Circuit knowledge

This project came about due to my interest in a new form of radio transmission called DRM, which stands for "Digital Radio Mondiale" (see www.drm.org). This is a new form of digital shortwave transmission. A few devices are available from Europe for decoding the digital signals but are expensive. I decided instead to modify an existing circuit, using a stable purpose-built 470kHz ceramic resonator as the oscillator, rather than the original unstable L/C version. The 455kHz IF signal from a shortwave receiver is fed into the input (pin 1) of a double-balanced mixer and oscillator (IC1) via a level adjustment pot (VR1). The NE506’s output (pin 4) is then AC-coupled to a PC’s sound card input for processing. With the capacitor between pins 5 & 7 set to 150pF, the oscillator frequency should be around 467.5kHz. You can check if the oscillator is working by putting it near a receiver tuned to 467kHz. You should hear a beat frequency.

Circuit diagram:

The IF signal of 455kHz is mixed with 467kHz, giving an output with a centre frequency of 12kHz. Sound cards should have no trouble sampling the 10kHz-wide DRM signal. A number of software-defined radio applications were found to work well with this converter. These applications perform all of the demodulation (SSB, AM, FM, etc) and various other DSP functions. If all is well, connect your 455kHz IF to the input and your computer sound card to the output. Run the Dream software (see http://drm.sourceforge.net), and tune to 6095Khz (RNZI), or 1440Khz (SBS). You should see the Dream software lock onto the DRM transmission and audio should start playing from the computer speakers. The NE602AN mixer/oscillator and 470kHz resonator are available for a cost of $12.50 - email the author for more details at jwtitmus@bigpond.com. A CD with various software defined receivers as well as the latest Dream software decoder is also available.