Ok, it became deafening silent overhere on this important subject, but fortunately our Russian friends still are active and via a PM i got some info on how to continue.
Not sure if he appreciate to be named, so i will call him Mr S and only forward info and links.
I got the following link to a PDF describing in Russian how to measure the resonance of ferrite, that link is here:http://realstrannik.ru/media/kunena/attachments/6159/CE_ferrit.pdf
I have let google translate this and put it into a Word document which is attached and i will refer to this Word document in the video
This Word document info together with the following info should lead to a method on how to determine the resonance frequency of a ferrite core.
Received extra info via PM:For exaple you want to find ferro-resonace of core from your HV flyback transformer.
1. Remove the core from Flyback
2. It has quadratic form. Now wind 2 test coils X and Y coil with smae number of turns and same wire diammeter. For example 10 turns in X axis than continue winding 10 turns in a Y-axis. As you will see it is in fact one coil electricaly but not magneticaly. 1/2 coil is positioning in X-axis and 1/2 in Y-axis.
3. Use generator from 50 Hz- up to 1..2 MHz for square wave impulses. You can try your generator on TL494.
4. Main goal here is time for opening MOSFET must be shortest as possible. Also time for closing them must be shortest as possible. You need 10ns to open output transistor and not higher 50ns for closing him. Both parameters for ezxample on 200kHz or upper (it's better).
5. Put 75 oms (non-inductive) resistor at output of MOSFET and measure turn-off time on your 3054C. For example MOSFET has gave 50ns turn-off time which is what you need.
6. Remove resistor and conect coil to MOSFET between VPlus supply rail and Drain of your MOSFET.
7. Wind 1/2 winding on your core for connecting your 3054C. Winding should be placed farther away from your excitation coil. So put it on the opposite corner of your quadratic core. Opposite side is maximum longer from your excitation coil.
8. One channel probe on Gate of MOSFET - (if you have signal ringing from TL494 generator put a several small ferrite beads in your gate terminal)/
9. Second channel signal from 1/2 winding.
10. Sweep frequency and stretch the time base to get nice big oscillogram. For example you get 400ns from excitation pulse-width 10ns....200ns. Now you have got impulse resonse of your ferrite core. As you carefully look at oscillogram you will see how ferrite is establishing self-oscillations on exactly one frequency but strictly at the moment after turn-off your MOSFET. Read pulse width response and check it on spectral analyser. You should get nice big peak eactly on Resonant frequency of your ferrite core. As shortest excitation impulse is better. If you have prfessional signal generator try excitation from 1nS up to 200 ns. Just turn on peak detector and sweep generator. Very simple if you have equipment.
I did follow the instructions in the Word document together with the above info, but the result is not as mentioned in the Word document.
The pickup signal is a nice copy of the input frequency, so a nice square wave.
See the video here: https://www.youtube.com/watch?v=muC-UlyDBcw&feature=youtu.be
Not sure what went wrong as i also sweeped a large frequency range (so not only at 173KHz) and varied the duty cycle between 10 and 90%.
Any comments are appreciated, regards Itsu