ADS Layout Tutorial
Note: While it is possible to create a PCB layout in ADS, other programs are available that make the layout process much easier. Eagle, for instance, provides many tools and libraries that are either more complicated to u or simply unavailable in ADS. The
current PCB layout for the mini MRI transmitter and receiver were designed in ADS and thus would likely have to be edited using ADS. It is for that reason that this tutorial was written.
Creating a new layout project
The first step in creating an ADS PCB layout is to open ADS (Advanced Design System). At BYU this program is available on the fifth floor of the Clyde in the CAEDM Lab, or in the Biomedical Imaging Lab.
1) Select File New Project , choo a name and a directory for the new project.
Select one of the ADS Standards under the Project Technology Files option.
2) ADS will then bring up a schematic window. This window allows urs to create
and simulate various types of schematics using built in library components. A layout can be generated directly from this schematic window by lecting几天不吃饭会晕倒
Layout Generate/Update Layout . This option is valuable for microstrip and other designs, but for component bad PCB layouts it is easier to clo this
window and go straight to the layout window.
3) Select File New Design , give the file a name, and under the Create New
Design in option choo New Layout Window .
Creating the footprints
The next step in PCB layout design is to create the footprints for each component that will be ud in the design. This implies that the designer knows which components will be ud before the design is begun. Most companies will provide spec sheets for their products. The spec sheets often contain suggested PCB layout footprints. For example, figure 1 shows the suggested footprint and dimensions for the Minicircuits JTOS-50 Voltage Controlled Oscillator. If part dimensions are not available, the
component dimensions will need to be measured. To allow for error, the footprints on the PCB are usually designed to be a bit larger than the actual measurements. Note: Preci measurements are very important for creating a good layout.
Figure 1: Suggested footprint for the Minicircuits JTOS-50 VCO Top View Bottom View
taken from:
/cgi-bin/spec?cat=vco&model=JTOS-50&pix=bk276.gif&bv=4)
For this specific component (figure 1) it is important to notice that the top-left picture is a top view of the VCO while the bottom-left picture is a bottom view of the same VCO. Notice particularly that pin 1 is the lower-left pin in the top view but is the upper-left pin in the bottom view. When creating the layout it is important to u the top view for the pin definitions. If the bottom view is ud, the pins will be mirrored and will not be connected to the correct pads.
1)Once the suggested footprints (or preci measurements) are obtained, the
footprints can be created. The first step is to choo what layer(s) to u. Most basic PCB layouts will consist of a top plane (where the components will be
soldered) and a bottom plane (which is usually a ground plane). A layer can be lected by using the pull-down menu on the upper right of the ADS layout screen as shown in figure 2.
Figure 2: ADS Layout screen displaying layer pull-down menu.
The easiest layers to u are cond for the top plane, hole for any holes and vias, cond2 for the bottom plane, and silk_screen for any text the designer wants to appear on the finished board. However, for the current mini-MRI layout the pc1 layer was ud for the top plane, the pc2 layer was ud for the bottom plane, and the pcvia1 layer was ud for the vias.
2)The footprint for the component in figure 1 is straightforward. As can be en
from the figure, the footprint consists of 14 pads (where the component’s pins will be soldered) of equal dimension and spacing. For the VCO shown in figure 1 the pad dimensions are D x K or 100mils x 65mils (e figure 1). The pads can
easily be created using the Inrt Rectangle command. As the rectangle is
placed, the dimensions will be shown as en in figure 3. By default, ADS will u increments of 5 mils (if mils are being ud) for placing and moving elements.
The dimensions can be made finer by lecting Options Grid Spacing and then lecting the desired dimension resolution. The first of the three numbers corresponds to the increment size (i.e. f
or <5-1-100> the increment size will be 5mils). For footprints that don’t u rectangular pads, the circle or polygon tools provided by ADS can be ud. If the components are leaded instead of surface mount, the pads will have to involve three layers. For example, if the layers
闹够了没有
suggested in step 1 were ud for a leaded component, the hole, cond,and cond2 layers would be required. The hole dimensions would need to be just slightly
larger than the component’s lead dimensions so that the lead can fit easily through
the hole. The cond and cond2 dimensions should be at least 20% larger than the hole dimensions on all sides. This is important so that the metal surface area
around the hole is sufficient to solder the lead to the PC board.
高中地理必修一Figure 3: ADS Layout screen displaying rectangle placement Note: It is important to know that the footprint pads are usually made larger then the actual component pins to ensure enough metal area for soldering. Most
虎妞人物形象分析
companies will u this principle when suggesting footprint dimensions, but for measured dimensions the designer will need to do this.
3)One advantage of ADS is its ability to copy, paste, rotate, and so forth. This
makes the creation of footprints much easier. For the footprint shown in figure 1, the next step would be to copy the rectangle 14 times (once for each of the pads).
When copying a part, ADS will prompt for the paste coordinates. Rather than inputting coordinates it is easier to lect the Default option. The G dimension shown in figure 1 gives the pitch which is important for pad spacing. The finished footprint for the VCO is shown in figure 4. As can be en in the figure,
construction lines can be ud to define spacing and other dimensions. A
construction line can be placed by lecting Inrt Construction Line.
Construction Lines
Figure 4: Finished VCO footprint with construction lines
4)Once the footprint is created, the design should be saved. This is done by
lecting File Save Design As and by entering the desired name. All designs will be saved in the Networks directory of the layout project.
Interconnecting the components
When all of the footprints are generated, the next step is to connect the components together.
1)The first step in component interconnection is to connect each component to its
necessary circuitry. For example, many components require capacitors in shunt
with their power supply inputs. In addition, all components will require
connections to ground. A new layout design should be opened and the component footprint should be inrted by lecting Inrt Component Component
Library,choosing the current project under the Libraries window, and choosing
the correct footprint (the footprint will have the same name you gave it when you saved it). Any construction lines ud for the footprint creation will not appear.
The trace tool should be ud to create connections between components. The
traces should overlap onto the components a bit to ensure that there are no gaps.
The minimum trace width will be dependent on the frequency at which the circuit will be ud. A minimum trace width of 20 – 25 mils is fairly safe. The method
for connecting the component to ground will depend on the PC board fabrication
process. If the bottom plane will be a ground plane (strongly advid), the
破绽百出component can be connected to ground by placing vias (holes) in the correct
places. When creating ground connections, only a top (i.e. cond) and via (i.e.
hole) layer will be needed. If the bottom layer (i.e. cond2) is ud, the vias will be parated from the
ground plane which will, in effect, isolate the ground vias from the ground plane. This is a bad thing.
Most spec sheets will suggest biasing and other circuitry for the component. For
the VCO the following information was provided for connecting the pins (e
figure 5a). Bad on the given pin connections, the various ground pads were小学生法制教育
connected together and traces were nt out from the pins that can be connected to other components (e figure 5b.)
a)
GND vias
Pin 1
b)
Figure 5: VCO circuitry layout (shown in 5b) bad on the required pin connections (shown in 5a)
Note: For a fabrication process that can create drill plated holes, it would be better to place a ground via on every pad that needs to be connected to ground.
This would eliminate the need to interconnect all of the ground pads.
Once all of the appropriate ground, capacitor, and other connections are made any desired text should be added using the silk screen layer. A minimum font size of
30 should generally be ud for this text. Be sure not to silk screen on top of the
footprint layer. Finally, the design should be saved (under a different name than the footprint).
2)At this point the footprint for each component should be created, connections to
党员的要求ground should be included, and the footprints should be connected to all circuitry suggested by the manufacturer (i.e. bias resistors, shunt capacitors, input and
output capacitors, etc.). The next step is to connect the various components
together. A new layout design should be opened and each component required in the design should be inrted by again lecting Inrt Component
Component Library.The components should then be connected according to the circuit design. This is a straightforward process and is done using the trace tool.
Note: Allowing two or more traces to run for long distances in parallel is poor design technique. At certain frequencies parallel traces will have coupling effects that can effect the operation of the circuit. It is also good to avoid sharp corners when possible. To do this the curve option under the Corner Type lection in the trace window can be chon. A Curve Radius of at least 20 is suggested.
3)The final step in component interconnection is to connect the circuit to power,
input/output, and ground. For the frequencies at which the mini-MRI circuit will be operating (0 – 42.5MHz) BNC connectors are suggested for input and output.
The standard CON5 design is easy to u (e figure 6a). For power and ground connections banana connectors (e figure 6b) are effective. If the bottom plane is
a ground plane it will only be necessary to have one banana connection to that
plane. Again make certain to u only the top and via layers when creating this footprint. The power
source(s) will need to be connected to all appropriate
components. The final MRI transmitter/receiver layout is shown in figure 7.
a) b) Figure 6: CON5 BNC and Banana connector footprints
>青城派
