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Signal termination - RF Cafe Forums

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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.


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


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.


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!


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


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.


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.


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