Re: Means of determining microstrip impedance
- From: Mac <foo@xxxxxxx>
- Date: Wed, 13 Apr 2005 03:21:45 GMT
On Tue, 12 Apr 2005 15:30:20 -0700, Joel Kolstad wrote:
> I have some microstrip traces that were designed to be 50 ohms, but I'd like
> to find out what they really are. I have a network analyzer good to 6GHz at
> my disposal. Does anyone have a favorite means of doing this? This was my
> thought:
>
> -- Terminate the trace in 50 ohm loads
> -- Have the network analyzer plot S11 up to whatever frequency corresponds
> to at least half a wavelength; this will ideally display a circle that has
> one edge at the center of the Smith chart (Zin=50 ohms) and the other edge
> either greater than Z=50 if Z0 of the microstrip is >50 ohms or less than
> Z=50 if Z0 of the microstrip is <50 ohms.
> -- With some straightforward calculations, one can show that Z0 of the
> microstrip is the geometric mean of these left and right 'edges' of the
> circle. E.g., if the circle intersects Z=32 ohms and Z=50 ohms, Z0 is 30
> ohms.
>
> My problem is that my microstrips start behaving 'badly' well before the
> trace is half a wavelength long (this occurs around 2GHz on my test board),
> so it's difficult to decide what the edge of the 'circle' (which is really
> more of an arbitrary curve) should be. I am planning on making some longer
> test microstrips, but I'm curious if the 'practical' people out there have a
> well-known method for performing this measurement.
>
> ---Joel Kolstad
>
> (...who hasn't quite gotten around to reading Rober Witte's book yet...)
If the trace is long enough, and you have a scope fast enough, and if you
can put a fast edge on it, you can measure Zo using the voltage divider
approach.
The idea is that you would put a fast-edge in series with, say, a 47 Ohm
resistor, then measure the ground referenced voltages (using an
oscilloscope with active probes) on both sides of the resistor.
You want to use the first plateau voltage, and ignore the step caused by
the reflection from the end of the line.
For some period of time after the edge starts to rise, what you have is a
Voltage divider formed by Rseries and Zo. So the rise time of the edge
must be significantly shorter than the two-way flight time down the trace
and back for this to work.
Note, I've never actually done this with a circuit board, although I have
done it with coax, just for fun.
--Mac
.
- References:
- Means of determining microstrip impedance
- From: Joel Kolstad
- Means of determining microstrip impedance
- Prev by Date: Re: Prefered resistor range
- Next by Date: Re: Tire Pressure Sensor
- Previous by thread: Re: Means of determining microstrip impedance
- Next by thread: Re: Means of determining microstrip impedance
- Index(es):
Relevant Pages
|
