Can I Correlate This Spice Total Noise to My Actual Oscilloscope Observation?

[PeteC]

I used TINA-TI to model a circuit and it predicted a Total Noise that is negligible below 300 KHz and flattens out at 300 mV above 60 MHz.

I built the circuit and want to correlate what I see on my scope with what the model predicted.

My analog scope has a 250 MHz bandwidth so I believe it should have negligible effect on the readings.

With the input grounded (through a large capacitor) I observe a "white noise" pattern on the scope. The peak to peak amplitude is about 1.5 V.

Is that a reasonable match to the prediction or do I have a circuit problem?

Thanks In Advance,

Pete

 

[keith1200rs]

That sounds about right. I would have to look up the maths (I cannot do so at the moment) unless someone else can do so, but as a rule of thumb a peak to peak noise will roughly correspond to an RMS value of one fifth. You do need to consider the bandwidth you are observing over in each case though, to make sure you are comparing like with like. The value of a fifth (or peak to peak is 5 times the RMS) comes from the probability of a noise peak of a particular Gaussian RMS noise source exceeding a certain value. From memory, 99% of the time it will be below 2.5 times the RMS value (so 5 times the peak to peak).

I hope I have remembered it correctly. A quick simulation with a transient noise source (if Tina supports it) should confirm it.

Keith