Re: 453 calibrator
Fabio Trevisan
Hello Lop Pol, and all the gurus,
toggle quoted messageShow quoted text
Here's one that got me thinking... Looking at the output stage, fed by 12V through a 12R Resistor and a saturated switching transistor... Isn't it too marginal to expect 1V across 200 Ohms, through a 2k19 resistor? Does it really ever reach 1V? Just making the math: 200R + 2k19 = 2k39 x 5mA = 11.95V! It leaves 50mV for the voltage drop on the 12R resistor AND the Vce(SAT) of the transistor! Only the average current of the output stage (5mA @ 50% duty cycle = 2.5mA,) through the 12R resistor already eatsup 30mV! and the same resistor still have to feed the oscillator... What's left for the transistor voltage drop? The smaller voltage drop I've seen, for any saturation current curve, for this type of transistor is around 70~80mV... and that's the end of the line of the lowest current graphic! And Tek claims the calibrator specs to be + 1% (over a normal temperature range)? I know Tek wouldn't usually screw up on such a basic thing but... Didn't they stretch it a little bit too far here? And yet, 2k19 is not a standard value even for E192 series... and Tek lists this resistor as a 0.25% precision resistor !? I can't understand they went this far in choosing a hi precision non standard resistor, for a design that can't make it onspec even on paper, let alone real world. Am I missing something here? Rgrds, Fabio
On Tue, May 8, 2018 at 09:26 am, lop pol wrote:

