Using equivalent circuit models for discontinuity in layouts

microstrip lyout

Hello

Why is it necessary to use equivalent circuit models for discontinuities occuring in microstrip layout?
I want an example of one such discontinuity and the circuit topology that is used to model it..

Thanks

Re: microstrip lyout

When a wave travelling down a line hits a discontinuity, some of that power is distributed to higher "evanescent" modes of propagation. The power typically can not stay propagating in these higher modes, so it recombines with the main power a little further down the line. If you modelled this effect (by looking far away from the discontinuity), it would look a lot like a lumped element reactance had occured at the discontinuity. "Far away" means a couple of wavelengths away.

Sometimes, but not always, the physical shape correlates to the lumped element discontinuity model. A change of width in microstrip, for example looks like a shunt capacitor on the wider side (excess capacitance from the fringing fields on the sharp corners) and a series inductor (as the currents get "squeezed" into the smaller width). Sometimes the model has weird things like negative capacitances, just because they curve fit the data best.


You need to use these models if you are operating at a high enough frequency for them to have an effect, or if you have a frequency-critical function (like a narrowband filter).

If you have a number of discontinuities that are "close" together, the lumped element models break down. You then have to do a full Emag field analysis of the structure.

Re: microstrip lyout

Hi,

there is a practical reason for introducing lumped element equivalent circuits for discontinuity analysis. Circuit or network theory was devoloped long before distributed circuit were in use. One tends to explain and analyse the novel things by bringing them back to the well known fields.

So, e.g. you join your real transmissinn lines (e.g. microstrip) some extra effect on such a composite is present apart from the junction of two ideal lines having primary characteristics same as microstrips. If you put some lumped elements of apropriate values to model the junction, you can describe this effect to some extent accuretely and then work again with ideal lines and lumped elements as there are a vast experience how to handle this situation.

Things are not that simple in practice, as described above element values come with limited accuracy and range of validity and are generally models of isolated discontinuities. In real world designs with lot of discontinuities inevitably present, small errors present in each model might lead alltogether to big errors in final circuit response.

flyhigh

microstrip lyout

which is similar to the water flow on the river,if face a discontinuity,it does not flow smooly.So as the microwave circuits.
For the microstrip,when face the discontinuity,the EM flow not only by Main mode(TEM mode),but also there i high mode,so we must eccount the high mode effect.

Re: microstrip lyout

basically it becomes very easy to understand the behaviour of the microstrip structure if its layout can be expressed in terms of lumped elements.

then by finding the transfer function of this structure its overall characteristics can be found.

for example a microstrip line having a gap in its upper conductor can be considered as a capacitance or a shorted microstrip line as a conductor in loose sense(with some restricions)

rgrds