The New Ham Radio Receiver

The ham radio receiver of the 21st century is mostly software. It is called a Software Defined Radio or SDR for short.

Out of the many features and advantages of a SDR receiver, I particularly like its effectiveness in picking extremely weak signals out of the noise and surrounding interference. Simply astounding!

How does SDR work?

First, as with any other radio receiver, the antenna is connected to the SDR's "hardware" RF front-end. Its purpose is to...

1. Interface physically with the antenna for optimum RF energy transfer to the receiver.

2. Serve as low-pass or band-pass filter.

3. Amplify the signals.

4. Convert the frequency of signals down to an intermediate frequency (IF) suitable for the ADC stage that follows.

A SDR Block Diagram

The departure from a conventional receiver starts here.

Prelude To A Soft Process

The amplified IF analog signal produced by the RF front-end is fed to an analog-to-digital-converter (ADC).

The digital output of the ADC is then fed to a Field Programmable Gate Array (FPGA).

1. The FPGA extracts the "I" and "Q" components of the signal.
   
   The "I" and "Q" signal pair is called a complex signal. It is produced in the FPGA by two frequency mixers having a phase shift of 90° between them.

2. The I/Q output of the FPGA is then fed to the USB 2 programmable controller.

3. The software defined radio, running on the PC, takes its I/Q data from the USB 2 controller. The SDR software...

   • ... extracts the information from the signal for audio output.

   • ... displays a graphical user interface giving the user access to control functions and a variety of selectable visual outputs.

PC O/S Critical!

All of the signal demodulation and spectral functions are done by the SDR software on your PC.

Most SDR ham radio receiver implementations will usually (at least) support AM, WFM, USB, LSB, N-FM, DSB and CW with fully adjustable DSP filter bandwidths ... down to below 1 Hz in some cases!

The SDR is hosted by the O/S of your computer, just like any other application. The majority of the SDR implementations run on some flavor of Microsoft Windows®.

However, an increasing number of SDR projects are now turning to the Linux open source operating system which offers...

• much faster signal handling and process-to-process communications,
• far greater stability,
• much more flexibility.

No Cost Receiver Upgrades!

On top of its higher performance and flexibility, a SDR offers ham radio operators the neat advantage of being able to add new functions and features to their ham radio receiver at no cost!

All you have to do is download and install
the latest version of the SDR software and presto!
You have a brand new radio.
Neat! ;-)

SDR Implementations For Ham Radio

Noteworthy SDR implementations are:

• WinRadio WR-G303i®
• RF Space SDR-IQ®
• SRL QuickSilver QS1R VERB®
• WoodBoxRadio Perseus®
• WeakSignals WinRad®
• Ettus Research LLC USRP1 & USRP2 Designed for GNU Radio software. The most advanced, flexible, open source implementation for multiple O/S!

I have to mention here the exceptional transceivers produced by FlexRadio Systems™. You have to hear the signals you receive with them to believe their published outstanding specs!

For more on SDR types and for links to research and development projects, see this fine SDR resource maintained by F4DAN.

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The New Ham Radio Receiver

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