[SI-LIST] Re: Question about termination of transmission line
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: "SI-list" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 21 Dec 2005 13:24:37 -0500
> When we consider the load termination, we only need to guarantee the
> resistor chosen equal to the impedance of the transmission line, just as the
> AC load termination which uses a series capacitor and resistor to eliminate
> the DC power consumption.
I got stuck already on that part. If you use RC termination to reduce DC
power consumption, you do need to make sure that the capacitor's reactance
is negligible, at those frequencies for which transmission line termination
is needed. Otherwise, the complex load impedance isn't a good match.
At low frequencies or slow rise times (compared to the electrical length of
the line), a match might not be needed. That's what makes RC termination
possible.
> Most of time, we use the step function signal to
> analyse the transmission line effect, that is the reflection phenomenon. But
> in my opinion, we also can use other kinds of signal, just as sin function,
> because we can use fourier series to disassemble the step signal to a series
> of variety of frequency sin signal, right?
Yes.
> If what I mention before is right, we can use a sin signal with
> frequency w to reconsider the AC load termination, under this situation the
> load impedance is R+jwc,
Actually, R+1/(jwC).
> which isn't equal to the impedance of transmission
> line, right?
True. For low frequencies and small C, the 1/(jwC) component becomes
significant and causes mismatch.
> Then we can assemble all the frequency sin signal together, which is
> the step signal. So I could get the consequence that the capacitor make
> the function in any of them , but lost the function when considered the
> integration. Why?
> who can point my mistake? I'm so confused.
The effect of the capacitor is there (or should be there) in both cases.
The capacitor causes a frequency-dependent (or time- and signal-dependent)
mismatch even in the step function case.
Both analyses (time-domain or Fourier) must lead to the same results.
Regards,
Andy
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