What are 5dB 50 Ohm pads? Can they be home brewed?

[Kevin Doyle]

Hi all,
Just read that an RF oscillator out put should go to a 5-10 db 50Ohm pad
before going to the following Power amp.

What is a pad and can I home brew one?

Cheers,
Kevin.

 

[John Woodgate]

I read in sci.electronics.design that Kevin Doyle <irishdude79@yahoo.ie>
wrote (in <cj9sl9$v37$1@kermit.esat.net&gt about 'What are 5dB 50 Ohm
pads? Can they be home brewed?', on Mon, 27 Sep 2004:

Hi all,
Just read that an RF oscillator out put should go to a 5-10 db 50Ohm pad
before going to the following Power amp.

What is a pad and can I home brew one?

An attenuator, and yes, you can make them, at least if you don't want to

go over 500 MHz or so, and you are not looking for high precision.

It's usual to make 6 dB pads, or multiples of 6 dB, but 10 dB pads are
also used. You need good quality input and output connectors (BNC are
OK) and a *small* die-cast box, probably the smallest you can get. Don't
economise by using anything else. (OK, people use bits of copper pipe
and such; it's more work, though.) You need three 1% tolerance
resistors. Values for 50 ohm attenuators are low, and the inductance of
a resistor is usually negligible. No PC board is necessary or desirable.

There are tables of values in reference books, but you can calculate the
values using Ohm's Law. I expect there are free calculation programs on
the web. The object is to get the attenuation you want, while making
the input and output resistances still 50 ohms. There are two possible
configurations, and one often gives you more convenient resistor values
than the other:

Use Courier font:


In o----R1---+---R1----o Out In o--+---Rb---+--o Out
| | |
R2 Ra Ra
| | |
o---------+---------o Common o--+--------+--o Common

T-type Pi-type

For 6 dB, R1 = 16.6 ohms, R2 = 66.9 ohms
Ra = 150 ohms, Rb = 37.4 ohms
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Joerg]

Hi Kevin,

What is a pad and can I home brew one?


Pad? Something to alleviate back pain?

Just kidding. To add to John's reply, not all resistors are low
inductance. A pretty safe bet would be SMT resistor. Otherwise take
regular ones and in case of doubt take one of similar value that you can
sacrifice and sand it down to see that it isn't comprised of "turns".

Regards, Joerg

http://www.analogconsultants.com

 

[Kevin Doyle]

OK that link worked a treat thanks clarence.
I'm doing my reading from Cotter W. Sayre "complete wireless design" but as
usual with technicial books not all the terms are explained!

Cheers,
Kevin.
"Clarence" <no@No.com> wrote in message
news:YC06d.20537$QJ3.10044@newssvr21.news.prodigy.com...

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:RpuBcAN3HIWBFwve@jmwa.demon.co.uk...
I read in sci.electronics.design that Kevin Doyle <irishdude79@yahoo.ie
wrote (in <cj9sl9$v37$1@kermit.esat.net&gt about 'What are 5dB 50 Ohm
pads? Can they be home brewed?', on Mon, 27 Sep 2004:
Hi all,
Just read that an RF oscillator out put should go to a 5-10 db 50Ohm pad
before going to the following Power amp.

What is a pad and can I home brew one?

An attenuator, and yes, you can make them, at least if you don't want to
go over 500 MHz or so, and you are not looking for high precision.


Try this:

http://www.microwaves101.com/encyclopedia/calcattenuator.cfm

 

[Joerg]

Hi Kevin,

Here is one link that may have a home brew instruction but I can't read
that old post script file. Also has another calculator:

http://www.qsl.net/n9zia/wireless/appendixF.html#4

Scroll down to "Attenuator Pads".

Regards, Joerg

http://www.analogconsultants.com

 

[Jim Yanik]

"Clarence" <no@No.com> wrote in
news:YC06d.20537$QJ3.10044@newssvr21.news.prodigy.com:

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:RpuBcAN3HIWBFwve@jmwa.demon.co.uk...
I read in sci.electronics.design that Kevin Doyle
irishdude79@yahoo.ie> wrote (in <cj9sl9$v37$1@kermit.esat.net&gt
about 'What are 5dB 50 Ohm pads? Can they be home brewed?', on Mon,
27 Sep 2004:
Hi all,
Just read that an RF oscillator out put should go to a 5-10 db 50Ohm
pad before going to the following Power amp.

What is a pad and can I home brew one?

An attenuator, and yes, you can make them, at least if you don't want
to go over 500 MHz or so, and you are not looking for high precision.


Try this:

http://www.microwaves101.com/encyclopedia/calcattenuator.cfm

use film-type chip resistors (SMDs) a PCB,and good layout and you can
probably get to 1 Ghz.

--
Jim Yanik
jyanik-at-kua.net