poniedziałek, 2 lipca 2012

Microstrip Calc & Ansoft Designer

If you don't know how to use Microstrip Calc for your circuits, you should read it! :) I will be using Microstrip Calc to calculate impedances, wavelength and microstrips for one stage Wilkinson divider (working at 3Ghz, Z0 = 50 Ohm) and Ansoft Designer (PC) to plot transmission and insulation between ports. On start we have to know what PCB we will use. It determines permeability (Er), trace/dielectric thickness. Listed parameters impact on impedances and wavelengths.

Normal FR4 PCB has:
- Er around 4.7
- trace thickness : 34 um (1.34 mil)
- dielectric thickness : 1.55 mm (62 mil)

I chose special PCB (RT Duroid 5870), it has Er - 2.33, dielectric thickness - 0.787 and trace thickness equal to 17 um (0.017 mm) as you can see below (from datasheet).

 Now we are ready to use Microstrip Calc. First selsect Wilkinson divider and write in parameters:

Now we have to remember our impedances (Z1, Z0) and λ. In the next step we can calculate our microstrips from given impedances.

Fill in Er, B, T and Z0, click Synthesize and we will get W (width of microstrip). Do it again with Z0 = 71 Ohm. This is the moment to run Ansoft Designer (Getting Started). Ansoft will open new project. Do fallowing steps:

1. Add Cicruit Design: Click Project > Insert Circuit Design, in new window (Choose Layout) click none.

2. Add microstrip: Click Circuit > TRL > Microstrip > Single, at first time we have to add our PCB. It should look like in the picture:

In next window write in sizes of microstrip and click Analyze then Add. Now we have one microstrip. Copy it four times (3 ports - Z0: 50 Ohm, and 2 impedances Z1: 71 Ohm for Wilkinson). 

3. Add Resistor: Click Search tab in left panel. Search for RES, then insert it. 

4. Add Ports: Click Draw > Interface Port. Now we have to change port to microwave port. Right click on port, select Properties > Edit (Edit Port) and select Microwave.

Our circuit should look like in the picture below:

5. Add and run Analyze: Click Circuit > Add Solution Setup, in window click next. In right window click F (frequency), go to Edit and choose start/stop frequency and step (smaller step cause that plot will be more accurate). After go to Circuit > Analyze. Right click on Results and create two reports, first with S11, S22, S33 and second with S21, S31, S32. I got fallowing characteristics:

As you can see our 3Ghz frequency has offset around 300Mhz. We can correct it by extending length of our microstrips (smaller frequency = more physical length). After it I got:

Now we have great characteristics related with insulation (S11, S22, S33) and transmission (S21, S31). You can do it with other couplers/dividers from Microstrip Calc :)

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