# Difference between electric and microwave circuit?

1. Apr 14, 2016

### JaneHall89

I understand in an electric circuit electrons in metallic materials move around by being directed by potential differences (fields!)

I guess in a microwave circuit photons propagate as waves and are directed around and manipulated throughout the circuit by its geometry?

Or have I got this completely wrong...

2. Apr 14, 2016

### axmls

No. The difference is that in microwave circuits, the frequencies of the voltages and currents are generally high enough that regular circuit laws don't work anymore, and it becomes necessary to use Maxwell's equations in full.

3. Apr 14, 2016

### JaneHall89

So its still electrons that are being used to move around an RF/Micro circuit?

4. Apr 14, 2016

### axmls

Correct. The distinction is the frequency that the circuit operates at.

5. Apr 14, 2016

### analogdesign

As a caveat, sometimes people refer to systems using such components as resonant cavities and waveguides as "circuits". In this context, yes, it is a matter of geometry.

For the most part, however, axmls is correct here. Although, again, I would add a distinction. If the circuit somehow relates to radio transmission or reception it is more likely to be called a microwave circuit. I have worked on a lot of systems with multi-GHz clocks and serial data ports and no one would call them microwave circuits.

Context is everything.

6. Apr 14, 2016

### Baluncore

Yes, the presence of electrons is irrelevant in that they are used to do the same job in both cases. All “electric” circuits guide signals along transmission lines, usually designed to keep those signals apart. A signal on a transmission line is propagated as an EM field between two, (sometimes more), conductive surfaces. A wire is just half a transmission line being operated at a low frequency. The return wire of the “circuit” makes the other half and so forms the complete transmission line.

"Microwave" circuits are a sub-class of "electric" circuits.
The term "microwave circuit" is usually restricted to the reception, generation, processing or transmission of a microwave wireless signal. In a microwave circuit the circuit is tuned to a particular microwave frequency or band. There is often a cut-off frequency below which the circuit will not function.

Like "return circuits", or two wire "transmission lines", a microwave signal in a "waveguide" is also being guided between two conductive surfaces. Those surfaces are held apart geometrically by a “metal insulator” which is dimensioned to be resonant at those microwave frequencies and therefore appear reflective, so invisible to the wave. The geometry and dimensions of components in a microwave circuit greatly restrict the frequency of operation, but they also significantly reduce losses and so increase the efficiency of operation.

7. Apr 15, 2016

### davenn

just a small correction in your misunderstanding ..... microwave RF( radio) is generally referred to for frequencies over ~ 1 GHz
signals below 1GHz are still radio transmissions
ELF - Extra Low Freq
SLF - Super Low Frequency
ULF - Ultra Low Frequency
VLF - Very Low Frequency
LF - Low Frequency
MF - Medium Frequency
HF - High Frequency
VHF - Very High Frequency
UHF - Ultra High Frequency 300MHz to 3000MHz ( 3GHz)
SHF - Super High Frequency
EHF - Extra High Frequency

NOTE, they can all be used for radio transmissions

see here for the generally recognised freq coverage of each of those band ranges

some more band designation info here

Context IS everything

cheers
Dave

8. Apr 15, 2016

### Jeff Rosenbury

Charges (ex. electrons) produce fields. But fields can exist on their own as well (photons). From an EE point of view, photons are (we rarely use just one) an electric field feeding on a magnetic field which in turn feeds on the electric field. (A changing electric field produces a changing magnetic field which in turn produces a changing electric field.)

So let's look inside that microwave oven. Inside there a magnetron which is a tube like in old tube radios. The tube is set between strong magnets. In the tube is a beam of electrons zipping around a circle (due to the magnet). There is a metal plate near the circle with some circles (holes we will be reusing this word, so type I holesI) in it with gaps to the main circle.

The electrons form a field which causes positive charges (also confusingly called holes, type II, holeII) to travel to exactly counteract the electrons. This is called an image current. This is because the current produces an image of the electrons in the metal.

But when the image current reaches the gap the holesII needs to quickly run around the holeI to catch up to the electrons which don't need to follow the metal. To do this they accelerate, then decelerate. Since accelerating charges emit photons due to special relativity, lots of microwaves jump into the tube.

The tube is shaped to guide the microwave through a waveguide into the oven.

The point is that we have an easier time manipulating charges than photons. There are some quantum mechanical reasons for this, but basically charges act as handles to move themselves. (They also don't "stack up" in one place very well, i.e. are fermions; photons are bosons and do "stack".)

So most of electronics uses charges (currents) to manipulate fields, but sometimes it is easier to use photons more directly like in waveguides.