lambda/4 line philosophy

[young.microwave.eng]

hi
would you please describe me about the lambda/4 line philosophy that is used to isolate DC circuit and RF parts in microwave circuit design.my main question is about the length of the line.why it should be lambda/4.what happened if length reach the value less or grater that specified traditional value?
thanks in advance

 

[amihomo]

The input impedance of a transmission line of length l terminated with load ZL is given by:
Zin=Z0(ZL+jZ0tan(Bl))/(Z0+jZLtan(Bl))

where, B=2*pi/λ.


the λ/4 line is known as the quarter-wave transformer and the input impedance of this line is given by (letting l=λ/4 in the above formula) :
Zin=Z0²/ZL

if you do short-circuit the end of the λ/4 line, its input impedance will be OPEN-CIRCUIT and vice versa.

Using this theoretical basis, we can use the λ/4 line to isolate the DC bias from RF section.

regards

 

[young.microwave.eng]

thanks a lot
your explanation was veryusful

 

[Junaid151]

Hi, I am a student and on a project... I am having a hard time understanding how lambda/4 transformers are used for the separation of DC biasing and AC signal. As lambda/4 transformers are used for impedance matching which basically just ensures maximum power transfer.
Any help would be welcome and could you also point me to a direction where I can do my research. Thanks in advance

 

[venu.gongal]

Hi, lambda/4 section is used to isolate the Ac signal from the flowing into DC power supplies. check the configuration of a bias tee..the AC signal(RF) should see a high impedance (current always takes the least resistance path) towards the DC connection. So open ckt lambda/4 line looks like a short ckt at the point where it is connected.

 

[Junaid151]

Thank you so much was really helpful. I am also given to understand that seperating DC bias and Ac signal is also done by a standing wave method at high frequencies can you please explain how is done or again guide me in the right direction so that I can do the reasearch... again your help is greatly appreciated.

 

[venu.gongal]

Check this weblink

**broken link removed**

It would be useful.

Thanks
venu

 

[Junaid151]

Thankyou so much for your help, I have grasped the concept now.
cheers!