paszczakojad
Newbie level 6
![Newbie level 6](/styles/images/ranks/blevel3.gif)
Hi,
I designed a few parallel coupled line filters (at 2.1 GHz and 10 GHz) and after manufacturing they have very high insertion loss, much larger than in simulations. Simulation in HFSS gave 2 dB loss, while the real filter has 7-8 dB loss!
S11 is quite good - between 15 and 20 dB, so I don't think it's caused by connector soldering or other reflections. I thought it's because of radiation or substrate loss, but I'm using Arlon 25FR with loss tangent of 0.0035 and whole filter is shielded (top cover is 5 mm above the substrate).
I noticed that simulations using ADS Momentum gave S21 closer to what I measured (4.5-5dB) and I was unable to optimize it to perform better. But I thought HFSS would be more accurate.
I found an article (**broken link removed**) about increasing coupling between coupled lines by cutting out ground plane under the coupling sections, but that doesn't change much in the HFSS simulation (because the S21 is already good there).
I attached my HFSS project - could you take a look at it? The 'cutouts' of the enclosure are meant to reduce design complexity and prevent box resonances (the real-life enclosure looks more or less the same). There is also mesh seeding used but the result is the same with regular adaptive meshing (but the simulations takes much longer to finish).
I used 0.254mm thick Arlon25FR substrate (because in other projects 50 ohm line width was narrow enough to connect to MMICs without any tapers) - maybe this thickness contributes to the loss? (the ground plane is closer to microstrips than coupled microstrips). But in such case why HFSS didn't show that? And in case of other structures - like stepped-impedance filter or directional coupler - measurements match HFSS simulations.
P.
I designed a few parallel coupled line filters (at 2.1 GHz and 10 GHz) and after manufacturing they have very high insertion loss, much larger than in simulations. Simulation in HFSS gave 2 dB loss, while the real filter has 7-8 dB loss!
S11 is quite good - between 15 and 20 dB, so I don't think it's caused by connector soldering or other reflections. I thought it's because of radiation or substrate loss, but I'm using Arlon 25FR with loss tangent of 0.0035 and whole filter is shielded (top cover is 5 mm above the substrate).
I noticed that simulations using ADS Momentum gave S21 closer to what I measured (4.5-5dB) and I was unable to optimize it to perform better. But I thought HFSS would be more accurate.
I found an article (**broken link removed**) about increasing coupling between coupled lines by cutting out ground plane under the coupling sections, but that doesn't change much in the HFSS simulation (because the S21 is already good there).
I attached my HFSS project - could you take a look at it? The 'cutouts' of the enclosure are meant to reduce design complexity and prevent box resonances (the real-life enclosure looks more or less the same). There is also mesh seeding used but the result is the same with regular adaptive meshing (but the simulations takes much longer to finish).
I used 0.254mm thick Arlon25FR substrate (because in other projects 50 ohm line width was narrow enough to connect to MMICs without any tapers) - maybe this thickness contributes to the loss? (the ground plane is closer to microstrips than coupled microstrips). But in such case why HFSS didn't show that? And in case of other structures - like stepped-impedance filter or directional coupler - measurements match HFSS simulations.
P.