I purchased a MIcsig DP2003 High Voltage Differential Probe (Approx. $240). 

 It has 2 ranges 560V(200x) and 5600V(200x) as I want to be able to check cells with higher. 

The have another model the Micsig DP1003 with 50x/500x up to 1300V would have been a better match to my O-scope as it has a 50x range. The one I purchased works great, but the voltage displayed is low by a factor of 2.  When I was looking for probe,

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

​

I saw reviews that said O-scope would just to range of probes or you could download new ranges, however, that did not work for my older scope.  I will try to make note of this when I post scope pictures, so people know value for it on screen is incorrect.

As the differential probed does not use the system ground it give a true reading on the voltage across the interface to the cell and very clean signal as all the common mode noise is removed.  In screen shots I set the probes to show the pulses up to match the signal going into primary coil which I am using as frequency reference, you can do this by either moving the probes or using invert function of scope. This is for display purpose only as the only signal the cell sees it the one on cell interface.

The pictures below show my next series of test. I repeated the above testing but added a few more tests points at I wanted to capture points where the signal made a change.   Input analog signal set to minimum gain and offset.

Analog frequency and gate is approximately 41hz and gate has 50% duty cycle. Only the frequency was changed pictures show the digital input to primary in yellow on CH1 which I used to set frequency and provide a reference signal for the CH2 blue which is from the differential probe on the cell interface

(In this case a capacitor and resistors to provide load, no cells so sync reading will be off from complete system).

NOTE: The differential probe reading on scope is low by a factor of times 2 as probe scale does match scopes.

Goal is to see how the differential signal changes as frequency changes.  In most case the table show the signal zoomed in on one digital pulse in the left table cell and the right show same signal zoomed out so you can see either complete gate pulse or multiple pulses with gates shown. 

Couple of exceptions P1 and P24 are in the first row of one table and last few rows of second table show a few special items.  Dan P24 is close to the signal in the YouTube videos you recommended I look at to see what resonance signal looks like.

The shape of the signal on the cell changes with frequency as expected.  It ramps up as the capacitor charges, but shape of ramp varies depending on the phasing of the digital and analog inputs. 

The zoomed signal shows that the number of pulses in each digital pulse varies with frequency in pictures below they varies from a high of 6 at 700hz to 1 at 11khz. The position of the pulses also changes which I believe also effects the shape of each Gate Pulse.  You can see this it the second picture in each row.

In some case you can see the charge of the capacitor ramp up then level off for the rest of the gate pulse.  At select points the ramp is a straight slope P23 but getting that is very touchy and usually happen just before signal is in resonance.  Usually shortly before that happens there is sag in the startup ramp see P23.

When the signal is resonance the differential signal is a straight line P20 and P21. 

Also, at 5khz P14 and P15 while not complete zero the differential signal is flat with no ramp up charge.

P18 and P19 show what happens when I slightly increased frequency above 5khz where the output signal was flat.  Look closely at these 2 pictures in P18 you can see the input frequency jumping around and in P19 where I change it slightly input frequency levels off, but you can see a ramp in top of blue signal.

Ronnie has stated you do not want to be in exact resonance but slight off, I wonder it the point where you have the straight slope could be the point he is talking about (P13 and P22). 

I also wonder is you are at point where the signal ramps up then levels off would like they charge you can get on the cells, see P23 it show the leveling off that I am talking about.  This happens at several other points as frequency increases.

While I do not show it here, I did do a quick check to we what would happen if changed offset.

  Did this test at 5khz. To see this, I turned DC coupling on both scope channels, and moved CH1 to analog input to primary and put scope sync on CH2 as it had a nice clean pulse. 

By doing this I was able to see both the analog input and differential output change.  The levels on both stayed in sync and moved the same amount on the scope.  I plan on doing more testing on to very nothing else changes, but this is what I expected to happen.
 
While this is what I expect the signals should look like the values I am getting are for my test setup and I expect they will be different for an operational system.  At this point I am still trying to understand what the controls do. 

​

NOTE

Nice to see some cleaner signals etc, but we should all note  we  must stay under 10kz in my opinion
Stan did mention this i will try to find the reference. Note that capacitance and smoothing can be gain in the cell design rather than circuit

so be careful not to over engineer the circuit when no cell attached