Thank you for visiting RF Cafe! Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes Wireless System Designer RF Stencils for Visio Shapes for Word Search RF Cafe Sitemap Advertising Facebook RF Cafe Forums RF Cafe Homepage Thank you for visiting RF Cafe!

Signal Termination - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


antenna_ant
Post subject: Signal termination Posted: Tue Nov 15, 2005 8:22 pm
I have a few questions about signal termination-

1) At what frequency is termination required.
2) what effects would you see if the output signal is not terminated by a 50ohm impedance?
3) If you are measuring the output signal of an op amp at F=6Mhz, how should I terminate the signal. I am measuring the output on a digital oscilloscope.

Thank you.


Top

Kirt Blattenberger
Post subject: Posted: Tue Nov 15, 2005 10:09 pm

Site Admin


Joined: Sun Aug 03, 2003 2:02 pm
Posts: 308
Location: Erie, PA
Greetings antenna_ant:

In answer to your questions:

1) A properly matched termination is required whenever you want maximum power transfer from the source to the load - regardless of frequency.

2) If any source does not "see" its complex conjugate impedance, part of the incident power is reflected back to the load. The result is standing waves on the transmission line, and less than maximum efficiency. In worst case conditions, the amplitude of the standing wave(s) can exceed the voltage breakdown level of the line and/or source driver and result in a catastrophic failure.

3) A rule of thumb is that standing waves become a design concern when the transmission path between the source and load is greater than about 1/10 of a wavelength. The wavelength of 6 MHz in air is 1,976 inches, and on a typical substrate or in coax cable would be around 1,100 inches, so 1/10 wavelength would be about 200 inches and 110 inches, respectively. If the separation between your source and load is less than those values, then standing waves are of no real concern. At 6 MHz, you are most likely targeting a specific voltage or current to a load and not maximum power transfer, and would choose a load impedance to achieve your goal if the source/load distance is less than 1/10 wavelength. So, if you are designing a PCB at 6 MHz, use any load value that gives you the magnitude & I/V phase relationship that works. If the 6 MHz will be driven down a long cable, definitely match source & load impedances to minimize reflections, then impedance transform and amplify at the load as(if) required.

_________________
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster


Top

antenna_ant
Post subject: Posted: Tue Nov 15, 2005 11:57 pm
Kirt, thank you very much.
The reason I asked is because my phase shifter is not working properly @ 6MHz.

It works fine at in the KHz range. It works 'okay' at 1 or 2MHz.
The ckt is an active all-pass filter. Look at fig. 1 here - http://www.maxim-ic.com.cn/appnotes.cfm ... number/559

R1 is a 10k pot and R is 1k and C is 150pF.

At 6 Mhz and above, the output amplitude is higher than the input. As I increase the phase difference, both the input and output amplitude decrease and increase gradually by a small amount.

The op amp I am using is LM7171 (200MHz).

Input and ouput are terminated by 50ohm resistors.

Any idea why its acting this way?

Thanks in advance.


Top

Guest
Post subject: Termination?Posted: Fri Nov 18, 2005 8:10 pm
It's not the termination that's causing trouble, but the phase response of the opamp and the output impedance as a function of frequency.

Have you tried National's "Webench" simulations on your circuit?

Good LucK!


Top

attenuator
Post subject: Posted: Tue Dec 20, 2005 8:28 am

Captain

Joined: Mon Sep 26, 2005 7:29 pm
Posts: 13
Your making assumptions of a 50 ohm system. This could be some of your trouble. As the 1771 has a very Hi input Z and a almost 0 output. Actually 15 ohms according to the data sheet. All the test data for the 1771 uses 100 ohm or 1kohm loads. Perhaps you might consider that.


Top

Dan R
Post subject: Posted: Thu Dec 22, 2005 9:53 am
Also, make sure your oscilloscope probes are high impedance, so as not to change the load the circuit sees (significantly) when measuring voltage.

--Dan



Posted  11/12/2012

RF Cafe Software

   Wireless System Designer - RF Cafe
Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
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 Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

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

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !

height-line