth of SDR is determined by alterable digital filters [1].

An example of a receive path in a software-defined RF data link
  • Receivers

SDR receivers have a superheterodyne architecture which contains a combination of amplification with frequency mixing. It allows performance across large frequencies and different RF bands. Therefore, SDRs are compatible with multicarrier applications (when more than one channel is required).

A superheterodyne receiver

Mixers are used to convert the RF spectrum to the IF frequency. More than one mixers can be used in a design. At each stage of mixing, process filtering should be used and it must cover the maximum possible spectrum. The main purpose of using a demodulator is to separate I and Q components. Digital quadrature demodulator is used to achieve perfectly matched signal paths and to avoid I/Q imbalances. LO (local oscillator) can be programmed via the PLL or DDS technique[2].

  • Transmitters

Transmit path is also based on superheterodyne architecture. Modulated baseband data is obtained by DSP or baseband ASIC. The data must be converted to desired IF with an FPGA/ASIC or with a mixer/modulator. Digital Up Converter (DUC) block transfers the baseband signal to IF. Then, translation channels can be summed and interpolated with filter and after that, the data is sent to a DAC. If IF is used, final conversion to RF frequency is applied by mixer and LO optimizes the distortion. Lastly, a power amplifier is used to achieve power gain.

  • Challenges

The two main problems with the SDR are cost and power. The power issue is due to the requirement of high-performance devices which means high-linearity devices[3].

  • Things to do with SDR

-Receive broadcast radio (88Mhz-108Mhz)

-FM listening application

-It can be used as ADS-B aircraft air radar

-Listening to aircraft traffic control conversations (around 118Mhz)

-Listening to DAB broadcast radio(with a decoding software like SDR-J) and so on

An inexpensive way to get started with SDR is the RTL-SDR dongle. An application of SDR can be implemented by using analog TV receiver with RTL2832U chip and R820T Tuner which has a frequency range between 24 Mhz and 1850 Mhz. RTL device can be updated with a program called Zadig to make it compatible with SDR. For detailed guidance:

A video about RTL-SDR




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