Software Defined Radio (SDR) Basics

A collection of stuff I think I know and understand about Software Defined Radio (SDR) at a basic (simple) level

  • The Antenna and Band Pass Filter

    Antenna and band pass filter

    An antenna is resonant (tuned) to a specific part of the electromagnetic (radio) spectrum providing the optimal collection of the electric field at that specific frequency. This voltage in the form of an alternating sign wave is passed down the coax cable to a band pass filter (BPF) the blocks out (filters out) the unwanted parts of the radio spectrum leaving the wanted signal

  • Analogue to Digital Conversion (ADC)

    Analogue to Digital Conversion (ADC)

    The analogue sign wave is sampled at regular (time) intervals and the voltage value is collected / stored in a range of 0 to 255 (if using 8 bits (couple of nibbles) [e.g. 1010 1010] of computer values)

  • The Signal Mixer

    A classic quadrature approach

    The signal mixer takes in the real radio signal received from the antenna and combines that with the local oscillator (which articulates the signal wanted). The mixer then produces a suite of output signals which form the intermediate frequency (baseband) which is passed along to the computer via USB. This is done twice once for the signal in phase "I" and then again for the signal out of phase "Q". This quadrature technique is done to fix the problem of not knowing if the wanted signal is above or below the (wanted) frequency that is articulated by the local oscillator

  • The USB SDR Radio

    SDR Radio Receiver outputting in phase (I) and out phase (Q) samples via USB to the computer for decoding

    A typical SDR radio will sample the analogue signal voltages post filtering into the digital domain using ADC techniques. The radio will typically do this twice for the signal in phase "I" and out phase "Q" which is then sent to the computer for further processing and demodulation (decoding). After demodulation the encoded signal will be ready for the radio listener

  • Under and Over Sampling

    Under and over sampling of the ADC

    Undersampling the ADC would potentially give a wider bandwidth in situations where the ADC is limited by the number of samples (per second) it can handle

    Oversampling the ADC can give a larger dynamic range between the signal and noise i.e. it increases the detail (or resolution) of the captured signal

  • Nyquist Theorem

    Nyquist Theorem

    The theory that you need twice as much bandwidth to sample a given bandwidth accurately i.e. for 30MHz of bandwidth spectrum capture (at the same time) you need 60MHz (samples per second) of sampling bandwidth