Lowering cockpit RF interference

I had the same problem, though with a 12v T&B, and used a ferrit in the
power lead to solve it.


The only electrical noise problem I've had was when I installed a 28v T&B
which was powered of the glider's 12v supply with a solid state 12v-28v
DC-DC converter. The solution was to put an electrolytic capacitor across
the 12V supply to the converter. This was a specialized noise cancelling
capacitor and IIRC it is 150 uF with a 1.5A transient current capacity.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Sat, 15 Dec 2012 11:49:06 -0700, Dan Marotta wrote:

I had the same problem, though with a 12v T&B, and used a ferrit in the
power lead to solve it.

I think I tried a ferrite core on the 12v side of the voltage converter
without success and then remembered that Maplins (the UK equivalent of
RadioShack) had these capacitors at a reasonable price. Better yet it was
easy to fit: I just soldered it across the T&B power switch.


The only electrical noise problem I've had was when I installed a 28v
T&B which was powered of the glider's 12v supply with a solid state
12v-28v DC-DC converter. The solution was to put an electrolytic
capacitor across the 12V supply to the converter. This was a
specialized noise cancelling capacitor and IIRC it is 150 uF with a
1.5A transient current capacity.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Saturday, December 15, 2012 2:32:26 PM UTC-8, Martin Gregorie wrote:
On Sat, 15 Dec 2012 11:49:06 -0700, Dan Marotta wrote:

I think I tried a ferrite core on the 12v side of the voltage converter

without success and then remembered that Maplins (the UK equivalent of

RadioShack) had these capacitors at a reasonable price. Better yet it was

easy to fit: I just soldered it across the T&B power switch.

martin@ | Martin Gregorie

gregorie. | Essex, UK

org


A couple of thoughts here . . .

When you say you soldered it "across" the power switch, that might lead one to assume you soldered it to the two terminals of your switch, i.e. in parallel with the switch contacts). Normally a filter capacitor would be connected across the power leads, i.e. from + to -. While a filter inductor (such as a ferrite) is connected in series.

When trouble shooting RFI, it helps to be able to quantify results. If it's interferance on comm freqencies, a hand held radio can be useful - squelch off, and distance away from suspected RFI source as appropriate one can use the handheld as a field strength meter for radiated interferance.

This is not so much use when dealing with closely spaced instruments on the panel where one is causing problems with another (LNAV did that to Comm on one of previous glider). There, just slapping in clamp-ferrites did the job.

If you have enough slack in wiring leads, and the size of the ferrite allows, you get almost double the iductance for loopiing the wire/s through the ferrite twice, (or even more for 3 times etc). Sometimes adding a capacitor in concert with the ferrite helps even more, especially when noise is coupling via power leads (ferrite or inductor in series and cap across the wires in parallel to form a classic pi filter).

Ferrites are not all the same, they have a frequency band at which they will be most effective. Same thing with the size of capacitors used in filtering apps. (do a search on inductive and capacitive reactance and resonant frequecy). In the U.S., Digi-Key is one good source for ferrites etc.

bumper
zz
Minden, NV

bumper

On Sun, 16 Dec 2012 11:02:57 -0800, bumper wrote:


When you say you soldered it "across" the power switch, that might lead
one to assume you soldered it to the two terminals of your switch, i.e.
in parallel with the switch contacts). Normally a filter capacitor would
be connected across the power leads, i.e. from + to -. While a filter
inductor (such as a ferrite) is connected in series.

The switch is a double pole push-on push-off type, so one pole each for
ground and 12v I was able to use it as a two position tag strip, which
made wiring easier. The capacitor is across the +12 and ground connectors
on the T&B side of the switch.

When trouble shooting RFI, it helps to be able to quantify results. If
it's interferance on comm freqencies, a hand held radio can be useful -
squelch off, and distance away from suspected RFI source as appropriate
one can use the handheld as a field strength meter for radiated
interferance.

It was purely electrical noise that cut in when the T&B was nearly up to
speed. I don't think it was RF because:

(a)ferrites didn't have any effect

(b) if it was coming from the DC-DC 12-28v solid state adapter I'd have
expected to hear it loudest when the T&B started because thats when you
see the biggest current drain from an electric motor.

(c) an old, mechanical T&B shouldn't be an RF source unless its sparking
like hell. This is a Mil-spec R C Allen type MD-4A, so possibly ex-USAF,
that has been modified to suit a glider's rate of turn rather than a
power plane. I'm told that these devices have a centrifugal switch that
sets the RPM and has the side effect of switching in and out at several
tens of cycles a second when its up to speed. That is what the racket
sounded like too.

This is not so much use when dealing with closely spaced instruments on
the panel where one is causing problems with another (LNAV did that to
Comm on one of previous glider). There, just slapping in clamp-ferrites
did the job.

This was very easily localized: T&B off: no problem. T&B turned on: nasty
noise on the radio once it had spun up. No effect from turning any other
stuff on or off.

....other good stuff snipped.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Been a long time since EE school, but, IIRC, "switching on and off" creates
a LOT of RF noise.


"Martin Gregorie" wrote in message
...
On Sun, 16 Dec 2012 11:02:57 -0800, bumper wrote:


When you say you soldered it "across" the power switch, that might lead
one to assume you soldered it to the two terminals of your switch, i.e.
in parallel with the switch contacts). Normally a filter capacitor would
be connected across the power leads, i.e. from + to -. While a filter
inductor (such as a ferrite) is connected in series.

The switch is a double pole push-on push-off type, so one pole each for
ground and 12v I was able to use it as a two position tag strip, which
made wiring easier. The capacitor is across the +12 and ground connectors
on the T&B side of the switch.

When trouble shooting RFI, it helps to be able to quantify results. If
it's interferance on comm freqencies, a hand held radio can be useful -
squelch off, and distance away from suspected RFI source as appropriate
one can use the handheld as a field strength meter for radiated
interferance.

It was purely electrical noise that cut in when the T&B was nearly up to
speed. I don't think it was RF because:

(a)ferrites didn't have any effect

(b) if it was coming from the DC-DC 12-28v solid state adapter I'd have
expected to hear it loudest when the T&B started because thats when you
see the biggest current drain from an electric motor.

(c) an old, mechanical T&B shouldn't be an RF source unless its sparking
like hell. This is a Mil-spec R C Allen type MD-4A, so possibly ex-USAF,
that has been modified to suit a glider's rate of turn rather than a
power plane. I'm told that these devices have a centrifugal switch that
sets the RPM and has the side effect of switching in and out at several
tens of cycles a second when its up to speed. That is what the racket
sounded like too.

This is not so much use when dealing with closely spaced instruments on
the panel where one is causing problems with another (LNAV did that to
Comm on one of previous glider). There, just slapping in clamp-ferrites
did the job.

This was very easily localized: T&B off: no problem. T&B turned on: nasty
noise on the radio once it had spun up. No effect from turning any other
stuff on or off.

...other good stuff snipped.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Sun, 16 Dec 2012 17:12:24 -0700, Dan Marotta wrote:

Been a long time since EE school, but, IIRC, "switching on and off"
creates a LOT of RF noise.

Even at those low voltages and currents?

But what does solving the problem by fitting a simple capacitor across
the supply say about the relative amounts of RV vs. electrical noise?
Bear in mind that there's around 50cm of unshielded cable between the
capacitor on the switch and the T&B - cable that was supplied with it.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

You're making my brain hurt thinking about these things! Switching DC on
and off amounts to a square wave which, as we all know, is made up of an
infinite series of sine waves, i.e., DC to light, and somewhere in there is
the RF spectrum.

From my days as an Air Force radio tecnhician, before becoming an edumacated
AF ossifer and pilot, my experience with receivers indicated that just a
couple of micro volts at the antenna input would break squelch. Now that
was from a 5 watt transmitter many, many miles away (line of sight); and
your source is merely inches away. A capacitor of the correct value acts as
a short circuit to RF at the tuned frequency whereas an inductor (RF choke)
acts like a high resistance (at the tuned frequency). Either or both of
these of the proper values and connected in the proper way will greatly
reduce or eliminate RF interference.

In my case, with the turn indicator causing the interference, I rummaged
through an electronics parts bin (actually a jelly jar) and found a
ready-made RF choke which consisted of a torroid with two wraps of wire
giving 4 loose ends. I connected one wrap (about 4 or 5 turns) in series
with +12v and the other in series with the ground to the instrument.
Problem solved.


"Martin Gregorie" wrote in message
...
On Sun, 16 Dec 2012 17:12:24 -0700, Dan Marotta wrote:

Been a long time since EE school, but, IIRC, "switching on and off"
creates a LOT of RF noise.

Even at those low voltages and currents?

But what does solving the problem by fitting a simple capacitor across
the supply say about the relative amounts of RV vs. electrical noise?
Bear in mind that there's around 50cm of unshielded cable between the
capacitor on the switch and the T&B - cable that was supplied with it.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |