Software Defined Radios (SDR) have become popular with hams over the last few years. Many own one, and those who don’t have certainly heard of them. What is an SDR receiver, and why might you want one over a traditional radio? Or maybe you want to hedge your bets with a superheterodyne or hybrid SDR? We’ll help you navigate the pros and cons of each.
Different Strokes for Different Folks
To understand each type of receiver, we need to look at their components and characteristics. Here are the three main varieties, though some don’t fit neatly into a specific category.
Superheterodyne: A superheterodyne receiver (superhet) is a type of radio receiver that uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more easily processed than the original carrier frequency. The traditional superhetrodyne radio works using conventional radio components rather than software.
Here’s a block diagram of a typical superhetrodyne (superhet) receiver:
The diagram above shows a dual conversion superheterodyne receiver. They may have more conversion stages and additional circuitry to provide the required levels of performance, like Kenwood’s TS-590SG.
A basic superheterodyne starts with the antenna connected to an RF amplifier, which amplifies the very weak signals picked up by the antenna. Some high-end radios put bandpass filters between the antenna and RF amplifier to block strong out-of-band signals. It mixes the incoming signal with a single frequency from the local oscillator to convert the signals to a new frequency.
Next comes the demodulator. This could be for amplitude modulation (AM), single sideband (SSB), frequency modulation (FM), or any other mode. It is also possible to switch between different demodulators according to the mode being received. The final component in the superheterodyne receiver block diagram is an audio amplifier.
Direct Sampling SDR: With a software-defined receiver (SDR), all of the radio control is done via software. Functions such as changing frequency, electing filters, and changing bands are no longer done by the radio hardware itself. The hardware becomes less complex due to the elimination of circuits that would normally be in a traditional superhet radio. With digital circuitry, you reduce noise, distortion, and signal loss found in each successive stage of a superheterodyne. The picture below shows a block diagram of an SDR radio.
The IC-7300 employs an RF direct sampling system where RF signals are directly converted to digital data. Looking over the diagram, you can see that the incoming signal wastes no time going from analog to digital. Received signals are filtered, amplified, and then sent to an analog-to-digital converter (ADC). Then they are fed to a field-programmable gate array (FPGA) for conversion.
Because frequency conversion is not required, the overall design of a direct RF-sampling receiver is much simpler in comparison to a superheterodyne receiver. The ADC essentially replaces the mixer, oscillator, and the entire IF signal chain found in superheterodyne configurations. Because they have fewer components, direct RF-sampling receivers can be built into more compact radios.
The digital signal processor (DSP) portion of the SDR, which is software operated, does the mixing, filtering, and demodulation that’s done by analog hardware in a traditional radio. If you looked at a block diagram of the DSP functions, they would be basically the same as found in a traditional superhet radio. The big advantage is that you can change the various parameters on the fly, such as IF filter width and shape, automatic gain control (AGC), etc.
Hybrids: A hybrid SDR is a radio system that uses a combination of conventional analog components as well as software and digital technologies to process a radio signal. The picture below shows a block diagram of a hybrid radio, the Yaesu FTDX10.
With a hybrid SDR, the RF signal is first amplified and then transformed using an analog circuit to a range that can be processed by the ADC. An amplifier might be added before the ADC to provide some gain. The FPGA then processes the digital output from the ADC, and passes it on to the DSP.
Pros and Cons: Superheterodyne
Superheterodyne receivers still have a place in the market. The better ones offer excellent selectivity and sensitivity that rival SDR radios. They also are immune to some problems that occur in direct-conversion receivers. But they’re slowly being squeezed out of the market due to the added features and continuing price reductions in SDR/Hybrid transceivers.
Pros
- IF stages convert high frequency to low frequency, so all signal processing takes place at the lower frequencies–better results.
- It is easier to filter an IF signal compared to an RF signal.
- It offers excellent receiver sensitivity.
Cons
- Requires multiple local oscillators and RF mixers to convert signals from RF to IF before conversion to baseband. This increases overall cost.
- Receivers are generally larger in size because they require more components.
Hybrid SDR Pros and Cons
Hybrid SDR design is a practical option given the current state of RF digital technology. High speed and high resolution A/D and D/A converters are still somewhat expensive, and the hybrid architecture is a budget-conscious way to improve receiver performance. This design also combines the strengths of both superheterodyne and SDR receivers, as well as some limited shortcomings.
Pros
- A superhet down conversion stage before the A/D conversion stage can provide better dynamic range than a simple direct conversion SDR.
Cons
- All analog processing will introduce some noise, and the direct sampling approach eliminates that. On the other hand, the hybrid approach does allow insertion of a roofing filter in front of the A/D converter, which can improve performance.
- It does not sample the full spectrum like a direct sampling SDR radio.
SDR Direct RF-Sampling Pros and Cons
The SDR direct RF-sampling receiver basically consists of a low-noise amplifier (LNA), the required filters, and the ADC. The ADC digitizes the RF signal directly and sends it to a processor. An ADC essentially replaces the mixer, oscillator, and the entire IF signal chain found in superheterodyne configurations. Because they have fewer components, direct RF-sampling receivers can be built in smaller packages.
Pros
- The ability to receive and transmit various modulation methods using a common set of hardware.
- They can be quickly and easily upgraded with enhanced features via software/firmware.
- Direct conversion SDRs are exceptionally good at isolating weak signals from adjacent strong signals.
- Elimination of higher-cost analog hardware.
- Can view a real-time waterfall display of an entire band and see all of the stations operating at one glance.
- Recent advances in analog-to-digital converters have enabled them to directly digitize signals at RF frequencies. While operating at high frequencies, they can maintain low noise and good linearity.
- SDRs continue to benefit from the simplification of radio designs. Cost of digital parts will continue to fall, yet digital devices will continue to get more powerful.
Cons
- If you’re used to knobs and dials on a traditional receiver, SDR will require some changes in your operating routine, especially if it’s controlled by a PC. However, several manufacturers have models with traditional controls and integrated touch-screen menus.
- Receivers still have some limitations regarding the range of frequencies and receiver bandwidth they can support.
- Software bugs–fortunately, updates can be easily added.
Flexibility
The idea of a more flexible radio becomes the most obvious and important advantage of SDR/Hybrid radios over classic superheterodynes. The ability to upgrade your system for better performance and for new features means endless possibilities for SDR radios as they are developed.
Another related benefit to SDR is that by making a less complicated RF front-end, there are fewer total parts needed. With digital components like the DSP and FPGA taking the place of many passive and active components, costs become cheaper in the long run.
Using both superheterodyne and SDR hybrid technology together offers the best of both worlds. If you look at the current Sherwood Labs receiver ratings, two of the top three are Yaesu hybrids, the other is a Flex direct conversion SDR. A significant number of others in the top 20 are also various flavors of SDR or hybrid. It’s clear that SDR technology adds to the performance of current transceivers, whether alone or in combination with more traditional technology.
