Products > Test Equipment
DIY Transformer for use with Bode Plots.
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mawyatt:

--- Quote from: joeqsmith on May 24, 2022, 09:59:42 am ---I have no idea if that's a problem with the setup or if its real.  Seemed very impressive.   As I posted, my first attempt was around 252Hz.  Hardly in the ball park.   

--- End quote ---

No worries, the setup is fine :-+

Measured the DSO input Ch2 as 50.07 ohms with input cable and connector and added a 100.08 ohm series resistor as a Thru, both measured with a KS34465A. The DSO Ch1 is High Z and reads Vin, Ch2 is 50 ohms and reads Vo, and DSO set to Bode Mode.

The results should show 20*log{50.07/(50.07+100.08)} or -9.539dBV, and shows -9.56dBV @ 100Hz, -9.59dBV @ 100KHz and -9.61dBV @ 1MHz. Certainly fits within our present needs for this capability ;)

This is with the DSO at the highest resolution available on Bode Mode and using 10Bit Mode yielding 0.1dBV/div and 1 degree/div. Hopefully the new true 12bit ADC version (the present SDS2104X + uses an 8bit ADC and extends performance to 10 bits with DSP but limits scope BW to 100MHz) will allow higher resolution. Keep in mind this DSO does range scale the inputs to allow much larger Dynamic Range than possible with 8 or 10 bits digitation, basically dynamically scaling to fit the input within the "sweet spot" of the base core ADC.

Best,
joeqsmith:
Thanks.    For the phase, is the reference a manual offset you can add?   

mawyatt:
The phase, amplitude reference & scaling is just settings for the display.

Best,

Mike
joeqsmith:
Makes sense.  Thanks.   I'm still not sure about your test setup.   Assuming you had followed the threads you had linked (what a rabbit hole), what your thoughts are about the following comment and the three that follow:

https://www.eevblog.com/forum/blog/eevblog-1104-omicron-labs-bode-100-teardown/msg3038468/#msg3038468
mawyatt:
We're using the transformer as an injection device for Closed Loop measurements and the built-in Bode function of the SDS2104X + measures the input across the transformer primary with Ch1 and the output is Ch2 which can be set to 50 or 1M ohms. The absolute amplitude and phase of the injection signal isn't critical for Closed Loop measurements as long as the amplitude is not high enough to cause issues with the system under test or saturate the transformer, and not low enough for the DSO to have trouble digitizing. The later becomes an issue with high loop gain systems where the feedback attempts to cancel the injected signal, this can be seen in some of the Close Loop plots shown earlier at the low frequency ends of the plots.

Edit: With Closed Loop Bode the transformer secondary is placed in series with the feedback loop and DSO channels are connected across the secondary. Here's a video showing how the setup is configured for this Bode Mode and also shows how the Isolation Transformer for Bode use is built and connected.



Another link.

http://www.simprojects.nl/images/Gain_Phase_measurements.pdf

As mentioned a couple times, the DSO samples the Ch1 input {Vin} and Ch2 output {Vout}.

Since the Bode plot is Vout/Vin and Vo = T(f)* Vin, where T(f) is the Transfer Function as a function of frequency of the DUT,

then Bode Plot = Vout/Vin = (T(f)*Vin)/Vin or simply T(f)

Note the Bode Plot is independent (ideally) of input signal level as it should be for an ideal linear system transfer function.

This is the way the Siglent DSO computes the Bode Plot I believe.

Note: On measuring equivalent magnetizing and leakage inductance of the transformer, one must be careful not to saturate the transformer core. It's a good idea to monitor the excitation current, as just a few milliamps can cause issues with these small cores. Also, the pair of equivalent inductances need to be measured close to the intended equivalent model use frequency for the estimation of transformer performance, this means at low frequencies for the magnetizing inductance and higher frequencies for the leakage inductance. Using an LCR meter with variable amplitude and selectable test frequency is helpful here. For a detailed model one should also include the effects of distributed winding capacitance, core loss, wiring loss and so on.

Anyway, hope this helps to understand how these cores and transformers are used and behave as Injection devices for Bode Plot usage.

Best,
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