Direct Conversion Software Defined Receiver

I recently purchased and built a low cost Soft66Lite kit which is a Direct Conversion (DC) Software Defined Receiver (SDR) the simple cousin of the well known Software Defined Receivers and unlike the early Direct Conversion Receivers of the past, as the mixer stages are based on a Quadrature Sampling Detector (QSD).

In the Direct Conversion SDR version, the radio frequency (RF) signal is first down converted to an audio frequency (AF) where it is then sampled by a high performance stereo audio card or Analogue-to-digital converter (ADC). Then through the use of digital signal processing (DSP) it can be filtered and enhance to demodulate many modulation systems including AM, CW, SSB, FM and a variety of digital modes.

Quadrature sampling detector (QSD)

A QSD is a system that switches the incoming RF signals into in-phase signals (I) and quadrature signals (Q) by the frequency of the local oscillator. The in-phase signal is the first 90º of the RF signals waveform (I) and the quadrature signal is the second 90º segment of the RF signals waveform (Q).

Mathematical functions can be used by the software to calculate the phase and amplitude of the original signal by measuring the values of I and Q simultaneously which has all the information contained about the original RF signal in it.

SDRs have become increasingly popular in recent years due to their relative low cost; the ubiquitous availably of high speed computers and they are significant flexible in terms of bandwidth and demodulation in comparison with traditional superheterodyne receivers. So I thought I might be good to build a few different units to gain an understanding of these receivers and see if these low cost radios can be employed in a Radio Telescope project at least in the latter IF and Detector stages.

Examples of software used include:

• Rocky
• Winrad
• SDRadio