ADS， LPF filter question

Hello friends,

I need a LPF around 850 MHz corner and I found this is quite large as parallel coupled lines so I modified to a 9'th Chebyshev lumped version. Inductors are in 10nH range and caps in 3...10pF and I decided to made inductors on PCB and keep caps ceramics 0805.

I choose for inductors W=10 mils (for Z>100) built from MLIN, MTEE and MSABND and I tune them along with caps for best insertion and return loss. For capacitors I choosed CQ_PAD model from ADS with estimated Q=2000 @ 1 MHz.

LPF looks nice at simulation. Few questions from your ADS experience:

1. When I complete ADS momentum on the same project, results are aberrant. Perhaps momentum ignore my physically existing caps. Any ideas?

2. I haven't found how to set a printed inductor with given (calculated) inductance. I also haven't found a printed inductor without vias :) I need both legs on the same side and no jumpers.

3. Project will be on ordinary 1.6mm FR4. From your experience, er=4.35 will be close enough? Also I choose after long google study :) tan loss=0.02.

Thanks in advance,

ADS Momentum doesn't recognize pysical components which are placed from library.Momentum simulates only pysical layout,vias etc..
So, layout should be transferred onto schematic view to do a co-simulation.In this case ,you should put a port element to connect for each capacitor.Then you can connect your capacitors in schematic view and s-parameters of your layout will already be there..

Layout inductors can be considered short circuited transmission lines and you can find input impedance of a short circuited transmission line in the literature.After that, it's easy to find inductance of a short circuited transmission line.

If you increase your er, you will obtain smaller dimensions for layout components..But it's more expensive...

BigBoss, tnx for fast reply,

I've already done this, looks perfect to me (check attached imgs) but I wonder how accurate will be in practice.

Momentum give me some warning that I should consider using coupled lines in design because "some ports are parallel". This applies to my case (W=10 mils and drawing is at scale)?

I wish, but I make my own boards :) I haven't found (yet) a good supplier of quality photo laminates (Arlon, Rogers, etc). I use 1.6 FR4 because this is the only one I can buy. Even 0.8 thick photo coated is impossible to buy here.

Tnx,[img][/img]

You have connected the capacitors directly to the ground but in the reality, the capacitors will be connected via by thru holes that will have some attenuations at that frequency.Take care of this...
And also, connector connections should be carefully considered due to discontinuities between the circuit and connectors.
Some fine tunings will be needed ( shifting the capacitors left and right) because no component is ideal.And us only RFMW type capacitors from Murata or AVX or ATC to obtain the accuray because using standard chip capacitors will have low Q factors and poor temperature coefficients.

Momentum warns you about couplings between the ports that should be avoided to make even mode transmission.It can be neglected if the distance is enoughly high compare to 1/8 wavelength.

FR4 can be used but don't use solder mask to cover the TLs, instead if it's possible, TL should be plated with chemical Au...
If you intend to make your own board, i suggest you use DRY FILM technique with a photolotter to obtain more accuracy but the materials are quite expensive...But accurate.

Well, tnx for remembering simulating cap vias, I completely miss that part :)

No solder mask, I cannot do this at home nor true "vias". Imagine Au plating is almost impossible, I'll be happy at least to chemical silver but hope to find those substances in EU.

I don't know exactly how DRY FILM method works but I use UV factory impressed laminates and ordinarily laser printer (Lexmark E120). Print is done from OrCad gerber tool on semi-transparent paper (I don't know how to translate in english but I can post paper code later), then put some black-covering spray on that paper (also spray is from the same company) and expose to UV light. Probably you know the method...

Cheers,

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