Signal Termination - RF Cafe Forums
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
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.
Post subject: Posted: Tue
Nov 15, 2005 10:09 pm
Aug 03, 2003 2:02 pm
Location: Erie, PA
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.
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)
- Kirt Blattenberger
Cafe Progenitor & Webmaster
Post subject: Posted: Tue Nov 15, 2005 11:57 pm
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
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
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
Have you tried National's "Webench" simulations
on your circuit?
Post subject: Posted: Tue Dec 20, 2005 8:28 am
Joined: Mon Sep 26, 2005 7:29 pm
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.
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.