On Tue, Nov 12, 2024 at 01:42 AM, Donald S Brant Jr wrote:
Only if the PC sound card provides phase information. I do not know whether or
not they do.
A basic PC sound interface is simply a group of analog-digital and digital-analog converters. They operate at least in 16 bit resolution, while many have 24 bits. The sample rate is settable, at least to 48kHz, often to 96kHz or more. Usually sampling is coherent between channels, so that accurate phase information can be obtained. The signal/noise ratio usually is somewhere between 80 and 120dB.
The (very inexpensive) sound chip that comes on the mainboard of my computer offers 10 audio channels, which can be configured quite freely as inputs or outputs, with lots of options. A lot of measuring can be done with that, including full VNA functionality. It's just a matter of what the software does.
I have set up my audio chip to provide a stereo line input, a stereo line output, a mono microphone input with separate power supply, and a quadrophonic output for speakers. These four logical audio devices operate completely separate and independently. For example, I can use my line input and output to measure the impedance of an RLC circuit over the audio range with LIMP software, while at the same time listening to music through the speaker outputs with a music player program.
In contrast to desktop computers, the internal audio support of laptop computers tends to be very limited, mainly due to lack of enough audio connectors. But cheap USB audio interfaces are available, and many of them offer a stereo output and a stereo input, so an impedance analyzer can be built around one of those interfaces, using just an extremely simple circuit, which could be as basic as a single resistor and a few connectors. If more input channels are wanted, to make a full two-port VNA with reference sampling, one can use two of those cheap USB audio interfaces, or a single, more expensive one, that has enough inputs in a single unit.
More expensive sound interfaces usually include a DSP chip, differential audio connections, high quality connectors, etc.
Most of the audio measurement software available seems to be centered around speaker and room acoustics measurements, but can also be used in general electronics work. ARTA-LIMP is an old, good, well-known example. Daqarta is also worth looking into, but it's payware whose free features are too limited to be really useful. EasyLCR is a soundcard LCR meter software that works well, but just on a single frequency. And there is a very good program for those who need an audio spectrum analyzer, with a real heap of bells and whistles included: Spectrum Lab, by DL4YHF. There are also many audio oscilloscope programs, but any plain and simple music editing software that allows viewing the waveform can be used as an audio storage oscilloscope with almost unlimited memory. Since I'm old fashioned, I use Cool Edit Pro...
People using the PC sound system for measurements are often at first hindered by Windows mixing input and output audio channels in software, without asking the user. This "feature" can be turned off, but the relevant settings were very well hidden by Microsoft. For those who have stumbled across this problem, here is the recipe, which works fine in my setup of Windows 10:
Right-click on loudspeaker icon in taskbar.
Open sound settings.
At Output, Speakers, choose Device properties.
Choose Additional device properties.
Select Levels.
There you can set up the mix of sources for the speaker outputs. Set to zero all the ones you do not want to directly feed through into the audio output.
My feeling is that with PCs being so common, and almost all of them having audio support, there is little point in building any sophisticated special hardware for audio measurements. Just using an existing PC and suitable software provides high quality measurements, on the cheap. So that's what I use: The PC sound chip in the audio range, and the NanoVNA in the RF range, starting right above audio.
Donald S Brant Jr
Warren Allgyer
Bruce Akhurst