Oscilloscope measurment error?

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*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...
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That is listed in the Oscilloscope specs, and then you have to eyeball it, so that can be 3% too.
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...

Many oscilloscopes have specifications for their error. But often these depend on your having had your scope calibrated recently, having had your scope turned on and warmed up for a sufficient period of time, etc.
See if you have had your scope calibrated recently and if there are accuracy specifications available for it.
Sometimes you can get a less accurate estimate of this by using a known good amplitude calibration signal of about the same waveform and amplitude, measure that and determine the error, and assume the same error will be present in your actual measurements.
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Thanks to all for replies...

Its calibrated and i have the specifications. But there is small complication about the measurment. Ill describe shortly measurment process: Im increasing manualy E (AC voltage source: E*sin(2Pi*f*t)) and im looking current wave form at oscilloscope display and waiting to see that the frequency of current equals 2*f (double source frequency) and then im reading the value of E (at the display of some other instrument). Now should i write result something like this:
EV+-0.1V.
I know that all instruments have influence on measurment error, but i want to know what is the influence of Oscilloscope because im not using any "numeric" values from oscilloscope, im only "looking" at display and waiting to recognize "frequency of current" = 2*"source frequency"...
Should i simply use error-data from oscilloscope-manual or is it more complicated because of subjective nature of measurment: "...im only "looking" at display and waiting to recognize"... hmmm.. im waiting to recognize... what is my "error"....? maybe is this the main question...
I hope its more clear now...
Thanks...
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On Wed, 25 Aug 2004 00:14:35 +0200, Kruno Milicevic wrote:

What is the bacndwith of the scope and at which frequency are you measering? Do not forget that the bandwith of a scope is specified at the -3db point. So the amplitude error could be 30% ....
-bent
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