FAQs from Rohde & Schwarz

NRP - Usage of averaging in combination with chopper stabilization


I found a passage in the NRP (Z81) manual that describes that a kind of chopper stabilization works in averaging mode.

So there seems to be an improvement in the average here, but it's not described in detail. I have two questions about this:

- How is the average value collected when using a single sampling window?

- How does chopper stabilization work?


First of all, it is important not to confuse the aperture window (which is called 'sampling window' here) with hardware sampling.

The NRP-Z8x sampler runs at 80 MHz. This means that even with the smallest possible measurement window of 50 ns, the sensor's ADC already captures 4 samples.

As an example, a measurement window of 1 µs results in 80 samples. Then the chopper is switched (i.e. the polarity is inverted) and

records another 80 samples. From these two chopper phases you then get an (internal) measured value, which due to the different

polarity of the chopper has already been freed from the common-mode components.

After that comes the averaging. This means that an average factor of e.g. 8 runs through the aforementioned cycle 8 times.

And when all that is finally done, the sensor provides a single reading.