Re: soft source question - TDR simulation

[BYERS ANDREW CLARK <byersa@ucsub.Colorado.EDU>]

Good idea, but I cannot specify a hard source with a resistance. I might
be able to insert a spice resistor as a port in series with a hard source,
though. I will try it and let the list know ...

Thanks though Ian!

Andy

On Mon, 11 Oct 1999, RUMSEY  IAN SCOTT wrote:

> andy-
> 	Try a hard resistive source.  Part of the magic of the soft source
> is that it's always matched to any kind of line.  That effect may be more
> significant than the series resistor, and I think that a hard source may
> be more realistic for a voltage-source-in-series-with-resitor type of
> driver.
> 
> 
> -Ian Rumsey
> 
> On Mon, 11 Oct 1999, BYERS  ANDREW CLARK wrote:
> 
> > Hello all,
> > 	I have noticed a funny effect when I am using a resistive soft
> > source on a microstrip line in lc. What I want is a TDR-type of a source,
> > which is a driver with a series resistor that is approximately matched to
> > the impedance of the line. The initial voltage launched on the line is
> > then one half of the voltage source. This initial voltage is simply a
> > voltage divider between the line impedance and the source voltage. 
> > 	For example, if you had a 1V source waveform, a 50 ohm source
> > resistor (Rs) and a 50 ohm line (Zo), the initial voltage is 
> > 	Vinitial = (1V)*(50/100) = 0.5 V
> > This works fine in lc, for when I match the resistive source to the
> > (estimated) line impedance, the initial voltage is about 0.5V. 
> > 
> > 	Another example is when the line is overdriven, or the source
> > resistor is lower than the line impedance. If Rs=10, Z0=50, V=1V,
> > 		Vinitial = (1V)*(50/60) = 0.833 V
> > The same should be true for underdriven lines (Rs > Zo), where the initial
> > voltage is less that the desired 0.5V.
> > 
> > 	Here is the problem: when I vary the resistance of the soft source
> > in lc, I do not see this variation in the initial voltage launched onto my
> > transmission line. I see some weak variation, but it appears that the
> > resistor is not working like I had suspected. I have not delved into the
> > theory too hard yet, but can anyone tell me if they have got this working
> > and how? Or why mine is not working? I am source the end of a microstrip
> > line that is open (ie not terminated into the PML). The source is 4-5
> > cells from the boundary. The source is the width of the line, etc, etc. 
> > 
> > 	Thanks in advance, Andy Byers
> > 
>