Even though broadcast services are desert- ing the AM domains in the long, medium and short wavebands, there is still plenty of interest to be found surﬁng the radio waves with a home-constructed receiver. Now more than ever you might say, because many distant stations now come up far more clearly because they are no longer swamped by stronger signals. In fact it is often so quiet on the short waves that it’s easy to imagine your receiver has gone deaf. On some bands it is the radio ama- teurs who produce the strongest signals. And there is always something new to ﬁnd, from pirate radio stations through SSB radiotelephony to the new digital modes.
That just has to make you curious! Elektor has already published many radio and receiver projects. A Software Deﬁned Radio with USB interface was introduced as long ago as 2007 . In the mean- time much thought has been devoted to conceiving updates for this design. How- ever, the PLL chip we used originally is no longer made, making it necessary to ﬁnd a new solution. This has arrived in the form of the Silicon Lab SI5351 chip, a CMOS clock generator from 8 kHz to 160 MHz with I. And then there came an idea: why not simply build the entire receiver as an Arduino Shield?
This decided the power supply requirements, using the USB interface already available on the PC. The Arduino would look after controlling the VFO and could be addressed in plain language so to speak (6030 kHz please). And what is perhaps even more exciting, this even gives you a real chance to build a totally standalo ne receiver. Operation could be migrated from the PC to the Arduino relatively simply.
And who knows, perhaps one day the decoding of the IQ signal as well? You simply connect a large coil as an antenna and immediately you’re able to receive the VLF band. Down there are plenty of interesting signals, even trans- mitters installed on submarines.
GQRX: GQRX is a software defined radio receiver for Funcube Dongle (FCD), RTL2832U-based DVB-T devices (RTL-SDR), Universal Software Radio Peripherals (USRP) and Osmo SDR devices. It is powered by GNU Radio and the Qt GUI toolkit. This page describes an updated version of the SDR receiver. Elektor Academy - AVR Software Defined Radio in. Medium with loggings, schematics, software.
If you want to use the sound card for higher frequencies, you must ﬁrst down- convert the signals. The process is akin to a superhet with a lower intermediate frequency (IF).
The PC handles the IF stages, ﬁltering, automatic gain control (AGC) and demodulation. In principle a simple direct mixer with a diode ring mixer or the well-known NE612 would be adequate for this. Only a stable variable oscillator (VFO) would be needed in addi- tion. For special applications you could.
First, let’s go back to basics. What fun- damentally is a Software Deﬁned Radio?
For quite some time the development of digital electronics left radios entirely untouched. When home computers ﬁrst became available, most radios were still analog. Then development began to take place, at least for digitizing their tuning. Today’s radios are often equipped with a PLL synthesizer that simpliﬁes tuning and guarantees precise conformity to channel spacing. The rest of the circuitry remains analog as previously. Subsequently digital electronics appeared inside commercial RF equipment and amateur radio gear. Ever more of the analog functions in devices were replaced by software.
In most cases a digital signal processor (DSP) with appropriate soft- ware operates out of sight from the user and takes care of optimal ﬁlter curves, variable bandwidth, signal decoding, interfere nce suppression and much more. The equipment is altogether improved and with less hardware overhead. Further examples of this kind of development can be found in smartphones and other por- table end-devices. At the same time it’s evident that hobby constructors can no longer keep pace with this technology. In fact things don’t need to be so sophis- ticated, however.