How can I measure conducted RF ?

[Roland]

Hi

I have a circuit board and 5W VHF radio in a plastic enclosure. When I key
the radio is sometimes screws up operation of my DTMF decoder/ FSK chip
and adds noise to the synthesized audio I am sending out over the radio.

Is there a way to measure the conducted RF on my board. When I try looking
with my scope, there is RF on every trace including ground. My scope leads/
scope probes, everything has RF on it.

I have up to 20 lines coming in from sensors,battery,radio....


Roland

 

[James Meyer]

On Fri, 21 May 2004 15:52:49 GMT, "Roland" <REMOVETHISrsdeluca@shaw.ca> posted
this:

Hi

I have a circuit board and 5W VHF radio in a plastic enclosure. When I key
the radio is sometimes screws up operation of my DTMF decoder/ FSK chip
and adds noise to the synthesized audio I am sending out over the radio.

Is there a way to measure the conducted RF on my board. When I try looking
with my scope, there is RF on every trace including ground. My scope leads/
scope probes, everything has RF on it.

I have up to 20 lines coming in from sensors,battery,radio....


Roland

OK, suppose you *could* measure the conducted RF. The next step would
be to reduce the conducted RF.

Why not take the easy way out and just start by reducing the conducted
RF without measuring it?

The first step would be to put both the radio and the circuit board into
individual *metal* boxes. Then both boxes could go in your plastic box. That
may be enough to cure the problem and you won't have to measure anything.

Jim

 

[Paul Mathews]

"Roland" <REMOVETHISrsdeluca@shaw.ca> wrote in message news:<lBprc.527001$Pk3.315561@pd7tw1no>...

Hi

I have a circuit board and 5W VHF radio in a plastic enclosure. When I key
the radio is sometimes screws up operation of my DTMF decoder/ FSK chip
and adds noise to the synthesized audio I am sending out over the radio.

Is there a way to measure the conducted RF on my board. When I try looking
with my scope, there is RF on every trace including ground. My scope leads/
scope probes, everything has RF on it.

I have up to 20 lines coming in from sensors,battery,radio....


Roland

Just do a google search on 'EMI sniffer' and you'll turn up lots of
info on how to build your own. Very easy. Very effective.

Paul Mathews

 

[Roland DeLuca]

"Paul Mathews" <optoeng@pioneernet.net> wrote in message
news:ed443a10.0405211525.2e09cf20@posting.google.com...

"Roland" <REMOVETHISrsdeluca@shaw.ca> wrote in message
news:<lBprc.527001$Pk3.315561@pd7tw1no>...
Hi

I have a circuit board and 5W VHF radio in a plastic enclosure. When I
key
the radio is sometimes screws up operation of my DTMF decoder/ FSK chip
and adds noise to the synthesized audio I am sending out over the radio.

Is there a way to measure the conducted RF on my board. When I try
looking
with my scope, there is RF on every trace including ground. My scope
leads/
scope probes, everything has RF on it.

I have up to 20 lines coming in from sensors,battery,radio....


Roland

Just do a google search on 'EMI sniffer' and you'll turn up lots of
info on how to build your own. Very easy. Very effective.

Paul Mathews

Thanks Paul

I will try building it. Here is the link if others are interested
http://www.reed-electronics.com/ednmag/archives/1998/060498/12DF_04.pdf


As far as putting everything in a metal box, I tried it with limited success
.. I think my problem is that the RF is conducted in the wires and gets in
that way. I could add ferrite beads/ caps to all lines and better shielding
but it gets costly. So I want to try and find the worst offenders first.


Roland DeLuca

 

[Terry Given]

"Roland DeLuca" <REMOVETHISrsdeluca@shaw.ca> wrote in message
news:Wuwrc.529161$Pk3.72616@pd7tw1no...

"Paul Mathews" <optoeng@pioneernet.net> wrote in message
news:ed443a10.0405211525.2e09cf20@posting.google.com...
"Roland" <REMOVETHISrsdeluca@shaw.ca> wrote in message
news:<lBprc.527001$Pk3.315561@pd7tw1no>...
Hi

I have a circuit board and 5W VHF radio in a plastic enclosure. When I
key
the radio is sometimes screws up operation of my DTMF decoder/ FSK
chip
and adds noise to the synthesized audio I am sending out over the
radio.

Is there a way to measure the conducted RF on my board. When I try
looking
with my scope, there is RF on every trace including ground. My scope
leads/
scope probes, everything has RF on it.

I have up to 20 lines coming in from sensors,battery,radio....


Roland

Just do a google search on 'EMI sniffer' and you'll turn up lots of
info on how to build your own. Very easy. Very effective.

Paul Mathews


Thanks Paul

I will try building it. Here is the link if others are interested
http://www.reed-electronics.com/ednmag/archives/1998/060498/12DF_04.pdf


As far as putting everything in a metal box, I tried it with limited
success
. I think my problem is that the RF is conducted in the wires and gets in
that way. I could add ferrite beads/ caps to all lines and better
shielding
but it gets costly. So I want to try and find the worst offenders first.


Roland DeLuca

Hi Roland,

In practice it is very difficult to measure such RF interference (be it self
inflicted or otherwise). As soon as you attach a dangly wire (read as:
aerial), you will change the behaviour of your circuit, often making it
worse.

In general you would be better off to read a book or two on EMI suppression
techniques, then implement all the easy-to-do solutions. Henry Ott's book is
excellent, but I prefer these two:

Controlling radiated emissions by design, Michel Mardiguan, and
Controlling conducted emissions by design, John Fluke jr (all this book
really says is include all of the parasitics in your design, often easier
said than done

Step 1 is generally to throw out your pcb layout, and do it again with a
solid 0V plane - absolutely no slots! this is usually quite achievable on 2
layers - I once had a 1500W PFC pcb re-designed in this way. we deleted the
existing 0V traces, which freed up lots of room on the pcb. we then moved
every bottom-layer trace to the top layer that we could - about 90% of them
were simply layer changes! Then we poured an 0V plane, and spent about 4hrs
reducing the size of the inevitable slots (where there are either traces on
the bottom layer, OR adjacent vias which form a slot (these just get moved
apart a bit, to keep the slots small, say 1cm max). The entire re-layout
took our PADS guy 1 day, and the PFC worked beautifully afterwards (it used
to explode), all without changing a single component!

This is usually enough to solve your problem completely, all without
measuring a damn thing.

Step 2 is then to band-limit all input and output signals, right at the
connectors - capacitors are the go here. If a signal is low-speed, then
filter the shit out of it - dont provide more bandwidth than you have to.

Then you can look at cables - twisted pair for H-field attenuation, an
electrostatic screen for E-field attenuation, etc.

Then start thinking about ferrites etc, which are usually used to try and
compensate for lousy pcb layout - invariably it is far cheaper to just re-do
the pcb layout.

Hell, often a 4-layer PCB is a cheaper solution than wanky cables, ferrites
etc - I did an EMI analysis on a bearing temperature monitor for a 400kW
rock crusher (AC motor drives generate metric shitloads of EMI, they werent
using $80/m screened cables etc) a while back. The controller was a 2-layer
pcb with a long, skinny 0V trace, and it sucked majorly. The component
routing was sufficiently complex that a 2-layer pcb was unlikely to work
well, and would take a bloody long time to do. We deleted the 0V and Vcc
traces, added 2 internal layers and made mid-layer1 = 0V plane, mid-layer2 =
Vcc plane - again, only took an hour or two. It added about $10 to the cost
of the controller (2x original pcb price) but instantly solved dozens of
weird intermittent problems. Again, no measurements were taken. It was also
about $10,000 cheaper than re-wiring the ac motor drives with appropriate
cabling to reduce the noise source. Needless to say, the customer was happy.

You are dead right about the cables - internal radiated noise invariably
enters your cables, shoots out the box as conducted EMI, then radiates away
again - this is why the filters etc. should be placed as close as possible
to the edge of the box. Often when you place a ferrite on a cable, it turns
conducted emi into radiated emi, which leaps around the "filter" and back
into the cable - especially if your cable then runs across your pcb.....

cheers
Terry

 

[Scott Stephens]

Roland wrote:

Is there a way to measure the conducted RF on my board.

Think about what you are doing. You are trying to measure a electric or
magnetic fields, without your measurement device becoming either a
source or sink.

The sniffer probe may work, but when you move it near the board, it too
may well affect any measurement, or pick up electric field radiating off
one part of the board, and put it on another part.

It would be nice if there were some kind of aerosol LCD mist that could
be sprayed, and stoboscopically illuminated with modulated laser light
to image the RF.

Practically, the best thing is to design or shield. Keep a ground-plane
near the board for the RF to have a (relatively) low-capacitance path
to. If you have coils, see they are shielded. Put a ground plane under
your sensitive circuit, and ground the plane with short wires at many
points until your troubles go away.

with my scope, there is RF on every trace including ground. My scope leads/
scope probes, everything has RF on it.

Think of hot RF traces as glowing. You don't want to put anything near
that will reflect the light around, you want to absorb it.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************

 

[Paul Mathews]

"Roland DeLuca" <REMOVETHISrsdeluca@shaw.ca> wrote in message news:<Wuwrc.529161$Pk3.72616@pd7tw1no>...

"Paul Mathews" <optoeng@pioneernet.net> wrote in message
news:ed443a10.0405211525.2e09cf20@posting.google.com...
"Roland" <REMOVETHISrsdeluca@shaw.ca> wrote in message
news:<lBprc.527001$Pk3.315561@pd7tw1no>...
Hi

I have a circuit board and 5W VHF radio in a plastic enclosure. When I
key
the radio is sometimes screws up operation of my DTMF decoder/ FSK chip
and adds noise to the synthesized audio I am sending out over the radio.

Is there a way to measure the conducted RF on my board. When I try
looking
with my scope, there is RF on every trace including ground. My scope
leads/
scope probes, everything has RF on it.

I have up to 20 lines coming in from sensors,battery,radio....


Roland

Just do a google search on 'EMI sniffer' and you'll turn up lots of
info on how to build your own. Very easy. Very effective.

Paul Mathews


Thanks Paul

I will try building it. Here is the link if others are interested
http://www.reed-electronics.com/ednmag/archives/1998/060498/12DF_04.pdf


As far as putting everything in a metal box, I tried it with limited success
. I think my problem is that the RF is conducted in the wires and gets in
that way. I could add ferrite beads/ caps to all lines and better shielding
but it gets costly. So I want to try and find the worst offenders first.


Roland DeLuca

The other posters are right to suggest that EMI can be baffling, that
filters should be put near the boundaries of the enclosure, and that a
comprehensive design to minimize the generation and transmission of
EMI is best. However, you can learn a lot by sniffing, and it's not
true that things change so much when you use a probe that it's
useless.

There are many books and publications on the subject, some of them on
the web. You might start with a book by Montrose:
http://www.amazon.com/exec/obidos/tg/detail/-/078034703X/qid=1085241594/sr=1-1/ref=sr_1_1/102-3016581-3485711?v=glance&s=books

Meanwhile, here are a couple of universal rules for EMI minimization:

All signal paths have a corresponding 'return path', which is often
thought of as flowing in a 'grounding system'. These return paths are
just as important as the the signal conductors themselves. In
general, signal and return paths should be close together at all
points, thereby minimizing magnetic loop area. This is the single
most important aspect of EMI minimization, both for susceptibility and
radiation minimization.

Shielding is generally effective to the extent that shielded circuitry
is completely enclosed in a continuous conductive structure. Any
openings or conductors exiting this structure require special
consideration.

Have fun.

Paul Mathews

 

[Terry Given]

"Paul Mathews" <optoeng@pioneernet.net> wrote in message
news:ed443a10.0405220804.40b0757@posting.google.com...

"Roland DeLuca" <REMOVETHISrsdeluca@shaw.ca> wrote in message
news:<Wuwrc.529161$Pk3.72616@pd7tw1no>...
"Paul Mathews" <optoeng@pioneernet.net> wrote in message
news:ed443a10.0405211525.2e09cf20@posting.google.com...
"Roland" <REMOVETHISrsdeluca@shaw.ca> wrote in message
news:<lBprc.527001$Pk3.315561@pd7tw1no>...
Hi

I have a circuit board and 5W VHF radio in a plastic enclosure. When
I
key
the radio is sometimes screws up operation of my DTMF decoder/ FSK
chip
and adds noise to the synthesized audio I am sending out over the
radio.

Is there a way to measure the conducted RF on my board. When I try
looking
with my scope, there is RF on every trace including ground. My scope
leads/
scope probes, everything has RF on it.

I have up to 20 lines coming in from sensors,battery,radio....


Roland

Just do a google search on 'EMI sniffer' and you'll turn up lots of
info on how to build your own. Very easy. Very effective.

Paul Mathews


Thanks Paul

I will try building it. Here is the link if others are interested
http://www.reed-electronics.com/ednmag/archives/1998/060498/12DF_04.pdf


As far as putting everything in a metal box, I tried it with limited
success
. I think my problem is that the RF is conducted in the wires and gets
in
that way. I could add ferrite beads/ caps to all lines and better
shielding
but it gets costly. So I want to try and find the worst offenders first.


Roland DeLuca


The other posters are right to suggest that EMI can be baffling, that
filters should be put near the boundaries of the enclosure, and that a
comprehensive design to minimize the generation and transmission of
EMI is best. However, you can learn a lot by sniffing, and it's not
true that things change so much when you use a probe that it's
useless.

There are many books and publications on the subject, some of them on
the web. You might start with a book by Montrose:

http://www.amazon.com/exec/obidos/tg/detail/-/078034703X/qid=1085241594/sr=1

-1/ref=sr_1_1/102-3016581-3485711?v=glance&s=books

Meanwhile, here are a couple of universal rules for EMI minimization:

All signal paths have a corresponding 'return path', which is often
thought of as flowing in a 'grounding system'. These return paths are
just as important as the the signal conductors themselves. In
general, signal and return paths should be close together at all
points, thereby minimizing magnetic loop area. This is the single
most important aspect of EMI minimization, both for susceptibility and
radiation minimization.

Shielding is generally effective to the extent that shielded circuitry
is completely enclosed in a continuous conductive structure. Any
openings or conductors exiting this structure require special
consideration.

Have fun.

Paul Mathews

Nicely put, Paul. Montrose's book is IMO pretty good - I have read it but
didnt buy it, as I already have all the info elsewhere.

How about a nice, terse summary:

Current flows in loops. minimise them.

Alas, the ubiquitous "ground" symbol in schematics, seems to be designed
specifically to hide this fact from all and sundry, who seem to have
forgotten KCL.

Cheers
Terry.

 

[James Meyer]

On Sun, 23 May 2004 12:58:10 -0700, "Terry Given" <the_domes@xtra.co.nz> posted
this:

Alas, the ubiquitous "ground" symbol in schematics, seems to be designed
specifically to hide this fact from all and sundry, who seem to have
forgotten KCL.

Cheers
Terry.

What does potassium chloride have to do with conducted RF?

Jim

 

[Terry Given]

"James Meyer" <jmeyer@nowhere.com> wrote in message
news:1581b016enbrd0j4tmqqpjmmj6rspe0qf3@4ax.com...

On Sun, 23 May 2004 12:58:10 -0700, "Terry Given" <the_domes@xtra.co.nz
posted
this:


Alas, the ubiquitous "ground" symbol in schematics, seems to be designed
specifically to hide this fact from all and sundry, who seem to have
forgotten KCL.

Cheers
Terry.


What does potassium chloride have to do with conducted RF?

Jim

think dead white guy. I said KCL not KCl.

Terry