PDA

View Full Version : Radio Interference



Andrewpv01
26th March 2008, 04:28 AM
I have finally bought a "proper" GPS instead of the bluetooth mouse type I was usng before.

It is a Logic Gear XL488. I am having problems with radio interference. It is mounted to the windscreen on the drivers side at the bottom in a Falcon wagon (about 2 years old).

The problem I am having is when I start to get a little way form town I get a lot more static on the radio (FM). I only worked out it was the GPS causing it because I turned it off about 5-10 k's from home. There was an instant improvement in the radio reception.

I can duplicate these results anytime I am more than about 30+k's from the radio transmitter by turning the GPS on/off.

I didn't think these were allowed to cause any radio interference. Does anybody have any similar occurences with the Logic Gear? Should I move it somewhere else on the windscreen or do I just have a faulty one?

Thanks
Andrew

bshwckr
26th March 2008, 11:06 AM
G'Day Andrew
If possible, try the unit in another vehicle. If you get the same result, point the blame at the gps unit. This is not to say that it is faulty, it just proves that it is not your cars wiring system.
If it is the gps unit, it may be that the frequency it operates on is close to the frequency of the radio station you are listening to. Not neccessarily the processor speed but maybe the screen refresh rate etc. I would be surprised if this can be changed in the options of the GPS unit. Try tuning into a different radio station that has a very different frequency and see how that goes. There should be several stations you can chose from on Canobolas.
You can get supressors to reduce this RFI but you should'nt have to. As you say, these things should not interfer.
Just some ideas on how to pinpoint the problem.
Cheers
bshwckr

Andrewpv01
27th March 2008, 04:13 AM
I will try it in a different car in the next few days.

I think I have checked a couple of different stations and it did the same thing, but I will try a couple more too.

Thanks
Andrew

julianh
27th March 2008, 07:51 AM
A couple of things to try, to see if you can narrow down the exact source of interference:

a) Disconnect the power lead from the cigarette lighter, so that the GPS is running on its internal battery. If the interference goes away, this suggests an issue with the power cord and / or the cigarette light socket.

b) If the interference goes away when on battery power, plug the unit back in, and try moving the unit and the power cord around, to see if the drape of the cable is close to a sensitive part of the radio receiver. Also, try jiggling the sockets in the cigarette lighter and in the GPS, and flexing the cable - could be a poor contact at one end of the cable, or a bad wire in the cable itself.

c) If the interference remains when on battery power, try moving the unit around in the car - could be that the unit is located very close to a sensitive part of the antenna or similar.

Hope this helps!

festy
27th March 2008, 12:35 PM
A couple of things to try, to see if you can narrow down the exact source of interference:

a) Disconnect the power lead from the cigarette lighter, so that the GPS is running on its internal battery. If the interference goes away, this suggests an issue with the power cord and / or the cigarette light socket.
That's a great place to start - i'd imagine you'd be much more likely to get rfi at around 100MHz from a noisy switchmode PSU than from a GPSr.

Andrewpv01
1st April 2008, 03:32 AM
It turns out it is the psu for it.

I have unplugged it and the interference disappears.

Now to find where I can move it.

Thanks
Andrew

Nicko
1st April 2008, 11:13 AM
PSU as in Cigarette Lighter Cable?

festy
1st April 2008, 12:56 PM
Yes, the cigarette lighter cable is actually a DC-DC switchmode power supply unit (or SMPSU) that converts 12v DC to 5v DC.
The old style of 5v power supplies were linear converters, and converted the additional power to heat. This was not very efficient, and needed big heatsinks.
Switchmode power supplies are far more efficient and produce very little heat, but operate internally at high frequencies so require decent filtering. They're often overlooked in favour of linear supplies in radio applications, because of their tendency to produce radio frequency interference (rfi) - as in this case.

I used to use a linear supply to convert 12v into 5v (for pda) and 3.3v (for GPSr) but could not cool it well enough - supplying 1 amp @ 5v kept it hot enough to burn my leg on.
After redesigning the PSU to use a switchmode regulator instead, I've had no hassles drawing 2+ amps @5v with no heating issues.

There's various ways to reduce the interference, but one of the easier methods is to move the PSU away from the radio ;)