STEORN DEMO LIVE & STREAM in Dublin, December 15th, 10 AM

[X00013]

@omni, try a watt meter
              http://www.youtube.com/watch?v=_eLa2BiiOa0&playnext=1&videos=NhIkX24TzkM

[Omnibus]

@omni, try a watt meter
              http://www.youtube.com/watch?v=_eLa2BiiOa0&playnext=1&videos=NhIkX24TzkM

Thanks, buddy. In this case, however, a very detailed measurement is needed whereby multiplication of the momentary I and V values is to be carried out at time intervals as short as possible. The wattmeter only gives average values based on rms.

The song is kewl. The vid reminds me of when I have to go to Mass through CT.

[gyulasun]

The C value in my RC circuit was 115pF. The 1:1 probe, like I said, was 110pF. So, you consider these two capacitances to be connected in parallel, correct?

Yes.

And, so what I measure isn't the phase shift caused by the 115pF of the capacitance in the measured circuit but is the phase shift caused by 110 + 115 = 225pF capacitance, right? Now, if I use an active probe I'll measure 115 + 5 = 120pF?

Yes.

Isn't there a way to eliminate the capacitance of the voltage probe altogether?

In your particular case the cap values are known, this involves an embedding possibility whereby you could use a 5pF cap instead of the 115pF and the 1:1 probe’s 110pF will add to the C value in the RC circuit. This way the phase shift effectively will be caused by 115pF what you wish to use.

Also, I see these low capacitance probes have even lower impedance than 1MOhm and that will cause even higher current flowing through the circuit of the probe. Somehow, a sticky wicked situation. Wish there could be probes working like electrometers, with practically infinite input impedance, having no capacitance. Do you think such probes exist? 

Yes, there are active probes with 1 MegaOhm input impedance @ 2pF self capacitance only, like Tektronix P6205 (list price is about EU1000)
Unfortunately they are designed for certain scope families to plug in like TDS1000 series but then these are also 8-bit scopes…

Another question -- suppose one finds the right probe, do you think an 8-bit scope (having somewhere around +-100mV accuracy) has enough accuracy for these studies or I should look for a higher end?

I am afraid you should.  For instance the TDS3000 series are with 9-bit resolution, though this may still not give you the needed precision. I have not searched further Tektronix digital scope families for their possible 10-bit feature, you may wish to inquire, keeping an eye for an active probe connection possibility too.

Gyula

[Omnibus]

@gyulasun,

In your particular case the cap values are known, this involves an embedding possibility whereby you could use a 5pF cap instead of the 115pF and the 1:1 probe’s 110pF will add to the C value in the RC circuit. This way the phase shift effectively will be caused by 115pF what you wish to use.

The RC circuit is with C = 115pF and R = 10Ohm. This is what I have in the circuit under study (say, these RadioShack components I used in some of the measurements). Suppose I do the measurements with the 1:1 voltage probe having 110pF input capacitance and I'm measuring the voltage before the current probe. In this way the current through the probe is not included. In such a case I should have I and V corresponding to the RadioShack components (115pF and 10Ohm) but the I-V phase shift will correspond to the 110 + 115 = 225pF, if I understand you correctly. But then wouldn't it be better to measure current after the voltage probe? In this way I will have all the current (through the circuit and through the probe), the correct voltage across the studied circuit but the I-V phase shift will be correctly attributed to the true current corresponding to the 225pF? In such a case, if I now know for sure that what I measure as I and as V does correspond to the known capacitance 225pF then my only issue would be the accuracy of the scope. Does that make sense?

Of course, I may use the 1:10 probe which has lower input capacitance (don't remember how much it was but probably around 50pF would be somewhat close) and then the known capacitance will be lower -- 115 + 50 = 165pF. I would prefer the 1:1 probe though because the 1:10 probe adds to the inaccuracy of the scope.

Now, if the above is correct I should really focus on upgrading the scope. What do you think about the 14-bit scope cards, instead of using Tektronix scopes (unfortunately, however, my current probe only works with Tektronix and that's another issue I have to take into account).

[gyulasun]

Somewhere you surely uploaded a schematic on your circuit with the probes included, or an arrangement of your setup.

Would you refer to it with a link please? It would be easier to speak of it.