Computers and Digital Signal Processing already play a big role in recent Amateur Radio transceivers. Many HAMs have a good understanding of these features and regularly use them for all manner of filtering, noise reduction and signal processing tasks while on the air. We’ve also seen more and more radios with Spectrum Scopes which make it easier to visualize what is on a given band in real time. Thanks to increasing volumes in color displays, Digital Signal Processor (DSP) applications and low-cost processors, these capabilities are now common – even on entry-level HF transceivers.
Software Defined Radios (SDRs) are the next logical step in this evolution. SDRs are not new, they have been around for some time now. SDR technology has continued to improve as the cost and performance of Analog to Digital Converters, Programmable Logic Devices and other processors that make up the hardware side of SDRs have improved. We are now to the point where it is possible to build an SDR for Amateur Radio applications which can directly sample RF at frequencies as high as 150 MHz.
Direct Sampling SDR receiver designs have some important advantages over more conventional single conversion and super-heterodyne receiver (i.e. multiple conversion) designs. These include:
- Higher dynamic range
- Low phase noise
- Ability to cover multiple bands simultaneously with multiple receivers
- Very high-quality spectrum displays
- Flexible, high-performance filters
- The ability to add new modulation schemes and other features via software updates
The first two items above (dynamic range and phase noise) are particularly important as they result in receiver performance which is significantly better than that which can be achieved with the best direct and superhet designs. Take for example a busy contest environment when a band is very crowded (ex. 40m at night in a worldwide DX phone contest). There are many strong signals crowded closely together on the band. Even the best conventional design receivers will have trouble hearing moderate and weak signals in this environment. The problem is that the strong signals tend to overload the analog circuitry in the conversion stages of conventional radios which produces a great deal of Intermodulation Distortion Design products. Phase noise also compounds this problem.
A direct sampling SDR converts the incoming RF signals with high dynamic range Analog to Digital conversion and then performs all of the filtering and demodulation of the incoming signals in software. This approach limits the potential for Intermodulation Distortion with an end result that all of the signals on the band (including the weaker ones) are much clearer. This approach also allows very high order filtering to be applied in the RF domain which results in greatly improved selectivity and rejection of closely spaced adjacent signals with minimal distortion.
By now some may be think that this all sounds great but I don’t want to have to use my computer to make QSOs. There is good news on this front as well. We are beginning to see the major transceiver manufacturers introduce direct sampling SDR technology in radios with conventional “buttons and knobs” interfaces.
New designs like the Icom IC-7300 can provide a way to gain the performance and feature advantages of an SDR in a radio which has a more conventional interface. The entry of the major manufacturers into the direct sampling space and the resulting competition should help to lower prices for all types of SDRs.
Want to give SDR technology a try without spending a lot of $? There are several very good SDR Dongles available along with SDR software at a minimal cost. Dongles are typically receive-only but some can also transmit as very lower power. The use of this technology in digital TV receivers and set-top boxes has made the cost of SDR Dongles very low and there is some very good SDR software available for free on the web. Dongles are generally broad coverage receivers and they can also be used to listen to signal outside the Amateur Bands.
It is interesting to follow the rapid evolution of SDR technology. We recently integrated a FlexRadio-6700 SDR into our station to enable us to operate remotely via the internet. You can read more about this project on our blog.
– Fred (AB1OC)