Design considerations.... 1

[rfsuperman]

I am just about to buy the parts necessary to build an 11 channel signal strentgth indicator for Wireless LAN (802.11b).

The basics of the circuit are the following:

Antenna
-to possibly an LNA(low noise amp)
-to a Mixer (with a 700-800 MHz frequency synthesizer... controlling the Local oscillator) (this changes the channels and also keeps the IF constant at about 1.7 GHz)
-to a Power Detector (MAX 2015, from maxim-ic.com)...which bascially takes in the AC signal and outputs a DC voltage.
-this will then be read and processed and outputted to an LCD via a microcontroller.

The difficult part, I foresee, will lie in obtaining the signal and processing it with as little noise as possible. I obviously need a Printed Circuit Board for these high frequencies, but the companies that make them require a final,exact design for all the components which doesn't make it easy to test/change components if necessary. So is there some sort of PRACTICE PCB board?

Also, what type of wire, cable can I attach to the antenna and not lose the 2.4GHz signal? I.E...what wire has bandwidth to hold that high of frequency.

Any help AT ALL related to this would be helpful. I am so interested in RF/WLAN but just beginning in design.

Thanks
rfsuperman

 

[BrianR]

What do you mean by practice PCBs. At these frequencies you will use striplines for impedance matching and they need to be constructed to close tolerances. If you hope to be able to remove components, modify the board and replace them for another test, I think you'll be disappointed with their life expectancy.

Do your PCB design and order a prototype PCB from the manufacturer but you'll probably need new components for each board.

 

[rfsuperman]

Thanks for responding. By "practice PCB", I meant something like a protoboard minus the destructive capacitances that would destroy the signal.

I found something that I think will work, called Plugboard, to "practice" so I can try a variety of components with each other with out having a specific design layout at

http://www.vectorelect.com/Plugbord.htm

I'm new to RF and don't understand what you mean by stripline. I realize the impedances must match and that most components use a 50 ohm impedance. Could you please explain a bit more about this stripline and how to implement it.

Thanks
rfsuperman

 

[VEBill]

Don't even think about using plugboards for RF, especially at 2.4 or 1.7 GHz.

This is a rather ambitious project for someone without a reasonable amount of experience. What is the background ?

 

[logbook]

Quit now before you get disappointed!

You are not going to be able to make this idea work at 2.4GHz on that prototype board. You may just about succeed on standard FR4 if you keep the traces short, but you would typically need to use a more expensive low dielectric constant material at those frequencies.

PCBs are very cheap these days. I just ordered two double sided FR4 pcbs, 3.5 inches x 2.4 inches, for £40, including artwork (pcb-pool).

Microstrip is pcb traces over a ground plane. Strip line is traces with ground planes either side. Either will be ok for this frequency.

 

[BrianR]

rfsuperman,

A knowledge of impedance matching techniques is essential for a design like this one. The noise, gain and dynamic performance of each device depends on each port seeing the correct impedance. Using lumped components like capacitors and inductors is impractical at these frequencies as stray values are large and difficult to control. Using a stripline or microstrip is how you achieve accurate impedances. It is much more difficult to do any other way.

If you are new to RF this is not the project to learn on.

 

[Higgler]

A really good and easy to use Impedance Calculator is at

http://www.rogers-corp.com/mwu/mwi_java/mwij_vp.html

from Rogers corporation, check it out. If you want printed lines for 50 ohms, it'll tell you what size to make them for any configuration. It's loss calculation is pretty accurate too.

At 2.4 gHz, the wavelength is 11.803/2.4 inches= 4.9 inches, and on Microwave boards you can usually accept imperfections that are 1/50th of a wavelength (maybe 1/20th), or 0.1 inches, so you could probably get away with hand building things with connections from one IC to another using thin copper strips cut to the proper width based on the Rogers calculator, and have them soldered to the IC pins and glued down to the circuit board. Getting ground to the chips is very important at RF, Normally a chip mounted on the top of the board needs Via holes down through the board to the ground plane bottom. Or some other form of low inductance connection to ground.

 

[rfsuperman]

Ok, Thanks to all who replied. I have done more research, spoken to more people and bought a few books. Those of you who said to pack up and don't try this, you're right that this is tough stuff, but it's also fun!

I am still going through with the build.

I am using a VectorBord Plugboard and adapters to mount the small IC's. The leads will be short as possible.

The frequency synthesizer I hope to use will have a L.O. at apprx. 2210 MHz. The ISM band is about 2400-2500 MHz. I know that if I do a direct conversion down to baseband with a 2400 MHz L.O., there will be some "coupling" "interference" or whatever because both the RF and L.O. are nearly the same. With a gap of around 200 MHz from my anticipated L.O. of 2210 and the RF of 2400, do you foresee a problem with this "interference"?

Also, I have learned alot about microstrip but still have no visual of how it is implemented. Are there any pictures of project that show either microstrip or stripline?

Finally, is anyone familiar with a tunable bandpass filter? Perhaps I can adjust the desired L.O. and at the same time, adjust the RF pre filter befor the signal goes to the mixer based upon the L.O. I have selected.

Thanks again everyone,
rfsuperman

 

[IRstuff]

You've been given fair warning already, but you're already going against expert advice to not use vectorboard, you should be also clear that using adapters is an even worse circuit sin.

The lead inductance of the adapter alone, not even counting the interconnect, will kill operation at the frequencies you're trying to use.

TTFN

 

[VEBill]

At the very least, have a Spectrum Analyzer on the bench connected to a small portable antenna so that you can detect any unwanted L-band oscillations.

 

[BrianR]

I checked the VectorBord Plugboard on their web site and note that its designed for wirewrap and eyelets. I was going to reply in kind but thought it might be a waste of time. There is no chance it will work at 2GHz.