I measure some behaviours of ferroresonant circuit and the main goal of
measurment is to
determine for which amplitude of AC voltage source i get current with
doubled frequency compared
with frequency of source.

Well, i must determine for which E of voltage source E

I only can detect this behaviour with looking on oscilloscop to see waveform of current and with increasing of E to determine the moment (and, of course, value of E) in which frequency of current becomes 2*f...

I made this measurments but my problem is to calculate measurment error (unrealibility?).. If i measure something with voltmeter i have in its specifications data about its inrealibility/error and then i can write measurment result something like this:

measured voltage= 5V +- 10%, or measured voltage= 5V +- 0.5V...

But how to determine this value (+-10% or +-0.5V) for oscilloscope at which im looking and waiting to see doubled frequency of current...?

Thanks...

P.S. It would be easier if i had "Frequencymeter" but i dont have it and because of that i must determine wanted value E only with Oscilloscop-display and with my eyes waiting for 2f frequency of current...

Well, i must determine for which E of voltage source E

***sin(2***pi***f***t) i get current with frequency 2*f...I only can detect this behaviour with looking on oscilloscop to see waveform of current and with increasing of E to determine the moment (and, of course, value of E) in which frequency of current becomes 2*f...

I made this measurments but my problem is to calculate measurment error (unrealibility?).. If i measure something with voltmeter i have in its specifications data about its inrealibility/error and then i can write measurment result something like this:

measured voltage= 5V +- 10%, or measured voltage= 5V +- 0.5V...

But how to determine this value (+-10% or +-0.5V) for oscilloscope at which im looking and waiting to see doubled frequency of current...?

Thanks...

P.S. It would be easier if i had "Frequencymeter" but i dont have it and because of that i must determine wanted value E only with Oscilloscop-display and with my eyes waiting for 2f frequency of current...