What's wrong with my CRO ?

I'm wondering if you're seeing something like an aliasing effect due to the different sampling rates for digital storage on the two timebase speeds??

The first picture of the sine wave has a small amplitude effect while the 500mS picture has almost a 100% modulated effect... So I would guess its to do with the timebase speed and sampling rates...

I'm not familiar with this scope so that's all I can say... Normally this effect is caused by the 50Hz signal 'beating' with the mains 50Hz as they're not quite in 'synch' but not to the extent you're seeing...

John.

How are you inputting the 50Hz signal? Have you got you're 'scope input hooked up straight into the mains???

(Remind me not to try that)

Nowt wrong with that Wrenched, a x10 scope probe is good up to 400Volts...

Anyway it might be on a transformer output or something.

I hope your scope is earthed OP..?

John.

Look at your sampling rates. In your first picture you have 500 S/s (Samples per second), or 10 samples per cycle. Only 10 samples gives a rough portrayal of the true waveshape. In your second sample you have only 100 S/s, or only 2 samples per cycle - a terrible approximation of the true waveshape. Since the 50Hz of your power is not synchronized with the sampling of the 'scope, each subsequent sample is taken at a slightly different phase of the original wave.

There is nothing wrong with the CRO, you just need a MUCH higher sampling rate.

Dick

You`re completely right, Dick. 2 samples per period is the absolute minimum sample rate according to Shannons theorem. lets assume the frequency of the scope is crystal-based and the 50Hz signal has a little deviation then you see just these kind of modulation because 50.1Hz already hurts this theorem (= less than 2sps).

The minimum sample rate zo get a usable in minimal 10 times higher than the observes frequency - better 50 or 100 when you are interested in exact details.

So the scope is ok - it`s only a handling error.
Regards Uwe

Shannon may have had a theory but my ears tell me another story when I listen to a CD. Unsatisfying high frequency reproduction - or is that something to do with another part of the processing (anti-aliasing?)

Wrenched, your observation is right. Shannons theorem does not tell you anything about the quality of information. With 2 samples per period the only minimal info you get is a plus or minus sign per half wave - no correct magnitude(because you have only 1 magnitude value per half wave, thats like a rectangular waveform). The more samples you have the better the quality of info will be - besides the polarity you get the magnitude info in a number of steps.

A sampling frequency of, lets say 96kHz, makes an oversampling of 4.8 related to the upper audio frequency of 20kHz (which is bad), but 4800 related to the lower audio frequency of 20Hz (which is much better).
Regards Uwe

its a trigger proplem its not triggering properly if its ok then check its internal power supply may be filter problem service it from auth person