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Help on UWB project. Help selectiong components.

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Sink0

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Hi, i am trying to build this project: **broken link removed**

I am very noob to RF area so o got few questions. First, which components are critical? I mean i should use 1% precison or less? On the noise modulated pulse generator he used some old transistors with Hfe of 50. Any recomedation about this specific transistor? Any cheap partnumeber that would do the job? On the transmitter i was not able to find the 2N5109. I think it is a very old power RF transistor. Any good substitute that would work on this project? I am getting all parts from Farnell Spain, so if in stock i would apreciate :) .

I was looking for a substitute for 2N2369 becouse its very expensive. I could find BSV52 , but the power dissipation and max curretn is lower. Is it a good choice? Any good SMD sugestion?

Thank you!
 

jiripolivka

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It is good to follow the original paper in details. However, in some cases you can try to replace the "old expensive" or difficult to get transistors with newer ones.
The transistors used were simply good old RF transistors. Try to find replacements capable of RF operation: Ft > 1 GHz if possible, the dissipation is not so important.
I guess the author simply took transistors from a shelf and tested their performance. He also had instrumentation to test his circuits; not everybody has it.
The system design is experimental, so be ready to experiment. Critical elements are the switching diodes in Figs. 5 and 8, and short connections around. The RF switching transistors in those two figures should be as fast as possible; I would advise to use types used in UHF tuners or satellite IF tuners/amplifiers.
Have fun! And do not give up if your system does not work at once!
 
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Thank you!.

Yea i just fount the 25109 and decided using the 2N2369. I will leave for experimenting new components as soon as i get this working.

Hmm how about the precion of components and power rate? Any comment? I could not find any detail about the power related to resistors. And neither about precion of any component. Actually i got some difficult to find 1pf and 0.5pf capacitors with any precision. Just got 0.5pf capacitors with +/-0.25pf. How critical are theses values?

I would advise to use types used in UHF tuners or satellite IF tuners/amplifiers.

What does that means?

Haha, yea i will not give up. Dont worry, i am building this for learning purposes, i know i wont get any results easily hehe. About the instrumentation, i got a 4GS/s scope but not a 9GS as described on the circuit so i wont be able to take a look at the pulse. So i will have belive it is working and try to perform other debug tatics haha. But thats in the learning plan hehe.

Thank you!
 

jiripolivka

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I think the capacitor values are not critical. A good small capacitor can be made by twisting two short pieces (~2 cm long each) of Teflon-insulated wire. Two-three twists can make 1-2 pF, depending on insulation thickness.
My advice to use components from old TV tuners comes from my experience; I used a number of transistors, varicaps and trimmers quite successfully. With the transfer to digital television, many old TV sets are now discarded while components can be reused.
All resistors in the described circuits are 1/8W, only collector resistors in the fast switches may need 1/4 or 1/2W rating. To get a fast strong pulses to sampling diodes, the transistor should pass up to 0.1 A when open.
The only critical sections in the circuits are those around the sampling diodes and switching transistors. Use as short wires as possible, follow the photographs. As you have a quite good oscilloscope, you can see at least the short pulses like narrow "needles" if not like the waveforms in the paper.
Good luck!
 
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Thank you very much!! I will report here if i have results, hehe for other people that might try making it too. I am not going to build it for 2 weeks becouse i am on a trip, but when i come back i will build it for sure! Thank you very much!
 

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