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).

Direct Conversion Software Defined Receiver

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.

Direct Conversion Software Defined Receiver

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.

Direct Conversion Software Defined Receiver

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:


174Mhz Receiver with 10.7Mhz SDR IF Detector

I've always thought a low-cost single band Software Defined Radio (SDR) might be used to convert a VHF or UHF FM Radio into an "All Mode" Receiver. Recently I came a across a low cost PLL VHF 174Mhz FM 0.2uV telemetry receiver card on ebay. So I have removed a few component to disconnect the 455khz down converter and FM limiter and then wired in the software defined radio tuned 10.7Mhz into the IF. I'm pleased to report this actually works.

Direct Conversion Software Defined Receiver

VHF SDR 173Mhz

Using a 48Khz USB Sound card wired in directly and the WRplus SDR Software and a Signal Generator tuned to 173.308Mhz

SDR 173.308Mhz

Signal Generator tuned to 173.328Mhz giving this reciver a bandwidth or tuning range of 24khz

SDR 173.328Mhz

10.7Mhz Crystal Oscillator

10.7Mhz Local Oscillator on the SDR PCB