Radio EMI problem...

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

nospam@nowhere.nil - 05 Jul 2004 21:32 GMT

I'm trying to track down an EMI problem.
This is with my solar powered RC plane.

Specifics:
I'm using standard 6M RC gear from JR on ch 04, FM on 50.880 Mhz.
I've got two receivers that exhibit exactly the same problem.
A JR FM receiver and a berg FM receiver.

Both work fine until I turn on my switching solar maximum power point
tracker.

The power tracker runs 6 seperate channels at about 25W each channel.
The switcher runs at ~ 150Khz +/- 20Khz

I've filered, shielded and added caps with very little effect.

The receiver is in the tail of the airplane the converter in the
front,. There is currently no electrical connection from the receiver
to the other electronics on the plane. I'm currently powering the
receiver from battries. The only connection is capactive to the
carbon fiber airframe.

I'm using an Icom R3 to hunt for the EMI.

The Icom receivers shows the 50.880 signal clear as a bell.
When I turn the TX off the icom sees no no difference in background
signal with the power converter either on or off.

When I run the Icom receiver tuned to 50.880 near the RC receiver I
see a significant reduction in range, but not as much reduction as
when I run the power converter.

My first theory is that I am generating IF or image noise that is
clobbering the receiver.
To test this I set a signal generator out next to the receiver and ran
it at 450Khz, 455Khz and 10.7 Mhz with no effect on range.
(signal generator is an old digital fluke runing 13dbm into a 2 ft
peice of wire)

Any ideas on what I can do to solve this... it is driving me crazy,
I've been working on it for more than a month.

Paul (Kl7JG)

Reply to this Message

Doug McLaren - 06 Jul 2004 04:24 GMT

| Specifics:
| I'm using standard 6M RC gear from JR on ch 04, FM on 50.880 Mhz.
| I've got two receivers that exhibit exactly the same problem.

Speaking of the problem, what is the problem? Jittering? Loss of
range? Something else?

| A JR FM receiver and a berg FM receiver.

Note that both of these are single conversion receivers. But they're
the good single conversion receivers -- in theory, as good as the dual
conversion receives out there, but the duals might still be better for
some applications, perhaps yours.

You might want to try a dual conversion receiver -- they might be
better able to reject the interference you're seeing.

| The power tracker runs 6 seperate channels at about 25W each channel.
| The switcher runs at ~ 150Khz +/- 20Khz

You're probably seeing harmonics of that, somewhere. I'm not sure
what this thing is, but switching power supplies are well known for
generating interference. ESCs do too, but the capacitors used on them
filter it out pretty effectively.

| I've filered, shielded and added caps with very little effect.

Ferrite beads around all the wires to it perhaps? The servo wires
might also be bringing it into the receiver -- I've seen people
putting small chokes in their servo wires, perhaps that would help?
(I think it was mentioned in the most recent Model Airplane News.)

| I'm using an Icom R3 to hunt for the EMI.
|
| The Icom receivers shows the 50.880 signal clear as a bell.

Unfortunately, RC receivers tend to be cheap, and will get birdies and
such. Just because there's no interference at 50.880 mhz that doesn't
mean that there's nothing to interfere with the RC receiver.

| When I run the Icom receiver tuned to 50.880 near the RC receiver I
| see a significant reduction in range, but not as much reduction as
| when I run the power converter.

The Icom receiver causes interference by itself?

| My first theory is that I am generating IF or image noise that is
| clobbering the receiver.
| To test this I set a signal generator out next to the receiver and ran
| it at 450Khz, 455Khz and 10.7 Mhz with no effect on range.

I believe that 10.7 mHz is a dual conversion thing, and neither of
your receivers is dual conversion. Well, JR does make some, but most
of their receivers are not.

| Any ideas on what I can do to solve this... it is driving me crazy,
| I've been working on it for more than a month.

Well, if it's a solar plane, it's probably costs enough that $50-$100
more for a receiver is well spent if it makes the problem go away. :)

And you could try 72 mHz equipment ... maybe it would work better? In
theory it should work the same, but it's a possibility.

Signature

Doug McLaren, dougmc@frenzy.com
These pills can't be habit-forming; I've been taking them for years!

Reply to this Message

nospam@nowhere.nil - 06 Jul 2004 05:18 GMT

>Speaking of the problem, what is the problem? Jittering? Loss of
>range? Something else?

Loss of range. (60% reduction),

>You might want to try a dual conversion receiver -- they might be
>better able to reject the interference you're seeing.

Who makes a good 6M dual conversion receiver? (See the end for why it
gopt to be 6M)

>| The power tracker runs 6 seperate channels at about 25W each channel.
>| The switcher runs at ~ 150Khz +/- 20Khz
[quoted text clipped - 8 lines]
>putting small chokes in their servo wires, perhaps that would help?
>(I think it was mentioned in the most recent Model Airplane News.)

I've got both common mode and normal mode inductive, ferite filters
on all leads going to the receiver.
The problem still exists with the receiver completly disconnected from
the other aircraft wiring. The servo leads are short (6 inches) and
filtered.

>| I'm using an Icom R3 to hunt for the EMI.
>|
[quoted text clipped - 9 lines]
>
>The Icom receiver causes interference by itself?

Not uncommon for two receivers tuned to the same area to have LO image
problems.

>| My first theory is that I am generating IF or image noise that is
>| clobbering the receiver.
[quoted text clipped - 10 lines]
>Well, if it's a solar plane, it's probably costs enough that $50-$100
>more for a receiver is well spent if it makes the problem go away. :)

I'd gladly spend $500 for a receiver if it made the problem go away.
6M receivers are hard to come by, JR, Berg and FMA are the only ones I
know of.
I had some real bad problems with an FMA a while a go and I'm a bit
gun shy.

>And you could try 72 mHz equipment ... maybe it would work better? In
>theory it should work the same, but it's a possibility.

One of the purposes of the airplane is to set some FAI records,
I've built a 1W 6M amplifier and put it on a 4 element 6M yagi so I
have positive control. I can't do that on 72Mhz.
My higher power rig also limits me to straight FM, not PCM as I drive
my 1W transmitter with a trainer cord. This problem is with the normal
JR Tx, not my super Tx. As a side note I've put my Super TX on a
spectrum analizer and it looks good well within normal Ham tx purity
limits.

P.S.
Before anyone freaks out wrt this project and the AMA presidents
last note about being nice with the FAA, I've been talking with the
local FAA FSDO (Flight Standards District Office ) and we have an
understanding as to where I can fly this for alttitude attempts how
high, notification requirements etc....

Reply to this Message

Doug McLaren - 06 Jul 2004 06:27 GMT

| >Speaking of the problem, what is the problem? Jittering? Loss of
| >range? Something else?
|
| Loss of range. (60% reduction),

Hmm, not good. Any sort of strong RF intereference can desense a
receiver, even if it doesn't directly interfere.

How far are you trying to go anyways? I'd think that with good
directional antennas, pointed in the right direction, you could do 5
or so miles even with just one watt.

| >You might want to try a dual conversion receiver -- they might be
| >better able to reject the interference you're seeing.
| Who makes a good 6M dual conversion receiver? (See the end for why it
| gopt to be 6M)

| I'd gladly spend $500 for a receiver if it made the problem go away.
| 6M receivers are hard to come by, JR, Berg and FMA are the only ones
| I know of.

Futaba does too. I've got a few of them. They seem to work fine,
though I don't have the challenges that you do.

I don't think there's any shift issues with the 50 mhz equipment, so a
Futaba 50 mhz receiver should work fine with your JR transmitter.
Were you close, I'd let you try one of them, but you can order one
from Tower Hobbies easily enough for under $100.

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXAKL8&P=ML
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXAKM4&P=7

You might be able to get some more range by properly tuning the
receiver antenna, making it the right length to be a 1/4 wave antenna?
Perhaps cracking open the antenna and finding the other lead, turning
it into a 1/2 wave dipole? (This all depends on your receiver
internals, of course -- there might be a coil inside to make the
antenna resonant even though it's less than 1/4 wavelength, especially
for 50 mhz. Maybe making a 5/8 wave antenna? (Of course, that would
be very long.) Sounds like you know what you're doing, so you can try
that.

| I had some real bad problems with an FMA a while a go and I'm a bit
| gun shy.

I've had fine luck with FMA receivers, though I've not tried a 50 mhz one.

| >And you could try 72 mHz equipment ... maybe it would work better? In
| >theory it should work the same, but it's a possibility.
|
| One of the purposes of the airplane is to set some FAI records,
| I've built a 1W 6M amplifier and put it on a 4 element 6M yagi so I
| have positive control. I can't do that on 72Mhz.

Most R/C transmitters transmit at one watt, whatever the band. So why
would you need a one watt amplifier? Or was 1W a typo, and the actual
figure is higher?

The FCC regulations only permit one watt for R/C control on the ham
bands, so I don't see where they let you use more power than the 72
mhz band anyways.

And is there any reason you couldn't use a directional antenna on 72
mhz? I don't think the 72 mhz band would let you make your own
transmitter, but I don't think it would prevent you from making your
own antennas?

| My higher power rig also limits me to straight FM, not PCM as I
| drive my 1W transmitter with a trainer cord.

I'm not sure that's the case. PCM uses the same FM transmission mode
as FM (PPM) does, it's just that the signal being sent is different,
PCM vs PPM. Your transmitter should happily transmit the PCM signal
just like the FM signal.

... though some transmitters do send a PPM signal over the buddy box
cord, even when in PCM mode. But this would only be the high end
transmitters ... the lower end ones send the same signal as they
transmit.

The Futaba 6XAS radio I have definately sends PCM both ways. But the
9C I have doesn't -- or, at least it has the option of not doing so.
If your radio has a module, you might be able to pull the PCM signal
from the wires going to the module if you need to.

Not that I think PCM will help -- it will have exactly the same range
problems as PPM.

Signature

Doug McLaren, dougmc@frenzy.com, AD5RH
`I never entertain wicked thoughts -- but sometimes they entertain me.'
-- Laurence Peter

Reply to this Message

potifar - 06 Jul 2004 08:40 GMT

after reading the complete dialog, the odds are that a harmonic of th
switching frequency gets into the IF of the receiver(s).

if this is true, then dual conversion would not help here since it ha
its highest gain part in its second IF which is essentially identica
to that in a single conversion receiver (but it would be worth th
test).

you will probably wind up having to completely shield the switcher
using feed-through capacitors at its inputs and outputs.

secondly, you may want to build a metal can for the receiver to shiel
its IF strip which uses non-shielded filters in most receivers. an
ground the crystal can. if the switcher is operating with those hig
currents it most likely puts out a nice picket fence on the spectru
analyzer - if you could buy, borrow or steal a spectrum analyzer tha
would save you a whole bunch of time.

you would probably also benefit from a coaxial antena connection an
use a 'tuned' (appropiately loaded) dipole-like antenna, as far away a
possible from switcher.

where are you located?

peter berg
kq6a

--
potifa

Reply to this Message

nospam@nowhere.nil - 06 Jul 2004 14:28 GMT

Several comments:

1W Tx......
When an RC manufactuor actually publishes TX watage values it is
500mw or 750mw. I know that 1W is not much more, and may not be worth
the effort. My 1W tx delivers 1W 3 inches at the terminals of a 3 el
yagi. That is probably a different discussion.

Antennas...and noise source I have room to put a full resonant
dipole in the wings, but the wings are covered with solar cells,
The wing also has an Al spar that conducts the return current from the
cells back to the center, so I'm not sure how well the antenna would
work that other conductive noisy components. ~12A of current in each
wing The switcher peak to peak ripple current is on the order of 1/2A
at the imput to the switcher, I've got a big cap on each of the
switcher in0put leads before they leave the foil enclosure.
I've already put the switcher in an Al foil enclosure
The switcher ripple current is very hard to reduce as power inductors
are heavy, I can either increse the Sw freq, or add bigger
inductors...

Note to Peter:
The first Rx I tried was one of your Berg rx's
I've also tried the JR, both seem to behave identically, the JR is
actually more sensitive to the interpherance as it does not try to
make sure the signal makes sense it just passes what it receives
straight through.....thus I've done most of the troubleshooting with
the JR.

I don't have a spectrum analizer I can take to the plane, but the Icom
R3 has a spectrum display with 5K,10K,20K,and 100K wide bins that I
can use to look at 1Mhz at a time. I've spent some time looking for
image problems, but I'm not sure where to look.

Is the Berg Rx IF at 455Khz?

One last note, The plane also has a 900Mhz telmetry radio tranmitting
at ~100mw, but turning that on and off has absolutly no detectable
effect on the RX range.

>where are you located?

I'm in San Diego, specifically in Solana beach.
Troubleshooting this problem is complicated by the fact that I can
only trouble shoot when it is sunny. We've had a lot of costal low
clouds in June...

Note to Peter I belive I have your personal E-Mail, if it has not
changed in the last 2 years I'll send you an E-Mail and try to set up
a phone conversation.

Paul KL7JG

>after reading the complete dialog, the odds are that a harmonic of the
>switching frequency gets into the IF of the receiver(s).
[quoted text clipped - 20 lines]
>peter berg
>kq6aa

Reply to this Message

Vogan - 13 Jul 2004 11:03 GMT

> Several comments:
>
[quoted text clipped - 41 lines]
> only trouble shoot when it is sunny. We've had a lot of costal low
> clouds in June...

<SNIP>

Just a thought, are you sure it is being caused by radiated emissions from
the switcher? Is the RX powered from the output of the switcher, if it is
then it could be conducted emissions getting into the RX that way. To test
this you could try powering the RX from a seperate battery pack while the
switcher is running. Like someone else said you could try making a shielded
box for the switcher.

Thinking about it the third harmonic of the switching frequency is almost
bang on the IF frequency (455kHz).

Thats about all I can think of at the moment, I know your pain I had to
track down a similar problem with a product at work a while back, noise from
a switching power supply getting into the RX of a radio, I fixed that and
then had problems on transmit :)

Best of luck
Vaughan

Reply to this Message

Olin K. McDaniel - 22 Jul 2004 04:54 GMT

>> Several comments:
>>
[quoted text clipped - 61 lines]
>Best of luck
>Vaughan

This subject attracted my interest, and am sorry it's about run dry.
I do have two 50.920 MHz dual conversion receivers, made by HiTec or
RCD, with Futaba J servo sockets. They are marked FM/HFD-08RD,
whatever that means. I had them in different planes some time ago,
and at present am not using them. I also still have the transmit
module and the rubber duck for 6 meters, so I can run any sort of test
you might like me to try. Or will lend/sell you one of these
receivers if you prefer.

What piques my curiosity though is - why do you need a 25 watt
switching power supply on board. I've fitted out planes with ATV
transmitter and mini video camera, but don't need anything more than
battery power here. Just curious, but willing to help if I can.

Olin McDaniel, W4PFZ, AMA 30932, Leader Member
To reply by email, please remove "abcd" from Return address
-----------------------------------------------------
"Ignorance is treatable, Stupidity is incurable. Sometimes
the difference is hardly distinguishable, however."

Reply to this Message

Model Forum / Radio Controlled / Air Models / July 2004

Radio EMI problem...