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aeajr

475 Posts

Posted - 10/03/2006 :  11:16:06 PM  Show Profile
WHAT YOU NEED TO KNOW ABOUT RECEIVERS
by Ed Anderson
aeajr on the forums
Revised 7/2008
Revised 10/2010
Updated 2/2013


You control the plane by moving controls on the radio, but it is the receiver that "hears" the radio and directs those commands to the proper servos to move them according to your wishes. So, what do you need to know about receivers when preparing and flying your plane?

Note that this is written for North America.

Also be aware that there are two parts to the thing we normally call the radio. There is the part that we touch, the box/controller/encoder, that has the sticks. If it is a computer radio it has a program that we use to "program" our models and save in model memories.

Then there is the RF, radio frequency, deck. This part sets the RF channel, whether it is 72 MHz or 2.4 GHz. This is the part that also handles the "protocol". More on that later.


FREQUENCY AND CHANNEL

Receivers are specific to a given frequency. For example, in North America, NA, our planes are flown most commonly on 27 MHz, 72 MHz, or 2.4 GHz and perhaps one or two others. Your receiver has to match the frequency of your radio (transmitter) in order to be able to hear it.

Until recently the primary frequency for RC aircraft was 72 MHz. Today 2.4 GHz has been added and is growing rapidly. Because both are still very much in use I will cover them both. By contrast 75 MHz is dedicated to surface use, but 2.4 GHz can also be used for surface craft or aircraft.


72 MHZ

The 72 MHz frequency band is split into 50 sub frequencies, or channels so that we can have more than one person flying a plane at any given time. If two pilots are on the same channel as the same time the aircraft would not know which to listen to, leading to loss of control of the aircraft.

Normally you need to get a crystal with your receiver that matches the channel of your 72 MHz radio. In RTF packages, this is already done, so you don't need to worry about it. However if you are buying your own receivers, you must match them to the frequency and channel of your radio when you buy them. Your supplier can help you with the details. One suggestion is that you not mix crystal brands. They may work but this introduces a risk that you are better off avoiding. If you get a Hitec receiver, get a Hitec crystal.


AM and FM and FM SHIFT

Just like your car radio, RC radios can use AM or FM to transmit their instructions to the plane. AM is an older technology but it is still in use, primarily in low end 2 and 3 channel radios. However most new 72 MHz radios are FM. Both work!

In North America, FM radios are grouped by those using positive shift and those that use negative shift. Typically we speak of JR and Airtronics as positive shift. Hitec and Futaba are negative shift. In some cases these brands can be made to change shift through a function called shift select or
reverse shift.

Shift refers to how the radio codes instructions for the receiver. One is not better than the other, they are just different. This is only important when you are buying a new receiver as you need to be sure that your FM receiver and your FM radio are using the same shift. Shift does not apply to AM radios.

Crystals are not specific to shift, but they may be specific to AM vs. FM. Be sure you get the right type of crystal for your receiver.

FM/PPM and FM/PCM

PPM and PCM further define how the radio codes commands to the receiver. We normally speak of PPM and PCM in the context of FM radio/receiver combinations. If you are buying an AM receiver/radio, you don't need to take this into consideration.

FM receivers can be either PPM or PCM. When people say FM, they typically mean FM/PPM. If they say PCM, they mean FM/PCM.

As long as the shift is right, you can mix brands of FM/PPM radios and FM/PPM receivers. On the other hand, FM/PCM receivers are highly brand specific. If you have a Futaba radio capable of PCM transmission and you wish to use a PCM receiver, you must have a Futaba PCM receiver that is compatible with that model radio. No mixing brands in PCM.

As far as I know, all FM radios can transmit in FM/PPM. Some can transmit in FM/PCM also. I don't know of any that are FM/PCM only, but there may be one out there. If PCM is listed, it is normally an extra feature, not a requirement you use PCM.

Some will say that PCM is better and more reliable. I can neither confirm or dispute this point. I will point you to a couple of articles that discusses PCM, how it works and their opinion of the advantages.

Futaba FAQ on Advantages of FM/PCM over FM/PPM
http://www.futabarc.com/faq/product-faq.html#q102

Article on PCM vs. PPM
http://www.aerodesign.de/peter/2000/PCM/PCM_PPM_eng.html#Anker143602

PCM receivers tend to be more expensive, larger and heavier. From what I gather FM/PPM is what the overwhelming majority of flyers use. FM/PCM seems to be most popular in the high performance world, giant scale and competition planes. Choose whichever you like as either will fly your plane.


2.4 GHz AKA Spread Spectrum

A few years ago radios were introduced on the 2.4 GHz band. These systems are also referred to as spread spectrum systems because they can operate using more than one frequency in the 2.4 GHz band in order to avoid channel conflict and interference from other 2.4 GHz sources.

First they were available for surface craft and later for aircraft. This discussion will focus on the aircraft use of 2.4 GHz and how it deferrers from the older but still widely used 72 MHz systems.
The greatest benefit to 2.4 GHz systems offer over 72 MHz systems is that we no longer have to be concerned with frequency control. The radios on the 2.4 GHz band automatically resolve any conflicts so that multiple radios can be on 2.4 GHz without conflict. This is done by binding the radio and the receiver. The receiver records the ID of the radio and will only listen to signals coming from that radio. In this way it will ignore other 2.4 GHz signals coming from other radios, as well as other 2.4 GHz sources.

If for no other reason, this makes 2.4 GHz very desirable. Most new radio systems for RC use are being introduced on the 2.4 GHz band.
In most cases 2.4 GHz receivers are brand specific. That is to say that if you fly a Spektrum radio you need to purchase a Spektrum receiver. Even if your Futaba receiver is 2.4 GHz it won't work with the Spektrum radio. There have been some recent introductions of compatible receivers. I will not discuss these. For the most part you will want to use the same brand radio and receiver for 2.4 GHz systems.


RANGE

For practical purposes, range is determined by the receiver, not the radio. It is a function of sensitivity of the receiver and its ability to pick out the radio signal and filter out noise. Many brands state the rated range of their receivers.
Some radio makers will give you a working range for their receiver; say 1000 feet or one mile or something like that. Others may not indicate range in feet but will classify their receivers as full range�, indoor or parkflyer receivers. The latter two suggest short range receivers. While absolute numbers cannot be provided I am going to give you what I use.

Full range receivers are typically considered to be line of sight receivers. That means if you can see the aircraft you can control it. If you are not provided with a specific range by the maker, I would use 1 KM or about 3000 feet as a safe working range. It might work farther away than that but I would expect a full range receiver to have at least this range.

Likewise parkflyer has no specific meaning, so I use about 700 feet as a working range for these receivers, about two football fields end to end. For indoor I would use about 150 feet as a safe range. Again, these are just ballpark estimates on my part but these are probably safe working ranges.

How much range is enough? That depends on the application. You can NEVER have too much range, but you can have too little. If the plane gets out of range it will crash or fly away. More range is always better.

Here are my suggestions for minimums:

Indoors

Indoor planes are usually very weight sensitive, every gram counts.
To get extremely light weight, sometimes range has to be sacrificed but that
is OK indoors as long as you know what it is. I suggest 100' minimum and
more is better but you may be fine with less. Many indoor flying spaces are
less than 100 feet along any span and you are not going to accidentally fly
past the walls.


Outdoor - Planes

Slowflyers, micro helis and small electric planes less than 36" wing spans can
often get by with ultra light receivers with ranges of as little as 500
feet. This is adequate if you have a small model or fly in a small field of
under 500 feet in span. Many of these small models can be hard to see at
ranges of more than 350 feet, approximately the length of a football field.
I prefer more range, but many people do fine with 500 foot receivers.

Today there are plenty of micro receivers with 1000' or greater rated range
that are under 1/3 ounce, about 9 grams. I have a large field that is 1600
feet long so it is easy for me to get a plane out beyond 500 feet without
realizing it. While it can become hard to see them at that range, I don't
want to lose it because I ran out of receiver range.

If you can tolerate up to .7 ounce, about 20 grams, for your receiver, then there is no reason to use a receiver with a 500 foot range limit, except price. Why limit yourself with short range receivers and take a chance of losing you model?

For gliders, sailplanes, fast electrics or glow planes with wing spans of 72 inches or more you would be well advised to use a full range receiver. Some of these planes can be quite large and can be easily flown at distances approacing 1 KM.


SIGNAL PROCESSING - Single and Dual Conversion, DSP and more

This discussion is specific to the non-2.4 GHz receivers as you don’t see these designations on 2.4 GHz receivers. In addition to range, receivers will usually specify if they are single
conversion, dual conversion, or that they use some other method of signal processing. I will leave it to the engineers to go into depth here. However, as a general rule, dual conversion is better than single but there are excellent single conversion receivers that have digital signal processing and other ways of making sure they pick up the right signal.

Some receiver brands offer 72 MHz receivers in single conversion, dual conversion and perhaps other types of receivers. Be sure you get the right kind of crystal based on the receiver. For example, Hitec dual conversion receivers and single conversion receivers take different types of crystals. I don't know what makes them different but you cannot interchange them. They won't work.


CHANNELS

We spoke of channels above in terms of frequency. We also use the word channels to describe how many servos/devices you can control. So a 4 channel radio can control up to 4 devices, for example. It is OK to have more channels in the receiver than your radio as some slots are used for things other than channel control. For example, if we have a 4 channel radio and are flying a 4 channel plane your slots might be used like this:

1 per control channel = 4
1 receiver battery
1 for plane locator or battery monitor

In this case you might want a 6 channel receiver to give you 6 slots. Or you can use one or more Y cables to share slots. However I prefer to have a receiver with extra slots rather than use Y cables. I feel it will give me greater reliability. Rather than putting money into Y cables I would rather put the money into the receiver.

If you have a 3 channel electric plane, you need a minimum of a 3 channel receiver. You don't typically need a separate slot for a receiver battery as your electronic speed control normally provides the receiver with battery power from your motor battery. You can use a 3, 4, 5, X channel receiver, but it must have at least 3 channels.

You can also use a 2 or 3 channel receiver with a 4 or more channel radio, but you will only have 2 or 3 channels of control available. An example might be to use a 3 channel receiver for your R/E/T plane but use a 4 channel radio to fly it. That works!


COMPUTER RADIO AND CHANNEL MIXES

If you are splitting functions using mixes in a computer radio your receiver may need more channels. For example, if you have a computer radio, you might be able to use two servos for your ailerons and have each work from its own channel. Each aileron will be controlled its own channel. Some radios can put the second aileron on any channel and some require they be on specific channels. Consult your manual for guidance here.

Here is an example where we use more than one slot for a function because we have individual servos on each surface. This is the layout of one of my gliders and is controlled from my Futaba 9C computer radio. I use an 8 channel Futaba radio and receiver and 7 servos.

Ailerons - channels 1 & 7
Flaps - channels 5 & 6
Elevator - channel 2
Rudder - channel 4
Tow hook release Channel 8
Battery - uses channel 3 slot
Plane Locator - Shares channel 8 slot with the tow hood release servo
via a Y cable


POWER TO THE RECEIVER

Note that most receivers operate at 4.8 to 6 Volts. This is usually supplied by a 4-5 cell NiCD or NiMh receiver pack. In planes using glow or gas power, or in gliders, this is a battery pack that plugs into the receiver or into a switch that goes into the receiver. There are some new receivers that can work on a two cell lithium pack of 7.4V. There are some tiny receivers, made for indoor flight that can operate one lipo cell at 3.7 V. Always read your manual, but in general, never directly plug a battery pack of more than 5 cells, or 6 volts into your receiver unless you are sure it can take that voltage.

If this plane has an electric motor, the receiver will most likely get its power from the ESC, electronic speed control. Note that even though your flight battery might be 7.2V or higher, the ESC has a circuit that steps this down to 5 volts to power the receiver. This circuit, called the BEC, battery eliminator circuit, eliminates the need for a separate receiver battery.

If you look at your manual for your ESC, it probably indicates that, if you use more than a certain voltage for your motor pack, you will need to go to a separate receiver battery. This is because the BEC can only step the voltage down so far. Or it may say the BEC can handle up to 4 servos on the receiver up to a 9.6V motor battery, for example, but you are restricted to 3 servos if you go above that. After that it has to be bypassed, you need
a separate receiver pack or external BEC, battery elimination circuit.


Summary

The receiver is the most critical of all the electronics you will put in your plane. The most expensive radio with the wildest features is just a paperweight without a good receiver to decode and deliver the instructions to the servos. While the terms can be confusing at first, you should now be prepared to choose a receiver with confidence. Remember to always consult your radio manual for any specific needs of your radio system.

A key point is that it is the receiver and not the radio that really dictates the range you can expect. I encourage you to be very aware of the range rating of your receivers so you don't lose a plane by exceeding your safe range.

Your receiver has to have enough channels to accept commands from your radio and to accommodate the number of servos/devices you have in the plane. However the number of channels in the receiver does not have to match the number in your radio.

If you are on 72 MHz, your receiver needs to match your radio in the areas of shift, frequency and channel as well as FM/PPM or FM/PCM features. For FM/PPM you can mix and match receiver brands. On 2.4 GHz and FM/PCM you probably need to stay with the same brand for transmitter.

That's about it. Treat your receivers with care and they will take care of your planes for years to come!

Best regards,
Ed Anderson
Long Island Silent Flyers
ESL Newsletter Editor

Edited by - aeajr on 11/09/2010 5:18:37 PM
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