Newbie in RF with a crazy project, a few questions - RF Cafe Forums
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Post subject: Newbie in RF with a crazy project, a few questions
Unread postPosted: Mon Sep 13, 2004 3:42 pm
Joined: Mon Sep 13, 2004 2:37 pm
I am working in my thesis project which at the
moment consists in a Ultra Wide Band (UWB) Impulse radio Tx.
idea is implement and test a simple UWB Impulse transmitter using available
RF components. So far, a first system design simulation with ADS 2003C
using SRD diodes to generate very short impulses has been carried out
succesfully. Now I have to move to the circuit level and PCB design.
I would want to know suggestions to start the Layout. I thougth to use
the ADS Layout tool to make the PCB, but my first impression is that
it will be a hell to place SMD components such as voltage regulators,
amplifiers, attenuators, filters, etc... I dont know if there are libraries
for the schematics parts and footprints for the layout.
was thinking to use Orcad Schematics/Layout or other similar tools,
and once the PCB layout is ready, export it to the ADS and simulate
Do you know if it is possible to do this codesign using
a Layout tool to generate the PCB tracks and simulate back in ADS?
Do you think that this procedure is correct? If not, what tools
do you suggest I should use?
Thanks in advance,
Unread postPosted: Tue Sep 14, 2004 5:21 am
You should separate between the RF layout to the layout of other
parts of your circuits (for example as you mentioned voltage regulators
The right way from my experience to do this task is to
simulate the layout of critical RF sections in ADS or other RF CAD tool,
and to be accurate as much as possible in the definition of the layout
parameters i.e: Substrate type, er, Tand, thickness etc. This helps
a lot to get accurate results. You can export the layout from your PCB
tool to ADS (using DXF format or similar) and then simulate the layout
structure and see the effect of your layout on the circuit performance.
By this trial and error you can reach to the optimal layout of your
circuit. Once you have reached to the optimal layout you can transfer
it back to the PCB tool and this would be your final layout on the board.
The impact of the layout on the circuit goes hand in hand with the
increase of the frequency, this is especially right for getting the
optimal miter, diamaeter of vias etc.
The first step to start
with is of course to calculate the track width for obtaining the desired
Zo. You can do it with LineCalc, which is an excellent tool for many
RF structures (microstrip, stripline etc) and you can synthesize and
simulate physical structures and see the electrical outcome and vice
All after all RF PCB layout requires some epxerience and
CAD tools can be a good way to predict various phenomenas but you should
jump to the water and gain this experience this is the best way to learn.
Good luck, and keep us posted.
Unread postPosted: Tue Sep 14, 2004
Thank you very much for your help Itay.
At this moment
I am learning how to export Orcad Layout Plus designs to DXF format,
so I can import them in ADS afterwards.
I am sure I will have
a lot of questions soon. Thanks for sharing your experience.
postPosted: Tue Sep 14, 2004 10:44 am
You have two circuits
here, the RF and the DC. You will hve to design chokes between them
to keep them separate, that can be tricky for UWB but any approach that
considers interactions will be immensly more complicated so forget about
What are you planning to do with ADS, obviously you have
done a nonlinear network simulation? ADS can create a Layout from this
network with some effort but so can OrCad. It would be difficult to
layout the DC part of the circuit with ADS. I think ADS is the qurikiest
layout editor of all the major simulators, I also think OrCad is the
best of the PCB tool I have used.
In practice, I send the DXF
to our PCB layout group. The generate the schematics and goto layout.
I just approve the final design.
Post subject: new questions
Thu Sep 30, 2004 6:10 am
Sep 13, 2004 2:37 pm
I am here again to kindly ask for your help. First, thank for the previous
feedback. I succesfully exported a DXF test file and imported it in
Non-linear simulations with ADS were succesful, so I spent
the last weeks working on schematics in Orcad Capture. In addition,
I have worked on the footprints for the layout.
My design uses 2
Minicitcuits RF amplifiers, 3 RF switches, a 100MHz oscilator, a few
PI attenuator built using resistors, a few PECL drivers, and connectors,
etc. I designed the power supply for this circuits with 3 Voltage regulators
Now I am working on the layout, and I have the following
The UWB pulses have a frequency spectrum from more
or less 3 to 6GHz.
1. What kind of substrate should I use?
2. How many layers do you suggest?
3. how can I distribute the power?
4. Which SMD resistors do you suggest for this application? Where can
I find the models for them in order to simulate at least the critical
5) I am using SMA connectors for the whole design, but I dont
know if it is correct for the output, is it?
Unread postPosted: Thu Sep 30, 2004 8:27 am
I am happy to hear that your project is going well.
To answer your questions:
1. What kind of substrate should I
Well this is all depends on your frequency range, which
as you stated is between 3-6GHz, for this I highly recommend the RO4350
(ROGERS) family of substrates which have repetitive and accurate Er
of 3.48 and low losses (low Tand). You should define the paramaters
of this substrate in the simulations of transmission lines and critical
2. How many layers do you suggest?
Since you stated
that your design consists of 3 kind of circuits: Power, logic and RF,
I would suggest using 6 layers in the following order:
layer with all the RF componenets and related power supplies)
Voltage supplies layer
Logic signals layer
GND layer (in this layer place the PECL drivers and related power supply)
Make sure that your GND layers are complete, use 1 Oz copper clading
for each layer. the RF traces in the top layer should be free of solder
mask at this frequency range to minimize insertion loss. Put plated
via holes near each pad of componenet that is connected to GND, stitch
the edge of the board with as many via holes as you can to form a good
GND connection and to prevent bouncing of the RF signal. the distance
between the via holes should be less than 1/10 lambda.
recommend using shielded cans for isolation between the blocks and preventing
radiations and spurious signals from one block to the other. The diamater
of the via holes should be 10 mil.
3. how can I distribute the
Use wide traces in the power supply layer plane to ditribute
the supplies to the components. then you will connect each supply as
close as possible to the device through a plated via hole. The diamater
of the via holes for power connection should be between 12-16 mil.
4. Which SMD resistors do you suggest for this application? Where
can I find the models for them in order to simulate at least the critical
It all depends on the power levels your application is
using. I assume that 0805 would be enough. 0805 are built for 1/8W,
0603 are built for 1/16W. calaculate the power levels and decide.
No need for simulating the resistors as their parasistics start
to effect above 10GHz.
5. 5) I am using SMA connectors for the
whole design, but I dont know if it is correct for the output, is it?
It is the best choice. However there are many vendors for SMA connectors.
Y ou should do some checks and find the best for your application. Some
of the popular companies are M/A-COM, Huber-Suhner etc.
this helps. Please keep in touch I will be keen to help you with your
Post subject: Thanks Itay!!
Unread postPosted: Fri Oct 01, 2004
Joined: Mon Sep 13, 2004 2:37
I am happy
to receive your answer. Thank you very much for your suggestions and
ideas, they are very valuable. I am going to work on the layout these
days and will post any news.