Logo

Bandwidth Considerations

Digital to Analog Converters (DAC's), whether part of independent circuits or built into an Arbitrary Waveform Generator (AWG), output stair stepped voltages over time to represent the smooth continuous analog signal. This stair-stepped voltage waveform or 'zero order hold time' in the time domain does not produce a flat frequency spectrum, but a sinc or sin(x)/x spectrum.

DAC Rolloff

The sinc 'spectral roll-off' of the DAC has important consequences when one is attempting to get the best possible performance out of a system. This spectral roll-off decreases the achievable dynamic range possible with any given DAC since the smallest possible signal change is quantized by the DAC's least significant bit.

When it comes to up-converting a DAC, the best dynamic range portion of the spectrum is closest to DC, suffering the least roll-off. However, when making an up-converter, it is not possible use the DAC's baseband signal all the way down to DC. The problem lies in the mixing process where the baseband signal from the DAC is mixed with a Local Oscillator (LO), and the unwanted LO and side-band must be filtered away after mixing to obtain only the desired frequency converted signal. The question then becomes: How 'steeply' can the filter skirts rise up to remove the unwanted LO but pass the desired sideband? Unfortunately, no one has yet created the so-called 'brick wall filter.' Thus, some amount of the best of the DAC's signal spectrum must be wasted in the up-conversion process to allow for less than perfect filters.

Bandwidth

We believe that our filter technology allows us to make filters with an industry-leading shape factor for the broadband converter application. This means less wasted spectrum and maximum dynamic range in the up-converted spectrum. It also means that for the same up-converter AWG (or DAC) bandwidth, our up-converters actually deliver better system dynamic range using the best part of the baseband spectrum. (At the current time, we do not sell our filter technology separately from our converters.)

Think of it this way... You purchase an $85,000 arbitray waveform generator that can effectively produce signals from DC to 3.2 GHz with an acceptable dynamic range for your application. The cost of the useful bandwidth 'thrown away' by using filtering that can only work down to within 500 MHz of DC versus within 200 MHz of DC as our units can, is $7,968! Worse yet, only getting within 500 MHz of DC also lowers the overall SFDR of the system due to greater roll-off.