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DIY Transformer for use with Bode Plots.
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mawyatt:
Yes our smaller core has 25 turns, and noted in the video the long ~7M wire, ours was only ~2 meters. Did note your larger inductance and assumed you had a much higher permeability core, so a metal tape core seems reasonable. Have no idea about the core we used since it was part of the CM Filter. One could do tests and figure out some core parameters, but no time budget for that at the moment.

Best,
Jay_Diddy_B:
Hi,

I am late to the party. I missed the start of this thread. Do you have any questions for me?

Jay_Diddy_B
mawyatt:
We used the PicoScope Frequency Response Analysis tool with a PicoScope 4262 (16 Bit ADC) to see if we could improve the Bode Plot results with the Closed Loop measurements with a OP-07 in a non-inverting 11X Gain configuration. Using the Noise Reject Mode with a Sample Rate of 500KHz and Noise Rejection Bandwidth of 1Hz (64X oversampling) we attempted to improve the Close Loop Bode Plot results hoping the PicoScope 16bit ADC and the FRA DSP Noise Reduction effects would pull out Ch1 signal and smooth out the randomness below 100Hz. As you can see that was not the case.

Conclusion is that the Isolation transformer needs to have a much higher Primary inductance to keep the low frequency signal level reasonable for the scope to capture accurately since the high gain feedback loop is effectively nulling the scope Ch1 Vin signal, so we'll be getting a larger core and winding another transformer with larger primary inductance for these low frequency Closed Loop Bode measurements with high gain Op-amps like the OP-07.

Others with larger primary inductance Isolation transformers might want to give this Close Loop Bode Plot a try, very interesting and useful technique.

Best,
Jay_Diddy_B:
Hi,

Let me use LTspice to illustrate some of the features of the measuring the open loop gain of an op-amp. I have used the universal op-amp in LTspice so that I have a known answer. I have configured the op-amp to have an open loop gain of 1E6 and and a GBW product of 500K.

AC domain Model




In SPICE I can have a floating source so I do not need to include the transformer. The open loop gain is obtained by plotting V(a)/V(b).
The results show that I have an amplifier with a single pole slope, a gain of 1E6 and a GBW of 500kHz. This is the correct answer for the universal op-amp used in the model.
The gain is 94dB at 10Hz.

Time Domain Model



Switching to the time domain, transient analysis, and using a 10Hz 2V p-p signal the signals at nodes A and B can be examined.
The voltage between nodes A and B is 2V p-p, as defined by the source. The voltage from A to ground is 2V p-p. The voltage on node B with respect to ground is only 40uV p-p. This is the inject signal divided by the loop gain at the injection frequency.

The signal on Node B is very small. It is -94dB, 50000 times smaller than the injection signal.

Coupling Transformer LF bandwidth



The coupling transformer LF -3dB point is given by:

F = (Rsource // Rload) / 2 x Pi x Lmag

I have chosen Lmag = 68mH

This gives a LF (-3dB) point at 58.5 Hz


Adding the Transformer to the open loop measurement




Adding the transformer with a -3dB point of 60Hz does not impact the open loop response at all as shown in this model. It does reduce the signal amplitude at 10Hz by 16dB.

It is the signal to noise ratio and the noise floor of the FRA that impacts the measurement, not the bandwidth of the isolation transformer directly.

Regards,
Jay_Diddy_B
Jay_Diddy_B:
Hi,

Here is a very simple test to circuit to determine if your FRA is capable of making the measurement:




It is a 100dB attenuator.

Jay_Diddy_B
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