Anti Alias Filtering

Using the raw ADC input makes it necessary to add an anti alias filter. If one of the amplification stages in in the signal path the amplifier itself will cut all high frequencies. But the direct path, gain one, needs a dedicated low pass filter. What are the criteria for selection?

Of course it needs a reasonable cutoff frequency. But low pass filters have additional properties that might be important to consider, so lets shed some light on the different possibilities.

Things to consider include

• filter order
• resistive vs reflective
• phase shifts
• pass band gain constantness

Reflection Domains

How can we design the signal path and what are the consequences for signal integrity. Apart from bandwidth, rise time, and dispersion one important issue is to manage the reflections. Reflections occur on all boundaries, i.e. connectors of cables, passive, and active elements.

The goal is to minimize reflections as seen by the user. Reflections that are not seen are not as bad. They will attenuate the useful signal (because part of the signal energy is reflected), but this is just attenuation. You can distinguish input port reflections (its coefficient in the S parameter matrix is called S11) and output port reflections (S22). Note that S parameters are complex numbers.

Rise Times and Reflections

Did a quick check of the preamplifier rise times, while I had the coax relay on my test-bench. The results are a bit slower than expected. The reflections are also measured and then some of the low pass filters were also checked.

Signal Switching

For diverse signal switching applications we need a coaxial relay. After some investigation we settled on Radiall's R572 series. In evaluation is now a R572.332.000. The rated bandwidth is only 3 GHz, but the series goes up to 26 GHz and the lower bandwidth variants are just rejects (my guess) designed differently, better performance over a broader frequency band at the dispense of performance in the lowest frequency region.

Price of Bandwidth

We need to construct our own variable gain front end amplifier, aka preamp, for the ADQ412. But this gives us some freedom in design and the trade offs and makes it possible to separate the actual amplifier from the ADC.