passive RF attenuator - RF Cafe Forums

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layla
Post subject: passive RF attenuator Posted: Thu Aug 10, 2006 7:29 pm

Captain

Joined: Thu Aug 10, 2006 7:09 pm
Posts: 7
I want to design a 50 ohm variable attenuator (0 -100 db) that attenuates a 315 MHz signal, i know the design would be either R-R or R-C attenuator but i cant seem to find proper pdfs or websites to start learning how to design, can anyone help me?

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IR
Post subject: Posted: Fri Aug 11, 2006 4:30 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello

Most probably the attenuator you will use will be a pure resistive one. In this case there is no meaning to the frequency raneg you will use (Up to a frequency in which the resistors have parasitics).

You need to consider few things beforhand:

1. What is the maximal power that this attenuator has to handle?
2. How will you change the attenuation value (Mechanicaly or electrically controlled?)

These things will effect your design...

You can find calculators to calculate the resistors values:

http://home.sandiego.edu/~ekim/e194rfs0 ... atten.html

If you need more information, please let me know.

_________________
Best regards,

- IR

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nubbage
Post subject: Passive RF AttenuatorPosted: Fri Aug 11, 2006 5:01 am

General

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Layla
The values you will need to choose from the ref IR mentions, are 1dB, 2dB, 4dB and 5dB
With combinations of those values switched in or out of circuit, you go from 0dB thru 10dB

Then you need to cascade 10 of these modules.

Use PIN switches or at 315MHz toggle switches with very short connections to switch the values in and out.

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layla
Post subject: passive RF attenuatorPosted: Fri Aug 11, 2006 5:01 am

Captain

Joined: Thu Aug 10, 2006 7:09 pm
Posts: 7
Hello

Thanks a lot for your reply, power considerations will not be a critical issue since I think I will be dealing with low power values in general.
what will be the difference in design if I wanted the design to be mechanically controlled or electrically controlled?
However, I'll think I'll go with mechanical switches, does that mean I have to calculate the values of resistors for each attenuation value then put these resistors in a cascaded manner where I change the input output ports?

Just an additional thought, is there any tips or advices for how to be a good Rf enginner and where to start reading or how?

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layla
Post subject: Re: Passive RF AttenuatorPosted: Fri Aug 11, 2006 5:13 am

Captain

Joined: Thu Aug 10, 2006 7:09 pm
Posts: 7
nubbage wrote:
Hi Layla
The values you will need to choose from the ref IR mentions, are 1dB, 2dB, 4dB and 5dB
With combinations of those values switched in or out of circuit, you go from 0dB thru 10dB

Then you need to cascade 10 of these modules.

Use PIN switches or at 315MHz toggle switches with very short connections to switch the values in and out.

Thanks for the reply, but can u give me a rough diagram of the connections coz i have a vague idea of how the connections will be to get the range from 0-100 dB.
Sorry for the trouble

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nubbage
Post subject: passive RF attenuatorPosted: Fri Aug 11, 2006 8:01 am

General

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Layla

Diagrams are a sore point on this site.

Unkonwn Ed???

I will be able to consult my circuits library this weekend, and get back to you next Monday, if by then we have a diagram facility.

What is the possibility UE?
IMHO diagrams are kind of essential to circuit explanations.

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IR
Post subject: Posted: Fri Aug 11, 2006 9:06 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

As nubbage suggested you will have to build a group consisted of 1dB-5dB attentuators which are cascaded.

Then in each group for example you will have to match the respective attenuators to the value of attenuation. For example:

9dB=4dB+5dB
8dB=3dB+5dB

That means you will have to connect the discrete attenuators separately. And on top of that to connect 9 groups of 10dB attenuators (tens of dB).

_________________
Best regards,

- IR

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fred47
Post subject: Variable AttenuatorPosted: Tue Aug 15, 2006 11:42 am

General

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi!

A few points:
1. At 315 MHz, the switches are almost guaranteed to be a problem - ordinary off-the-shelf miniature toggle switches might be good enough, but the only way to find out would be construction and test.

2. Small TO-5 can relays (like those made by Teledyne) can have good RF characteristics up to at least 500 MHz, but are a bit pricey.

3. 50-Ohm microstrip on an appropriate PCB would be a good idea.

4. Use surface mount resistors on a PCB if you want good accuracy at VHF - the leads for ordinary resistors can form unwanted inductors.

5. I'd like to emphasize the wisdom in some of the previous replies: it's much easier to get 10 dB of attenuation accurately at VHF than 20, 40, or 80 dB. The advice from IR and nubbage this way (use 9 or 10 10-dB variable stages) is top-quality. Ignore it only if you don't need precision.

6. I presume that you're looking for something that can be produced in some quantities, so that the attenuators made by Agilent (at the high end) and MiniCircuits Labs (at the low end) etc. are too costly. If that assumption is wrong, and you're doing a one-off project, unless you're going for the experience and knowledge gained in construction, you'd do better buying rather than building.

Good Luck!
Fred

Posted 11/12/2012