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RE: Primary Scoping


>
>>I have thought about using a 1000:1 divider ratio using 4 of the dale
>>100Megohm resistors in series to a  5 parallel combination of 2megohm 
>carbon
>>comp resistors. 
>
>You've just built a lowpass filter with a cutoff frequency of 2 Hz!  So you 
>won't see too much of the signal you are trying to measure (100's KHz).
>
>Fc = 1/(2*pi*R*C) = 1/(2*3.14*400e6*200e-12) = 2 Hz.
>
>With such large resistors, any capacitance is going to severely reduce the 
>frequency response.  This includes the cables and scope input capacitance.
>

Not exactly correct.  The cutoff is actually about 2KHz not 2Hz because the 
impedance seen by the 200pf is the 5 parallel 2M resistors.  However, this is 
still too low.  400M is not necessary for the series resistance.  15-50M 
should do fine (it will dissipate about 15W).  That way, the shunt resistor 
can be 1.5-5K for 10,000:1 ratio.  The shunt R should be kept low for several 
reasons.  First, it keeps interference out, and second, it preserves 
frequency response even with high values of series resistance.  Please don't 
anyone argue this last point, I'm right.  For the capacitance given, the 
frequency cutoff in this case would be 160KHz (for 5K).

Zap,
Mark