| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 710.1 | 3IM experience | LEDS::LEWIS | | Wed May 11 1988 02:06 | 52 |
|
An interesting situation occurred at the CMRCM field sunday. We
had a gorgeous day, complete with the virgin flight of Charlie Watt's
Aeromaster (I'm sure Charlie's gonna give us a report). Even saw
Anker down there, I guess CRRCM was too crowded? Now's the time
we can get back at Al - while we fly in 70 degree perfection he's
baking in the desert! (that outa provoke one of those "God's country"
comebacks!).
Well, we started using a new plan, I guess it's called the Pacific
plan? We have two flight stations 60' apart and you can have two
pilots per station. The allowed frequencies at each station are
picked to minimize 3IM.
It failed, but our conclusion is that it was most likely a faulty
receiver. The circumstances... Charlie Nelson was flying his OMH
biplane on channel 46. I went up on 50, next to Charlie. At the
other station, someone went up on 48. Our positions...
Charlie (46) Me (50) Other guy (48)
!------ 20'------|
|---------------60'----------------|
Charlie got hit on final approach and the OMH got banged up. At
first we thought it was because I was standing too far away from
Charlie (almost even spacing between the 3, a real no-no. So we
did a ground experiment. His plane was getting hit even worse when
I stood closer to him. Even when we moved farther away from the other
guy, the OMH was getting hit. It was definitely 3IM, because if I
turned off or the other guy turned off the interference went away.
The interesting thing is that we tried the same experiment with
another plane on 46 and there was no problem. Charlie had done
a frequency change via crystal swapping some time ago. Apparently
Charlie's Airtronics receiver doesn't reject 3IM as well, perhaps
because of the crystal swap.
From this I've learned to be cautious about 3IM no matter what special
frequency control plans are in place; I will not go up if two adjacent
channels are in use. Were we doing something wrong? Should we
expect this new plan to prevent 3IM? For example, should the following
spacing have worked if Charlie's receiver was ok?...
(46) (50) (48) (??)
|-5'-|--------------- 60+'-------------------|
Bill
P.S. the damage to the OMH wasn't bad. Not nearly as bad as Harvey
Thomasian's 1/4 scale cub, which had the engine quit 20' over the
runway. It went into the tall grass fairly hard and snapped in
half behind the wing.
|
| 710.2 | Is this how 3IM works? | CTHULU::YERAZUNIS | I will give you bodies beyond your wildest imaginings. | Wed May 11 1988 15:03 | 113 |
| Let me see if I have this straight (on how to calculate 3IM
interference)...
NOTE - I AM MAKING UP THESE FREQUENCIES- THEY ARE _NOT_ REAL FREQS
1st transmitter on 72.060 MHz
- standing right next to each other
- and the runway
2nd transmitter on 72.080 MHz
causes --> sidebands at 72.040, 72.100 (base
freq's + and - the beat freq.
3rd transmitter on 72.100 MHz <- this guy get shot down...
What happens (check me on this): The antennas on transmitters 1
and 2 also act as recievers; they feed a certain amount of RF energy
backwards into the power transistors in the transmitter finals.
For example, let's look at transmitter #2. It's antenna picks up
some of transmitter #1's RF, and this is coupled back into the
transistor final. This RF signal pulls the emitter up and down
in voltage at 72.060 MHz. Meanwhile, transmitter #2's RF driver
is pulling the base up and down at 72.080 MHz. Result is that
transmitter 2 puts out most of it's power at 72.080- but a little
at 72.080 +- (72.080 - 76.060) = 72.040, 72.100. (because the emitter
voltage excursions _look_ to the output transistor as base voltage
excursions!) Let's assume that the output on these sidebands
is just 1% of the transmitter power output.
(or maybe transmitter #2 is wired common-emitter- but the RF couples
across into the base. It doesn't matter how it gets there, really)
Well, we all know that RF power falls off with the square of the
distance between transmitter and reciever. So, if transmitter 2
was 10 feet from the ship, and transmitter 3 was 100 feet from the
ship, the "correct" and "interfering" signals at the ship would
be equal in strength- instant shoot-down.
But wait! There's more! No reciever is so good that it can tell
the difference between "correct" and "interfering" signals that
are equal in power. Typically, recievers need significantly more
power in the "correct" signal for them to pick out the "correct"
instead of "interfering" (or some noisy mix of both). The minimum
ratio of "correct" versus "interfering" is called the _capture ratio_
of the reciever. It's usually in dB (3 dB ~= doubling of voltage).
A _super_quality_ audiophile FM reciever might have a capture ratio
of .9 dB . I have such an FM reciever at home- it's about 3" high,
19" wide, 12" deep, weighs 10 lbs and cost $500. "Walkman"-type
radios typically have capture ratios around 3-10 dB. An RC reciever
(since it costs less than a walkman, and must be _much_ smaller
and lighter) probably has a worse capture ratio than that. But
let's assume 10 dB.
10 dB is about 2 x 2 x 2 x 1.25 or roughly a ratio of 10 in voltage
required for the reciever to separate "correct" versus "interfering"
signals. In our above example, transmitter 3 (the "correct"
transmitter) was 100 feet from the ship. The signal from #3 has
to be 10x stronger than the interfering signal from #2- so (square
root law again) transmitter #2 will again shoot down the ship if
it's closer than roughly 32 feet. That can concieveably happen.
(don't forget to add in such vagarities as polarization phenomena,
weird ground wave propagation, etc).
------------
This explains a lot of things:
Why 3IM requires 3 transmitters, equally spaced in frequency,
Why 3IM happens mostly during takeoff, landing, and ground ops
(much better chance to get close to the interfering
transmitter)
Why 3IM is greatly alleviated by increasing the distance
_between_ transmitters (greater distance -> less induced
voltage in transmitter #2's final -> decreased energy put
out in the +- sidebands. Remember, induced voltage decreases
with square of distance!)
Why 3IM risk is increased to everyone on the field whenever
anybody on the field uses a crappy transmitter- except to the guy
with the crap transmitter. (3IM from a given transmitter will
never be on the same channel as the primary freq. of the
interfering transmitter.)
Why FM is less susceptible to 3IM than AM - but when they go
they go big-time. That's because FM has a very sharp threshold
between fully-capturing the correct signal versus just getting
noise. AM deteriorates more quickly- but never has the sharp
drop-off that FM has.
Why better (more expensive) radios are less susceptible to 3IM-
a better reciever will have a better capture ratio. Every
6 dB better (or worse) in capture ratio doubles/halves the
3IM danger radius- which multiplies/divides the danger air
_volume_ by EIGHT!
-------------
Do I have this right? It seems to make sense...
\__ -Bill
{((___O===--0' Yerazunis
|
| 710.3 | More on 3IM? | LEDS::LEWIS | | Fri May 13 1988 21:35 | 33 |
|
RE: .766
> Let me see if I have this straight (on how to calculate 3IM
> interference)...
Bill, to date I haven't heard any theories like yours... my
understanding of 3IM was that one of the end frequencies beats with
the second harmonic of the middle frequency to produce a
fairly strong signal at the third frequency. For example...
Channel 50 : 72.790
Channel 48 : 72.750 (these are the actual numbers)
Channel 46 : 72.710
Channel 48's second harmonic is at 145.500.
145.500 - 72.290 = 72.710
145.500 - 72.710 = 72.290
This gives interference patterns for channels 46 and 50 (3IM is
known to affect the end channels, not the middle ones). The
interference patterns appear only at certain physical orientations
of the plane (I think when the plane is eqidistant from the two
interfering transmitters). I'm not sure how your theory can explain
the positional aspect of the interference, unless I missed something.
I always thought the "mixing" was done in free air, not at the
transmitter output stages.
One of these days I'm gonna brush up (relearn?) my communications
theory and really try to understand the mechanism for 3IM.
Bill
|
| 710.4 | ANother possibility | SNDCSL::SMITH | William P.N. (WOOKIE::) Smith | Sat May 14 1988 10:31 | 17 |
| I'll have to ask around, but _I_ always thought 3IM (what we used
to call 'intermod') was when 2 channels would beat against each
other and the result would beat against one of them, producing the
third. In the example above:
Ch. 46 (72.710) and Ch. 48 (72.750) would beat together to make
sum (don't worry about it) and difference (40 KHz) signals. Then
the 40 KHz would beat with Ch. 48 (72.750) to make a sum of 72.790
(which wipes out Ch. 50) and a difference of 72.710 [but that won't
wipe out Ch. 46 cuz it's phase-locked to the Ch. 46 transmitter].
I'll ask my EM Field Theory professor if I think of it. Don't suppose
it really makes much difference _how_ it happens, other than being
able to understand and predict it....
Willie
|
| 710.5 | I choose hamburger "B"... | MIDEVL::YERAZUNIS | Where do those things come from, anyway? | Sat May 14 1988 11:22 | 62 |
| Hmmm. This is getting fun!
Seems like we have 3 competing theories:
All theories agree that having ANY 3 frequencies equally spaced
and in use at the same time is an invitation to trouble...
Theory X: 3IM happens when a strong nearby transmitter leaks
RF into a transmitter driver, thereby causing a beat
frequency and causing the transmitter to pump out energy
at f0 +- beat-freq.
Theory Y: 3IM happens when two transmitters, both broadcasting
into "free space", at frequencies f1 anf f2, produce
in free space a signal at F=(f1+f2). This f3 signal
then beats against one of the original two signals
producing f4= f1 + f3 and f5 = f1 - f3.
Theory Z: 3IM happens when two transmitters, both broadcasting
into "free space" at frequencies f1 and f2, produce
in free space a signal at f = (f1 - f2). This f3 signal
then beats against one of the original two signals
producing f4 = f1 + f3 and f5 = f1 - f3.
Now, let's be sort-of scientific and see what each theory predicts:
Theory X says that the interference frequencies will be at each
interferer's frequency plus and minus the difference in their
frequency, relative power at each interference frequency depending on
the level of shielding of transmitter and proximity of other
interferer. If theory X is true, the "shoot-down" frequencies may be
either higher or lower than both interferers.
Theory Y says that it's the sum of the two interferers that beat
against an original interferer, producing (for the higher interferer)
the lower interferer and (for the lower interferer), the lower
interferer minus the difference. So, the shoot-down frequency should
always be at a LOWER frequency if theory Y is true.
Theory Z says that the difference of the two interferers that beat
against an original interferer, producing (for the lower interferer)
the higher interferer, and (for the higher interferer) the higher
interferer minus the difference. So, the shoot-down frequency should
always be at a HIGHER frequency if theory Z is true.
Theories Y and Z say 3IM is a "space effect" and don't depend on
the distances between transmitters, only their orientations. The
Y and Z effects should have "shoot-down" radial zones extending
out to infinity.
Theory X says 3IM is a "local effect" and should rapidly fall off
with increasing distance between the two interfering transmitters
and increasing distance between the interfering transmitters and
the ship. The "shoot-down" zone should be roughly spherical and
centered on the interfering transmitters.
-------------
Well, how does 3IM behave? Or does it behave like all 3, at various
times of the year, phase of the moon, etc?
|
| 710.6 | Just muddying the waters a bit. :+) | SNDCSL::SMITH | William P.N. (WOOKIE::) Smith | Sun May 15 1988 01:46 | 45 |
| Actually, it turns out not to be quite so simple! :+) I talked
to someone_who_should_know and he gave me the following:
Any two signals can mix, in a mixer or free space, to provide sum
_AND_ difference signals.
Any real transmitter has a signal not only at the desired transmitter
frequency, but at integral multiples thereof (though at reduced
power levels). Ideally these harmonics are infinitely small, but
in the real world they can be significant.
Therefore, 'intermod' is very complicated:
Take transmitter one. It has a carrier and one or two significant
harmonics.
Take transmitter two with it's associated carrier and one or two
significant harmonics. [Note this gives us around 6 frequencies
to play with].
Take any pair of frequencies from the above list, they will mix
and generate sum and difference frequencies. Take all possible
combinations of frequencies from the above list and you will get
a large number of additional unwanted frequencies. Now that you
have this expanded list, start mixing the sum/difference frequencies
back into the original list. As you can see, this gets complicated
very quickly, but all of these frequencies is generated. Fortunately,
some of the results are so small in amplitude as not to matter.
So, you have it all figured out? Well, not quite. How about high
power transmitters in the area, paging channels, cellular phones,
airplane control towers, TV stations, etc, etc, etc. Even if the
frequencies are far away from the ones you are worried about, if
the power levels are high enough, Bad Things can still happen.
Oh, did we mention receiver overload from high-power adjacent
transmitters, intermod that produces a frequency at your receiver's
IF, or other bizarre effects? Don't forget that some transmitters
are only keyed when in use (like pagers and 2-way radios and such)
and the interference may come and go.....
It's almost a miracle anything works at all!
Willie
|
| 710.7 | intermodulation in space is a myth | BRAT::RYDER | perpetually the bewildered beginner | Wed Dec 26 1990 06:30 | 14 |
| re Note 710.6
>> Any two signals can mix in ..... free space, to provide sum
>> _AND_ difference signals.
This is not correct. The "someone_who_should_know" gave you mostly correct
information but not that regarding mixing in free space. The process
requires a non-linearity, either as a non-linear transfer function
operating on the simple sum of the two signals or as a device multiplying
one by the other to create a product. If the non-linearity is occurring in
the *test equipment being used*, then it can *appear* that it is happening
in free space, but it isn't.
Alton
|
| 710.8 | theoretical basis for intermodulation | BRAT::RYDER | perpetually the bewildered beginner | Wed Dec 26 1990 08:18 | 44 |
| Following the reasoning in Krauss, Bostian, and Raab, "Solid State Radio
Engineering", ISBN 0-471-03018-x:
If the transfer function of the non-linear device is expressed as a Taylor
series,
vout = k0 + k1 * vin + k2 * vin**2 + k3 * vin**3 + .......
and if the input is a Fourier sum of sinusoids, then the terms
kn * vin**n
represent the n-th order intermodulation products. (Hence the "3" in "3IM",
etc.) This *can be* evaluated for any Fourier sum, and it can be reasonable
in certain cases for an input of a simple sum of two sinusoids.
If all coefficients above k2 are zero, we have a square law detector, and
the output consists of the two inputs, their second harmonics, a DC term,
and the expected sum and difference frequencies. In this case there are
*no* other output frequencies. It is another myth that the spurious
frequencies mix again to create yet other frequencies and those mix again
etc.; if there is one stage of non-linearity, there is one set of
frequencies generated. However, real non-linear devices will indeed have
higher order terms in the transfer function and will, therefor, generate
the more complex higher order intermodulation products in that one stage.
If the non-linearity is severe, e.g. overloading of an amplifier, the
resulting cacophony can appear to support the myth. In the common case of
a non-linearity followed by another non-linearity, harmonics from the first
can mix in the second to generate harmonic intermodulation distortion.
The referenced book doesn't expand the third order terms, and I'm not sure
I have the patience for it. It is easier to observe intermodulation and
measure it. I've done that, and I'll report the data in my next reply.
However, measurements of reality are measurements of complex distortions
with mixtures of 3IM, 4IM, 5IM, etc. all munged together. For convenience
we label the mess "3IM". I think that strictly speaking 3IM has only the
two original frequencies plus one frequency below the lower by the
difference and another above the higher by the difference, but the
measurements consistently show a tail of lower and a tail of higher
spurious frequencies. e.g. Channels 28 and 30 generate 26, 24, 22, etc.
plus 32, 34, 36, etc. Neither analysis nor measurements indicate anything
*between* the two inputs. More one that in the measurement note.
Alton
|
| 710.9 | experiments with intermodulation and egg nog | BRAT::RYDER | perpetually the bewildered beginner | Thu Dec 27 1990 09:29 | 130 |
| 3IM can and does occur in a solitary receiver where, in our case, it
evidently does no harm. It can also occur in a solitary transmitter;
that, too, is harmless. But when a receiver picks up transmissions
from two or more foreign transmitters and creates 3IM in its RF
amplifier or first mixer, that receiver may be hit --- the 3rd order
intercept point number is a measure of this likelihood. And when a
transmitter acts like a receiver, picks up another transmitters
emissions, creates 3IM with the other carrier, and then transmits the
results, planes can drop from the sky; the receiver's 3OIP doesn't
help. So we have four cases: solitary and gregarious, both with
receiver-generated and transmitter-generated possibilities. Both
solitary cases seem harmless. A receiver like the Airtronics 92765
with a high 3OIP offers some protection to a receiver near widely
separated transmitters. But if the foreign transmitters are near each
other and nearer as a group to the receiver than the receiver's
matching transmitter, no receiver is safe.
Glenn Schrader did some homework on how this last effect might happen,
and his analysis seems to be on the mark. I spent Christmas day with
an old friend that happens to be in the electronic countermeasures
business (people in that business understand this stuff very well), and
he supported Glenn's reasoning. They even have a term, "back
intermod", for one of Glenn's conjectures. I'll leave the details to
Glenn, but it seems that there are two ways for a transmitter to act
like a receiver: 1) when the circuitry of the transmitter (e.g. the
encoder) picks up another's RF, and 2) when the transmitting antenna
acts like a receiving antenna and picks up the foreign transmission
while it, itself, is still transmitting. (Now you know why I'll leave
the details to Glenn.) This in-and-out-the-antenna distortion is the
effect called "back intermod". I have the AMA District I ICOM, so my
friend and I ran some tests for 3IM. I had also run some tests myself,
and both sets of results are reported in this note. All tests were run
inside my house in the evening.
For my early tests I put an RC20 and an RC24 together and observed
signals at RC12, 16, [20, 24,] 28, 32, and 36. My tests were
curtailed by my wife. I jammed TV5 which she was trying to watch at
the time. I presume that I also jammed TV4's audio, but I didn't
check. She had no patience for a scientific curiosity that wiped out
her viewing pleasure.
The tests with the professional were better organized, better
documented, and run while no one was watching TV. Most of the tests
were run with only the ICOM as an intended receiver. Because the tests
were run inside an old farmhouse with a wood stove, Lally columns, and
wiring, etc., there was some variation in signal strength as we moved
the transmitters about. But the results were consistent enough to be
valuable.
To establish a base line, we set two transmitters back-to-back and
vertical with the antennas about two inches apart; it may not have been
realistic, but it was useful. The ICOM was about 30 feet away and was
used as a poor man's spectrum analyzer. (This guy has been spoiled
with $40,000 test instruments, and I have him jury rigging with a
borrowed scanner! The plight of the peasants.) The ICOM R7000 was set
to FMn, stepping at 10 Kc, and with the squelch at 9 o'clock. We
checked out each transmitter singly and observed only the desired
signal at the ICOM. RC32, an Airtronics Vanguard FM with a gold
sticker, came in at S9.5 at 72.430. (If you have ever used an ICOM,
you understand that there is nil significance between S9.2 and S9.8,
let alone S9.5.) RC30, a Futaba Attack AM with a gold sticker, came in
at S9 at 72.390. There were weak PRS signals at 72.220, 72.360,
72.460, and 72.700; even with both transmitters quiet, there were very
weak signals at RC49, RC56, and RC57 coming from somewhere; we ignored
all these in all of our tests.
We then turned both on and measured the intermodulation products:
RCnn S (S is the signal strength reading)
24 <1
26 1
28 5
30 9 the Futaba
32 9+ the Vanguard
34 4
36 1
38 <1
40 <1
42 <1
Then both were set flat on their backs on the wooden table with the
antennas in line and pointing away from each other. The bottoms of the
plastic cases were touching. The ICOM antenna was left vertical. This
test was intended to compare antenna-to-antenna coupling vs coupling
from circuit-directly-to-circuit. I may have changed the squelch.
RCnn S
24
24.5 <1
26 <1 but could hear the RC modulation
28 3.5 and 3.0
28.5 <1
30 9
31.5 1.5
32 9- the antenna was now shadowed by a stove
32.5 2+
33.5 <1
34 3.5
34.5 <1
36 <1
38 <1
45 <1
45.5 <1
Then leaving the orientation the same, we separated the cases by about
a meter. The spurious signals at RC28 and RC34 both dropped below S1.
To check the ICOM, we set the transmitters vertical but separated by 30
feet --- one still on the table, one about 40 feet from the ICOM. All
spurious signals disappeared. We also tried a 20 foot separation, but
the documentation was suffering from the egg nog syndrome by then.
Conclusions:
None of the spurious signals of interest were generated in the ICOM;
all were generated in the transmitters.
Back intermod happens and can dominate the spectrum. Front intermod
also happens.
Sensible separation of the pilots can eliminate transmitter generated
intermodulation.
High order transmitter intermodulation generates signals at multiples
of the transmitter RF difference; these signals drop off with the RF
distance from the transmitters. i.e. If two pilots are near each
other and flying on RC44 and RC48, don't fly at RC40 or RC52 and beware
at RC36 and RC56.
|
| 710.10 | Ask Mr. Wizard | KAY::FISHER | Stop and smell the balsa. | Thu Dec 27 1990 10:54 | 7 |
| How can you tell - based on observation alone - no fare quoting
books - that the mixing didn't occur in the air?
Bye --+--
Kay R. Fisher |
---------------O---------------
################################################################################
|
| 710.11 | my 0.02$ | SHTGUN::SCHRADER | | Thu Dec 27 1990 15:52 | 237 |
| RE -0.2 Al,
What follows is the mail that I sent to you that summarized our
discussions about 3IM. It does get a little techincal in spots...
RE -0.1 Kay,
If the signals were mixing in the air then the transmitter
separation would not matter. Since 3IM appears when transmitters are close,
the cause has to be an interaction between the transmitters.
One more point i'll throw in. Just because things "mix" doesn't mean that
they intermodulate. The signals from two transmitters will, of course, mix
in the air but this is a linear mixing which does not produce intermodulation.
The sum and difference frequencies only show up when mixing happens in the
presence of a non-linearity. Al's entry a few notes ago was a good explanation
of why a non-linearity creates intermodulation products.
GS
---------------------------------------------------------------------------
From: SHTGUN::SCHRADER 18-DEC-1990 17:29:18.43
To: CSS::BRAT::RYDER
CC: SCHRADER
Subj: RE: birth of the blues
>From: CSS::BRAT::RYDER "perpetually the bewildered beginner"
12-DEC-1990 07:06:21.64
>To: CSS::SCHRADER
>CC: LEDS::WATT,RYDER
>Subj: birth of the blues
>
> Glenn, when talking to Charlie Watt last night, I recalled our chat
> about the origin of spurious signals in the transmitters of others.
> Charlie understands about as much as I do about interference in general
> and as little as I do about the details of how signals entering a
> foreign transmitter cause spurious transmissions. Can you
>
> 1. Contact Charlie about this
>
> 2. Write up the information so others like me can share the results?
>
> Alton, the theorist who plugs his scope probes into the wall socket and
> touches the AC plug to the circuit under test. That's OK; evidently
> the trace was off the screen inasmuch as I couldn't find it.
>
I'm not exactly sure of the technical level of the target audience here
so i'll start at the beginning...
This description of what 3IM interference is and how it happens is
based upon some field observations, assorted reading, a fair amount of
knowledge about how RF circuits work, and a lot of deduction. I haven't
been able to verify that what follows is really what happens since I
don't have the proper test equipment in my basement. However, this
explanation both accounts for what happens at the field and is correct
from a circuit theory point of view. Since I haven't seen a better
explanation ANYWHERE, I use this as my working hypothesis.
3IM seems to occur when transmitters are closer than about 30'.
Maintaining a 30' separation is one of the preventative measures that
the AMA recommends. I have also observed this at the field. The
interference happens on channels which are multiples of the channel
spacing between two transmitting channels. If for instance channels 30
and 34 are transmitting then the channel spacing is 4 and 3IM might
occur on channels 26 (30-4), 38 (34+4), 22 (30-2*4), 42 (30+2*4), etc.
Since the interference occurs at multiples of a frequency difference,
it must be caused by some sort non-linear mixing of the two
transmitting channels. This is similar to what happens the the
receiver's mixer to produce the IF frequency.
The interference occurs when the transmitters are close. This is an
important point. From this I deduce that whatever the source of the
interference is, it is not at the receiver. If the interference were
occurring at the receiver then the closeness of the transmitters would
not matter. Also, since the "ether" is linear, signals mixing in "the
air" >CAN< >NOT< be creating the interference. The last place which is
left is the transmitter, which makes sense since it is the relative
position of the transmitters which seem to cause the problem.
The closeness of the transmitters must be making them interfere with
each other. I can only see two ways that this could happen. The wiring
in a transmitter could be directly picking up the signal from the other
transmitter or the antenna can be receiving the signal from the other
transmitter. I do not think that it is the wiring. The reason is that
the RF field around a transmitter is pretty intense even in normal
operation if the wiring was such that it could pick up very much RF
then the channel encoders would probably have trouble just from the
transmitters own RF field. My guess is that the wiring already has
enough bypass caps and/or ferrite beads so that RF on the internal
wiring won't cause a problem.
So, my guess is that a transmitter picks up the signal from another
nearby transmitter and something happens which causes 3IM. Note that on
a flight line, with people standing in the same direction, the antennas
will be parallel in a lot of cases. This condition maximizes the RF
coupling between the antennas. When the transmitters get close a
transmitter could be picking up quite a bit of signal from it's
neighbor.
There are two things which might be happening to cause 3IM. Their
mechanism is a little different but the effects are the same. One thing
that is common is that the transmitter's output gets modulated with the
channel difference frequency. In order to modulate with the difference
frequency it first has to be created by a non-linear mixing of the two
channels. I think that this happens at the transmitter's output stage
in one of two ways depending on the transmitter's design. If the
transmitter output stage is a class C amplifier then it is by
definition non-linear. A class C output stage should do a pretty good
job of mixing the transmitter's output with the received signal and
produce sum and difference frequencies. The other possibility is that
the received signal "overloads" the output stage (maybe by making the
output try to swing outside of the supply voltage range) and causes it
to clip. The clipping makes the output stage nicely non-linear and once
again sum and difference frequencies will be created.
The sum frequency will be up around 144Mhz and probably isn't a
factor. The difference frequency is the spacing between the transmitter
frequency and the frequency of the adjacent transmitter. The problem is
probably created by the low frequency difference component. One of two
things could happen depending on the design of the transmitter.
One is that the low frequency signal is pulling the output stage bias
point around and thereby modulating the transmitted signal with the
channel difference frequency.
The other is that the bypass capacitors in the transmitter output stage
are probably picked to shunt off the 72Mhz which is normally the only
thing which should be at the antenna. The difference frequency is down
in the tens or hundreds of Khz and could probably go right past the
bypass caps and feed back into the other circuits in the transmitter.
Depending on where the signal wound up, it could modulate the signal
>going to< the output stage with the channel difference frequency. This
is a much much worse case scenario than the former one since the
transmitter's output stage would then happily amplify and transmit the
signal modulated with the difference component.
Another thing to keep in mind is that the low frequency difference
signal has it's own harmonics. If the mixing of the two transmitter's
center frequencies produced a pure sine wave then the difference
frequency would have a single frequency component. It will not be. It
will be clipped and/or heavily distorted so it will have potentially a
lot of harmonics in it. The harmonics of the difference signal will
occur at multiples of the frequency difference between the
transmitters. This is why 3IM can show up at any multiple of the
frequency difference. When the difference frequency plus harmonics mix
with the transmitter center frequency the signal picks up frequency
components at the center frequency +/- the frequency of each harmonic.
Since all of this is connected to an antenna all of this radiates back
out the antenna. At first it may seem a little strange that an antenna
is radiating, receiving, and re-radiating stuff all at the same time.
An antenna is not a one way valve. The flow of electricity is NOT like
the flow of water in a pipe. It is more like ripples on the surface of
a pond in that the ripples can be going in different directions, pass
through each other, and keep going. This is a little like what is
happening on the antenna.
The following uses channels 30 and 34 as an example but any two channels
will do the same thing.
I would expect the antenna on the transmitter to radiate a spectrum
something like:
|
|
|
|
|
| | |
| | |
| | | | |
Etc<-------------------------------------->Etc
F1-2*D F1-D F1 F1+D F1+2*D
CHANNEL-> 22 26 30 34 38
BUT, this same thing is happening in the other transmitter so it will
be picking up and re-radiating at it's frequency as follows:
|
|
|
|
|
| | |
| | |
| | | | |
Etc<-------------------------------------->Etc
F2-2*D F2-D F2 F2+D F2+2*D
CHANNEL-> 26 30 34 38 42
And the receivers will get both:
| |
| |
| |
| |
| |
| |
| | | |
| | | |
| | | |
| | | | | |
Etc<--------------------------------------------->Etc
F2-3*D F2-2*D F2-D F2 F2+D F2+2*D
F1-2*D F1-D F1 F1+D F1+2*D F1+3*D
CHANNEL-> 22 26 30 34 38 42
In either case the transmitter's antenna will now radiate on the
intended frequency plus frequencies which are multiples of the
frequency difference between the two transmitters.
Let's suppose that there is somebody flying on channel 26. Maybe
he is either further from his plane than you are or his antenna
orientation with respect to his model is such that he isn't getting
much signal through. If the amount of his signal that he gets at this
receiver isn't enough to drown out the interference that is coming off of
the two transmitters that are close together then he gets a 3IM hit.
Note that the interfering signal is exactly on his center frequency. It
makes no difference how narrow band or how tightly tuned or how may
stages of conversion he has. These things ONLY affect the rejection of
signals which are not on the receiver's center frequency. Under these
conditions the receiver gets hit, period. The problem is not with the
receiver, it is with the transmitter.
Well, that's what I know about 3IM. One of these days, I'm going to
have to find somebody with enough test equipment to either verify or
disprove it.
G. Schrader
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| 710.12 | change everything you can | ABACUS::RYDER | perpetually the bewildered beginner | Thu Dec 27 1990 17:58 | 11 |
| re Note 710.10 by Kay
>> How can you tell that the mixing didn't occur in the air?
By changing receivers and transmitters in every combination.
Intermodulation in space would show at least some invariance with equipment
changes. Glenn's separation method is also valid, but some disbeliever
would claim that the separation changed space somehow.
This problem is akin to proving that ghosts do not exist.
|
| 710.13 | Xmtr 3IM and Cars?? | DAVE::MITTON | DECnet is not just for Ethernet anymore | Fri Dec 28 1990 19:18 | 12 |
| RE: .10
Al, this is most interesting (I'm going to print it out and study it)
but my question is: If 3IM is so easy to induce by transmitters next
to each other, then why to Car Racers not see it that often?
Typical RC Car races have the competitors (6-8 or more) standing next
to each other in a grandstand over the track. (typically 10-15 ft long)
Are the surface frequencies special? Are surface radios better? ;-}
wondering.... Dave.
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| 710.14 | car people must live right ;-) | BRAT::RYDER | perpetually the bewildered beginner | Fri Dec 28 1990 21:56 | 26 |
| re Note 710.13 by DAVE::MITTON
>> then why to Car Racers not see it that often?
I don't know. This question was raised twice before in this file, once in
105.53 and another place where I think there was an opinion offered.
The opinion, where ever it is hiding, mentioned that car people change
frequencies routinely, but I have no experience with cars myself.
It is possible that by clustering they ensure that the jamming transmitters
cannot have a range advantage over the victim transmitter. By spreading
over a flight line, we can experience having a plane on or over the runway
being much closer to the jammers than to the owner.
>> Are the surface frequencies special?
No.
>> Are surface radios better? ;-}
I doubt it.
Alton
wondering.... Dave.
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| 710.15 | half and half an experiment, that is | BRAT::RYDER | perpetually the bewildered beginner | Fri Dec 28 1990 22:26 | 20 |
| Note 710.9 reported on transmitter generation of 3IM, but it didn't
discuss the effects of this on a plane. Tests of any dangerous
situations and the resulting vulnerability are needed to put this into
perspective. Well documented field experience would be even better.
We actually ran one such test that evening. I set up two [different]
transmitters to generate 3IM, put a plane next to the ICOM, and took
the victim transmitter outside to simulate a flight line situation. But
by that time my colleagues were either too tired or too full of eggnog
to document what was happening to the plane, and from outside I
couldn't see it myself. I didn't notice the lack of documentation
until I started writing that note.
In the interests of a well documented experiment, I should document the
egg nog contents. Each Thanksgiving, Betty mixes 8 eggs with 1.5 cups
of sugar, 10 ounces of good rum, 26 ounces of brandy and enough cream
and milk (1/2 + 1/2) to make a gallon in total. This is refrigerated,
*untouched and untasted* until Christmas. It starts out harsh and
becomes quite smooth by January. I have had it as late as April. No
3IM experiment should be run at night without it. But don't fly.
|
| 710.16 | 3IM | LEDS::WATT | | Wed Jan 02 1991 08:14 | 16 |
| I'm sure that cars do sometimes suffer from 3IM problems, but they
probably are never as far from the Transmitter as planes typically are.
The 3IM signal is usually down considerably from the good signal unless
the good transmitter is not close to the two transmitters that are
causing the 3IM product. If we all stood very close together and flew,
we would be generating 3IM but the plane would always 'see' the good
signal at a much higher amplitude than the 3IM signal. The problem is
worsened when two transmitters close together generate 3IM and the
channel that is getting 'HIT' has it's transmitter far from the other
two. (like at another flight station) Now, when the plane flies near
the two offending transmitters, it can 'see' the 3IM signal as larger
then the signal from it's own transmitter. I've seen this on landing
situations at our field.
Charlie
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| 710.17 | cars & 3IM | SHTGUN::SCHRADER | | Wed Jan 02 1991 08:46 | 24 |
| There is nothing special about the surface frequencies. Given similar equipment
the same problem should exist. But the equipment IS different. One thing that
comes to mind is the amount of RF power being transmitted. The range at which
you control surface stuff is quite small compared to the range at which
aircraft are operated so the transmitter doesn't need nearly as much power.
It could be that the lower power doesn't affect the adjacent transmitters
enough to cause a 3IM problem. Could somebody look on the label of
a car transmitter and see if it says what the radiated power is? My airplane
radios radiate in the 0.5 watt to 0.75 watt range and that is supposedly
enough for about 1/2 mile of range.
It could also be that when car radios were first being developed they had
enough 3IM problems so that they designed 3IM out of the output stage. It
wouldn't be that hard to do for a low power transmitter if you knew about
the problem up front. On the surface (pun intended 8^) cars should be a
3IM event waiting to happen so this seems possible.
Then on the other hand the "clustering" could be eliminating the effect RE-.?
Al, is that ICOM scanner able to cover the surface frequencies? It would be
interesting to check a couple of car radios just to see what they do.
Bubble bubble, toil and trouble. Fire burn, cauldron bubble.
Glenn Schrader
|
| 710.18 | | SNAX::SMITH | I FEEL THE NEED | Wed Jan 02 1991 10:10 | 18 |
| When I was racing my RC10, I ran into plenty of interference at
various times. Cars suffer the same symptoms as planes if someone
turns on on your channel. I've been racing down the straight away and
been driven into the wall on several occasions.
There were also times when driving past a certain location on the
track, the steering would glitch as you went through. One of the nice
things about cars, is that with such low power, it's common practice to
change channels. So, if you had a problem on one, you could just plug
in a new set of crystals and go off on your merry way on a different
channel.
One of the most common occurences of either 3IM or 2IM was at the start
of the race. All drivers, usually anywhere from 6 to 10, all stand
shoulder to shoulder on the drivers stand. Couple that with 10 cars all
within 2 feet of each other on the track, and cars were jumping around
all over the place. We usually needed 2 or 3 guys on the track holding
cars at the start of each race.
|