Filter for a transmitter?

[PVenkman]

I have a transmitter (a Royal Evo 9 with a synth module) that's suffering from interference. At a particular flying site, close to a military base, everything I try to fly with the Evo has severe glitching. The same planes with a different transmitter are fine at that site, and the Evo with the same planes is fine at every other site I've tried. I wouldn't blame anyone for being skeptical on this, but please for this exercise just accept that the problem really is with the transmitter.

I've tried shielding the transmitter a few different ways with no success. However I'm open to suggestions on better ways to do this.

What I'm thinking now is the signal causing the problem might be coming in through the antenna of the transmitter (that certainly seems reasonable, at least as far as a path into the transmitter). The antenna connects to the circuit board in the transmitter via a simple spade connector. That means it would be possible to add something in the circuit between the antenna and the circuit board without cutting any wires or doing any soldering.

So would it be possible to construct some kind of filter that would let the 72 MHz signal of the transmitter pass through with no problems but would stop signals of different frequencies from getting through? In particular is it possible to do this without affecting the transmitter in any way (causes it to be off frequency, or overheat, or whatever)?

If it's possible, could one of you bright people explain how to do it? I can read circuit diagrams and am reasonable with a soldering iron so I could build something simple, I just don't know what it would be!

Thanks in advance.

[Jim McPherson]

It is highly unlikely that a signal is coming in on your TX antenna strong enough to cause interference in the transmitted wave. It's more likely that your transmitter is not transmitting at an acceptable power level. In more RF quiet areas you don't experience any problems becuase there is less noise and lower power levels are not noticed. I don't think a filter will do anything for you. More likely you have a problem with your amplifier section, it may need repair.

-Jim

[PVenkman]

Jim, that could be but I doubt it. I sent the transmitter back to Hitec, they checked it and tuned it and said it was fine. I still had problems, so they replaced the transmitter, and I still have the same problems. The radio range checks very well at other sites.

Even assuming I'm completely wrong and a filter won't help, I'd at least like to try that to eliminate it as a possibility. Any ideas?

[vintage1]

This came up somewhere else didn't it?

ISTR that I reckoned there were a few possibilities, taking what you sday as true.

(i) the site is saturated with some heavy RF that is messing with the digital side.

(ii) ditto,but upsetting the synthesiser.

(iii) its some kind of unusual transmirssion spectrum from our transmitter that is mixing with something else there, and upsetting the reciever.


IF you can reproduce it on the ground, first of all try the transmitter in a biscuit tin with a hole punched through for the antenna (insulate that!) ..if you haven't got a biscuit tin just wrap it in kitchen foil and see what happens.

If that fixes it, is (i) or (ii) not (iii).

If you can beg borrow or steal a crystal module, try that - if that fixes it, its (ii).

It might be worthhwile knocking up a 'chanel monitor' (see my thread in here on that) and wandering about seeing what kind of sound you get.

Does teh TX interefer with other channels at all? And more so there than elsewhere?

[PVenkman]

I've had a couple threads on this, but so far no resolution.

I can easily recreate this on the ground; in fact except for one instance it's glitched so badly on the ground I've been afraid to fly.

At one point I did try the 'biscuit tin', and it seemed to greatly reduce the problem. However I haven't been able to do much past that. I tried lining the back half of the case with foil, and that didn't help. I wrapped the RF module in foil (insulated from the module!), and that didn't help.

I finally did meet another person with an Evo at this site; he had a crystal module, and DIDN'T have any problems! In retrospect I should have asked if we could swap modules to see what happened. I'm trying to get Hitec to lend me a crystal module to try.

As far as I can tell my transmitter doesn't interfere with other channels. Without a channel monitor though I can't say that's 100% true; I can say I've been there with other fliers a number of times on a variety of channels and I've never caused problems for someone else. And at any other site it doesn't cause any problems for me!

So my best guess from your list is (ii); I haven't had any luck with shielding, so that's why I'm pursuing a filter of some kind.

[vintage1]

Ok. I think we went round this loop before...but it might spark someone else here anyway. If it IS the synth module, its properly Multiplexes problem.

Proving it may be difficult tho, and fixing it almost certainly violates FCC regs on modding the RF side of things.

My guess is that the part of the circuitry that picks up and amplifies the oscillator frequency (and divides down to compare with the reference frequency) is picking up something else as well. It needs to be in a little tin box on the board itself. Adding that box without being able to measure and test things is fraught.

One possible idea is to get a frequency counter, and test by wrapping a turn of wire round the antenna. It should show the same on a clean site as your dirty site. If it doesn't you can 'prove' to multiplex that something is awry. Whether they will fix it or simply offer you the money back is another issue.

I certainly think that borrowing briefly a crystal module and testing with that would also be a nail in the coffin of the problem. If that works you also have very conclusive evidence to pass back to multiplex.

So..I think hat the way forward is first to narrow down the scope of the problem. This will command respect from the engineers at Multiplex, and tell them where to look, and then get firm with them. And, if possible, ask anyone who might know (maybe peter berg) if he knows who designs them. Then try and get to high-ups in the organisation. I had similar sorts of issues when re-selling software. We found bugs: No one would believe us until we could get through to the real techies behind the scenes with a clear report. Then the bugs got fixed (mostly).

Bear in mind tho, that if you are the only one, they are not going to want to re-tool an entire production line just to fix your problem. Its cheaper for them to e.g. give you a free Xtal module and say ' have that and use it there and shut up'.

Unless of course, you happen to make a big fuss publicly and people start to get put of buying the kit, when simple cost benefit analysis may make them reconsider the cost of a fix versus the cost of suing you for libel versus the cost of losing sales... :cool:

[PVenkman]

I'm working on getting a crystal module to test now; hopefully that pans out and I'll have some more info. If I can get a crystal module and it works I have at least some kind of solution although I would really hate to give up the synth capability at that site.

I gather from the fact nobody is jumping in here that actually adding some kind of filter isn't all that simple. It SEEMS like it should be simple - just select a filter that passes the 72MHz band and insert it between the antenna and the circuit board - but apparently there's more to it than that.

[vintage1]

You are assuming the crud is coming in down the antenna, and that there isn't one there already...

[foofighter]

I gather from the fact nobody is jumping in here that actually adding some kind of filter isn't all that simple.

Correct.
Between the RF output stage and the antenna is a impedance matching circuit that transform the transistor output impedance to antenna impedance. It will probably do this in the form of a low-pass filter to remove harmonics at the same time. To recalculate this as a band-pass filter is no trivial excercise!
In any case it is possible that being a military field that the RF causing the problem is much higher in freq and possibly pulsed? In which case the filter does not have adequate stop band at freqs so far away from its nominal operating point.



I'm working on getting a crystal module to test now; hopefully that pans out and I'll have some more info. If I can get a crystal module and it works I have at least some kind of solution although I would really hate to give up the synth capability at that site.

This is the best 1st step.

If the xtal module is ok then it is probably that the synth module does not have enough reverse isolation between the RF stages and the VCO. Significant levels of (pulsed) RF arriving at the antenna can be coupled back to the VCO and the carrier freq will be disturbed. Probably not enough (or for long enough) to measure any freq change but certainly long enough to corrupt the transmitted frame.


If you can enlist the help of the other EVO user, try using your synth module in his Tx and his xtal module in yours.

If the problem always follows the synth module then you will need to identify what freq is causing the problem. It will then be possible to design a series trap circuit to connect between the antenna and ground to reduce the level of the interfering signal(#)

In any case the RF levels needed to create such a problem must be pretty high? (Maybe you should be inside the 'biscuit tin' :eek: )


(#) See sticky about Tx mods.....

[PVenkman]

I have an update. I got my hands on a crystal RF module, and it is glitch-free at that site! Same plane, same reciever, etc. the only change is the RF module, and it went from unflyable to rock solid.

I've passed this info back to MPX, but since I'm one person at one location I don't know that they'll do anything about it.

Now that it's for sure narrowed down to the RF module, is there anything I can do? I guess I'm back to the filter idea since shielding hasn't worked. The problem is I don't have the knowledge needed to design such a beast (although I could build it given a schematic).

[foofighter]

OK, back to the 'test' site then.

Find a location where the problem seems worst.
Take something to stand the Tx on while checking (wooden stool etc.)

For EVERY test place the Tx in EXACTLY the same position (within 1/2 inch)
Also try and position yourself in EXACTLY the same position for every test.
(If the problem is caused by UHF/radar etc then any position error must be much smaller than the wavelength causing the problem...)

Do a range check with the synth unit with the antenna removed.
Do a range check with the xtal unit with the antenna removed.

If the problem is still there, the RF causing the problem is most probably coming through the case of Tx. [A]

If the problem is gone then the interfering RF is probably coming via the antenna so continue:

Repeat with the antenna installed but collapsed.

Check level of glitching with antenna collapsed.
Extend antenna about 1/10 inch and check glitching - repeat this test about 20-30 times. This will check if there is a resonant length of antenna where the problem is worst. [B]

[A] Synth shield could be the problem. Obtain an aerosol can of nickel spray paint for EMI/EMC. Remove the synth unit PCB from its case and spray the inside of the case with a coat or two of the nickel paint. Make sure that any contact points between the synth PCB and the case a ground points on the PCB. (Otherwise remove/scrape the nickel paint away at this point)

Reassemble the synth unit and retest.

If the problem is still there, the RF is probably coming through the Tx case into the synth. Install ferrite beads on leads to the synth unit (especially the modulation input)

[foofighter]

[B] In this case the RF interferer is passing via the antenna and it is probably a lack of reverse isolation [S12 in terms of S parameters] in the Tx buffer/driver stages that causes the problem.

You will need to identify the frequency of the interferer.
Borrow a scanner (30 - 2000 MHz would be a good start) etc and see if you can find something (use wideband FM mode for searching - its quicker :) )

From the freq of interfering signal we can calculate a suitable series trap filter that can be installed between the antenna output and ground.

If you find nothing below 2GHz, then some intuitive 'guesswork' can be used for a possible filter design.

[foofighter]

For EMI/RFI shielding, there are many suitable aerosol sprays available for treating the insides of plastic cases.

A typical example would be http://www.mgchemicals.com/products/841.html

Either that or anything equivalent will do.

For cables and wires there are many types of ferrite beads too. Any single hole ferrite bead will be OK. Just buy according to suitable size.


All in all it aint going to be easy......


Good luck :)

[PVenkman]

foofighter, thanks! You given me lots of good things to try.

Let me give you some more information in case it triggers any other ideas.

The RF module unfortunately isn't in a seperate case, it's just an exposed PCB. I did try wrapping it completely in foil (with an insulating layer between the foil and the PCB) and grounding the foil, and it didn't help.

The RF module has two connector blocks, so I did have to leave two holes in the foil. I made them as small as possible but couldn't eliminate them entirely.

There are straight pins that stick out of the main board of the transmitter. The RF module has two connector blocks that just slide down over these pins. Unfortunately I don't see a way to put beads on them, unless I can find some really really tiny beads.

But again thanks for the suggestions - you've been very helpful!

[Mr.RC-CAM]

Regardless of how you do it, fixing this sort of problem will require exceptional RF expertise and a good post-modification evaluation of the Tx's RF performance. If you are sure the interference is entering via the antenna, then an RF isolator may help. For example: http://www.raditek.com/isolators.htm

Frankly, any external interference that is strong enough to affect a popular Tx's RF deck would concern me enough to fly elsewhere. Even if the glitches could be tamed.

RC-CAM

[PVenkman]

RC-CAM, I'm beginning to suspect more and more that this is simply over my head and the only possible hope I have is convincing someone a whole lot smarter that I am that it's an interesting problem to tackle. The frustrating thing for me is lots of other people fly there with all different kinds of equipment with absolutely no trouble at all.

The isolator idea is interesting; do they have the same issue with impedance matching as a filter?

And just curious, are you worried about the strength of the signal for health reasons or because it may still cause a problem even if it looks like everything's fixed?

[Mr.RC-CAM]

The isolator idea is interesting; do they have the same issue with impedance matching as a filter?
Impedance matching needs to be met or RF efficiency will drop. The short whips on our Tx's suggests that they are below 50 ohms, so a 50 ohm isolator may work with some minor antenna coil tweaks. But, it is really hard to say. And at this point, the violating RF may be entering through other areas too, so the isolator may not fully solve the problem.

...are you worried about the strength of the signal for health reasons or because it may still cause a problem even if it looks like everything's fixed?
From looking at your reported symptoms and what you have tried, this appears to be a serious external RF source. Frankly, it is VERY unusual to have RF enter through the antenna of a handheld Tx device and cause improper operation of its RF deck. {For sure, this is the first time I have ever heard it reported by an R/C'er.}

The RF source would need to be VERY strong. Strong enough to suggest that other elements of the R/C Tx's and Rx's RF chain could become compromised too. That is where I get uncomfortable.

One course of action is skip working on a "fix" and instead concentrate on identifying the interference itself. With useful data, you may be able to garner technical cooperation with the radio mfg. At a minimum, you would need to determine the frequency and the RF field strength. If possible, obtain the ID of the operating station.

If you have a buddy that belongs to a ham club, have him present this problem in their next meeting. Then challenge the club members to find out what is going on. They live for that sort of fun. ;)

RC-CAM

[foofighter]

Impedance matching needs to be met or RF efficiency will drop. The short whips on our Tx's suggests that they are below 50 ohms, so a 50 ohm isolator may work with some minor antenna coil tweaks. But, it is really hard to say. And at this point, the violating RF may be entering through other areas too, so the isolator may not fully solve the problem.

The isolator will need to be correctly terminated to operate properly. Electrically 'short' Tx whip antenna will be fairly high impedance. RF power loss could be significant and the quoted isolation may even drop to zero in extreme cases. This may result in 'apparently' worse performance than you have now.


From looking at your reported symptoms and what you have tried, this appears to be a serious external RF source. Frankly, it is VERY unusual to have RF enter through the antenna of a handheld Tx device and cause improper operation of its RF deck. {For sure, this is the first time I have ever heard it reported by an R/C'er.}

The unit is approved in Europe so it should have immunity from 80 - 1000MHz at a 3 V/m field strength. BUT I suspect that the interferer is MUCH higher in frequency. If this is a military airfield I would suspect ground radar or MLS as the culprit. Actual RF level may be very high but only in short pulses. The pulses would not disturb a xtal circuit but can cause havoc with unscreened or poorly isolated VCO circuits in the synth unit.


If you have a buddy that belongs to a ham club, have him present this problem in their next meeting. Then challenge the club members to find out what is going on. They live for that sort of fun. ;)

RC-CAM

Now that should qualify for 'idea of the week' :D

[DesertDawg]

If the synthesiser is poorly shielded; the interference frequency could be nearly anywhere in the RF spectrum; in fact, I'd look for signals in the MF or low-HF end of the spectrum. Questions:

1.) Does this occur on any TX channel? If so, what is the synth's clock oscillator frequency? Are there any AM BCB Stations nearby? Maybe an aircraft nav beacon? Ham radio operators? (No flames 'til you read the whole post!)

2.) Describe the layout of your field: Fences, buildings, etc. Anything composed of multiple metallic parts that may corrode.


For question #1; I'm obviously looking for any transmitters that might be fairly near to the synth clock frequency. Close enough to slip through the rudimentary filters in the synthesizer; far enough away to detune the circuit. These oscillators are usually tripled (or more) to acheive the actual TZ frequency; the interference could also be tripled.

For question #2; I'm looking for something that could multiple the the interference's frequency. Corroded metal makes a good semiconductor different lengths or metal could interact to create a tank circuit that could effectively triple the interference frequency (or double, or a combination of these).

Anecdotal story: First, I previously asked for no flames on my Ham Radio comment because I'm known over in the FCC databases as A.R.O. KC7BDP! A few years ago, a bunch of us that hang out on 6 Meters noticed a weird signal generated locally; comparative signal strengths of the hash on various Ham's receivers indicated it was located somewhere North of my home, and very close to my pal K7ACS. Oh, and it was not heard by every Ham out here. We could even make out some audio, but not enough to identify it. Fortunately, I had a day off during the workweek (as I do today) and was lucky enough to pick out, of all things, the "bumper theme music" to the Rush Limbaugh show - BINGO! Listening to a local AM talk radio station confirmed my thought. Back to the calculator - the 23rd (!!!) multiple of the stations broadcast frequency, minus 455kHz was - guess what - the freq we were hearing this on. Called the station engineer a couple of times with my numbers, he blew me off - not his transmitter! Must be something screwed up on our (collective) end(s)! (I'm hearing the unsaid "Stupid Hams" in his tone of voice)
Turns out we were both right..... Directly south of the transmitter site is our local Boy Scout office; so I hopped in the truck, picked up some stuff I needed for my Troop and wandered around the back field with a frequency-dependent FS meter. Killer signal closet to the back chain-link fence and the adjoining TX site. (Well, duh!) Like a moron, I'm staring at my meter and also trying to dial my cellphone while walking around; right foot "finds" a gopher hole and I bounce off the fence - signal disappears COMPLETELY! Turns out that a joint in the fence"s top rail was acting as a detector and the associated tubing was acting as a mulitplier :eek: Squirted a little lube on the joint< gave it a twist or two and haven"t heard anything since! Weird stuff indeed!




Dawg

[PVenkman]

For 1:

I've tried channels 21, 30, 34, 50, and I think 18 and had the same behavior on each. Can't help you with any internals of the synth module - that may be available from MPX, but I don't have any of that info.

I don't know of any AM stations broadcasting nearby; there could be an aircraft beacon. There's a huge RADAR dome that's the closest visual antenna, but since this in on a military base there are probably any number of frequencies in use for various things. And I think there are HAM operators around as well - I seem to recall what sure looks like a HAM antenna on one of the house by the field.

And in regards to 2) there are miles of chain link fence in various states of rustiness, and since it's on a military base (the park part is decommissioned, but other parts are still active) there are any number of buildings and such around.

Basically this seems like one of the worst possible spots for RC, and when I first moved here it wouldn't have occurred to me to even try. But I saw others flying with no problem, and sure enough my 15-year-old gear worked just fine. I was extremely disappointed when I had trouble with my brand new Evo!

[vintage1]

Hmm. Radar dome. Ho humm.
Happens on all channels.
Happens with receivers that work FINE with other transmitters? Yes?

If you can reproduce it on the ground, try various points on the site and see whether that dome is implicated...