The main limitation with the N2PK VNA is that the inputs are transformer-coupled and response drops rapidly below the 10s of kHz, along with increasing distortion. Also the DDS outputs start rolling off around 3 kHz. The applications that I'm aware of don't have LF limits (MyVNA or the DG8SAQ VNWA software, which also supports N2PK, nor my own LabVIEW-based app). I did in fact get the think working down to 20 KHz for some audio testing but found that calibration was relatively unstable. By the way, the VNWA is not transformer-coupled and is rated to operate down to 1 kHz. The other hassle is that the VNA is designed around a 50-ohm environment, whereas most low-frequency audio and control systems are based on much higher impedances.
And yes your WB6DHW USB board will work...
There was an article in QEX about building a low-frequency adapter for VNAs (you need to be an ARRL member to download this):
What you really want for controls work is a dynamic signal analyzer (DSA). That typically would be an expensive commercial instrument from Keysight, B&K, SRS, etc. A basic DSA has an output (excitation) and two inputs. One input monitors the excitation signal, the other the response of the DUT. In the frequency domain, the ratio of output to input is the complex transfer function (magnitude and phase), and that will give you just about all you need to know for classical control system design.
A DSA can also be implemented on the cheap with a good sound card and the right software. Being a LabVIEW guy, I use the fact that sound I/O and DSA functionality are built-in (see Dual Channel Spectral Measurement). And there's now a free community version of LabVIEW :-) You could also acquire sound data using any app that saves standard audio files, and then use something like MatLab or Octave to do the math.
-Gary NA6O
