Testing the TK Tar Baby

[TinselKoala]

"Is your inverter not able to provide enough voltage for use with the 50R?"

When it's running off a 12-volt 5 amp-hour sealed lead-acid battery that has been used for many days of testing and that indicates 11.7 volts open circuit--- no, it's not.

[poynt99]

TK,

The "vertical offset" setting in the Tek scopes allows one to compensate for a large DC offset without having to resort to AC coupling. This allows you to have the trace on screen without having to adjust the trace vertical position, nor use AC coupling.

When you enter a value for the "offset", the zero-level (channel reference) bar on the left of the trace is now AT that offset level.

Useful for examining ripple on a relatively high voltage DC supply for instance.

[TinselKoala]

TK,

The "vertical offset" setting in the Tek scopes allows one to compensate for a large DC offset without having to resort to AC coupling. This allows you to have the trace on screen without having to adjust the trace vertical position, nor use AC coupling.

When you enter a value for the "offset", the zero-level (channel reference) bar on the left of the trace is now AT that offset level.

Useful for examining ripple on a relatively high voltage DC supply for instance.

OK, that makes sense to me and seems like a useful feature.

What effect does it have on the scope's math functions, and what effect does my having the CVR trace AC coupled have on the math?
(I  know that the math result is bogus (since it indicates the same thing that Ainslie's does) but I'd still like to understand just how all this affects the scope's math
calculations.)

And how does it jive with Ainslie's description of the LeCroy offset values given in her traces, since they seem to correspond strictly to "position" and not to the kind of vertical offset that you are describing?

What do you suppose the Green Trace is in the shot below? I think it's the common mosfet drains, shown AC coupled to get it on the screen where it is.

 

[poynt99]

OK, that makes sense to me and seems like a useful feature.

What effect does it have on the scope's math functions, and what effect does my having the CVR trace AC coupled have on the math?
(I  know that the math result is bogus (since it indicates the same thing that Ainslie's does) but I'd still like to understand just how all this affects the scope's math
calculations.)

Using the "offset" feature should have no effect on the MATH computations. It's simply a display feature.

However, if there is an observable change in the "position" of the trace when changing from AC to DC coupling, then there WILL (or at least should) be an effect on the MATH computation when switching between the two.

And how does it jive with Ainslie's description of the LeCroy offset values given in her traces, since they seem to correspond strictly to "position" and not to the kind of vertical offset that you are describing?

It is essentially the same thing. The LeCroy offset setting should also not have an effect on the MATH computations.

What do you suppose the Green Trace is in the shot below? I think it's the common mosfet drains, shown AC coupled to get it on the screen where it is.

Yes, of course. They clearly state what each trace is in the video. Yes, it was most likely set to AC coupling to make it easier to display. The presumption is that they multiplied the pink ("battery") and yellow (shunt) voltages, which are both on DC coupling, so there should be no problem with the MATH computation in that regard.

[picowatt]

TK,

From my read of the Tek manual I concur with .99's answers regarding the 'scope.

The Tek injects a DC offset at the preamp input to allow DC to be "nulled" prior to amplification.  This allows small AC details to be viewed on a signal with a large DC component without having to use AC coupling.  "Position" is separate and only a display function on the Tek.

Its a bummer the 'scope isn't a house guest!  So, you had to do all that driving just to get a few screen shots?  I can understand your apparent frustration over having to do so.

When I stated your replication was not exact, I did not mean it was materially different.  To do an exact replication, one would have to incorporate identical strays, i.e., layout, wire lengths, clip leads, wood or metal table for the circuit (grounded or not), equip locations, batteries on wood or metal table (grounded or not), equipment grounding, known bias current, etc.  But as far as I am concerned, even with the small differences in oscillation characteristics due to different strays, the two circuits perform similarly and provide similar measurements.

I suspect that any oscillator, connected to any battery (or pwr supply) with supply lead inductances that allow large AC excursions, measured similarly, would produce similar neg mean pwr results.  Even a simple 555 with a 9V battery, under similar conditions with loads scaled accordingly and measured similarly, would very likely produce a neg mean pwr measurement if the true battery voltage were not used in the calculations.

At this time, based on .99's analysis and the work you have done, I believe the negative mean power measurement to be fully explained and reproducible, and that it is not an indication that the circuit is operating without drawing power from the batteries, or recharging them.

PW