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The QS1R Communications Receiver

A bleeding edge direct sampling SDR.

LIMITS OF QUADRATURE SAMPLING SDRs

The bleeding edge of radio technology was once manifested in projects using quadrature sampling detectors feeding studio quality audio interfaces. Also known as "zero IF SDRs," these devices exhibited characteristics far superior to superheterodyne radios in the categories below:

1. Dynamic range, thanks to superior "H mode" mixers.
2. Stability, when the local oscillator is oven stabilized and / or GPS disciplined.
3. Signal purity, due to well designed A/D converters (often 24 bit studio quality audio interfaces).
4. Panoramic spectrum display, implemented in the software interface.
5. Point and click tuning, implemented in the software interface.

For example, the Softrock and Flexradio SDRs easily outperform most superhet radios in those five aspects. Innovative software makes these radios versatile and easy to use, but ultimately it is RF hardware with the first critical job: capturing a clear enough signal for manipulation in the digital domain.

Though the QSD based software defined radios typically have very high dynamic range, they are now reaching performance limits due to the local oscillator. These SDR's operating range is divided into a series of bands defined by the bandwidth of the A/D converter. Where a conventional transceiver may tune in 1, 10, or 100 Hz steps, a QSD based SDR may tune in 48, 96, or 192 kHz steps. Phase noise, spurs, and other problems arrived the moment designers moved away from simple crystal local oscillators. Superior general coverage performance, therefore, is a tough task requiring complex hardware designs, as seen in radios like the Flex-3000 or Flex-5000.

THE HPSDR MERCURY PROJECT

Consider a more advanced SDR design: directly sample the entire spectrum of interest, and implement all imaginable functions in a virtual transceiver. Very wide bandwidth A/D converters can digitize the entire DC through VHF range, and feed the data to a microprocessor. This method has fantastic potential! Consider a modest list of possibilities:

1. Very high performance multi-mode operation.
2. Versatile multifrequency operation.
3. Any manner of spread spectrum and high rate digital modes.
4. Multiple virtual transceivers using one or many antennas.
5. Synthetic phasing / beam forming for antennas.
6. Streaming RF data to remote receiver interfaces via the internet.
7. Very high definition reception and recording, limited by user's computing and storage.
8. Numerous specialized possibilities for RADAR and radio astronomy.

A group of nearly 1000 talented radio amateurs and short wave listeners have been working on a "High Performance Software Defined Radio (HPSDR)" project with the goal of ceating the sort of direct sampling SDR under discussion here. In fact, the HPSDR group is creating hardware and software which greatly advances radio's state of the art. Their Mercury module is an early example of movement toward direct sampling SDR designs.

THE QUICKSILVER QS1R - AMAZING AND ELEGANT

A ready-to use SDR currently available outside of military / government circles has potential to do all of this: the QS1R receiver (and soon a QS1T transmitter), made by the Software Radio Laboratory LLC. This radio is the result of intense brainstorming among numerous talented people, and more specifically, the work of a team led by Philip Covington (N8VB).

The QS1R has excellent specifications, and an in its most basic form, covers the range from 10 kHz to 62 Mhz, and through 300 MHz using undersampling. With proper front end equipment, it can receive VHF, UHF, or microwave signals in a manner far more advanced than conventional communications gear. The unit uses a USB 2.0 interface for connection to the user's computer, and companion software is available for Linux, MAC, and Windows. SDRMax is the open source software for this radio. SDRmax may be examined, analyzed, modified, and evaluated by the end user community. With each update, more capabilities are added, and the Quicksilver SDR system continues to improve.

LISTEN TO A QS1R ONLINE

For radio listeners curious about the Quicksilver SDR, graphical interface software makes it the best web controlled receiver on the internet. The QS1R can be remotely tuned throughout its range, in any mode, and with easily configurable settings. The panoramic display is excellent, and the audio quality is limited only by the end user's soundcard. It is possible to connect to an SDRMAX server on the other side of the world, using a modest broadband connection, and enjoy great sounding signals from the remote QS1R receiver. ESSB signals, for example, seem to jump into the foreground and hardly seem to originate hundreds or thousands of miles away.

Note that presently, when changing servers for SDRMAX, the server IP address should be manually set by editing the "GUILocalSettings.xml" file. It may be possible in the future to select servers from a dropdown list. It all depends on popular demand. If you think these remotely operated QS1Rs are fabulous, be sure to write to the hams hosting them and let them know.

Direct sampling software defined radio is certainly where the bleeding edge is found, and the QS1R reflects that state of the art quite nicely.