Proper frequency planning in a Synthetic Instrument RF system

As anyone that has tried to use RF in VXI or PXI platforms can tell you, the process is not as simple as bolting various modules together, loading software, and turning on the switch. Unlike the rack and stack legacy systems, a SI (synthetic instrument) requires a degree of preplanning for best system performance. Although there are many variables to consider, some companies have figured out how to obtain excellent RF performance in these platforms. Unlike legacy units where every block is self contained, SI systems by their nature try to reuse modules for different configurations. The same analog-to-digital converter (A/D) used for the main downconverter, for example, will be required to support a network analyzer, spectrum analyzer, and/or digital storage oscilloscope (DSO). The A/D´s sampling frequency that works well for the DSO might be too low for the downconverter´s IF (Intermediate Frequency) chain. The result could be a SI with poor noise and speed performance. What IF frequencies and bandwidths are ideal? These questions lead to others what is the input bandwidth of the Si´s A/D? Is it better to use the A/D in fundamental mode or will using it in the 2nd or 3rd Nyquist zone be a better choice? How much filtering if any will be required? What are the system noise implications for each of these choices? Many modules require clock and reference oscillator connections in addition to connections for the main signal and control paths. It is commonplace to interconnect a reference clock to all modules that have a PLL (phase locked loop). Has the module designer taken proper precautions to insure reasonable immunity to out of band spurious signals on these interconnects? Has the SI module designer made sure that each module doesn´t generate spurious signals on these lines and transfer them to the adjacent module? Operating frequencies and bandwidths continue to rise making it harder to keep the spurious signals from one module from radiating or conducting into- another. This paper will demonstrate that the proper selection of frequency plan in the SI RF subsystem plays a critical role in the performance of the SI. It is the mark of a properly designed SI to make these selections for best system performance. Spurious energy will be reduced, costs will be reduced, and dynamic range will be vastly enhanced.