GPS creating noise

[alpinelakespilot2000]

My panel is firing up this week and I've noticed a noise issue with my GPS that I could use some help troubleshooting:

Step 1: tested intercom system: it works fanastically with both headsets and with my mp3 music input-no unwanted noise anywhere.

Step 2: I added the Lowrance 2000c GPS and powered it up using AA batteries. No noise in system at all.

Step 3: when I powered up the GPS using the power cord attached to my avionics buss and avionics ground buss I noticed that it introduced a relatively quiet but high-pitched tone into my intercom/headphones.

Step 4: I found that if I powered the GPS through the panel mounted power port (cigarette lighter), also grounded at the av. buss, the noise remained as in Step 3, but got quieter.

Step 5: In experimenting further, I found that if I used the power coming from my av. buss but removed the ground from the avionics ground buss and grounded it against the airframe elsewhere, the tone remained but got really quiet. I almost wouldn't notice it if I wasn't looking for it.

Thus, the main issue seems to be with the GPS ground wiring. Any suggestions on how to remove the noise entirely?

Thanks.

[MrNomad]

I have a 9A and use a Lowrance 2000c to drive the AP74 autopilot connected to a D180. My good friend Chet (retired EE) masterminded most of the grounding with Gil & so I forwarded your question to them.

I recall we installed a large copper ground plate that was grounded directly to the battery & frame. Every device grounded to that plate individually connecting to screws into nut plates. We never grounded to the frame or piggybacked one ground onto another.

You can buy a cable from LEI that precludes the use of the cig liter altogether. I believe Spruce sells it too for fewer $$ than LEI.

I thought they were nuts but my airplane is noise free. Hope this helps.

Barry
Tucson

[alpinelakespilot2000]

Originally Posted by MrNomad

You can buy a cable from LEI that precludes the use of the cig liter altogether. I believe Spruce sells it too for fewer $$ than LEI.

Thanks for the reply, Barry. I do in fact have the cable from LEI and is what I intend to use. I only used the cigarette lighter power once to see if it made any difference. (Slightly quieter than using the LEI cable but still there.) That it was significantly quieter when I grounded the LEI cable directly to the to the airframe (back of the panel), rather than to the dedicated avionics ground block, did surprise me, but I'm not smart enough to know why that might have been the case, especially since ALL of my other avionics except the intercom were off when I checked.

One thing I'm curious about, does the shield on the LEI cable need to be grounded to anything? Initially I just trimmed back the black outside sheathing and shield to pull out the wires that had to go different directions (power, ground, Dynon, etc.) and then just clipped off the excess shield. Maybe I should have grounded it as well? (The rule of thumb I'd always been taught is that if you want to keep noise "IN" a wire, ground both ends of the shield. Not sure if the GPS end of the LEI cable shield is grounded, but I know the other end is not.)

Anyway, thanks for taking the time to respond. Maybe someone else will have other suggestions to try.

[Bryan Wood]

Steve,

A ground issue is likely your problem as already stated, but out of curiosity what did you use for your power source? Is there any chance that you used a noisy source such as a battery charger?

[L.Adamson]

Originally Posted by Bryan Wood

Steve,

A ground issue is likely your problem as already stated, but out of curiosity what did you use for your power source? Is there any chance that you used a noisy source such as a battery charger?

If so, that's most likely it. My charger certainly added noise.

L.Adamson --- RV6A

[alpinelakespilot2000]

Originally Posted by Bryan Wood

Steve,

A ground issue is likely your problem as already stated, but out of curiosity what did you use for your power source? Is there any chance that you used a noisy source such as a battery charger?

Thanks Bryan. Actually, no, I am running power to my avionics buss off my fully-charged firewall mounted battery (PC680), no charger attached.

[Kahuna]

Originally Posted by alpinelakespilot2000

Any suggestions on how to remove the noise entirely?

Thanks.

Install a 4700 PF capacitor across the GPS power and ground connection

[alpinelakespilot2000]

Originally Posted by Kahuna

Install a 4700 PF capacitor across the GPS power and ground connection

Thanks Kahuna. Any elaboration on what this is or where I can get it? Something I can just pick up at a local electronics store?

[MrNomad]

Originally Posted by alpinelakespilot2000

One thing I'm curious about, does the shield on the LEI cable need to be grounded to anything?

Gil said: Ground the shield at the power source end to the airframe - or single point ground. Keep the ground pigtail short, so go to the nearest good ground.