Spurious Free Dynamic Range (SFDR)

Ideally, when up-converting signals to higher frequencies, the converter should not add unwanted signals to the output. Unfortunately, however, when making a microwave converter, it is not possible to remove all spurious mixing products, such as Local Oscillator (LO) leakage, spurious signals from the synthesizers, InterModulation Distortion products (IMD), and other sources of stray signals.

The amount of stop-band filtering rejection necessary to eliminate undesirable spurious increases as SFDR increases. Most commercially available microwave filters are not built to maintain very high ultimate rejection in the stop-bands. Filter manufacturers typically specify 65 to 70 dB of 'tested' filter rejection, with theoretical designs of over 85 dB. In pratice, however, connectors, resonators, and tuning screws leak some amount of microwave signals. Thus, few commercially available filters today can exceed the 90 dB of stop band rejection necessary to provide 70 dB of SFDR. The illustration below shows the relative levels of desired and undesired signals in the typical microwave converter.


To make a better converter, we went back and redesigned the filters from scratch, so leakage signals were reduced to a level near commensurate with the coaxial cables used to interconnect components. Most of our internally built filters exceed 90 dBc of rejection in the stop-band. In addition, many of the filters are designed to have more symmetrical skirts by optimizing the internal coupling. Finally, to be environmentally friendly, the traditional internal plating was changed. All this doesn't reduce the cost of the filters, but it does make a converter with exceptionally good SFDR performance. (At the current time, we do not sell our filter technology separately from the converters.)

SFDR Graph

When designing a system that incorporates a frequency converter for use with digitally synthesized signals it is important to match the converter performance with the digitally generated signal. In our bandwidth discussion, the sinc roll-off was one significant factor to consider. Another factor is the Effective Number Of Bits (ENOB) which accounts for various imperfections in the digital to analog conversion process and further lowers the system dynamic range, particularly at the top of the band. Getting the most performance possible for the money spent is often critical to making a useful system because wideband Arbitrary Waveform Generators (AWG's) or Digital to Analog Converters (DAC's) usually have very limited dynamic range after accounting for sinc roll-off and ENOB. Further reduction of the dynamic range by poor frequency converter performance can result in a system that doesn't have sufficient signal fidelity to effectively perform as needed. When engineering a system, always carefully consider what SFDR you will really end up with and, if possible, leave a little extra margin.

SFDR Window