coupled LC resonance circuit

[toyonline]

Thanks Chris for detailed explanation and introduction to RF transmission line. Before talking to you, I even thought about using normal short circuit line to output that magnified signal. How innocent I am! I think I have a lot of homework to do.

But just for a benchmark try, I am not sure whether a very short RF coaxial cable would reduce effects from such as impendence mismatch or other kind of signal loss? For example, I read from a paper a 5m long RG8 cable has a typical time delay of 50ns. A spice simulation of such a cable has significant effect on output signal. But as long as time delay decreases to 5ns, that effect is negligible. Does this mean a RG8 cable of no longer than 0.5m has no significant effect on my output signal?

For the load of the circuit, the output is designed to be connected to a parallel planar stainless steel electrodes, or simply a capacitor of ~5pF. I guess this is not an ideal impedence for both the transmission line as well as the power supply. Maybe I can use some resistors to make a better balance. Am I on the right way?

Electronics, like most scientific fields of study, is made up of subsets, that in their own right can be viewed as their own field of study. RF Theory & Design is definitely a valid subset. Within the RF subset, Antenna / Transmission Line Theory & Design could and does fill textbooks. Fortunately, you can access all the information you can absorb by doing a Google search.

That said here are some transmission line tidbits:

(1) The reason 50Ω was chosen as the characteristic impedance of coax used in radio communications is because a quarter wave vertical ground plane antenna presents a natural 50Ω load impedance. It is also the simplest of all antennas and requires no matching networks at all.

(2) The term "Transmission Line Characteristic Impedance" is the most misunderstood and misapplied term in the electronics lexicon. I would venture a guess that only 1 out of every 100 people that think they understand it actually do! I've actually seen posts where the OP was trying to measure it with an Ohmmeter!

(3) I've forgotten more about transmission lines than I remember.

(4) Other than loss per foot; Transmission lines should play a transparent role but they can only do this when properly terminated at both ends. When it isn't it becomes something other than a transmission line. IE it's no longer transparent to the circuit driving it or the circuit it's piping to.

(5) Transmission lines are analogous to water pipes. Ideally we don't want them to leak at all. Except for resistive and dielectric loss (which we can't eliminate) we want all the RF fed into them to come out the other end. A mismatched load or source will cause the transmission to (radiate) leak. If the mismatch is severe enough it will resemble an antenna more than a transmission line.

(6) In a classroom environment there are few test setups that demonstrate the idiosyncrasies of transmission lines better than (TDR) Time Domain Reflectometry.


Linear Amplifier:

Yes, I understand that the author is using a linear as a driver but that circuit and the device it drives does not present a proper load for it. Since the ionizer does not appear to dissipate any real power it stands to reason that a real load won't be reflected back to the linear. Linear's or for that matter, any RF power amp does not like this and can destroy either. A linear with Fold-Back may protect it from destruction but would also prevent full power output. The only thing I see dissipating power is R1 @ <10W.

Hey, I'm not suggesting that his circuit doesn't work. I'm just concerned that there's no mention of the above.

Stress Analysis:

These are tests that manufacturers perform much like the auto industry does crash tests. All components have voltage, current, frequency, temperature and humidity maximums. Some may even have atmospheric pressure, vibration, shock impact maximums spec'd out. These are not tests that the home shop engineer gets involved in because it invariably results in the destruction of the (DUT) Device Under Test.

Chris

 

[toyonline]

No if the load is anything other than the characteristic impedance of the cable, there will be reflection of power back to the transmitter. ie. the worse the missmatch, the moer power that is reflected.

Dave

Thanks Dave for pointing out my misunderstanding!

So this means as long as the impedance mismatches the transmission line (e.g. 50 ohm), reflection effect will be significant. I am not sure whether this effect could be reduced for some extent, by either shorten the transmission line or add some resistors as loading impedance.

 

[davenn]

I am not sure whether this effect could be reduced for some extent, by either shorten the transmission line or add some resistors as loading impedance.

No and no

the mismatch will be there regardless of the transmission line length

there's no point in adding resistance to the load, all you will be doing is wasting transmitter power heating up the resistors

Your load, presumably an antenna,...
1) MUST be tuned to the freq of use, so it will radiate maximum power that it receives
2) MUST be matched to the transmission line so that maximum power is transferred to the antenna

Dont think you have specified you load yet ... type of antenna etc ?

Dave

 

[davenn]

Hey Dave, I investigated it a bit further. This link expands a bit on what you posted. http://en.wikipedia.org/wiki/Coaxial_cable#Choice_of_impedance

I want you to know that this revelation really sucks for me. I sure am glad that we didn't have internet forums for the entirety of the last 48 years!. After all, I've spent those years preaching the 1/4 wave ground plane theory!

Chris

we all live and learn, all cool

I havent done 40 something yrs of RF without learning a few things

sounds like you and I have been at this game for a similar time

cheers
Dave

 

[CDRIVE]

Dont think you have specified you load yet ... type of antenna etc ?

Dave

Dave, that's the hitch! There is NO antenna and virtually NO load! If he's reproducing the setup in his link the tuned circuit is being used to step up the RF voltage to (unloaded) extremes. This high RF voltage just sits on ionizing electrodes not intended to radiate like an antenna. At least that's how I understand it,.... but then ..once every 48 years I've been known to be wrong.

Chris

 

[davenn]

but then ..once every 48 years I've been known to be wrong.

you're too modest

I'm never wrong. One time, I thought I was wrong, but I was mistaken. haha

its sorta along the same lines of " I used to think I was indecisive, but now I'm not so sure"

Dave

 

[CDRIVE]

its sorta along the same lines of " I used to think I was indecisive, but now I'm not so sure"

Dave

Ha! I love that!!

Hey guys, as much as I loath reading directions it sometimes is necessary. I did a quick perusal of the PDF links in that article and things are much clearer now. I highly advise anyone interested in this thread to read them.

As it turns out the author is fully aware of proper loading of an RF power amp. How and where power is dissipated and resultant impedance reflection at the input is explained in detail. It would appear that R1 and resistive losses in the inductors provide the transformer action to provide a 50 Ohm input Z.

One thing is definitely obvious when reading the two PDF's. This article is not the typical crap you'll find on places like Instructables. It's a lab quality setup and environment. Cool stuff!!

Chris

 

[toyonline]

Ha! I love that!!

As it turns out the author is fully aware of proper loading of an RF power amp. How and where power is dissipated and resultant impedance reflection at the input is explained in detail. It would appear that R1 and resistive losses in the inductors provide the transformer action to provide a 50 Ohm input Z.

Chris

Hi, sorry for my late response. I have been to country for several days, where the internet is not available.

I have noticed the author tuning capacitors in the circuit for minimum power output. But I did not understand the purpose of this kind of tuning, until I have been discussed with you. So, if my understanding was right, that was actually tuning a right point for that 50 Ohm source impendance.

As I read from textbooks, for maximum power transmission, the impedance of source, transmission line, and load should all be matched. In this particular case, the match impedance is 50 Ohm. And that's why a 50Ohm coaxial cable was chosen.

Next, I need to characterize the source impedance and the load impedance (by Spice), for a better understanding and control of the circuit. What I am considering is, whether a variable resistor could be used for tuning purpose rather than a capacitor? Since the change in capacitance will alter the frequency of the circuit.

Felix

 

[toyonline]

No and no

the mismatch will be there regardless of the transmission line length

there's no point in adding resistance to the load, all you will be doing is wasting transmitter power heating up the resistors

Your load, presumably an antenna,...
1) MUST be tuned to the freq of use, so it will radiate maximum power that it receives
2) MUST be matched to the transmission line so that maximum power is transferred to the antenna

Dont think you have specified you load yet ... type of antenna etc ?

Dave

Thanks very much. And sorry for my late response.

I have read from the textbooks, and now I know for maximum power transmission, the impedance of source, transmission line, and the load should all matched. And usually a 50 Ohm impedance was chosen for impedance match purpose.

The load of this specific circuit was a parallel stainless steel electric plates, or simplified a capacitor. So there is no resistor part on loading.

 

[toyonline]

I have performed impedance analysis of the circuit. Hope it was right.

I have added a C5 capacitor to the output to mimic a parallel electrodes of ~2pF. The loading impedance of the whole LC circuit plus C5 are shown in figures. As I noticed, the impedance at two resonant frequencies are not exactly equal to 50 ohms (~20ohms instead). So does this means the overall loading of the linear amplifier is not matched with the source impedance of the amplifier (50ohms)?

According to the impedance analysis, the optimum condition is not the minimum impedance at two resonant frequencies. It should be a impedance of ~50ohms, which means frequencies nearby two resonant frequencies but not equal to (since the impedance analysis shows local minimum is ~20ohms). So I think the original tuning process is not that right since it says tuning for a local minimum. Am I right on this?

Secondly, considering all LC circuit and the linear amplifier as a lumped source will leave C5 as a loading impedance. The subsequent analysis shows the impedance of C5 will never match with a 50 ohms. So, is it reasonable to use a 50 ohms coax to transfer signal from node(004) to C5 (as what the author did in the article)?

Am I making mistake for the impedance analysis?


Felix

 

[toyonline]

I recently have built this LC circuit for voltage amplification purpose. Through frequency sweep, I observed two resonant frequencies of the circuit. So it seems my circuit works.

But the problem is the voltage increase is only ~8 times larger than the original input voltage, which is not enough for my purpose. And through Spice I know the simulated increase of voltage of this circuit is 50dB, i.e. 300 times larger.

I know Q factor maybe a parameter that influence voltage increase of my circuit. So maybe the limited amplification is due to low Q factor of some of electronic components in my circuit? If my hypothesis was right, maybe I need to change some old components in my circuit with brand new components.

Would any one help me to point out other possibilities? and suggestions to improve that voltage increase?

Thanks so much.