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Simulations show Currents do NOT cancel at the Parallel LC-resonance frequency, contrary to what we learn is one of the main characteristics of parallel resonance circuits. Why?

Testing various LC combinations, the two Currents always meet and cancel at a frequency lower than the XL = XC frequency.
?
Micro-Cap 12 (Transient Analysis; Oscilloscope) shows LC-resonans frequency / 1.034 ≈ frequency of current cancellation. Moving the input frequency up and down from this point you can find the perfect frequency where the two currents have the same amplitude = cancelling. Down in frequency makes the current through the inductor stronger, up makes the current through the capacitor stronger.

I mostly use RS = 10m Ohm in both the Capacitor and in the low induction Coil. Same is true with RS = 0 Ohm.

LC wise they are perfect(ly fake ;) 0 parasitic L and C. Anyway, the simulation makes no difference; Capacitance added to the inductor gives the same result as if the C was in a separate capacitor; It just moves the LC-resonance to a lower frequency. And the current-cancelling frequency also thereby moves down.

This also correlate to the readings on my nanoVNA; two different frequencies, and I do not understand how to combine them into an “official” Parallel LC-resonance circuit.

--
Simen Tobiassen

root (LC) is only true if R=0

If any R is present either in series with L or C, The root (LC) is modified... downward in freq...

Thanks, interesting! How is that computed?

But the result however, is the same with Serial Resistance = 0 Ohm in capacitor and in coil; Except I get beats on top of the 888630 Hz waves, probably modulated with the 918881 LC-resonance.
--
Simen Tobiassen

If you write the node equations for a parallel LC with an R in series with the L

wo= 1/√LC × √1-R^2 × C/L

Yes, the series R will perturb the resonance point from the expected value.

Many simulators apply default series R to L and perhaps C to prevent creating a singularity matrix. In some cases you can over ride the default.

Hello Simen,

Please let me know the name of the simulation software that you are use.

Thanks in advance.

Mark Wilkinson

________________________________
From: [email protected] <[email protected]> on behalf of Simen Tobiassen <simen@...>
Sent: Friday, March 26, 2021 7:52 AM
To: [email protected] <[email protected]>
Subject: [nanovna-users] Why does the two currents in a parallel LC-resonance circuit cancel on a lower frequency than the LC-resonance? #coils #charts #test #teaching #traps #simulation #rant #problem #measurement #learning #circuit

Simulations show Currents do NOT cancel at the Parallel LC-resonance frequency, contrary to what we learn is one of the main characteristics of parallel resonance circuits. Why?

Testing various LC combinations, the two Currents always meet and cancel at a frequency lower than the XL = XC frequency.
?
Micro-Cap 12 (Transient Analysis; Oscilloscope) shows LC-resonans frequency / 1.034 ≈ frequency of current cancellation. Moving the input frequency up and down from this point you can find the perfect frequency where the two currents have the same amplitude = cancelling. Down in frequency makes the current through the inductor stronger, up makes the current through the capacitor stronger.

I mostly use RS = 10m Ohm in both the Capacitor and in the low induction Coil. Same is true with RS = 0 Ohm.

LC wise they are perfect(ly fake ;) 0 parasitic L and C. Anyway, the simulation makes no difference; Capacitance added to the inductor gives the same result as if the C was in a separate capacitor; It just moves the LC-resonance to a lower frequency. And the current-cancelling frequency also thereby moves down.

This also correlate to the readings on my nanoVNA; two different frequencies, and I do not understand how to combine them into an “official” Parallel LC-resonance circuit.

--
Simen Tobiassen

He already gave this info, below:?? Micro-Cap 12
Google it - it's free windows s/w

This may have been stated in slightly different terms, but:

The circuit Q is the culprit. Lower Q results in dragging the resonant
frequency lower. Maybe get yourself some LN2 and immerse the parallel
tuned circuit and measure resonant frequency. Then progressively measeure
resonant frequency and temperature as the circuit warms. Of course, you
can take it the other way and increase the temperature while noting the
resonant frequency and temperature. You can also simulate this by adding /
subtracting loss - resistance - in series with each component.

Dave - W?LEV

On Fri, Mar 26, 2021 at 11:52 AM Simen Tobiassen <simen@...>
wrote:

Simulations show Currents do NOT cancel at the Parallel LC-resonance
frequency, contrary to what we learn is one of the main characteristics of
parallel resonance circuits. Why?

Testing various LC combinations, the two Currents always meet and cancel
at a frequency lower than the XL = XC frequency.
?
Micro-Cap 12 (Transient Analysis; Oscilloscope) shows LC-resonans
frequency / 1.034 ≈ frequency of current cancellation. Moving the input
frequency up and down from this point you can find the perfect frequency
where the two currents have the same amplitude = cancelling. Down in
frequency makes the current through the inductor stronger, up makes the
current through the capacitor stronger.

I mostly use RS = 10m Ohm in both the Capacitor and in the low induction
Coil. Same is true with RS = 0 Ohm.

LC wise they are perfect(ly fake ;) 0 parasitic L and C. Anyway, the
simulation makes no difference; Capacitance added to the inductor gives the
same result as if the C was in a separate capacitor; It just moves the
LC-resonance to a lower frequency. And the current-cancelling frequency
also thereby moves down.

This also correlate to the readings on my nanoVNA; two different
frequencies, and I do not understand how to combine them into an “official”
Parallel LC-resonance circuit.

--
Simen Tobiassen

--
*Dave - W?LEV*
*Just Let Darwin Work*

Simen,

Some of the other replies you have received are quite informative. If you want an in depth technical explanation on series and parallel resonant circuits and how the Q of the LC components affects the resonant frequency it was well covered by the Terman in his Radio Engineers handbook in 1943. You can download his book (out of copyright now) and find what you are looking for starting on page 135.



Roger

Dave: The ~1.53 dielectric constant of the LN2 will likely have an effect on the inductor value which may obscure the temperature effect. 73, Don N2VGU

Thank you for good ideas and info, but in this simulation case I guess it is the CPU and my head that need to be immersed in LN2 ;)

You say low Q will drag the frequency down - but the Q factor is high in this case: 4174.

--
Simen Tobiassen

On Fri, Mar 26, 2021 at 12:41 PM, Roger Need wrote:

If you want an in depth technical explanation on series and parallel resonant
circuits and how the Q of the LC components affects the resonant frequency it
was well covered by the Terman in his Radio Engineers handbook in 1943.

Great, Roger, I have downloaded the handbook and will read.

But this is about an abnormality in relation to what is though about parallel circuits. But often the guys of old knew other tings ... information are getting lost :(

--
Simen Tobiassen

On Fri, Mar 26, 2021 at 07:08 AM, alan victor wrote:


Many simulators apply default series R to L and perhaps C to prevent creating
a singularity matrix. In some cases you can over ride the default.
I use the now free Micro-Cap 12:

This is my Micro-Cap file “Parallel LC resonans - Current Cancel 888453 Hz, LC res 918881 Hz - remade from scratch.cir”:

Micro-Cap does apply a default high impedance R to connect parts of the circuit to prevent creating a singularity matrix, and it tells me when it does. I did not get a warning when I just remade this circuit to see if I would get a warning.

Reactance at LC Resonance frequency 918881 Hz is 86.603 Ohms. At 888453 Hz XC = 89.569 Ohms, XL = 83.735 Ohms. The Q-factor is 4174.

The small RS 10 mOhms in the coil and capacitor doesn’t seem to do much to do much to the current curves, except create beats on top of the 888630 Hz waves when I put RS in the coil and capacitor to 0 Ohms. Possibly being modulated by the 918881 LC-resonance frequency ??

[ ?IMAGE 0 ]

Seemingly no parallel Ohm from the Current Source, as I get the exact same result with a 1000 Mega Ohms resistor in series.

Input at frequency where Current Cancel, at 888453 Hz:
?
?[ ?IMAGE 1, 2 and 3 ]


Input at LC-Resonance frequency 918881 Hz:
?
[ ?IMAGE 3, 5 and 6 ]?
?

Note that on the LC resonance frequency the two Currents have mismatching amplitudes = NOT cancelling.

In real life I don’t recall any sign of a resonance / rise in voltage amplitude on a frequency right below the LC resonance frequency, but I do see some kind of resonance on the nanoVNA with the phase angle being 0 and hte reflex coefficient (imaginary) being 0 at that frequency.

It started out with me pondering why I have a problem achieving cancellation at the Self Resonant Frequency of coils, where it is very easy to locate the LC-resonance point as the Current is 0 degrees phase shifted related to the Voltage. But it is a problem getting the current flat.

--
Simen Tobiassen

Also on quora.com got a simular result.



--
Simen Tobiassen

On Fri, 26 Mar 2021 at 13:46, Simen Tobiassen <simen@...> wrote:

Thanks, interesting! How is that computed?

But the result however, is the same with Serial Resistance = 0 Ohm in
capacitor and in coil; Except I get beats on top of the 888630 Hz waves,
probably modulated with the 918881 LC-resonance.
--
Simen Tobiassen

The resonant frequency of a series tuned circuit is independent of any
series resistance. That’s not the case with a parallel tuned circuit.
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...

Telephone 01621-680100./ +44 1621 680100

Registered in England & Wales, company number 08914892.
Registered office:
Stokes Hall Lodge, Burnham Rd, Althorne, Chelmsford, Essex, CM3 6DT, United
Kingdom

On Fri, Mar 26, 2021 at 04:55 PM, Simen Tobiassen wrote:

It started out with me pondering why I have a problem achieving cancellation
at the Self Resonant Frequency of coils, where it is very easy to locate the
LC-resonance point as the Current is 0 degrees phase shifted related to the
Voltage. But it is a problem getting the current flat.

It is very easy to locate the Self Resonant Frequency of coils, as the Current is 0 degrees phase shifted related to the Voltage and easy to have moving left and right at the oscilloscope, but how do I get the current flat?

So my separate Real Life experiences, the Simulations and my NanoVNA experience seem to line up here - but I struggle to combine them into an “official” Parallel LC-resonance circuit. Should not be too complicated to get the current flat, right, ha ;) ...
--
Simen Tobiassen

The answers below contain the keys to answer your question. A thorough vector analysis that includes the effective series resistance of the coil will show that the point of maximum impedance shifts from the from the frequency where XL=Xc. So depending on how you interpret "resonance" you will get two different frequencies.
So is 'resonance' the point where XL =Xc and phase angle=0 degrees, or is it where you have maximum impedance?
Good question....... theory says it's the first, but from a practical standpoint, it is the second.

Just a-musing....

73
W0NRP

On Fri, Mar 26, 2021 at 07:34 PM, Neil Preston W0NRP wrote:

The answers below contain the keys to answer your question. A thorough vector
analysis that includes the effective series resistance of the coil will show
that the point of maximum impedance shifts from the from the frequency where
XL=Xc. So depending on how you interpret "resonance" you will get two
different frequencies.
So is 'resonance' the point where XL =Xc and phase angle=0 degrees, or is it
where you have maximum impedance?
Good question....... theory says it's the first, but from a practical
standpoint, it is the second.

Just a-musing....

??
I do the simulations with RS = 0 or 0.01 Ohms mostly, so even with a simulated 0 Ohm effective serial resistance, the currents still cancel at that lower frequency. So, the same results in regard to changes in the resistance, but I get beats when having no serial resistance at all.

Even though engineers and physicists tell me they are one and the same, you say this is actually a known phenomena? And more importantly, is there a known solution to getting what happens on these two frequencies together onto one frequency? How do I get them on top of each other - into an official parallel resonance circuit?

In addition to the Self Resonant Frequency of a coil, I see this lower frequency kind of resonance on my nanoVNA, having the Phase angle = 0 and the imaginary part of the Reflection coefficient = 0, but I am struggling to get the two frequencies to meet. Using a coil connected by a single lead, I can get these two phenomenons on top of each other by Earth grounding the coil.

But how do I accomplish this with a Source connected in parallel, as we will normally connect?

It started out with me pondering why I have a problem achieving cancellation of current at the Self Resonant Frequency of coils, where it is very easy to locate the LC-resonance point as the Current is 0 degrees phase shifted related to the Voltage and is easy to have moving left and right at the oscilloscope. But the problem is to also get the current flat at that frequency !#&$%

“Interpreting resonance" gets even harder when seeing what Micro-Cap does when I remove the source and have an Initial Condition = 100 Volts in the coil, and RS=0 in coil and Capacitor.
This puzzles me even more, as the 918881 Hz LC-resonance circuit now choose to freely oscillate at the measured frequencies of 866442, 870920, 870777, 871376 Hz etc; Even lower than the 888453 Hz “Currents-Cancel-resonance-frequency”, when using an 888453 Hz input Source. And these frequencies are indeed lower than the 918881 Hz LC-resonance frequency of the circuit.
?
[ IMAGE 1 ]

What kind of resonance is this? The FFT (AC Analysis) doesn't show me any spike at all, as it shows zero signs of any activity at any frequency, which is what I am used to when using IC.

When using a Source though, the FFT shows a Voltage Magnitude spike exactly where the LC-resonance is supposed to be; at 918881 Hz, and nothing shows at any other frequency.
?
[ IMAGE 2 ]

--
Simen Tobiassen

On Fri, Mar 26, 2021 at 07:34 PM, Neil Preston W0NRP wrote:

So is 'resonance' the point where XL =Xc and phase angle=0 degrees, or is it
where you have maximum impedance?
Good question....... theory says it's the first, but from a practical
standpoint, it is the second.

... or it is the first, as theory says, when having free oscillations in a Parallel Super Conductor LC-circuit ?
--
Simen Tobiassen

Are we talking two different problems here - or one and the same?

1) My measurement on a bifilar coil, where

LC-resonance / IMAGINARY Impedance (green) = 0 Ohm - being on a different frequency than ?
Maximum REAL Impedance (yellow) = 336 Ohm.

[ IMAGE 1 ]

It’s like the curves otherwise looking like a Coil’s Self Resonant Frequency point with 0 Ohm in the middle, are offset down into the Capacitive reactance region, so the IMAGINARY curve has become almost completely Capacitive.

How can there be Maximum REAL Impedance (yellow) = 336 Ohms at the frequency where
IMAGINARY Impedance (green) = about -144 Ohms?

Without understanding the dynamic REAL Resistance, I did think it was directly related to the IMAGINARY Reactance XL = XC point, thereby having to appear at the same frequency. REAL, as in being the REAL part of Impedance, being mathematically a Complex Number - but I do not understand it. What is this in reality in the world where I learned Resistance is what you have in a Resistor. This dynamic REAL Resistance can even be negative Resistance, which I understand as voltage.

The Klystron utilize positive resistance in its resonance cavity walls in resonance with a negative resistance of the same Ohm value, represented by the right level of Voltage, making it a Negative Resistance Oscillator. I find no explanation on how voltage relate to specific Ohm values.


2) I talk about
The Current cancelling on a frequency lower than the LC resonance frequency.

I wonder if this relates to what I experience as interesting activity on two different frequencies in a parallel resonance circuit, using my nanoVNA:

- Low frequency:?Phase angle = 0 degree, Reflection coefficient (imaginary part) = 0, IMAGINARY Impedance = 0 Ohm, capacitance/inductance changes ?
- High frequency:?SRF ( IMAGINARY Impedance = 0 Ohm, REAL Impedance = Maximum Ohm )

?[ IMAGE 3, 4 ]
?
Are the
- Lower frequency LC-resonance with Current Cancellation, but no Maximum Impedance, and the
- Higher frequency SRF LC-resonance with Maximum Impedance, but no Current cancellation?

How do I combine the two?

Only when I used a coil connected by a single lead, could I get these two phenomenons on top of each other by Earth grounding the coil. What happened? I did not tune, it just jumped into place.

I succeeded seemingly to get them together on the nanoVNA once, with a parallel resonance circuit connected to source in parallel, but I do not remember what I did to achieve this. And it was not at a real SRF point when I looked at it on the oscilloscope. And the current wasn’t flat, ha.

Are the lower frequency I see on the nanoVNA - the same frequency I see in the Micro-Cap simulation - and why I have trouble tuning the current flat on the oscilloscope?
--
Simen Tobiassen