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Newbie in RF with a crazy project, a few questions - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


Saul
Post subject: Newbie in RF with a crazy project, a few questions
Unread postPosted: Mon Sep 13, 2004 3:42 pm
Offline
Lieutenant

Joined: Mon Sep 13, 2004 2:37 pm
Posts: 3
Location: Sweden
Hi friends,

I am working in my thesis project which at the moment consists in a Ultra Wide Band (UWB) Impulse radio Tx.
The 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.
Instead I 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 again.

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,

Friendly regards

Saul


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Profile

Itay
Post subject:
Unread postPosted: Tue Sep 14, 2004 5:21 am

Greetings,

You should separate between the RF layout to the layout of other parts of your circuits (for example as you mentioned voltage regulators etc).

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 versa.

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.

Itay


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Guest
Post subject:
Unread postPosted: Tue Sep 14, 2004 7:47 am

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.

Saul


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Guesstimate
Post subject:
Unread 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 that.

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.

Good luck.


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Saul
Post subject: new questions
Unread postPosted: Thu Sep 30, 2004 6:10 am
Offline
Lieutenant

Joined: Mon Sep 13, 2004 2:37 pm
Posts: 3
Location: Sweden
Ok friends, 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 ADS.

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 (+7, +5,-5).

Now I am working on the layout, and I have the following questions:

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 paths?
5) I am using SMA connectors for the whole design, but I dont know if it is correct for the output, is it?

Thanks!!
Friendly regards,

Saul


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Profile

Itay
Post subject:
Unread postPosted: Thu Sep 30, 2004 8:27 am

Greetings Saul,


I am happy to hear that your project is going well. To answer your questions:

1. What kind of substrate should I use?

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 paths.

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:

Top (RF layer with all the RF componenets and related power supplies)
GND layer
Voltage supplies layer
GND 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.

I would 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 power?

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 paths?

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.

I hope this helps. Please keep in touch I will be keen to help you with your project.

Good luck,
Itay


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Saul
Post subject: Thanks Itay!!
Unread postPosted: Fri Oct 01, 2004 5:51 am
Offline
Lieutenant

Joined: Mon Sep 13, 2004 2:37 pm
Posts: 3
Location: Sweden
Hello Itay,

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.

Friendly regards,

Saul



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