What's the F̶r̶e̶q̶u̶e̶n̶c̶y̶ Bandwidth, Kenneth?

A project log for 8 GHz Sampling Oscilloscope

8 GHz BW, up to 1 TS/s ET Sampling

Ted YapoTed Yapo 08/23/2019 at 15:470 Comments

In the initial testing, I tried several approaches to estimating the bandwidth of the first sampler.

ADCMP606 Step

The ADCMP606 step generator was the first experiment over on #The Rise and Fall of Pulses , and in some ways the most successful. It just uses an ADCMP606 CML-output comparator to generate a moderately fast rise-time step, typically 160 ps according to the datasheet.

Here's the output measured with the sampler:

and with the Tek 11801B/SD-24:

The traces look similar, but again they were triggered slightly differently, which could cause some discrepancies. The differences right after the edge are most likely caused by some reflections from the sampler itself. Matching on the front-end needs work.

To estimate the sampler bandwidth from this, I fit a Gaussian step to the data points, and calculated the rise time of the fit function:

The Tek scope measures the same 10%-90% rise time at 140 ps.

The SD-24 head has a maximum rise time of 17 ps, so the step from the ADCMP606 is probably closer to sqrt(140^2 - 17^2) = 139 ps. From this, we can estimate the rise time of the prototype sampler = sqrt(149.4^2 - 139^2) = 55 ps.

A 55 ps rise time for a scope with a Gaussian response implies a bandwidth of 0.35/55e-12 = 6.4 GHz.