Windfreak Technologies
Windfreak Technologies
 

About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024
Webmaster:
    Kirt Blattenberger,

    BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:

AirplanesAndRockets.com

Electronics World articles Popular Electronics articles QST articles Radio & TV News articles Radio-Craft articles Radio-Electronics articles Short Wave Craft articles Wireless World articles Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs LinkedIn Crosswords Engineering Humor Kirt's Cogitations RF Engineering Quizzes Notable Quotes App Notes Calculators Education Engineering magazine articles Engineering software Engineering smorgasbord RF Cafe Archives RF Cascade Workbook 2018 RF Symbols for Visio - Word Advertising RF Cafe Homepage Thank you for visiting RF Cafe!
everythingRF RF & Microwave Parts Database (h1) - RF Cafe

question about "attenuation pad"... - RF Cafe Forums

RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. If the folks at phpBB would release a version with integrated sign-in from the major social media platforms, I would resurrect the RF Cafe Forums, but until then it is probably not worth the effort. Regardless, there are still lots of great posts in the archive that ware worth looking at.

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

-- Amateur Radio
-- Anecdotes, Gripes & Humor
-- Antennas
-- CAE, CAD, & Software
-- Circuits & Components
-- Employment & Interviews
-- Miscellany
-- Swap Shop
-- Systems
-- Test & Measurement
-- Webmaster

trashbox
Post subject: question about "attenuation pad"... Posted: Mon Jul 18, 2005 11:31 pm

Captain

Joined: Mon May 24, 2004 11:30 pm
Posts: 10
Hi all

During the RF testing,such as the path between 1.2GHz vco signal and divider, there are always an attenuation pad(pi or Tee resistor network),generally 3~5dB.

Some articles or posts mentioned that it is for isolation and the others think it is for impedance mismatch.

What is role of such attenuation pad? Would you recommend any URL,materials or references?

Thank you very much.


Top

IR
Post subject: Posted: Tue Jul 19, 2005 1:34 am

Site Admin


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

The functions of the attenuator are:

- To adjust the level of the signal from the VCO's output to the LO driver's input port or mixer. In many designs, there is a direct connection between the output of the LO driver to the mixer's LO port. Therefore, you will need to go with a specific power level to the LO driver's input (Which has its own gain). Thus, you will have to do adjustment by means of an attenuator.

- Impedance matching: Sometimes the VCO's output or the input of the LO driver have poor match (Mostly it will be the LO driver's input). The function of the attenuator is to provide matching. Since a pad is a resistive element is provides the best matching. One dB of attenuation takes the return loss 2dB down.

- Provide additional isolation between the output of the VCO to the LO driver stage.

_________________
Best regards,

- IR


Top

trashbox
Post subject: Posted: Tue Jul 19, 2005 3:07 am

Captain

Joined: Mon May 24, 2004 11:30 pm
Posts: 10
IR,
Thank you very much.


Top

trashbox
Post subject: Posted: Tue Jul 19, 2005 3:36 am

Captain

Joined: Mon May 24, 2004 11:30 pm
Posts: 10
Hi IR,
Thanks for your nice help.According to your answer,I think:

-- How to decide the input power requirement of a mixed-signal circuit? For example, My VCO is used in a PLL,i.e.,the VCO output is connected to a current-mode logic divider that require the input signal a specific DC voltage and peak-to-peak value(for example,1.0v<input signal<1.4v DC voltage=1.2v). I am puzzled how to decide the input power requirement of the divider if I want to select an attenuator?

-- As a matching pad, I think the attenuators' leg resistors are not equal(suppose the attenuator is Tee section).Am I right?

Thanks again.


Top

IR
Post subject: Posted: Tue Jul 19, 2005 4:07 am

Site Admin


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

I
Quote:
-- As a matching pad, I think the attenuators' leg resistors are not equal(suppose the attenuator is Tee section).Am I right?


Usually the attenuator is balanced (Has the same impedance on both sides). This is because you would like to terminate both sides of the attenuator to the same impedance (Normally 50 ohm)

Quote:
-- How to decide the input power requirement of a mixed-signal circuit? For example, My VCO is used in a PLL,i.e.,the VCO output is connected to a current-mode logic divider that require the input signal a specific DC voltage and peak-to-peak value(for example,1.0v<input signal<1.4v DC voltage=1.2v). I am puzzled how to decide the input power requirement of the divider if I want to select an attenuator?



The input power requirement is being calculated according to
P=V^2/R (Where R is the load impedance, again normally 50 ohm).
Regarding the DC levels you mentioned, you probably refer to the new standards as: LVPECL, PECL and similar. There is a series of buffers, dividers etc manufactured by MICREL www.micrel.com (And other IC manufacturers) that deal with converting sine signals (AC-Coupled) to these standards. I think that you should check their devices. An example to such device: SY89871U

Good luck!!

_________________
Best regards,

- IR


Top

Guest
Post subject: Posted: Tue Jul 19, 2005 6:50 am
IR,thank you sincerely.

What you mentioned that the attenuator can finish impedance match. And I also saw this conclusion on h**p://mrtmag.com/mag/radio_rf_attenuators_terminations/.What I can not undersand is:

How the pi-section pad,whose leg resistors are equal,smooth out the impedance mismatch?(In fact,it is explained in above URL,but I can not see it since the lack of mentioned figures)


Top

IR
Post subject: Posted: Tue Jul 19, 2005 7:42 am

Site Admin


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

I don't know how familiar you are with Smith Chart and impedance matching, so I would explain it as follows:

As I mentioned before, the pad is a resistive element. The calculation of resistor values in the pad is done to satisfy 2 conditions:

1) The desired attenuation value (it is done by means of a voltage divider)
2) The required impedance.

By achieving an impedance close to 50 ohm you will tighten the VSWR up to a point which is the center of the Smith Chart: 50+j0 (namely pure resistive 50 ohm without reactive component). The actual impedance based on reflection coefficient can be calculated by:

Zl=Z0*(1-gamma)/(1+gamma)

gamma=reflection coefficient.

_________________
Best regards,

- IR


Top

trashbox
Post subject: Posted: Tue Jul 19, 2005 9:21 pm

Captain

Joined: Mon May 24, 2004 11:30 pm
Posts: 10
IR,
Thanks. I think I understand it.
By the means of reducing gamma by adding an attenuator, we can have a better impedance match.(closer to the cener of Smith Chart).

Thank you again for your nice help. Please send me a message if you have any difficulty,I will try my best.



Posted  11/12/2012
Werbel Microwave - RF Cafe
Axiom Test Equipment - RF Cafe
 

Please Support RF Cafe by purchasing my  ridiculously low−priced products, all of which I created.

These Are Available for Free