400HZ Notch Filter how to.

[John Fields]

On Tue, 30 Sep 2003 07:21:33 -0800, "Dana" <dana.raffaniello@gci.net>
wrote:

Yes I tried that, and I found that to reduce the 400hz I needed to have my
cutoff at around 650hz and higher to obtain enough attenuation at 400hz.
The 400hz is the whine of the 400hz power that supplies the remote location
electronics, and as the equipment technology is old, the audio of the
operator is neing swamped out by this noise. Why thye did not go with some
kind of vox circuit like is used in aircraft I do not know.

---
If the offending signal is being coupled into the microphone
acoustically, then VOX won't help at all, since the 400Hz will still be
there when speech turns on the transmitter.

The solution is to notch out the 400Hz at the transmitter (at the mic or
downstream from the mic) and keep it from being transmitted in the first
place. A passive RC notch will do it, but the price you'll pay will be
quite a bit of attenuation of the audio you _want_ to send. Use an
active solution like the one described in the URL I posted for you
earlier and you can adjust the Q of the filter to pretty much leave most
of the audio alone except for the offending signal. Cheap, too. A dual
opamp and a handful of discretes and you're there...

If you'd rather buy than build, I can put something together for you for
about $20.00. Email me if you're interested.

--
John Fields

 

[Al Clark]

"Dana" <dana.raffaniello@gci.net> wrote in
news:vnj894gboc8k53@corp.supernews.com:

"Al Clark" <dsp@danvillesignal.com> wrote in message
news:Xns94065FEDDE68Caclarkdanvillesignal@66.133.128.150...
An analog notch filter may be fine for this application if the
following conditions are actually true:

1. Temperature is stable, so that the filter will not drift.

Once the site is turned up, the temp remains fairly constant.
Big fear here is latter on this winter when outside temps are -20 and
lower, may take a bit longer to stabilize.


2. The interfering signal is actually 400 Hz. My guess is that it has
a fundamental at 400 Hz and also some significant harmonics.

It is 400hz power that is making the whine. We use 400hz power on the
electronic equipment at the site, except of course the comm gear which
is your basic 120.


The harmonics
may be a bigger problem than the fundamental, since hearing is more
sensitive at higher frequencies (1-2kHz).

I would agree with this outlook in most other situations. But being
that this is the noise of the 400hz power, these harmonics are not so
bad.

I would look at the signal with an FFT analyzer (You can probably
find a PC program on the web). This information will suggest an
appropriate solution.

Looked at with a Spec A, as well as one of those level meters that
gives freq's and levels that are used to find rf hotspots to comply
with OSHA regs.

These might be:

1. A simple notch filter as discussed.
2. Multiple notch filters.

Dana:

I tried to send you an email reply but the address bounced. Call me if
you like. Our phone number is on our web site.


--
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com

 

[Dana]

"John Fields" <jfields@austininstruments.com> wrote in message
news:3s8jnvknfj32m1p0les47a34fr55l8a0ts@4ax.com...

On Tue, 30 Sep 2003 07:21:33 -0800, "Dana" <dana.raffaniello@gci.net
wrote:


Yes I tried that, and I found that to reduce the 400hz I needed to have
my
cutoff at around 650hz and higher to obtain enough attenuation at 400hz.
The 400hz is the whine of the 400hz power that supplies the remote
location
electronics, and as the equipment technology is old, the audio of the
operator is neing swamped out by this noise. Why thye did not go with
some
kind of vox circuit like is used in aircraft I do not know.

---
If the offending signal is being coupled into the microphone
acoustically, then VOX won't help at all, since the 400Hz will still be
there when speech turns on the transmitter.

Maybe I am missing something, but aircraft radio's tend to use vox to knock
out the noise from the cockpit. So is that not the noise that is being
acoustically picked up.
And maybe I should have clarified the com circuits are being transmitted
back on equipment that is powered by 120 line voltage, and not the 400hz
equipment. The 400hz noise in question is the 400hz converter in the
equipment shelter, that the mic is picking up.

The solution is to notch out the 400Hz at the transmitter (at the mic or
downstream from the mic) and keep it from being transmitted in the first
place. A passive RC notch will do it, but the price you'll pay will be
quite a bit of attenuation of the audio you _want_ to send. Use an
active solution like the one described in the URL I posted for you
earlier and you can adjust the Q of the filter to pretty much leave most
of the audio alone except for the offending signal. Cheap, too. A dual
opamp and a handful of discretes and you're there...

If you'd rather buy than build, I can put something together for you for
about $20.00. Email me if you're interested.

Thanks for the good tips. Will try both the active OPAMP circuit, and the
passive filter. Prefer just the passive, as I have quite the limited line
voltage supply.

--
John Fields

 

[John Fields]

On Tue, 30 Sep 2003 09:03:52 -0800, "Dana" <dana.raffaniello@gci.net>
wrote:

"John Fields" <jfields@austininstruments.com> wrote in message
news:3s8jnvknfj32m1p0les47a34fr55l8a0ts@4ax.com...
On Tue, 30 Sep 2003 07:21:33 -0800, "Dana" <dana.raffaniello@gci.net
wrote:


Yes I tried that, and I found that to reduce the 400hz I needed to have
my
cutoff at around 650hz and higher to obtain enough attenuation at 400hz.
The 400hz is the whine of the 400hz power that supplies the remote
location
electronics, and as the equipment technology is old, the audio of the
operator is neing swamped out by this noise. Why thye did not go with
some
kind of vox circuit like is used in aircraft I do not know.

---
If the offending signal is being coupled into the microphone
acoustically, then VOX won't help at all, since the 400Hz will still be
there when speech turns on the transmitter.

Maybe I am missing something, but aircraft radio's tend to use vox to knock
out the noise from the cockpit. So is that not the noise that is being
acoustically picked up.

---
VOX is an acronym for Voice Operated Xmit, and in and of itself has
nothing to do with anything other than turning the transmitter on and
off.

I believe what you may be thinking about is the microphone, which is
noise-cancelling and may, in reality, be an ideal solution for you if
you can switch mics.
---

And maybe I should have clarified the com circuits are being transmitted
back on equipment that is powered by 120 line voltage, and not the 400hz
equipment. The 400hz noise in question is the 400hz converter in the
equipment shelter, that the mic is picking up.

---
So, the 400Hz _is_ acoustical.
---

The solution is to notch out the 400Hz at the transmitter (at the mic or
downstream from the mic) and keep it from being transmitted in the first
place. A passive RC notch will do it, but the price you'll pay will be
quite a bit of attenuation of the audio you _want_ to send. Use an
active solution like the one described in the URL I posted for you
earlier and you can adjust the Q of the filter to pretty much leave most
of the audio alone except for the offending signal. Cheap, too. A dual
opamp and a handful of discretes and you're there...

If you'd rather buy than build, I can put something together for you for
about $20.00. Email me if you're interested.

Thanks for the good tips. Will try both the active OPAMP circuit, and the
passive filter. Prefer just the passive, as I have quite the limited line
voltage supply.

---
Since the transmitter will be ON when it's transmitting and the active
notch will only need to draw a few milliamps when it's working, chances
are you can get what you need from the radio or, if the mic is active,
from the mic itself.

--
John Fields

 

[Michael A. Terrell]

Dana wrote:

Up here In Fairbanks our choices are very limited.

Thanks.

They are probably better than when I was there. I spent a year at Ft
Greely, Aug. '73 to Aug. '74. We couldn't even get bulbs for the film
chain at the TV station, and all the equipment was classed as depot
level repair only.
--


Michael A. Terrell
Central Florida

 

[Jim Thompson]

On Mon, 29 Sep 2003 17:51:36 -0800, "Dana" <dana.raffaniello@gci.net>
wrote:

Looking to notch out a 400HZ whine from some audio circuits connecting
remote manned sites to a central location.
The audio is microwaved from remote location to the central location.
Would like to know the formula for determing the resistor and capacitor
values.
The overall bandpass is 300HZ to 3KHZ.
Or would it be better to just use an High Pass filter.
Problem with that is getting the inductors locally.
Up here In Fairbanks our choices are very limited.

Thanks.

See "GyratorFilterNotch.pdf" on the S.E.D/Schematics page of my
website.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[Gary Richardson]

"Dana" <dana.raffaniello@gci.net> wrote in message
news:vnj8oj49a2si1d@corp.supernews.com...

"John Fields" <jfields@austininstruments.com> wrote in message
news48jnvcsp26c8dhp0m05ink64i8tbi8gok@4ax.com...
On Tue, 30 Sep 2003 07:15:13 -0800, "Dana" <dana.raffaniello@gci.net
wrote:


That is more like what I am looking for, as I would prefer a passive
device
over an active. I originally came up with an LC notch filter, but
cannot
obtain any inducters locally, so I would like to look at just using an
rc
filter. But am having problems remembering from school the calculations
needed. That is the problem with technology going to board level and
even
box level repair, tend to forget the basics.
So does anyone remember how to determine component values for an rc
notch
filter.

---
If you'd take a look at the URL I referred you to you'd find that the
equations for everything you want to do are right there, whether you
want to use a passive or an active solution.


Domo arigato
Thanks.

--
John Fields

You might also read section 5.09 in A of E which has a good discussion of
twin-T and also the bridged differentiator, which is a tunable notch filter.

 

[its you]

Probably not helpful, but sometimes I use filter free, a pgm from
filter solutions, I think... ftp://ftp.nuhertz.com/filter/FF3_1.exe
does a variety of filters, gives you schematics, stuff like that.
Sorry if no help,...
On Tue, 30 Sep 2003 18:17:15 -0700, Jim Thompson
<invalid@invalid.invalid> wrote:

On Mon, 29 Sep 2003 17:51:36 -0800, "Dana" <dana.raffaniello@gci.net
wrote:

Looking to notch out a 400HZ whine from some audio circuits connecting
remote manned sites to a central location.
The audio is microwaved from remote location to the central location.
Would like to know the formula for determing the resistor and capacitor
values.
The overall bandpass is 300HZ to 3KHZ.
Or would it be better to just use an High Pass filter.
Problem with that is getting the inductors locally.
Up here In Fairbanks our choices are very limited.

Thanks.


See "GyratorFilterNotch.pdf" on the S.E.D/Schematics page of my
website.

...Jim Thompson

 

[John Fortier]

"Dana" <dana.raffaniello@gci.net> wrote in message
news:vnj73i3nf1mg89@corp.supernews.com...

jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in message
news:bk2invgfeku5396dqvohv0e4abel7eqku3@4ax.com...
On Tue, 30 Sep 2003 03:00:32 GMT, Al Clark <dsp@danvillesignal.com
wrote:

"Dana" <dana.raffaniello@gci.net> wrote in news:vnho0p7j179of7
@corp.supernews.com:

Looking to notch out a 400HZ whine from some audio circuits
connecting
remote manned sites to a central location.
The audio is microwaved from remote location to the central location.
Would like to know the formula for determing the resistor and
capacitor
values.
The overall bandpass is 300HZ to 3KHZ.
Or would it be better to just use an High Pass filter.
Problem with that is getting the inductors locally.
Up here In Fairbanks our choices are very limited.

Thanks.



This is an easy application for one of our small DSP boards. Our
DSP-8300
is flash programmable, and has a stereo 16 bit audio codec. You could
program the board with a 400 Hz notch and also a very good bandpass
filter if you want. We can help you with the code.

Details are on our web site.


Or use three capacitors and three resistors in a twin-tee notch. The
software is somewhat easier.

That is more like what I am looking for, as I would prefer a passive
device
over an active. I originally came up with an LC notch filter, but cannot
obtain any inducters locally, so I would like to look at just using an rc
filter. But am having problems remembering from school the calculations
needed. That is the problem with technology going to board level and even
box level repair, tend to forget the basics.
So does anyone remember how to determine component values for an rc notch
filter.

John

Fo = 1/(2*Pi*R*C)