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Dealing with an impedance mismatch

A

Adam Dickson

Jan 1, 1970
0
Hopefully, someone can offer some ideas..

I've got a board that is experiencing some impedance mismatch between
it and the function generator. The frequency is sitting about around
10 MHz (square wave), and I'm seeing a LOT of signal distortion (its a
simple isolator board). I measured the board's input impedance, and
came up with about 119 ohms. The gen, naturally, is expecting 50 ohms.

How can I deal with this nasty lil' situation?

Thanks in advance!
 
D

Don Pearce

Jan 1, 1970
0
Hopefully, someone can offer some ideas..

I've got a board that is experiencing some impedance mismatch between
it and the function generator. The frequency is sitting about around
10 MHz (square wave), and I'm seeing a LOT of signal distortion (its a
simple isolator board). I measured the board's input impedance, and
came up with about 119 ohms. The gen, naturally, is expecting 50 ohms.

How can I deal with this nasty lil' situation?

Thanks in advance!

Put an 86 ohm resistor in parallel with the board's input.

d
 
W

Walter Harley

Jan 1, 1970
0
Adam Dickson said:
Hopefully, someone can offer some ideas..

I've got a board that is experiencing some impedance mismatch between
it and the function generator. The frequency is sitting about around
10 MHz (square wave), and I'm seeing a LOT of signal distortion (its a
simple isolator board). I measured the board's input impedance, and
came up with about 119 ohms. The gen, naturally, is expecting 50 ohms.

If you feed the function generator's output straight into an oscilloscope
(1MEG input), do you see the same distortion?

If not, then your problem isn't impedance mismatch...
 
L

Leon Heller

Jan 1, 1970
0
Adam Dickson said:
Hopefully, someone can offer some ideas..

I've got a board that is experiencing some impedance mismatch between
it and the function generator. The frequency is sitting about around
10 MHz (square wave), and I'm seeing a LOT of signal distortion (its a
simple isolator board). I measured the board's input impedance, and
came up with about 119 ohms. The gen, naturally, is expecting 50 ohms.

How can I deal with this nasty lil' situation?

That sort of difference won't cause problems. You must have something wrong
somewhere.

Leon
 
J

Joerg

Jan 1, 1970
0
Adam, as Walter said, try the oscilloscope. If your function generator has
a true 50 ohm output impedance and the cable is truly 50 ohms your setup
would be "source terminated" and you should be ok. When using a TTL output
or other logic output they are usually much lower in impedance, you'd have
to check it's handbook for that.

Please mind that some cables might look like 50 ohms RG58 but in reality
may be a 75 ohm cable.

Regards, Joerg.
 
T

Tim Wescott

Jan 1, 1970
0
Walter said:
If you feed the function generator's output straight into an oscilloscope
(1MEG input), do you see the same distortion?

If not, then your problem isn't impedance mismatch...

Or it's a frequency-varying impedance mismatch. Just measuring the DC
resistance isn't going to tell you much.
 
R

Reg Edwards

Jan 1, 1970
0
How can I deal with this nasty lil' situation?=========================

Not a nasty situation at all.

An impedance mismatch usually causes no problems whatsoever. Just a change
in signal level.

Just carry on as if nothing has happened.

If by some chance something crops up, it will be only a trivial matter and
deal with it when something of consequence perhaps occurs.
 
P

Paul Burridge

Jan 1, 1970
0
That sort of difference won't cause problems. You must have something wrong
somewhere.

True enough. I wonder if there's any reactance present with the 119
Ohms he hasn't detected.
 
A

Active8

Jan 1, 1970
0
Adam, as Walter said, try the oscilloscope. If your function generator has
a true 50 ohm output impedance and the cable is truly 50 ohms your setup
would be "source terminated" and you should be ok. When using a TTL output
or other logic output they are usually much lower in impedance, you'd have
to check it's handbook for that.

Please mind that some cables might look like 50 ohms RG58 but in reality
may be a 75 ohm cable.

Regards, Joerg.

And just what does this ditortion look like? If that trimmer cap on
the scope isn't tweaked...

Connect the scope probe to the calibrate terminal and adjust the
prob for a square wave.
 
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