Bucking Coils

[Itsu]

A plot of input power against frequency might show a dip at the right point. Perhaps include a small load resistor in series with the coils so that you are measuring a serious quantity of power and not just circuit losses.

I put a small 51 Ohm SMD resistor in between the 2 bucking coils.

I feed in a 5Vpp sine wave sweeping from 1Hz to 10MHz during 1s, and monitoring this with the yellow probe, and this is the result:



So we have a 10MHz sweep from left to right, so each division is 1MHz
We see a dip around 5.7Mhz (and some further up, not visible).

Is that what you had in mind?

Itsu

[Smudge]

I put a small 51 Ohm SMD resistor in between the 2 bucking coils.

I feed in a 5Vpp sine wave sweeping from 1Hz to 10MHz during 1s, and monitoring this with the yellow probe, and this is the result:



So we have a 10MHz sweep from left to right, so each division is 1MHz
We see a dip around 5.7Mhz (and some further up, not visible).

Is that what you had in mind?

Itsu

Not quite, that is just input voltage.  (Interesting that the dip occurs at a frequency near where the delay time is one full cycle.)  Measuring current and voltage then doing the math to get input power at each frequency is what I suggested.  That voltage dip may not be a power dip.  I realize the power measurement can't be done using the frequency sweeping, it requires selecting whole cycles for the math.  I was expecting a dip in input power at a frequency where the delay line gives a 90 degree phase delay, like 1.25MHz for a 200nS delay.  So I would concentrate on looking around that frequency to find the dip (if it is real and not a figment of my imagination).

Edit.  Forget what I said!! Of course minimum voltage across a resistor is also minimum current.  I had a senior moment there.  Is that dip at 5.7MHz there without the 50 Ohms, i.e. the coils feeding an open circuit (just scope probe).  Then try monitoring current into a short circuit looking for a dip.

Smudge

[Itsu]

Ok, you asked for input power, sorry about that.

I am not sure why you say "power measurement can't be done using the frequency sweeping", does that mean that the following math function is not valid (voltage sweep x current sweep = power sweep?).

Anyway, put it up anyway, and got this result:



Sweep was from 1Hz to 20MHz as i did see another dip further on (19MHz).

Yellow: voltage across bucking coils
Green: current through bucking coils
Red: power into bucking coils (yellow x green).

I measured the shown power between the red vertical cursors (6.4MHz), and it was the biggest negative value (dip) besides the one further on.

Looking around 1.25MHz does not show any peak or dip.

Itsu

[Verpies]

At 1s/div I doubt that the voltage and current are sampled frequently enough to approximate a series of products that are representative of continuous power.

[Itsu]

OK,  but it could be an indication of it, right?

I am manually sampling a frequency range around the mentioned 1.25MHz (1Mhz to 1.5MHz) in 10kHz steps (no sweep) to see if there is something there and plot it in a graph, see here:



It is just a small part of the above shown sweep output, but the slope runs in a similar way as the same part of that sweep (2nd part of the first division).


Itsu

[Verpies]

To display the slow trend of instantaneous power vs. time, use you scope's histogram feature to display the average value of the math trace that is the product of current & voltage sampled at high rate. 
The i*v multiplication must occur at high sample rate but the averaging of the resulting products can be slow.

[Smudge]

Itsu,
I have looked at your sweep results and it seems to me you are measuring the input voltage to the series coils plus resistor.  You are not measuring the voltage across the resistor.  On that basis the results are quite revealing.

I disagree with Verpies, I think all the results are continuous values including the math.  What is not known with the voltage and currents values that all sit about the zero line is whether they create real or reactive power, and the math samples tell you this.  It is the peak values that we can easily discern and estimate on each waveform.  Staring at the 1Hz point we see by eye the voltage samples at 1.2V pk and the current samples at 28mA pk.  That ratio is 42.9 Ohms which is close to the actual 50 Ohm load, and at this frequency that is what the input sees.  The math tells us it is real power (all positive samples) reaching 35mW pk.  We would expect a 50 Ohm load to draw 28.8mW from a 1.2V input so the numbers from the waveforms are all in the right order.  Looking at higher frequencies where the inductive reactance takes over we see that the voltage reaches 2.4V pk so that seems to be the output from the unloaded 50 Ohm signal generator, hence the 1.2V figure is right for having a 50 Ohm load.

Moving along the waveform we see current reducing and voltage increasing as expected as the inductance of the coils take effect.  We also see the math moving towards having the same + and - peak values as we get into reactive power.  Minimum peak current is at about 4.5MHz where it is about 2mA pk with the voltage at about 2.4V pk, so the input is seeing about 120 Ohms impedance there.  The math tells us the power is again real at a value of 5mW peak.  At the 5.7MHz dip in voltage (now 1.4V pk) the current has increased to a maximum value of 20mA pk and the math is showing a real value of 30mW peak.  That is showing a 70 Ohm input resistance.

Beyond this 5.7MHz dip the negative power highlighted by Itsu could be a real effect so that needs investigating thoroughly.  A non-sweep test using a fixed frequency sine wave input should show more detail and allow better values to be determined.

Thanks for looking around the 1.2MHz region and not finding anything startling.  It seems something happens at higher frequencies.  That second dip at 19MHz is at a frequency 3.3333 times the first dip.  Are all those threes a coincidence?

Smudge 

[Itsu]

To display the slow trend of instantaneous power vs. time, use you scope's histogram feature to display the average value of the math trace that is the product of current & voltage sampled at high rate. 
The i*v multiplication must occur at high sample rate but the averaging of the resulting products can be slow.

I see what you mean with this "histogram feature", i have to play around with it some more.

Itsu

[Itsu]

Itsu,
I have looked at your sweep results and it seems to me you are measuring the input voltage to the series coils plus resistor.  You are not measuring the voltage across the resistor.  On that basis the results are quite revealing.

I disagree with Verpies, I think all the results are continuous values including the math.  What is not known with the voltage and currents values that all sit about the zero line is whether they create real or reactive power, and the math samples tell you this.  It is the peak values that we can easily discern and estimate on each waveform.  Staring at the 1Hz point we see by eye the voltage samples at 1.2V pk and the current samples at 28mA pk.  That ratio is 42.9 Ohms which is close to the actual 50 Ohm load, and at this frequency that is what the input sees.  The math tells us it is real power (all positive samples) reaching 35mW pk.  We would expect a 50 Ohm load to draw 28.8mW from a 1.2V input so the numbers from the waveforms are all in the right order.  Looking at higher frequencies where the inductive reactance takes over we see that the voltage reaches 2.4V pk so that seems to be the output from the unloaded 50 Ohm signal generator, hence the 1.2V figure is right for having a 50 Ohm load.

Moving along the waveform we see current reducing and voltage increasing as expected as the inductance of the coils take effect.  We also see the math moving towards having the same + and - peak values as we get into reactive power.  Minimum peak current is at about 4.5MHz where it is about 2mA pk with the voltage at about 2.4V pk, so the input is seeing about 120 Ohms impedance there.  The math tells us the power is again real at a value of 5mW peak.  At the 5.7MHz dip in voltage (now 1.4V pk) the current has increased to a maximum value of 20mA pk and the math is showing a real value of 30mW peak.  That is showing a 70 Ohm input resistance.

Beyond this 5.7MHz dip the negative power highlighted by Itsu could be a real effect so that needs investigating thoroughly.  A non-sweep test using a fixed frequency sine wave input should show more detail and allow better values to be determined.

Thanks for looking around the 1.2MHz region and not finding anything startling.  It seems something happens at higher frequencies.  That second dip at 19MHz is at a frequency 3.3333 times the first dip.  Are all those threes a coincidence?

Smudge

Thanks Smudge,

Nice info, to be clear about my present setup, this is how i have it right now:



If i need to make some measurements across the 51 Ohm resistor, please say so.

Beyond this 5.7MHz dip the negative power highlighted by Itsu could be a real effect so that needs investigating thoroughly.  A non-sweep test using a fixed frequency sine wave input should show more detail and allow better values to be determined.

OK, i will do that on that highlighted part. 

That second dip at 19MHz is at a frequency 3.3333 times the first dip.  Are all those threes a coincidence?

Well that 5.7MHz i mentioned for the 1st dip was an estimate as it is hard to determine the exact midpoint from that screenshot, same for the 2nd dip at 19MHz, so it very probably is a coincidence.

Itsu

[Verpies]

Nice info, to be clear about my present setup, this is how i have it right now:

Do you realize that your setup is equivalent to the following with the toroidal turns and delay line added ?


...and there is a classic transformer induction taking place between these two parallel half-overlapping current loops ?
You can find out how much this induction affects your measurements by making these current loops perpendicular to each other.

[Smudge]

ÿ
Do you realize that your setup is equivalent to the following with the toroidal turns and delay line added ?


...and there is a classic transformer induction taking place between these two parallel current loops ?
You can find out how much this induction affects your measurements by making these current loops parallel to each other.

That is not the case when feeding the coils in bucking mode since they drive currents in opposite directions around the toroidal loop.

[Smudge]

If Itsu did his sweep while using the voltage across the load his math channel should read positive mean power values.  If they also show negative values at the higher frequencies we know tbey are false.  This bench set out to look for negative resistance which shows up as negative power, but I did not expect it to show up so soon.  I was expecting a reduction in positive power.  It would be good if the negative values are genuine.

[Itsu]

Do you realize that your setup is equivalent to the following with the toroidal turns and delay line added ?


...and there is a classic transformer induction taking place between these two parallel half-overlapping current loops ?
You can find out how much this induction affects your measurements by making these current loops perpendicular to each other.

I did not realize that, but is this also the case when the delay line is not almost one full turn, but as it is really is, consists of two halves?



Itsu

[Verpies]

That is not the case when feeding the coils in bucking mode since they drive currents in opposite directions around the toroidal loop.

Yeah, "around" in which direction ?
If the currents are opposite and equal then their sum over the loop should be zero.

[Verpies]

I did not realize that, but is this also the case when the delay line is not almost one full turn, but as it is really is, consists of two halves?

Every current element will generate an inductive magnetic flux.  Even in loops that are not complete.  Currents must flow to charge and discharge these capacitors of the delay line.
If the magnetic flux generated by these currents integrates to zero over the area spanned by the other loop, then and only then the mutual axial induction will be zero.

Since you are an experimentalist and magnetic flux is a directional quantity, you can observe the influence of this unexpected induction on your measurements by gradually making one loop's plane perpendicular to the other loop's plane.

[Itsu]

Ok, but the problem is that my core with the coils and delay lines are sort of poured in concrete (hot glue), so it will be hard to "gradually making one loop's plane perpendicular to the other loop's plane".

[Itsu]

If Itsu did his sweep while using the voltage across the load his math channel should read positive mean power values.  If they also show negative values at the higher frequencies we know tbey are false.  This bench set out to look for negative resistance which shows up as negative power, but I did not expect it to show up so soon.  I was expecting a reduction in positive power.  It would be good if the negative values are genuine.

Well, i zoomed in on that 6 to 7MHz range which showed that -9mW Power by manually again measuring every 10kHz the voltage and current and let the math function calculate the power, with this as result:

 

So no negative power there as you expected, thus an artifact of the scope using this sweep method.

I do see that the power is continue to decrease and has a minimum of a few hundreds of uW around 12MHz.

I will make a similar 10kHz step power calculation there the next days or so.

Itsu 

[Verpies]

Ok, but the problem is that my core with the coils and delay lines are sort of poured in concrete (hot glue), so it will be hard to "gradually making one loop's plane perpendicular to the other loop's plane".

So don't move the core and its delay lines.
Move the plane of the other loop.

[Itsu]

Well, the "other loop" are the 2x 6 turns litz wire bucking coils which also are tightly secured with hot glue and even if i could move them, i have to manipulate them around or under or over the delay lines capacitors / return wire:



Perhaps when done with making the present measurements.

[Itsu]

I found an area in the sweep where the scope seems to measure negative power.

I had to switch over from the TDS3054B to the MDO3054 as with the increasing frequency (~18MHz) the unable to compensate 10ns deskew of the current probe poses increasing problems.

But the MDO3054 scope his current probe TCP0020 is less sensitive (10mA versus 1mA) compared to the P6302 current probes used on the TDS3054B, and we need to measure in uA's to make some sensible calculations.

So i settled for using the P6302 current probe on the MDO3054, which was properly deskewed, and seems happy to show negative average power (the TCP0020 never shows negative power when used on the MDO3054, but seems lost in dealing with uA's, which could be the reason).

Anyway, for what its worth, here is a frequency range from 16.5MHz to 18.5MHz where i manually measured in 100kHz steps the voltage and current and let the math calculate the mean power.
Note that in the orange area, i had to switch both the current and Pmean vertical settings to avoid clipping of the signals, and this influenced the calculations somehow.




So there is at least 1 area  in the sweep where my equipment has a hard time dealing with and displays a negative power.

Itsu 

[Centraflow]

At all, especially Barry (hope you are well).

What you should be looking at is not "a" coil fed from either end, but 2 coils (bifilar wound) with one fed from one end the the other from the other end.

In my case it is a trifilar coil where the two outers are fed one way and the center fed the other in a sequence of currents, and so magnetic fields.

The BUCKING is of the magnetic field and NOT as a standing wave within the wire.

Of course the magnetic fields must not meet head on, in the same place, you will not like it.

Mike