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Differential to common mode

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Junior Member level 3
May 4, 2001
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I have a differential output board at baseband (f<20MHz). How can I visualize the common mode in a spectrum analizer? Can I connect directly the inner conductors to the inner of the SA and the outer of the SA respectively? Is it really so simple?


If you are analyzing only 1 diff pair, this may be an option, you must make sure that the analyzer's AC line input ground does not interfere with the measurement via coupling to the shield of the test leads, it is best to have a diff input module.


When you talk about a diff module what are you talking about? A commercial one? How can I build it?

Thanks again.

In most cases it is not recomended to connect an unbalanced test equipment to a balanced output.
There are three possibilities:
1) Connect a load resistor from each output to ground, and connect the analyzer on one of the load resistors with a high impedance probe.
2) Connect the unbalanced analyzer to the balanced output with a BALUN (BALanced to UNbalanced) transformer. A good source of such BALUNS with good bandwidth is:
3) Build a differential amplifier with wide bandwidth and it will convert the balanced signal to unbalanced one. Current Feedback Amplifiers like THS3001 are excelent for such an application.

Well it isn't very healty, but if you have only one balanced output to measure, then you can set the Spectrum Analyser inputs to a float mode and go about your busyness. Naturally you have to make sure that no power grounds are shared between the source and the instrument.

the easy solution is to fit a switch between the earth to the analyser and the connection to the plug

and simply float the analyser

I always remove the earths and feed to earth via an optocoupler and isolate via an eld this way the bench floats but is still intrinsic

so no real problems with leekage

bad for rf above 1KW bit bit mouthwatering at a power up tx

with cling but for low power and test i use this method to get rid of the earth image pull

also watch your workbench it may be a static safe conductive number

radios dont like this nor does rf

good for cpu and static

so use a square of carpet... to sit the device on test on.....

for < 20 mhz a few turns of silver plated wire on a match
and a pickup is handy place a 33pf var cap across 8T .315 mm wire diameter and about 10 mm dia and connect in series with the match and the inner core of a probe lead from the analyzer

"to get rid of the earth image pull"

What is "earth image pull"...
Can somebody pls clarify, with some elaboration..?..

Thanks in advance...

earth image

in a spectrum analyser as with many scopes

they are earthed but not coupled

rather gnded by chasis

when you connect any form of oscilator that uses a psu that had 12 or 13 v from a psu also connected to earth

this can cause an earth loop
with the respective device chasis all being earthed direct
and the device undertest is earth floating

gnding or via earth the negative rail is a bad idea
becouse you end up bloing things up with the - of the probe shorting to 12 v psu etc etc

so everything floats but is coupled
is best quality pictures ?ask does a tv antenna connect to the outer of the coax cable direct to chassis yes and no

yes in newer tv sets where there is a large psu stage with coupled gnd
and no in older sets where lots of engineers kill them selfs shorting probes to a live chassis
becouse the scope \ variacs etc is directly connected to common earth {very dangerious in my learned opinion and i have some burn hole to prove it}

you get a voltage diferential that causes a dc shift in analyser display
as other users power up and down around you

esp at 20 or less mhz that is very annoying
in sympathy with the timebases of several devices running at the same bench or point in cicuit {mains plugs }

and therefore is a real problem

as destortion can creap into delicate pic up operations as described above

i was called from the topic before i could finnish

thanks for asking

but i would also concider a t pad attenuator in line
3 res 100 R T 50 R 50 R

harrysmith said:
I have a differential output board at baseband (f<20MHz). How can I visualize the common mode in a spectrum analizer? Can I connect directly the inner conductors to the inner of the SA and the outer of the SA respectively? Is it really so simple?


I can provide a schematic ti implement fast and broadband active Bal-Un. I think you could implent it with about 8 bjt.


don't directly connect - put a 1k between the differential outputs rated for the amount of power you can give. then just tag onto top or bottom for common mode, tag across resistor for diff mode. it sounds like you are trying to prove that you have little to no common mode spectrum?

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