 |
|
| Science Forum Index » Electronics - Basics Forum » EMI prevention / protection?... |
|
Page 1 of 2 Goto page 1, 2 Next |
|
| Author |
Message |
| dave.harper... |
 Posted: Fri Oct 30, 2009 4:43 am |
|
|
|
Guest
|
I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second). If I hold the transmitter
further away, it continues oscillating, but I'd prefer to reduce the
EMI to have a more robust design.
I have a few questions I was hoping some people with EMI experience
might be able to shed light on:
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
2. Would a low-pass filter on any of the op amp inputs be a good
option, or would shielding be a better option?
3. If I used shielding, would standard aluminum foil be adequate?
What general guidelines are there regarding form and allowable holes
in shielded enclosures?
Thanks in advance!
Dave |
|
|
| Back to top |
|
|
|
| George Herold... |
| Posted: Fri Oct 30, 2009 5:14 am |
|
|
|
Guest
|
On Oct 30, 10:43 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second). If I hold the transmitter
further away, it continues oscillating, but I'd prefer to reduce the
EMI to have a more robust design.
I have a few questions I was hoping some people with EMI experience
might be able to shed light on:
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
2. Would a low-pass filter on any of the op amp inputs be a good
option, or would shielding be a better option?
3. If I used shielding, would standard aluminum foil be adequate?
What general guidelines are there regarding form and allowable holes
in shielded enclosures?
Thanks in advance!
Dave
[/quote]
Dave, Are you using an opamp for the Wein oscillator? Does it have a
BJT front end? If so you might try switching to a FET input opamp.
(Though it never killed the oscillations I made a Wein oscillator that
would sometimes develop high frequency 'fuzz' as the oscillator
crossed certain voltages. Changing to a FET got rid of the 'fuzz'.)
George.
Oh a little shielding (putting it in a aluminim box) is a good idea
too. |
|
|
| Back to top |
|
|
|
| dave.harper... |
| Posted: Fri Oct 30, 2009 5:50 am |
|
|
|
Guest
|
It's a MCP601, which is a CMOS op amp. Would going to a J-FET op amp
be a big improvement over a CMOS?
On Oct 30, 10:14 am, George Herold <ggher... at (no spam) gmail.com> wrote:
[quote]Dave, Are you using an opamp for the Wein oscillator? Does it have a
BJT front end? If so you might try switching to a FET input opamp.
(Though it never killed the oscillations I made a Wein oscillator that
would sometimes develop high frequency 'fuzz' as the oscillator
crossed certain voltages. Changing to a FET got rid of the 'fuzz'.)
George.
Oh a little shielding (putting it in a aluminim box) is a good idea
too.[/quote] |
|
|
| Back to top |
|
|
|
| George Herold... |
| Posted: Fri Oct 30, 2009 7:06 am |
|
|
|
Guest
|
On Oct 30, 11:50 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]It's a MCP601, which is a CMOS op amp. Would going to a J-FET op amp
be a big improvement over a CMOS?
On Oct 30, 10:14 am, George Herold <ggher... at (no spam) gmail.com> wrote:
Dave, Are you using an opamp for the Wein oscillator? Does it have a
BJT front end? If so you might try switching to a FET input opamp.
(Though it never killed the oscillations I made a Wein oscillator that
would sometimes develop high frequency 'fuzz' as the oscillator
crossed certain voltages. Changing to a FET got rid of the 'fuzz'.)
George.
Oh a little shielding (putting it in a aluminim box) is a good idea
too.- Hide quoted text -
- Show quoted text -
[/quote]
Hmm I have no idea if that will make any difference. What are you
using to control the feedback? Can you use a more robust oscillator
(as John suggested) Some type of 'bang-bang' rather than sitting on
the 'knife-edge' of oscillation with the Wein bridge. I needed the
low harmonic distortion of the Wein bridge... Is that what you need?
George H. |
|
|
| Back to top |
|
|
|
| dave.harper... |
| Posted: Fri Oct 30, 2009 7:40 am |
|
|
|
Guest
|
I'm using the wein bridge to generate audio tones that are fed to the
transmitter for digital radio communication (basically a homebrew ASK
radio modem). I'd be open to using that, but I'm not aware of any
radio modems that output square waves.
This oscillator also has 2 digital pots attached: 1 to trim the space
tone, and 1 to trim the mark tone. There's a high speed switch
between the two pots to rapidly switch between mark and space tones.
I could use this same setup with a 555 and trim the resistor to vary
the pulse duration, but I'm not sure what impact a square wave would
have on transmission, reception, decoding, etc...?
Has anyone heard of a square wave being used as an audio tone for
digital radio communication?
Thanks in advance,
Dave
On Oct 30, 12:06 pm, George Herold <ggher... at (no spam) gmail.com> wrote:
[quote]Hmm I have no idea if that will make any difference. What are you
using to control the feedback? Can you use a more robust oscillator
(as John suggested) Some type of 'bang-bang' rather than sitting on
the 'knife-edge' of oscillation with the Wein bridge. I needed the
low harmonic distortion of the Wein bridge... Is that what you need?
George H.[/quote] |
|
|
| Back to top |
|
|
|
| George Herold... |
| Posted: Fri Oct 30, 2009 8:21 am |
|
|
|
Guest
|
On Oct 30, 1:40 pm, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]I'm using the wein bridge to generate audio tones that are fed to the
transmitter for digital radio communication (basically a homebrew ASK
radio modem). I'd be open to using that, but I'm not aware of any
radio modems that output square waves.
This oscillator also has 2 digital pots attached: 1 to trim the space
tone, and 1 to trim the mark tone. There's a high speed switch
between the two pots to rapidly switch between mark and space tones.
I could use this same setup with a 555 and trim the resistor to vary
the pulse duration, but I'm not sure what impact a square wave would
have on transmission, reception, decoding, etc...?
Has anyone heard of a square wave being used as an audio tone for
digital radio communication?
Thanks in advance,
Dave
On Oct 30, 12:06 pm, George Herold <ggher... at (no spam) gmail.com> wrote:
Hmm I have no idea if that will make any difference. What are you
using to control the feedback? Can you use a more robust oscillator
(as John suggested) Some type of 'bang-bang' rather than sitting on
the 'knife-edge' of oscillation with the Wein bridge. I needed the
low harmonic distortion of the Wein bridge... Is that what you need?
George H.- Hide quoted text -
- Show quoted text -
[/quote]
Hi Dave, I know nothing about ASK radio modem.
I think you can get a triangle wave out of a 555 (Or some other charge/
discharge osc.) You can then low pass filter the triangle to get rid
of the sharp peaks.. There are also some trianlge wave to sine wave
converter circuits that use diodes or transistors to 'smooth' the
tiangle wave. If you don't care too much about harmonic content then
either approach may work.
George H. |
|
|
| Back to top |
|
|
|
| whit3rd... |
| Posted: Fri Oct 30, 2009 8:26 am |
|
|
|
Guest
|
On Oct 30, 7:43 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second).
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or
[/quote]
The amplitude-stabilizing part of a Wein bridge oscillator has a diode
and capacitor. You've got a little crystal radio there, and it
detects
the transmitter and turns the oscillator down (WAY down) thinking
it has detected the oscillator amplitude. To check this, put a
voltmeter on that capacitor while you key the transmitter.
Put a bypass capacitor across the diode (to conduct RF) and
shield that sensitive part of the circuit. A ferrite bead in series
with
the diode won't do any harm, either. |
|
|
| Back to top |
|
|
|
| dave.harper... |
| Posted: Fri Oct 30, 2009 8:49 am |
|
|
|
Guest
|
On Oct 30, 1:26 pm, whit3rd <whit... at (no spam) gmail.com> wrote:
[quote]On Oct 30, 7:43 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second).
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or
The amplitude-stabilizing part of a Wein bridge oscillator has a diode
and capacitor. You've got a little crystal radio there, and it
detects
the transmitter and turns the oscillator down (WAY down) thinking
it has detected the oscillator amplitude. To check this, put a
voltmeter on that capacitor while you key the transmitter.
Put a bypass capacitor across the diode (to conduct RF) and
shield that sensitive part of the circuit. A ferrite bead in series
with
the diode won't do any harm, either.
[/quote]
The amplitude-stabilizing part of the wein bridge I constructed uses 2
diodes (opposite directions) to limit amplitude rather than a diode
and a cap. The amplitude limiting circuit I'm using is almost
identical to the one shown here:
http://radiocom.kunsan.ac.kr/lecture/ewb/analyses/wien_bridge.JPG
Would a bypass cap across or between the diodes help?
Thanks!
Dave |
|
|
| Back to top |
|
|
|
| Jan Panteltje... |
| Posted: Fri Oct 30, 2009 8:53 am |
|
|
|
Guest
|
On a sunny day (Fri, 30 Oct 2009 07:43:24 -0700 (PDT)) it happened
"dave.harper" <dave.harper at (no spam) gmail.com> wrote in
<2cf2f0ad-9c3e-4c8f-ad65-925d63725dfe at (no spam) m16g2000yqc.googlegroups.com>:
[quote]I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second). If I hold the transmitter
further away, it continues oscillating, but I'd prefer to reduce the
EMI to have a more robust design.
I have a few questions I was hoping some people with EMI experience
might be able to shed light on:
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
[/quote]
All of those possibly
Use metal housing,
Use feed through caps, perhaps some inductors, like in the form of ferrite beads,
on all in an out lines, including supply.
[quote]I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
[/quote]
10nF is close to a short at 144 MHz, but put some inductance in series and then it really attenuates. |
|
|
| Back to top |
|
|
|
| John Larkin... |
| Posted: Fri Oct 30, 2009 9:54 am |
|
|
|
Guest
|
On Fri, 30 Oct 2009 07:43:24 -0700 (PDT), "dave.harper"
<dave.harper at (no spam) gmail.com> wrote:
[quote]I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second). If I hold the transmitter
further away, it continues oscillating, but I'd prefer to reduce the
EMI to have a more robust design.
I have a few questions I was hoping some people with EMI experience
might be able to shed light on:
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
2. Would a low-pass filter on any of the op amp inputs be a good
option, or would shielding be a better option?
3. If I used shielding, would standard aluminum foil be adequate?
What general guidelines are there regarding form and allowable holes
in shielded enclosures?
Thanks in advance!
Dave
[/quote]
Most likely the RF is trashing the front end of the opamp. As George
suggests, try a jfet or mosfet opamp.
Layout matters a lot. Multilayer boards, with a solid ground plane and
a tight layout, help a lot.
Try adding a resistor, 1K or so, in series with the oscillator output,
close to the opamp, then maybe a cap to the ground plane. The output
lead can be an antenna feeding RF back into the opamp. Power supply
leads can conduct RF into the opamp as well; use ferrite beads and
bypass caps.
Or use some harder circuit, like a digital square-wave generator and a
passive lowpass filter.
You can't solder to aluminum foil. The best shield is a seamless
conductive box with emi filters on all leads that penetrate the walls.
You can make fun little boxes by cutting up copperclad FR4 and
soldering the seams.
The Bud/Hammond-type diecast aluminum boxes are good if the PCB inside
has a good ground plane, hard bolted to the box, and all i/o leads
have some sort of filtering or bypassing.
This thing has a thermocouple front-end amp inside an aluminum shield.
You can just see the unmasked copper ring on the pcb that the cover
sits on.
ftp://jjlarkin.lmi.net/Ferrite.JPG
The ferrite on the input leads bought another 20 dB or so of emi
rejection, specifically killing some narrowband resonant responses in
the 100-300 MHz range.
John |
|
|
| Back to top |
|
|
|
| Joerg... |
| Posted: Fri Oct 30, 2009 1:04 pm |
|
|
|
Guest
|
John Larkin wrote:
[quote]On Fri, 30 Oct 2009 07:43:24 -0700 (PDT), "dave.harper"
dave.harper at (no spam) gmail.com> wrote:
I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second). If I hold the transmitter
further away, it continues oscillating, but I'd prefer to reduce the
EMI to have a more robust design.
I have a few questions I was hoping some people with EMI experience
might be able to shed light on:
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
2. Would a low-pass filter on any of the op amp inputs be a good
option, or would shielding be a better option?
3. If I used shielding, would standard aluminum foil be adequate?
What general guidelines are there regarding form and allowable holes
in shielded enclosures?
Thanks in advance!
Dave
Most likely the RF is trashing the front end of the opamp. As George
suggests, try a jfet or mosfet opamp.
[/quote]
Yup. A cap across anything that even remotely looks like a BE junction
can also help.
[quote]Layout matters a lot. Multilayer boards, with a solid ground plane and
a tight layout, help a lot.
Try adding a resistor, 1K or so, in series with the oscillator output,
close to the opamp, then maybe a cap to the ground plane. The output
lead can be an antenna feeding RF back into the opamp. Power supply
leads can conduct RF into the opamp as well; use ferrite beads and
bypass caps.
[/quote]
Beads in front of opamp inputs or transistor bases also help. Must be as
close to the device as possible. And don't forget radiation smack-dab
into the chip itself.
[quote]Or use some harder circuit, like a digital square-wave generator and a
passive lowpass filter.
[/quote]
More current -> better.
[quote]You can't solder to aluminum foil. The best shield is a seamless
conductive box with emi filters on all leads that penetrate the walls.
You can make fun little boxes by cutting up copperclad FR4 and
soldering the seams.
[/quote]
Altoid cans work quite nicely. If the OP is in Europe (can't read his
posts because he uses Google) then Fisherman's Friends works well. But
don't let one of those mints sit in your mouth and fall asleep :-)
[...]
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM. |
|
|
| Back to top |
|
|
|
| Paul Keinanen... |
| Posted: Fri Oct 30, 2009 1:16 pm |
|
|
|
Guest
|
On Fri, 30 Oct 2009 07:43:24 -0700 (PDT), "dave.harper"
<dave.harper at (no spam) gmail.com> wrote:
[quote]1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
[/quote]
Sounds like a typical "audio rectification" problem, in which the
input transistor PN junction acts as an AM detector (crystal
detector). The other halfwave is removed and there is an biased audio
waveform depending on the RF amplitude and the audio can be heard with
headphones at the audio output.
However, if this a constant amplitude FM transmission, no audio will
be heard, but the rectified DC signal will upset the stage bias,
perhaps with several volts.
If this is a non-surface mount opamp, just insert small ferrite beads
on the input pins, before soldering it to the board.
Paul |
|
|
| Back to top |
|
|
|
| Robert Baer... |
| Posted: Fri Oct 30, 2009 11:41 pm |
|
|
|
Guest
|
Jan Panteltje wrote:
[quote]On a sunny day (Fri, 30 Oct 2009 07:43:24 -0700 (PDT)) it happened
"dave.harper" <dave.harper at (no spam) gmail.com> wrote in
2cf2f0ad-9c3e-4c8f-ad65-925d63725dfe at (no spam) m16g2000yqc.googlegroups.com>:
I'm using a wein bridge oscillator as an audio tone generator in a
radio modem (~2kHz). Ironically, when the transmitter is turned on,
RF from the transmitter (~144MHz) causes the oscillations to die
pretty quickly (a fraction of a second). If I hold the transmitter
further away, it continues oscillating, but I'd prefer to reduce the
EMI to have a more robust design.
I have a few questions I was hoping some people with EMI experience
might be able to shed light on:
1. Is the EMI likely cause by voltage fluctuation in the supply
voltage, or the input lines to the op amp? Or maybe in the IC itself?
All of those possibly
Use metal housing,
Use feed through caps, perhaps some inductors, like in the form of ferrite beads,
on all in an out lines, including supply.
I have a .01uF decoupling cap on the supply voltage (was out of 1uF
caps), but maybe this cap needs to be larger?
10nF is close to a short at 144 MHz, but put some inductance in series and then it really attenuates.
....or resonates if chosen "wisely"...best add some R for real damnping.[/quote] |
|
|
| Back to top |
|
|
|
| whit3rd... |
| Posted: Sat Oct 31, 2009 9:17 am |
|
|
|
Guest
|
On Oct 30, 11:49 am, "dave.harper" <dave.har... at (no spam) gmail.com> wrote:
[quote]On Oct 30, 1:26 pm, whit3rd <whit... at (no spam) gmail.com> wrote:
The amplitude-stabilizing part of a Wein bridge oscillator has a diode
and capacitor.
(I was thinking of a FET/voltage-controlled-resistor stabilizer)[/quote]
[quote]The amplitude-stabilizing part of the wein bridge I constructed uses 2
diodes (opposite directions) to limit amplitude
http://radiocom.kunsan.ac.kr/lecture/ewb/analyses/wien_bridge.JPG
Would a bypass cap across or between the diodes help?
[/quote]
Yes, the addition of 200 pF across the diodes might do some good;
RF reaching those diodes will modulate their ON threshold, which
will lower their impedance, and that's what turns the oscillator
amplitude
down. The sensitivity of this kind of network is lower than the one
I was thinking of, though.
Tin-plated soft steel (like tin cans) is an excellent shielding
material; I
think it's time to snip open some of the recycle-bin chili containers
and apply shielding. |
|
|
| Back to top |
|
|
|
| whit3rd... |
| Posted: Sat Oct 31, 2009 9:23 am |
|
|
|
Guest
|
On Oct 30, 12:04 pm, Joerg <inva... at (no spam) invalid.invalid> wrote:
[quote]Beads in front of opamp inputs or transistor bases also help.
[/quote]
Only as a last resort, surely!
The inputs and transistor bases are relatively HIGH impedance,
and beads are low-Z by comparison; I'd save those beads
for emitters where they have better matching... there has to
be RF current into clamp diodes before an op amp input
will benefit much from a bead. |
|
|
| Back to top |
|
|
|
|
|
All times are GMT - 5 Hours
The time now is Sun Dec 06, 2009 8:36 am
|
|