Induction and digital HF-circuits

Thread Starter

ZimmerJ

Joined Dec 9, 2020
55
Hello, i am currently studying oscilloscopes, specifically probe-design. My first hurdle so far seems to be the working principles of a coaxial cable. Long story short, i had to go back and refresh my memory regarding induction, skin-effect, capacitance, etc.

Now for my question:

The idea i have always had about induction and counter-electromotive force (back-EMF), is based on the fact that an alternating current changes direction. Now what i haven't really thought of before is high frequency digital signals, square waves. Usually these signals are 0 - 5/3.3 V so the current will always go one direction (DC) as it never goes below 0 V. Since it does not change direction; how can a back-EMF be produced, which is an absolute necessary event for skin-effect to occur?

I read that a varying intensity in current, which in turn makes a varying intensity in the magnetic field, is enough to produce a back-EMF but i don't understand how that is. I have always thought that the magnetic field as to go in reversed direction in order to produce a reversed induced current.

Any answers appreciated.
 

Papabravo

Joined Feb 24, 2006
17,300
A signal that looks like a square wave travels down the coaxial cable and reachs the other end where there is likely to be a connector that represents an impedance discontinuity between the characteristic impedance of the cable and the impedance of the connector, or it could be open, or it could be shouted, or it could be terminated in it's characteristic impedance. All of these situations except the proper termination will cause a portion of the incoming waveform to be reflected back to the source. This action may occur several times before the attenuation of the cable is enough to reduce the multiple reflections to an unmeasurable level.

As amateur radio operators we can measure these reflections to judge the suitability of a particular antenna. What we want in an antenna is to radiate most of the signal we send down the cable. Anything that comes back is wasted and if the magnitude is large enough can damage a transmitter. Naturally we would be eager to avoid that situation.
 

Thread Starter

ZimmerJ

Joined Dec 9, 2020
55
A signal that looks like a square wave travels down the coaxial cable and reachs the other end where there is likely to be a connector that represents an impedance discontinuity between the characteristic impedance of the cable and the impedance of the connector, or it could be open, or it could be shouted, or it could be terminated in it's characteristic impedance. All of these situations except the proper termination will cause a portion of the incoming waveform to be reflected back to the source. This action may occur several times before the attenuation of the cable is enough to reduce the multiple reflections to an unmeasurable level.

As amateur radio operators we can measure these reflections to judge the suitability of a particular antenna. What we want in an antenna is to radiate most of the signal we send down the cable. Anything that comes back is wasted and if the magnitude is large enough can damage a transmitter. Naturally we would be eager to avoid that situation.
Right, reflection! I was actually introduced to that just recently, yet i didn't think of it. That makes sense, reading about antennas will probably give me some more clarity so i'll do that. Sounds like you have the kind of job i would wanna do.

Thanks!
 

Wendy

Joined Mar 24, 2008
22,647
There is a piece of test equipment that puts this effect to good effect called a Time Domain Reflectometer (TDR for Short). It's used to find breaks in long transmission lines such as under the ocean.
 
Last edited:

Thread Starter

ZimmerJ

Joined Dec 9, 2020
55
A signal that looks like a square wave travels down the coaxial cable and reachs the other end where there is likely to be a connector that represents an impedance discontinuity between the characteristic impedance of the cable and the impedance of the connector, or it could be open, or it could be shouted, or it could be terminated in it's characteristic impedance. All of these situations except the proper termination will cause a portion of the incoming waveform to be reflected back to the source. This action may occur several times before the attenuation of the cable is enough to reduce the multiple reflections to an unmeasurable level.

As amateur radio operators we can measure these reflections to judge the suitability of a particular antenna. What we want in an antenna is to radiate most of the signal we send down the cable. Anything that comes back is wasted and if the magnitude is large enough can damage a transmitter. Naturally we would be eager to avoid that situation.
Aside from reflection i also read that a collapsing magnetic field induces a voltage, such that the current goes opposite. This i can recall from basic theory. A digital signal do seem like a good example of that, 3.3/5 Volt and then rapidly switches to 0 V and vice versa.

I guess i just want this confirmed to be accurate, there seems to be a lot of factors when it comes to induction and back-EMF in terms of transmission.
 

Deleted member 115935

Joined Dec 31, 1969
0
Aside from reflection i also read that a collapsing magnetic field induces a voltage, such that the current goes opposite. This i can recall from basic theory. A digital signal do seem like a good example of that, 3.3/5 Volt and then rapidly switches to 0 V and vice versa.

I guess i just want this confirmed to be accurate, there seems to be a lot of factors when it comes to induction and back-EMF in terms of transmission.
You have a good number of terms there, not necessarily in the correct order.

You mention

Skin effect. This is the effect where as frequencies increase, the current follows less in the core of the conductor, and more on the surface. This is the reason that higher frequency cables tend to have a better surface finish and / or a plating such as silver to give lower resistance.

Inductors and voltage generation.
A current flowing through a conductor, generates a field around it. A conductor crossing a field has a voltage induced in it.
In a inductor / coil with DC current flowing, the field is constant, so no voltage is generated in the inductor by the filed.
When you turn off the current in the inductor, then the field collapses back towards the coil,
this is a moving field, and a moving field that crosses a conductor ( coil ) generates a voltage,
Maxwels law, field direction change, means the the voltage generated is opposite direction to the voltage originally used to drive the coil. Electro magnetic field, an old word for voltage, so back emf is the back voltage generated across the coil.
The size of the voltage generated, is inversely proportional to the speed the field changes, which to a first degree "instant" so the back emf is "infinite". Hence the reason for the diodes across coils of things like relays,

Square waves and scope probes,
Are you the person who has been asking about the capacitor divider on scope probes ?

A scope probe has a few things in this context that you have touched on.

First you have to compensate for the effects of the cable and components of the cable, such that the frequency range is maximised, and is "flat"

Second , in a scope, you are not so worried about power transfer from the tip to the scope,
after all, you have a 10:1 attenuator there,

Reflections, the Bain of all "high frequency" work,
Now what constitutes "high frequency" you ask,
Good question, well it depends,

Look at a reflection, this is when a change in voltage, leaves a "transmiter" , travels along a wire and hit the far end.
Now the far end can be either , a short circuit, an open circuit , or some where in between,
If its that magical thing, called a matched termination at the receiver, then the voltage edge is absorbed 100 % into the receiver. If its not perfectly matched, then some of the voltage change will be reflected back down the wire. This reflection can either be +ve or -ve with respect to the input voltage.
this return voltage then goes back to the transmiter, where if its "matched" then the voltage is absorbed, if its not matched, the voltage again reflects, either +ve or -ve.

So a few things to take from this,
the time between the voltage leaving and coming back is set by the medium and the distance.
The voltage reflected and amplitude of the reflection, depends on how far away we are from the matched optimum.
Now transmitter's, tend to be good at absorbing this return, so we tend to only worry about the first pass.
If the time for the pulse to return is less than the time till the next voltage edge, then we tend not to need termination.

Now in RF, we always have a changing voltage, so we tend to need to terminate to keep good fidelity.

But even for lower frequency's , if you have Km of cable, and low loss, then you need to terminate,

So lots of bits you raise,
 

Thread Starter

ZimmerJ

Joined Dec 9, 2020
55
You have a good number of terms there, not necessarily in the correct order.

You mention

Skin effect. This is the effect where as frequencies increase, the current follows less in the core of the conductor, and more on the surface. This is the reason that higher frequency cables tend to have a better surface finish and / or a plating such as silver to give lower resistance.

Inductors and voltage generation.
A current flowing through a conductor, generates a field around it. A conductor crossing a field has a voltage induced in it.
In a inductor / coil with DC current flowing, the field is constant, so no voltage is generated in the inductor by the filed.
When you turn off the current in the inductor, then the field collapses back towards the coil,
this is a moving field, and a moving field that crosses a conductor ( coil ) generates a voltage,
Maxwels law, field direction change, means the the voltage generated is opposite direction to the voltage originally used to drive the coil. Electro magnetic field, an old word for voltage, so back emf is the back voltage generated across the coil.
The size of the voltage generated, is inversely proportional to the speed the field changes, which to a first degree "instant" so the back emf is "infinite". Hence the reason for the diodes across coils of things like relays,

Square waves and scope probes,
Are you the person who has been asking about the capacitor divider on scope probes ?

A scope probe has a few things in this context that you have touched on.

First you have to compensate for the effects of the cable and components of the cable, such that the frequency range is maximised, and is "flat"

Second , in a scope, you are not so worried about power transfer from the tip to the scope,
after all, you have a 10:1 attenuator there,

Reflections, the Bain of all "high frequency" work,
Now what constitutes "high frequency" you ask,
Good question, well it depends,

Look at a reflection, this is when a change in voltage, leaves a "transmiter" , travels along a wire and hit the far end.
Now the far end can be either , a short circuit, an open circuit , or some where in between,
If its that magical thing, called a matched termination at the receiver, then the voltage edge is absorbed 100 % into the receiver. If its not perfectly matched, then some of the voltage change will be reflected back down the wire. This reflection can either be +ve or -ve with respect to the input voltage.
this return voltage then goes back to the transmiter, where if its "matched" then the voltage is absorbed, if its not matched, the voltage again reflects, either +ve or -ve.

So a few things to take from this,
the time between the voltage leaving and coming back is set by the medium and the distance.
The voltage reflected and amplitude of the reflection, depends on how far away we are from the matched optimum.
Now transmitter's, tend to be good at absorbing this return, so we tend to only worry about the first pass.
If the time for the pulse to return is less than the time till the next voltage edge, then we tend not to need termination.

Now in RF, we always have a changing voltage, so we tend to need to terminate to keep good fidelity.

But even for lower frequency's , if you have Km of cable, and low loss, then you need to terminate,

So lots of bits you raise,
This is awesome, thanks!
Yes it's me. I realize as i've gotten deeper into the concept of high frequency, that there is a lot more to oscilloscopes than i initially thought. It's been a couple of days now just studying coaxial cable-design.
 
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