[Paparazzi-devel] Re: Interference problems part 1

[Helmut Wabnig]

On Sat, 29 Mar 2008 13:00:02 -0400, you wrote:

>Date: Sat, 29 Mar 2008 15:16:02 +0530
>From: "Mohit Mundhra" <address@hidden>
>Subject: [Paparazzi-devel]
>To: address@hidden
>Message-ID:
>       <address@hidden>
>Content-Type: text/plain; charset="iso-8859-1"
>
>I am using Futaba R156F ppm receiver with the Tiny 1.1 board. After tapping
>the PPM Signal from the receiver, I am able to move the servos and the
>throttle. But it is giving some interference problem which was not there
>before I have soldered the cable to the receiver for PPM signal tapping.
>
>Is there any special cable to be used for signal tapping or is it due to
>some other problem ??
>
>-- 
>mundhra

I cannot answer the question directly, but I will try to describe what
I suggest to do. Recently I had the chance to fix interference
problems on a model airplane. The plane has a wingspan of 2 m
and weighs about 8 Kg. The body is 1,2 m long with the motor
at the front. Directly behind the motor sits the 100 Ampere motor
driver, mounted on an aluminium bracket.  25 cm from the nose
sits the power  lithium polymer accumulator of 5000 mAh. Then 
follows an empty space of about 30 cm, after which the 35 MHz Graupner
receiver is mounted together with a separate servo lithium accumulator
of smaller size. From there wires run to the digital servos and the
radio antenna is mounted directly in front of the tail fin. One cable
runs the whole length to the front for delivering control pulses to
the motor servo power board.

The pilot noted two separate different problems, one which is natural
and cannot be fixed, and one which is deadly disturbing but can be
fixed.

1.)
Switch on the receiver, the motor power and the remote control.
The first and unavoidable interference occurs  when you try to scratch
the 35 MHz receiving antenna with a piece of metal you hold in your
hand. That induces pulse errors on the analog modulated pulse trains
which the remote control produces and sends to the receiver on the
plane. The receiver demodulates them and converts the pulses
back to DC level. The radio connection is FM modulated analog-wise
grandmother's steam radio and very susceptive to that kind of noise.
There is no real digital data coding on those outdated remote
controls!

What happenes when we rub the antenna with a metal is that the servos
go crazy and the flaps dance up and down wildly. Now try to scratch
the motor or other metal pars of the plane, that will induce the same
kind of errors. Of course, because the antenna and the other metal
parts of the plane together comprise the receiving systen, not only
the antenna wire as such. During flight nobody can touch the 
metal parts and the system will work normally. I advice you to
have a shortwave radio at hand where you may listen to the noise
on your remote conntrol channels. Those scanners are not expensive
and can be bought from every radio amateur equipment dealer.
I can easily bring your plane down by transmitting crackling noise
FM audio on your remote control frequency. Such noise may be
present from industrial sources or other remote controls.
Up to now I did not have the chance to test 2,4 GHz remotes.

(Guess that post grows too long, what do you think?) 

Time for  a break.
w.