Oscilloscope showing 49Hz ac as 99hz ? Can this be put to any use

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Joined Aug 15, 2011
I have made a small modified transformer, now when I pass only negative pulse of sine wave ac (49.67hz) to the primary of the transformer. When i check the op in the oscilloscope it shows op as 99.6 hz
On further checking the waveform I noticed that after the negative pulse is finished, during the positive period the waveform dips below the zero level to negative and then rises up again. I am guessing this is why the osc takes it as double the frequency (cause in 1 sec it switches up n down 100 times)
(attached image shows the waveform, with the anomaly encircled )

Can anyone confirm this and explain it
Can i put this design to some use.


Last edited:


Joined Oct 26, 2011
This is what I call a 'dirty' waveform--give the scope a break as is simply counting the additional transitions--if you mess with the trigger level, you may get it to read the frequency correctly, but ideally, it should go through a low pass filter so the scope can easily measure the fundamental--your horizontal trigger may have a low pass filter selection that may help. A resistive load on the transformer secondary may help substantially.


Joined Dec 26, 2010
This is useful, but I would say chiefly as an illustration of the limitations of the automated frequency measurement built into the oscilloscope. Depending on the complexity of the algorithm used, this is likely to give unreliable results with more complex waveforms.

As to the generation of harmonics (multiples of a fundamental frequency) by diodes or other non-linear devices, this is a well-known effect. For instance, the un-smoothed output from a full-wave rectifier fed with 50Hz input would have a considerable 100Hz content, and virtually none at 50Hz.

A transformer or coil fed via a half-wave rectifier could produce multiple transitions during the periods when the diode is turned off, due to resonance with its own self-capacitance or other capacitances. Note that an iron or ferrite cored transformer designed for AC input only may saturate if fed a signal with substantial DC content. Some complexity in the waveform you are seeing may reflect this