Hi,
most of you know former regular on this NG, David L Jones, who reinvented h imself as a video star with his EEVblog site. I stumbled on one of his many hundreds of blogs recently - please take a look.
The blog attempts to explode a "myth" about analogue and digital scopes in relation to display noise - particularly that DSOs are inherently noisier.
While I am not sure just what Dave is trying to prove, some of his assertio ns are very dubious.
The facts are that an analogue scope normally has a sharp trace revealing a great more detail of waveform being viewed. This is also a direct result o f having:
- Constant bandwidth ( 20,50,100MHz ) plus the same rise time at all sweep speeds.
- Continuous vertical screen resolution of around 1 part in a thousand.
- Absence of any artefacts due to sampling or quantising.
All this results in an accurate display of any continuous signal, including random noise. The clean trace shown on an analogue scope with no input is because there is no noise to be seen.
FYI: This might seem at odds with an input impedance of 1Mohm and bandwidth of 50MHz which has a calculated thermal noise of almost 1mV rms - with reg ular peaks over 4mV. The simple answer is that 1Mohm is in parallel with 20 pF, so the impedance falls steadily from 8kHz onwards down to 160 ohms at 5
0MHz. Thermal noise is reduced by this to about 15uV rms.The random noise spiking seen in the traces of the DSOs in the blog is main ly due to quantising errors plus the discontinuous, vertical screen resolut ion of only 200 pixels in case of two of them. When the sampled value falls between pixels, random toggling occurs.
Lastly, when Dave turns up the brightness on his Tek 2225, you see the trac e thicken ( due to mutual repulsion between electrons in the beam ) and als o some background pattern appears.
The background pattern is in the CRT's anti glare screen, which lights up w hen you do that revealing any imperfections in the plastic. He should have detached it.
Also when he uses the digital camera, the image becomes over exposed which again results in trace thickening. The story about "revealing missing noise " is a furphy.
BTW:
In the first few second of this blog on the Tek 2225, Dave seems to contrad ict himself.
.... Phil