EEVblog #1104 - Omicron Labs Bode 100 Teardown

[miro123]

Another theardown of Bode 100 TH test fixture

[miro123]

Then the question. How does this circuit works? Is it kind of direction coupler or LISN?
The answer of those questions can shed some light about the principle of working of this equipment.

[shabaz]

It simply uses that test fixture box to obtain the amplitude and phase difference compared with the stimulus signal (stimulus is measured from channel 1, and the DUT is measured with channel 2.  The resistors are used to form potential dividers.

You can't obtain much more about the implementation of the test equipment from just the test fixture box. But I imagine it internally may well contain a DSP chip or microcontroller to generate the stimulus (i.e. with a DAC), and use two ADCs to perform the measurements. From the amplitude and phase difference, it will be able to perform the calculation (and then repeat at the next frequency). At least, that's one possible implementation. I know nothing about that particular test instrument.

EDIT: Looking at the product page, it mentions there are superhet receivers internally. So, it's likely that the two channels (stimulus and DUT) are mixed down to either a very low frequency, or perhaps to DC, and then a couple of high-res ADCs) e.g. 24-bit can be used to perform the measurements.

[precaud]

Your question has already been answered:
https://www.eevblog.com/forum/testgear/mini-teardown-omicron-b-wic-impedance-test-adapter/?nowap

[shabaz]

You can better understand it by re-drawing the diagram to see the two potential dividers.

(Note: since I don't have access to the test fixture, the attached schematic may still have errors; if there are any, let me know, and I'll fix the diagram).

From the video, you can see how it approximately works; a stimulus is generated with a DDS, possibly switched in amplitude inside its metal can, and then that is received on the Source input on the test fixture, where it is applied to both potential dividers. The outputs from the potential dividers are the reference and DUT signals, and they will have an amplitude and phase difference depending on the DUT impedance.

Next, both of these signals are independently mixed (there are a lot of switches inside the metal cans to presumably do that!) with a second DDS-generated signal, which will have a precise relationship to the stimulus signal. That precise relationship will very likely be at least zero degrees and perhaps 90 degrees (alternated possibly) if mixed to DC. Your output signals from the mixer will then simply be digitized (there are two 24-bit ADCs, one per channel), and then all the data is sent into the PC (there may be some digital signal processing on the PCB too).