Giftina replied the topic: RF trace simulation in ADS

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.

Giftina replied the topic: RF trace simulation in ADS

Thank you Madengr.
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?

Dave replied the topic: RF trace simulation in ADS

To add a point to Madengr's list

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.

madengr replied the topic: RF trace simulation in ADS

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.

avantec replied the topic: Characteristic Impedance of Signal conductor

Hi Desert Sage,
It's me again. I am trying to find out how far the fields extend laterally for the two configurations of probes I had those questions on 7 months ago. Someone from Berkeley Labs wanted to know exactly how far the fields would extend in different mediums, soil and water etc. I remembered in college that about three radius lengths outside of the probes would suffice to encompass about 99% of the fields, but now I am uncertain as I can't find the exact topic anywhere. Do you happen to know if there is closed form solution for this parameter and its dependence on the dielectric constant of the medium? Sincerely, Brian

madengr replied the topic: Characteristic Impedance of Signal conductor

Thanks:

ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1125843

Desert Sage replied the topic: Characteristic Impedance of Signal conductor

"Coupled Circular Cylindrical Rods Between Parallel Ground Planes", Edward Cristal, IEEE-MTT July 1964 pp 428-439. These curves were used mostly to design combline filters but they can be used to design couplers (three rod dual directional for example). This paper is the logical follow up to Getsinger's paper (Coupled Rectangular Bars Between Parallel Plates, Jan 1962, IRE-MTT-10, pp 65-72).

madengr replied the topic: Characteristic Impedance of Signal conductor

What are Cristal's curves? I'll have to look that up.

Desert Sage replied the topic: Characteristic Impedance of Signal conductor

This boils down to the assumption used in designing combline filters based upon Cristal's curves. The charge distribution on one half of the conductor is assumed (to first order) to be unaffected by what is happening on the other side. It seems to be a relatively good assumption since I have designed many, many combline filters and never ran into a counter-example.

madengr replied the topic: Characteristic Impedance of Signal conductor

If the two wires are wide apart to begin with (i.e. high Z0 and the mutual capacitance <~ self capacitance), then it does not quite cut in half when adding the third wire. If they are already close together (i.e the mutual capacitance >> self capacitance) then it does. Try it with a microstrip calculator, then switch to stripline, keep decreasing the substrate thickness, and it approaches Z0/2.

avantec replied the topic: Characteristic Impedance of Signal conductor

Thanks, for response. I used that formula for my transmission line configuration but was uncertain of how to handle the second ground configuration in my other probe. I am in the process of trying to measure it using TDR. Are you sure it is halved? Brian

Desert Sage replied the topic: Characteristic Impedance of Signal conductor

I found this that might be useful: http://www.fcarc.club/dokuwiki/lib/exe/fetch.php?media=estimating_characteristic_impedances.pdf

It shows for two wires, Z=276 x Log (D/r), where D is center to center spacing, r is the wire radius. For your configuration I calculate 214.7 ohms. When you add the second ground wire the impedance will be halved or 107.4 ohms.

elee1l5 replied to the topic: Coax dielectric loss

The dielectric loss heats up the dielectric, in accordance with the loss tangent of the dielectric.. It seems to me that how much of this heat get transferred to the inner conductor will depend on the thermal properties of the dielectric and how the system is being cooled. Resistive heating heats up the inner conductor directly. So I don't see how we can directly compare the two effects.

Hadrien F4INX replied to the topic: Using a hammer instead of pliers

The additional spacing changes with thickness, but the multiplying factor does not. What influences the multiplying ratio are the ratio w/h (microstrip line over substrate thickness): larger microstrip lines will need lower multiplying factors, and dielectric constant: substrate with lower dielectric constants will need more spacing.

I typically use 5 for this ratio, but I already had to use at least 10 in some simulations where I needed high accuracy on S11.

Pro tip: always look at the mode patterns. When there is field on the port boundary, it is a sign you need to increase your box. If you want to be really sure, start from 5 and increase the value until impedance does not change anymore. In all cases, be careful not to introduce higher order modes by using too high spacings. I remember a tricky simulation where I had to reduce a lot the spacing to remove this effect.

Fahmi replied to the topic: Using a hammer instead of pliers

You are right, the box and the port surface just have to be large enough (in absolute terms not in frequency dependent fashion) for sufficient decay of the fields, but then I believe care must be taken when working with really thin substrates, as in absolute terms , the multiplier of substrate thickness for the space needed above the substrate for the fields to decay appreciably, will be different, say if we consider a 25 mil alumina vs. 100 micron GaAs substrates.

Hadrien F4INX replied to the topic: Using a hammer instead of pliers

The field decays exponentially, but wavelength has not so much influence in that process. The frequency dependence here is really low.

In the original question from Steve, I think that the main cause was the box influence. Integration line ambiguity has a much lower effect on microstrip lines sufficiently small.

Fahmi replied to the topic: Using a hammer instead of pliers

What I meant was that increasing the box size will in essence allow a better capture of the field distribution outside of the substrate, the field decays exponentially and one wants to allow the fields to decay enough such that the outer box boundaries do not disturb the field too much, the more wavelengths in air the better, and yes the port boundaries in this case are PEC, so the port size will have to extend enough above the substrate for the fields to decay. I recall Sonnet recommended something like 10 times the substrate thickness.
The safest bet is to see the actual housing where the circuit will be housed in and use those dimensions, as this will mimic the actual operating conditions.
And for the original question from Steve regarding the seemingly erratic calculations, the use of integration lines should take care of that and the results should be consistent.

Hadrien F4INX replied to the topic: Using a hammer instead of pliers

Fahmi wrote: which makes the box small as Hadrian pointed out, given that in air the
wavelength is the same, so my previous statementabout the box height need to be revised.


When I spoke about box size, it is not compared to the wavelength but to the substrate thickness. The conclusion would be the same for lower frequencies.

Fahmi wrote: 3-I also did not use radiation boundaries, again due to custom, the structures I
worked with were always housed in cavities.


For people using radiation boundaries, be careful: most often the simulator will use electric (metallic) or magnetic boundaries for the port calculation, even if radiation boundary conditions are used. And radiation boundaries don't like to be too close to the structure.

Hadrien

Fahmi replied to the topic: Using a hammer instead of pliers

Steve,
I played a bit with the model, and gave it a bit more thought, the following factors may be at play here
1- the trace thickness, I used 10 um , changing that will affect the impedance
2- the air box I used was a bit small increasing its height will increase the impedance,
I worked a lot with 15 mil alumina so 4 or 5 times that is about 75 mils much more than 5 times the
100 um GaAs substrate, which makes the box small as Hadrian pointed out, given that in air the
wavelength is the same, so my previous statementabout the box height need to be revised.
3-I also did not use radiation boundaries, again due to custom, the structures I
worked with were always housed in cavities.
As for lumped ports, I did not use them before, coming from the waveguide world, I always used wave ports,.
what I can tell you is that the simulations using wave ports, gave very good results (both in waveguide and microstrp) compared to measured results for filters which are finicky to start with .
Thanks Hadrien for the hints.

Hadrien F4INX replied to the topic: Using a hammer instead of pliers

Steve,

The lower impedance you get compared to what it should be is a telltale sign your port box is both 1/ too small and 2/ electrically shielded.

It is because the simulation method puts a box around the port, which is connected to ground, which reduces the impedance.

About the other tools: Momentum is cool for standard microstrip circuits but is awfully complex to use when you want to do fully custom parametrized layouts, both Sonnet and Momentum are slow when your metal filling factor is high (e.g. ground planes) due to the simulation method. Don't know Axiem.

Hadrien