[SOLVED] Help with mismatched circuit

Status
Not open for further replies.
L

LandLack

Member level 3
Joined
Aug 8, 2013
Messages
54
Helped
1
Reputation
2
Reaction score
1
Trophy points
8
Activity points
526
Hi, I've simulated a mismatched circuit: trace characteristic impedance=67.5 ohm, while all the devices have 50 ohms; and there are in total 6 devices in a row. The circuit has signals up to 1.4GHz.
The results were the same as with the circuit with traces' characteristic impedance of 50 ohm. How is this possible? I thought that I wold have some some losses due to the reflecting coefficient: R=(67-50)/(67+50), where R is the reflected voltage (and this is to be repeated by all 6 of the devices).
How is this possible? Am I forgetting something?
Please help understand this thing...

Best Regards,
LandLack
 

Matching at both line ends and electrical length has to be considered.What's the actual circuit?
 

going from 50 ohm to 75 ohm system will give a VSWR=1.5:1 (acceptable) , and about 4% of power will be lost due to this mismatch of (25 ohm) ..
u can do all the calculation using basic formulas...
the point is it is so low 0.04 (4%) part of the incident power that it becomes un measurable... in your case the difference is more low 17.5 ohm...
so i suggest for understanding the mismatch try increase the impedance of the trace so the result becomes more obvious...
if it didn't work too than u must be missing something else that needs to be bugged out!!!
Cheers
 
Maybe the line lengths between the components are quite short? In that case the line impedance mismatch will not have a major impact.
 

More generally, connection of a 50 ohm source and load by a line of different impedance won't be described by a single real reflection coefficient as considered in post #1 and #3. Instead you get a complex reflection coefficient as function of the line length. It will be zero for length of n*λ/2 and maximal for λ/4, 3*λ/4, ...
 
Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…