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Subject: Re: Replacing two microstrip stubs with one
Date: Tue, 26 Nov 2002 01:39:08 -0800
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References: <3DD4F2ED.DD32B937@niobiumfive.co.uk> <3DE22B0F.77DD6FC4@niobiumfive.co.uk> <3DE2E5EB.D5CDE49@sprintmail.com>
> The Technical Manager wrote:
> > Lizard Blizzard wrote:
> > > The Technical Manager wrote:
> > > > I have a microstrip transmission line that has two short circuited stubs
> > > > connected to it at the same point which radiate off in opposite
> > > > directions to each other so that the interconnection point is a cross
> > > > junction. Each stub has a different characteristic impedance and
> > > > electrical length. I want to replace these two stubs with a single short
> > > > circuited stub. How do I find the correct characteristic impedance and
> > > > electrical length of the replacement stub ? Does a formula exist to
> > > > calculate this ?
> > >
> > > There are formulas, of course, found in Reference Data for Radio
> > > Engineers, for example.
> > Where ? Which page ?
> > > But if you don't understand the reason why those were put there in the
> > > first place, it would be very unwise to meddle with them.
> > It is a concept, not some part of an an existing product.
> I think you ahve to sue a Smith Chart to find the Impedances.
> Are the Impedances... Resistive, Inductive, Capacitive ? Yes to all.
> The Smith Chart will really help you out in finding Z = R +/- jX.
> What will the signal do going into and reflecting back from the cross point?
> There alot to be done.
> A ----+---- B
> The signal goes into A and comes back out of A. The signal goes out of B, and
> reflects back into B. same goes for C and D. so you have 8 Reactance equations
> to look at. You have to look at S parameters vs. frequencey of your circuit.
> The stripline you design in will be 50 ohms? Most stripline will be 50 ohms.
> If the stripline is thinner, then maybe it'll be 120 ohm? There's a ton of
> engineering going on here. When you buy SMA and SMB connectors, semi-ridgid
> coax cable, you'll be buying 50 ohm hardware.
> But the way who sells 50 ohm SMB terminators? Pasternack ?
There is no free lunch.
Each stub provides some frequency dependent impedance at the junction.
I'm gonna go way out on a limb here and say that you can't get the same number
of poles and zeros if you remove some of the poles and zeros. You can sometimes
use a single
MORE COMPLEX stub, but you won't save anything...assuming you still need all those
poles and zeros.
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Tek 2465, TFT projector, ham radio, 30pS pulser
Tektronix Concept Books...
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