where exactly to put the noise filter

Hey Mike,

What's the cure for "bleed through" from one com radio to the other?

We usually keep "Com 2" tuned to our local AWOS frequency. When we're near
Iowa City, we can faintly hear it in the background through our
headphones -- even when "Com 2" or "Both" aren't selected on our audio
panel.

Of course, this isn't an issue without ANR headsets (couldn't hear it), or
when we're far enough away from the strong signal -- but it's one of those
little annoyances that always bugs us.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:

Hey Mike,

What's the cure for "bleed through" from one com radio to the other?

Usually "bleed through" refers to the effects of RF overload on Com2
while transmitting on Com1, or vice versa. Most audio panels contain
automatic muting circuits which completely block the audio from opposite
com reciever while the other's transmitter is keyed.

We usually keep "Com 2" tuned to our local AWOS frequency. When we're near
Iowa City, we can faintly hear it in the background through our
headphones -- even when "Com 2" or "Both" aren't selected on our audio
panel.

This sounds like inadequate isolation in the audio source switching of
the audio panel. Most of modern audio panels (King, PSE, RST) do not
intrisically have this problem. I think you have a PSE, which shouldn't
do this.

It is likely that your installer did something funky with the audio
wiring between Com1/Com2 and the audio panel to induce this. Could be
that the Speaker outputs of the Com radios are wired to 8 Ohm dummy load
resistors inside the audio panel. If the installer took a shortcut and
shared the ground connection from speaker-low output on the Com(s) to
the audio ground on the audio panel, it created a classic ground loop
between the speaker audio loop and the headphone output loop, thereby
creating the crosstalk. Better wiring technique might have prevented this.

Most modern Com radios dont care if the speaker outputs are terminated
into a dummy load resistor when not connected to the overhead speaker,
so it isn't even necessary to utilize those loads built into the audio
panel.

Of course, this isn't an issue without ANR headsets (couldn't hear it), or
when we're far enough away from the strong signal -- but it's one of those
little annoyances that always bugs us.

For further diagnosis, I'll bet that if you turn the volume control of
the non-selected comm radio down, then the bleedthrough will go away.
Also try switching the non-selected com receiver to the overhead speaker
while listening to the primary com on the ANR headsets, and report back.

MikeM
Skylane \1MM
Pacer \00Z

This sounds like inadequate isolation in the audio source switching of
the audio panel. Most of modern audio panels (King, PSE, RST) do not
intrisically have this problem. I think you have a PSE, which shouldn't
do this.

Actually, I have a Narco CP-136 audio panel.

(See a picture of this model at http://www.ronhahn.com/Hawk-XP/CP136.html )

For further diagnosis, I'll bet that if you turn the volume control of
the non-selected comm radio down, then the bleedthrough will go away.

Yep, I believe it does. So this means the audio panel is wired incorrectly?

Also try switching the non-selected com receiver to the overhead speaker
while listening to the primary com on the ANR headsets, and report back.

Okay, will do. I should expect to hear the AWOS broadcasting quietly on the
speaker, I presume, same as I hear in my headset?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:

This sounds like inadequate isolation in the audio source switching of
the audio panel. Most of modern audio panels (King, PSE, RST) do not
intrisically have this problem. I think you have a PSE, which shouldn't
do this.


Actually, I have a Narco CP-136 audio panel.

I do not have any experience with these, so the lack of isolation could be
a shortcoming of the design.

(See a picture of this model at http://www.ronhahn.com/Hawk-XP/CP136.html )


For further diagnosis, I'll bet that if you turn the volume control of
the non-selected comm radio down, then the bleedthrough will go away.


Yep, I believe it does. So this means the audio panel is wired incorrectly?

It just means that the bleed-through is happening at audio frequencies; not by
one IF coupling to the other IF as another poster postulated.


Also try switching the non-selected com receiver to the overhead speaker
while listening to the primary com on the ANR headsets, and report back.


Okay, will do. I should expect to hear the AWOS broadcasting quietly on the
speaker, I presume, same as I hear in my headset?

Yes, but try to determine if the faint feedthrough you hear in your headphones
is reduced by routing com2 to the speaker. It will be a subtle difference,
because the speaker audio will leak into your headphones accoustically. If the
feedthrough is reduced, then my speculation about the dummy load resistors would
warrant more investigation.

MikeM
Skylane '1MM
Pacer '00Z

On Fri, 20 Feb 2004 17:57:10 GMT, "Jay Honeck"
wrote:

Hey Mike,

What's the cure for "bleed through" from one com radio to the other?

We usually keep "Com 2" tuned to our local AWOS frequency. When we're near
Iowa City, we can faintly hear it in the background through our
headphones -- even when "Com 2" or "Both" aren't selected on our audio
panel.

Of course, this isn't an issue without ANR headsets (couldn't hear it), or
when we're far enough away from the strong signal -- but it's one of those
little annoyances that always bugs us.

Jay - what make/model are the COM radios in your plane?

-nathan

Jay - what make/model are the COM radios in your plane?

Com 1 is a brand new Narco 810-R, digital. (It's the slide-in replacement
for the Com 120)

Com 2 is an old-but-perfect Narco 120.

Both work flawlessly.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On Fri, 20 Feb 2004 23:11:27 GMT, "Jay Honeck"
wrote:

Jay - what make/model are the COM radios in your plane?

Com 1 is a brand new Narco 810-R, digital. (It's the slide-in replacement
for the Com 120)

Com 2 is an old-but-perfect Narco 120.

Hi Jay,

Take this with a caveat as I am a digital engineer who likes to learn
about RF stuff. I could be in left field here...

I asked because I was trying to determine if the radios utilized the
same IF frequency (which they would if it is the same radio). I don't
know enough about the Narco 810 or 120 to determine if they share the
same IF.

Where I was going with this...

The bleedthrough might be leakage from the IF section of one radio
coupling into the IF of the other radio. That would explain why the
audio is only noticeable as you are near the AWOS transmitter, where
the energy entering the IF stage on COM2 would be strongest.

An interesting test would be:

1. Shut off COM2 on the audio panel
2. Fly to a distant location where you can't hear the bleedthrough.
3. Turn on the audio for COM2
4. Can you hear the AWOS over the audio panel?

If you can hear the audio, then I suspect the audio problem is related
to the radio and not the audio panel (or audio wiring).

-Nathan

An interesting test would be:

1. Shut off COM2 on the audio panel
2. Fly to a distant location where you can't hear the bleedthrough.
3. Turn on the audio for COM2
4. Can you hear the AWOS over the audio panel?

If you can hear the audio, then I suspect the audio problem is related
to the radio and not the audio panel (or audio wiring).

I don't think I've got enough information here. What position would I have
the audio panel in for this test? Com 1, Com 2, or Both?

And, assuming Both, what do you mean when you say "Can I hear the AWOS over
the audio panel?" Do you mean on the speaker? Or in my headphones?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On Sat, 21 Feb 2004 02:24:12 GMT, "Jay Honeck"
wrote:

An interesting test would be:

1. Shut off COM2 on the audio panel
2. Fly to a distant location where you can't hear the bleedthrough.
3. Turn on the audio for COM2
4. Can you hear the AWOS over the audio panel?

If you can hear the audio, then I suspect the audio problem is related
to the radio and not the audio panel (or audio wiring).

I don't think I've got enough information here. What position would I have
the audio panel in for this test? Com 1, Com 2, or Both?

And, assuming Both, what do you mean when you say "Can I hear the AWOS over
the audio panel?" Do you mean on the speaker? Or in my headphones?

Ahhh, not exactly clear given your audio panel. I was debugging this
thinking of the KMA24 in my plane. I meant headphones only.

Going back a few steps (to make sure we're debugging the correct
problem) - the initial problem was with ONLY COM1 selected, you hear
the station that COM2 is tuned (to) through the headset?

For step 1 above, listen ONLY to COM1, but leave COM2 on and tuned to
the AWOS. You should still hear the AWOS on the bleedthrough on COM1.

Once you get far enough away that you no longer hear the bleedthrough,
deselect COM1 and select COM2 to be played through the headset. See
if you can hear the AWOS.

If you can hear the AWOS, I suspect the problem is due to IF coupling
between the radios. The reasoning being that if COM2 is still
receving the AWOS and producing audio - but the bleedthrough is gone,
than it is not a wiring/audio panel problem.

Anyway, Mike's idea is a bit easier to check. Vary the volume on COM2
and see if the volume on the bleedthrough changes. If it does, you
probably have some combination of a wiring/audio panel problem.

If it doesn't try my test.

Aren't avionics in 30 year old planes fun?

-Nathan

Going back a few steps (to make sure we're debugging the correct
problem) - the initial problem was with ONLY COM1 selected, you hear
the station that COM2 is tuned (to) through the headset?

Yes, very faintly. It's only noticeable on (or over) the field, with the
AWOS transmitter nearby.

Once you get far enough away that you no longer hear the bleedthrough,
deselect COM1 and select COM2 to be played through the headset. See
if you can hear the AWOS.

I already know the answer is "yes." The bleedthrough is only audible when
we're close to the transmitter.

If you can hear the AWOS, I suspect the problem is due to IF coupling
between the radios. The reasoning being that if COM2 is still
receving the AWOS and producing audio - but the bleedthrough is gone,
than it is not a wiring/audio panel problem.

Okay, so what would the "fix" be?

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"