SpectraChip™ IF is a digital replacement for analogue intermediate frequency (IF) filtering used in current spectrum analysers, and is supplied as an IP core for embedding on low cost FPGA devices. SpectraChip™ IF provides a digital implementation of all the standard features, such as resolution bandwidth filtering, video bandwidth filtering and conversion to log power.

Updating spectrum analyser designs to replace traditional analogue components with modern digital techniques helps to make the systems easier to manufacture, more reproducible and more reliable. The use of digital technology saves on the manufacturing alignment costs normally associated with analogue circuitry, and the guaranteed repeatability of the digital design reduces both the need for, and the costs of, ongoing calibration.

With the enhanced version of this first release core, new features also become possible. Most existing spectrum analyser designs are based on a swept approach to monitoring a band of the spectrum, which means that the system is only looking at each part of the spectrum for a finite period of time. By switching off the sweep function and using FFTs and other specialist techniques to perform the spectral analysis means that a wide band of interest can be analysed at all times, and that even very short duration pulses can be captured by the instrument. The further addition of complex spectral triggering allows storage of spectral data to occur upon a particular event in the frequency domain, this can then be presented to the user using 3-dimensional "waterfall" plots or 2-dimensional spectrograms, to show how frequency changes over time.

Basic block diagram of SpectraChip Digital IF

SpectraChip™ IF can be licensed from RFEL as an off-the-shelf design, and can be easily modified to meet particular interface requirements, as well as achieving different intermediate frequencies and ranges of RBW (Resolution Band Width) and VBW (Video Band Width) that may be required. The standard design will fit comfortably into the Spartan3 1000 FPGA device costing less than $100, and consumes a mere 0.5W in operation, or significantly less in the power save mode. This high specification design is available "off the shelf" fully characterised, documented and tested in silicon.

The principle behind the new product range is to simplify and de-risk the embedding and incorporation of high performance, digital designs into instrumentation for spectrum analysis, whether this is for general test instruments or equipment for specific applications. In many applications, this approach opens the opportunity for real-time monitoring of quality of service and pre-empting service degradation or total failure.

The designs provide a cost-effective and high performance solution wherever high quality spectral analysis is demanded. The flexible solutions can be upgraded with technological advances and user demands, as part of an ongoing development program. RFEL can also provide its proven design-in support services, with immediate access to its technical experts where design support is required. The comprehensive nature of the solutions available provides manufacturers with a platform upon which to design a whole family of products with different specifications to meet a variety of requirements, along with the ability to develop a clear roadmap of product development. Where appropriate, RFEL will provide bespoke solutions, incorporating specific device choices to meet the user requirement.

Future Developments
SpectraChip™ MHz and SpectraChip™ GHz are real-time designs, which are currently at an advanced development stage. In these designs large bandwidth signals can be presented to the core and one of RFEL's extensive range of filter-bank cores can be utilised to convert the signal to a frequency domain representation. Using the programmability benefit of the FPGA, a number of different filter banks can be supplied to meet different measurement requirements. Standard Gaussian shape filters can be used to produce a standard spectrum analyser effect, or higher specification, flat top, low transition band filters can be used to achieve power measurement accuracy and good frequency isolation.

In the real-time mode, an optional trigger function can be included in the SpectraChip™ design to allow masked triggering of spurious signals. This enables fleeting events to be stored and analysed on a time-frequency-power basis or passed via the secondary I&Q outputs to demodulation processes for characterisation.

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