Re: The 7CT1N sweep frequency...?


Don Black <donald_black@...>
 

Or perhaps Tektronix design was good enough that beating with the mains wasn't a problem. Maybe they just played Bingo and pulled out number 55 and thought "that'll do".

Don Black.

On 21-Aug-13 11:25 PM, cheater00 . wrote:
 

Hi Ed,

On 21 Aug 2013 10:10, "Ed Breya" <edbreya@...> wrote:
>
> Strictly speaking, the CT sweeping frequency could be arbitrary, as long as it's fast enough to avoid flicker, but slow enough to not have too much parasitic effects. Clocking frequencies in equipment are often selected so that they and their harmonics tend to land between those of the basic 50 and 60 Hz line frequencies, so that there is minimal interaction, and best line rejection.
>
> One possible reason for this in the CT may be to minimize the interaction of the sweeping load with the line frequency, within the scope mainframe power supply. Maybe the maximum power level needed for the CT is a bit too much, compared to "normal" plug-ins, so it would tend to cause stability problems and beat frequency artifacts in the display if it ran at or near line frequency.

Cool idea. I ask myself though if that is the case. Iirc a new branch in the graph is selected every osc cycle. If there were beating creating distortion, the distortion would modulate 11 branches at a time. What one wants is for the beating to be so fast that it is not visible as such. If the beating were affecting deflection then you want it to make a thicker line, rather than jitter the traces erratically. So the beating freq would need to be much higher but that is impractical. On the other hand you might want the beating to be really slow, so that it is just a slow drift so that you can still measure against the graticule.

D.


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