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July 20, 2018

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2 years 7 months ago #442 by Giftina
We are trying to simulate 42mm RF trace at 77GHz in ADS. The values that were obtained for return loss were +6.4 dB. But when the same trace was simulated for a length of 25mm, we are getting good return loss (-17 dB). We are using CPWG structure for RF traces. Is there any relation between the length of RF trace and return loss? Kindly let us know if there is any possible way to eliminate this. Thank you.

2 years 7 months ago - 2 years 7 months ago #443 by madengr
1) Is that return loss or |S11|? For a passive circuit, return loss is positive and |S11| is negative. If it is going positive to negative you have something fundamentally wrong with your simulation.

2) I assume this is EM simulation in Momentum? I don't use ADS, but maybe there are equivalents of Microwave Office's port impedance and passivity; check those. You have to be careful for port setup of CPWG. Typically the ground ports are "negative numbered" and of course you need two of them to evenly split the current. What do the currents look like? Do you have a proper mesh? Do the results converge as the mesh density is increased?

3) Finally, return loss does vary with electrical length. That's a fundamental property of a transmission line that is not exactly terminated with it's characteristic impedance. You can also have dispersion, but again I think you have something fundamentally wrong.
Last Edit: 2 years 7 months ago by madengr.

2 years 7 months ago #444 by Dave
Dave replied the topic: RF trace simulation in ADS

4) As well as port setup, be careful with via and/or ground strap spacing in the EM simulation; spacing relative to each other and distance from the first strap/vias to the ports can cause unexpected resonances in the result.

2 years 7 months ago #445 by Giftina
1.It is the S11 parameter.
2. It is the mometum simulation in ADS. We are getting proper S11 and S21 when the length of the RF trace is within 25mm. But if we increase the length beyound 25 mm, we are facing this issue. We are having two adjacent ground and one bottom ground and they are defined as negative in the port setup.Current distibution is as shown in the attachment. I have taken this snapshot from momentum visualization window. Mesh formed takes the colour of the ground conductor. Cells/Wavelength is 20 for this simulation.
3. These results are obtained from the layout simulation. Is it required to terminate it with 50 ohm in schematics?
Attachments:

2 years 7 months ago #446 by Giftina
Thank you Dave.
Is there any specification or formula to calculate the distance of the pin to the first via?
We have maintained a constant spacing between vias in this simulation.