I understand that certain receivers are better at glitch-free performance than others, and I want to know which one to get.

Someone told me the tendency to glitch is a problem of the receiver and not the Tx. Is this true? Any advice?

From : Don Stackhouse

Simple question, but no simple answer. Glitches are the result of the failure of the transmitter and receiver to communicate with each other. An entire book could be written on the subject. And, while some types of receivers can deal with certain problems better than others, none of them are completely immune, and none of them are totally unreliable either.

The other thing to keep in mind is that (especially with regards to airplanes) when you add safety in regards to one issue, you usually have to give up some safety and/or capability in another area. This is particularly true with regards to receivers for slow fliers. In general, the receivers that are especially good at rejecting some forms of interference tend to be heavier than the other choices. More weight means less performance, which means a greater probability of getting into a situation your plane can't fly itself out of, and also means more kinetic energy to be dissipated by splintering structure in a crash.

One of the big buzzwords you hear a lot about these days is "dual conversion", as opposed to "single conversion". This is a scheme (but not the only scheme) for combatting what's referred to as "third order intermodulation" or "3-IM". I'm sure some of the electronics gurus on this forum could explain it better than me, but I'll give it a shot.

Our receivers are typically based on the "superheterodyne" operating concept. These use a crystal oscillator in the receiver to generate a frequency that's slightly different from the frequency we're trying to pick up. The incoming signals from the antenna are mixed with the crystal oscillator frequency, creating a much lower intermediate frequency (typically 455 khz) that the receiver is tuned to pick up.

It's like that phenomenon you can hear when tuning a guitar or other stringed instrument. If you finger two strings to make the same note, with one string slightly out of tune, then pluck both of them simultaneously, the sound waves from the two strings will alternately reinforce, then cancel each other. You will hear a "beating" in the volume of the sound that corresponds exactly to the difference in the frequencies of the two strings. That "beating" of the volume is the intermediate frequency.

The frequency range (or "bandwidth") that a tuned circuit (like the one in a receiver) is sensitive to is a certain percentage of the nominal frequency it's set for. That percentage if something around 72 MEGAHERTZ is going to be a much bigger number than that same percentage of only 455 KILOHERTZ. Thus, this scheme of converting the signal to a much lower intermediate frequency results in a more precise receiver.

Unfortunately, the spacing of the frequencies used for model planes in the US since 1991 is just right so that if you have two other transmitters operating near you on just the right combination of frequencies, their signals could mix with each other and create an intermediate frequency that almost matches the intermediate frequency that your receiver is creating internally. The receiver can't tell the two intermediate frequencies apart, both get through into the controls, and you and your model proceed to have a very bad day.

For this to happen, you need to have just the right combination of transmitters operating in close proximity. If you fly by yourself or in a very small group, your odds of encountering that combination are very low (note, it takes at least two other transmitters in addition to your own to cause a 3-IM problem). Also, some frequencies in the R/C airplane band (in general the ones in the lower and middle portion of the band) have a greater number of possible transmitter combinations that will cause this than transmitters operating in other parts of the band. For example, most of my planes are on fairly high channels, and even at big fly-ins I rarely have problems with 3-IM. Meanwhile, Joe Hahn typically flies on frequencies in the middle of the band, and he's had significantly more problems with it. Because he flies a lot in large groups, Joe now uses dual-conversion receivers in many of his planes.

"Dual conversion" simply means that in addition to going through the first stage of conversion to an intermediate frequency, the signal goes through two of these to yet a second intermediate frequency. 3-IM can get through the first stage successfully, but not both. The down side is that now we have a bunch of extra circuitry we've added to the receiver's front end, some of which is bulky, heavy and relatively expensive.

There are other schemes. The intermediate frequency created by 3-IM is not exactly the same as the one generated by the receiver's crystal oscillator, so an extremely precise receiver can tell them apart. Many of the JR receivers use a system they call "ABC&W" to accomplish this. The Berg 6 receivers used a similar system, and there are others. The JR receivers have an excellent track record in high-density RF environments such as contests. Unfortunately, the problem with having a very precise receiver is that your transmitter must also be transmitting exactly on its assigned frequency. Some folks had problems with the Berg 6 in particular, and the problem turned out to be their transmitter (and in some cases these were high-dollar competition grade computer transmitters!). The receiver was listening to the correct frequency, but their transmitter was transmitting on a slightly different frequency, not where it was supposed to be. It's tough to convince a customer that the problem is his brand-new megabuck supercomputer transmitter and not this new receiver, especially when his other (less selective) receivers work fine with it. However, in nearly all cases that was indeed the problem. Some of the transmitter manufacturers even ended up publicly admitting that some of their transmitters weren't doing what they were supposed to and issuing a recall.

Other systems that seem to be getting quite popular lately are various forms of "digital signal processing". A microprocessor checks each incoming transmitter pulse train to see if it's within the parameters of a valid signal. If it doesn't (such as if the signal has been corrupted by 3-IM, or by other forms of interference as well), then the microprocessor rejects that pulse and holds the existing control settings until the next valid pulse train arrives. This can actually be smaller, cheaper and lighter than a dual conversion system, and it filters out all sorts of other interference besides just 3-IM. However, the software the microprocessor uses is very critical, and if not done just right it can cause more problems than it solves. Some of the early attempts at this strategy had this problem. However, I've heard good things about some of the new ones. I have a Berg Microstamp and a Plantraco DSP-4SC I'm planning to test soon, and I'll post the results here after I've accumulated some flight time with them.

The newest dual-conversion receivers seem to be improving with regards to size and weight as well. One of the receivers that very rapidly has made an outstanding reputation for itself is the FMA M-5. I have one of these in the prototype of our Roadkill Series Boeing B-17F, and I've been extremely happy with it. It's not terribly bigger or heavier than the GWS GWR-4P single conversion receivers I use in most of my planes, but it has dual conversion and full-size receiver range and performance. For small models where every gram is important I still like the GWS single conversion receivers, but for something that has as much blood, sweat and tears invested in it as that big Boeing, I like the extra "confidence factor" I get from knowing that M-5 is in there doing its job.

Some receivers got off to a rocky start, and some of them solved their problems and went on to develop a reasonably good reputation. JR came out with a sub-micro receiver called the 610-M that had some serious reliability problems. Supposedly they eventually got those fixed and the new ones are supposed to be reliable, although I've personally tried two of them, one early and one later one, and...well, let's just say that neither of them are in any of my airplanes right now. The Hitec Feather receiver initially had some problems with its electrical design and with the durability of the pc board (too thin and prone to cracking from what I heard). I got one of the early ones, with the short antenna, and the only change I made was to install a full-length antenna. I've had some glitches with it, but it's still in my Tiger Moth and generally performs well. I understand that the revised ones they came out with soon after the initial problems came to light have performed very well.

Then there's things like losses of signal, such as that rare case where your own transmitter signal bounces off of a nearby metal roof, and at one particular spot in the sky combines with the signal coming directly from the transmitter to the plane such that they momentarily cancel each other out. That's not a receiver problem; you can't expect it to receive a signal that's not even there! Typically the plane will make some sort of violent jerk in its flight path, then regain control (assuming terra firma or some other obstruction doesn't get in the way first). A receiver with digital signal processing presumably would eliminate the jerk, so you wouldn't even know the signal loss had occurred (which is why some of them like the Berg Microstamp actually log how many signal interruptions they filtered out during a flight and signal the count to you with a built-in LED). However, signal loss is control loss, and it's a small consolation if instead of convulsing itself into the ground, your model maintains smooth, stable, level flight right into the eagerly waiting branches of a nearby tree.

Then there's other RF ("Radio Frequency") noise sources, such as nearby arc welders, industrial applications (we are "secondary users" according to the FCC, and it is possible for primary users such as industrial cranes and such to be operating right on one of our model airplane frequencies, and with far more wattage), power line and other electrical grid effects. Us indoor fliers might even have to worry about lightning generated interference, which is not generally an issue for outdoor fliers, who generally have other concerns on their mind in that sort of weather (Oh, Boy, Here comes a thunderstorm, let's all go stand out in the middle of an open field and hold long, sharp metal rods!!). I once had an A6M2 Zero go in due to interference during an indoor fly-in with a thunderstorm outside, and we were all making jokes about it being just another case of "a Zero being shot down by Lightning". Actually, yes there were some of our P-38's at that event...

Antennas also can be a factor. The RFFS-100 system used in the really tiny and ultra-lightweight models is a good system, but not as interference resistant as most of the heavier systems, at least in my experience. The planes it flies in usually can't tolerate the bulk, weight and drag of a full length (39.5") trailing wire antenna, and the reception with the recommended 18" antenna is not enough for safe flying in some RF environments. I've found that using one of the little coiled-wire antennas such as the ones available from Azarr or from our website can give dramatic improvements with this radio.

The bottom line is that there is no receiver that can be absolutely perfect all the time, and even the ones that approach that usually cost something to achieve it, typically weight, size, price or all of the above. If you fly by yourself most of the time, dual conversion may not be an issue for you. Also, what works well in one RF environment might not do as well somewhere else. In the area of small lightweight receivers, I personally have done very well with the GWS GWR-4P single-conversion receivers, the FMA M-5 dual conversion, and some others. Other folks in other locations and flying situations might have different opinions. Probably the best solution is to check with the other fliers in your area and try to find a consensus on what seems to be working well for them, in airplanes with the same size and weight-carrying ability as yours.

The other thing to bear in mind is that by far, in the VAST majority of crashes that get blamed on "radio failure", the actual cause of the crash was the "nut" on the end of the control stick!

Don Stackhouse
DJ Aerotech