phase marker problem

[nd4448]

Hi
I have a strange behavior for the phase marker. I connected to a lock in amplifier and it detects frequency correctly. However, recently I measured some capacitors and noticed strange shape in Nyquist plot. So I suspected a phase problem. I checked with a scope and indeed it does not appear where it should... I put 0°,.. it appears somewere between 80 to 160 ° depending on the frequency. Has anyone encountered this problem ?
What troubles me is that is frequency dependent... As such this device is useless for impedance measurements.
Is my 6221 broken ? I have firmware A05.. I tried to upgrade the firmware but it is stated that one cannot uprage from A05 to D03.... unless we contact Keithley (nice !).
Here are my requests,
thanks for any help
Regards

[brian.d.smith]

How are you measuring phase? A hand drawing of the components and wiring, cables and adapters and fixture would help.

What frequencies are you applying? The value is the capacitor(s)?

If you want to measure phase accurately with a scope, you would need a resistor in series with the capacitor. Measure zero phase across the resistor and the unknown phase as voltage across the capacitor. Are you doing this? If so, what is the value of the resistor? What is the ESR of the capacitor?

The capacitor is not ideal, so you would have to calculate theoretical phase angle across the actual capacitor using the phase measurement resistor and the ESR of the capacitor resistor and the actual capacitance.

[nd4448]

Hi
I am sourcing a sine wave of amplitude 10 to 100 mA peak into a 10 Ohm "pure" resistance. I connected a Tektronix scope to the resistor on channel 1 and the phase marker output on channel 2. For 2 Hz the phase marker is around 16° , the voltage signal is good, the amplitude of the voltage signal is appropriate for the resistance. BUT, one can easily see on the scope that the phase marker is NOT where it should be.
I measured the phase marker position also with a Signal Recovery 7230 Lock in amplifier,.. same problem...16.32° phase shift
The phase marker angle for Keithley 6221 is set at 0.000 degrees.
Changing the frequency to other values changes the phase marker position...
This is not about measuring a capacitor, I want to make measurements of capacitors but I need of course phase angle !
any thoughts ?
thanks

[Dale C]

I ran a test with a 6221 with 10mA at 2Hz into a 10 ohm resistor.
The scope was monitoring the phase marker pulse and the sine wave.
I ran it at 0 degrees and 180 degrees.
The phase marker pulse looks on target to me.
The screen shots are attached.
Notice the orange triangle at the top of each screen shot. That is where the phase marker pulse appeared.
it is small because the sweep for a 2Hz signal is slow compared to the phase marker pulse is only 1usec.
It is there , just have to look for it.

[nd4448]

Hi
I have no problem in "seeing" the phase marker... i will make some - and attach here...


EDIT : checked again with the scope and lock in.... everything is fine,.. I don't know what happened...Mea culpa, next time I'll make -.
thanks
nd