6bit Attenuator Combining Problem

[moin26]

Hi,
I am designing a 6 bit attenuator and facing a problem when I combined them together.Each bit individual work properly it achieved proper attenuation proper S11 and S22 but unfortunately when combined them together cant get required attenuation and S11 S22. Plz help me in this regard what will be the possible causes.

Thanks in advance

 

[FvM]

Perfect attenuators with S11=S22=0 (e.g. pi-networks) can be infinitely cascaded without affecting input/output matching.

What's your attenuator topology? If you have multiple attenuators close together, e.g. in a chip, you might experience crosstalk and ground coupling.

 

[BigBoss]

Hi,
I am designing a 6 bit attenuator and facing a problem when I combined them together.Each bit individual work properly it achieved proper attenuation proper S11 and S22 but unfortunately when combined them together cant get required attenuation and S11 S22. Plz help me in this regard what will be the possible causes.

Thanks in advance

Have you combined them in parallel at their outputs ?? ( I guess so ) if you have done so, you did a mistake.You should use a proper combiner.

 

[moin26]

Thanks FVM and BigBoss for your reply.

I am designing chip and using T and pi topology and yes you right i have multiple attenuator in this combination in series (Pi+ T+T+T+T+Pi).Each attenuator individual work well each attenuator matching is good but when combine it becomes mess.

Thanks in advance.
Waiting for your kind reply.

- - - Updated - - -

Thanks FVM for your reply.

I am designing chip and using T and pi topology and yes you right i have multiple attenuator in this combination in series (Pi+ T+T+T+T+Pi).Each attenuator individual work well each attenuator matching is good but when combine it becomes mess.

Thanks in advance.
Waiting for your kind reply.

 

[BigBoss]

Thanks FVM and BigBoss for your reply.

I am designing chip and using T and pi topology and yes you right i have multiple attenuator in this combination in series (Pi+ T+T+T+T+Pi).Each attenuator individual work well each attenuator matching is good but when combine it becomes mess.

The independent attenuators work well but how much well ?? What are their independent return losses ?? The return loss for a simple attenuator should have very good figure, let say -15 dB or better while the second port is terminated by characteristic impedance.Otherwise the accumulated return loss may become worse when they are in cascade.

 

[moin26]

Thanks BigBoss,
I attached result figure of S11 and S22 in this post you can look on it.Return loss of independent attenuator is good.I attached image file if you not get kindly let me know.

Thanks in advance.