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  #1  
Old 09-19-2006, 11:29 AM
TomTreese TomTreese is offline
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PPM(FM) or PCM

I have been flying 16 months and have 950 flights and seven aircraft including an Avistar,Hanger-9 P-51 .40, GP PT-17 Stearman,Hanger-9 Cub,Shrike,Dave Patrick Ultimate .40 and most recently a Seagull Edge 540 .60. I have been flying with a Futaba 6EXA transmitter and Futaba & Hitec FM receivers.
I am planning on purchasing a Futaba 7CAF/P transmitter for more mixing capability.
My question is: What receiver, PPM(FM) or a PCM and why????

Thanks.
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  #2  
Old 09-19-2006, 10:28 PM
Dave Robelen Dave Robelen is offline
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Hi Tom,
Personally, I would spring for a PCM if any of your projects are powered with gas engines that have spark ignition. If they all are glow powered, the PPM version is completely adequate. Experience in our group has shown that PCM receivers are more likely to reject interference from spark systems.
Regards, Dave
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Old 11-04-2006, 09:53 AM
aeajr aeajr is offline
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WHAT YOU NEED TO KNOW ABOUT RECEIVERS
by Ed Anderson
aeajr on the forums
Revised 7/13/06

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?


FREQUENCY AND CHANNEL

Receivers are specific to a given frequency. For example, in North America,
NA, our planes can be flown on 27 MHz, 72 MHz, and perhaps others, but these
are the most common. Your receiver has to match the frequency of your radio
in order to be able to hear it. In NA 72 MHz is considered the RC aircraft
hobby channel it is split into 50 sub frequencies, or channels so that we
can have more than one person flying a plane at any given time. In NA, 27
MHz is typically only seen in low end RTF planes and is shared with low end
cars and boats and is limited to 6 channels.

Normally you need to get a crystal with your receiver that matches the
channel of your 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 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 as I have not used PCM receivers. 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/...ml#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.

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 do not. I suggest you stick with brands that state
their rated range or you could end up flying beyond the range of your
receiver.

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 weigh, sometimes range has to be sacrificed but that
is OK indoors as long as you know what it is. I suggest 200' 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 under 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. The
GWS pico 4 channel is a good example of this kind of receiver.

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 1/2 ounce, about 14 grams, for your receiver, then
there is no reason to use a receiver with a 500 foot range limit, except
price. The Spektrum DX6 receivers are good examples. Tiny in size they are
a safe working range of 1500 to 2000 feet. The Hitec Micro 05S at .3 oz,
about 8 grams, has a range of 1 mile. Berg, FMA Direct and others make tiny
receivers with over 1500' range ratings. 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 2
meters, about 80 inches or less, I recommend a minimum of 2600 feet, 1/2
mile or 1 KM depending on how your receiver specs are given. More is ALWAYS
better.

Planes with greater than 2 Meters or 80 inches, and especially thermal
duration sailplanes, I recommend you use a receiver with a 1 mile, 1.5 KM or
5000 foot + rating. It is quite easy to get these planes out 3/4 of a mile,
especially the larger sailplanes, and you don't want to have signal problems
with a plane this large that is out that far. This will give you good
signal strength for the likely distance you will fly the plane which is
probably no more than 75% of that range.


SIGNAL PROCESSING - Single and Dual Conversion, DSP and more

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.

I have no hesitation to use single conversion receivers with 2600 foot, (
1KM or .6 mile) rated ranges in my models that will be flown less than 1500
feet out. Most of my electric planes can't be easily flown further than
that and
since I am operating at less than 70% the raged range I feel comfortable
that good quality single conversion receivers should be fine.

For my 2 meter and greater wing span planes, I use only dual conversion
receivers. Here I am flying planes, typically sailplanes, that may be over
1/2 mile out and 1000 feet or more in altitude. I need every bit of signal
processing I can get to insure I get clean control. I can't afford even a
single glitch.

You make decisions based on your type of flying. This is what I do.

Some receiver brands offer 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 can not 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 has 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 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, but these are
rare. There are some tiny receivers, made for indoor flight that can
operate one lipo cell at 3.7 V, but these are also rare. Always read your
manual, but in general, never directly plug a battery pack of more than 5
cells, or 6 volts into your receiver or you will smoke it.

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.


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 carry out its instructions. 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.

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, but with FM/PCM you can't!

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

Good article on radios by the Torrey Pines Gulls Web Site.
http://www.torreypinesgulls.org/Radios.htm
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  #4  
Old 01-09-2007, 07:58 PM
Fwilson Fwilson is offline
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Smile best I have heard

AEAJR

That is one of the best discussions I have read on the electronics.

Thanks!!
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  #5  
Old 01-09-2007, 11:45 PM
aeajr aeajr is offline
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Glad you liked it.
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  #6  
Old 01-16-2007, 02:19 PM
frankyfly frankyfly is offline
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AEAJR,

What means a "full range" receiver ?

Frank
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  #7  
Old 01-16-2007, 02:30 PM
aeajr aeajr is offline
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Good question. There is no definition of "full range" As a result I do not buy receivers that do not give me an actual distance rating.

To some full range means 1500 feet. To others it is 1/2 mile/2600 feet. I consider full range 1 mile or over 5000 feet. As I fly sailplanes, some of which have 10 foot wing spans, I can get them out 3/4 mile so I want a receiver with that kind of range.

Typical parkflyers do fine on 1500 feet and lots are ok at 500, but I prefer more range, just to be sure. If the receiver can work at 1 mile, then I can be VERY confident it will do well at 1/4 mile.
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Old 01-16-2007, 04:28 PM
frankyfly frankyfly is offline
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AEAJR,
thank you for your fast replay.
What's your opinion about the Berg stamp microreceiver 4 ch ?
I'am going to start flying a couple zagi sorta wings, is it a good choise for them ?

T.I.A.

franky
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Old 02-11-2007, 11:29 AM
moster moster is offline
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I disagree (more radio info)

Additional info

1) all FM receivers are dual conversion....you have two intermediate frequencies 455Khz and 10.7Khz. All FM is this way including broadcast radio.
2)Range of a aircraft system is dependant on both the Tx and Rx. The range of most good quality 72Mhz radios is just over 1/2 mile. The FCC maximum output of a 72Mhz RC transmitter is 500 milliwatt (1/2 watt). (ie My futaba FP-7UAP Super PCM radio with a R129DP receiver has a range of 1000 meters per the specifications in manual). I know there are less sensitive lower quality receivers but i do not know of any better ones!
3)Theroretically, there is no range difference between running in PCM vs PPM modes. They both use the same 500 mWatt RF FM carrier signal. The only difference in the way the control info is sent / received by the radio equipment (basically a different data format).
4)I will never run PCM again!! The problem is that if a PCM receiver does not get the expected signal, it ignores everthing and goes into failsafe mode . There is a very low chance your plane will come back safely...it is completely out of control. When running PPM, a glitch (which is extremely rare) does not mean you are out of control. Think of it this way, would you rather watch TV and sometimes see a little static distortion in the picture, or have it blue screen every time something is not exactly correct. ( ie. when a plane flys over you house or a lightning strike ect).

Just food for thought.
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Old 02-11-2007, 05:56 PM
aeajr aeajr is offline
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Talking

QUOTE=moster]Additional info

1) all FM receivers are dual conversion....you have two intermediate frequencies 455Khz and 10.7Khz. All FM is this way including broadcast radio..[/QUOTE]

I do not know the context of your comment, as I am not a radio engineer, but in the context of RC receivers on 72 MHz, a very large number are advertised to be single conversion vs. dual conversion. So you statement makes no sense to me. Perhaps you can explain.

Quote:
Originally Posted by moster
2)Range of a aircraft system is dependant on both the Tx and Rx. The range of most good quality 72Mhz radios is just over 1/2 mile. The FCC maximum output of a 72Mhz RC transmitter is 500 milliwatt (1/2 watt). (ie My futaba FP-7UAP Super PCM radio with a R129DP receiver has a range of 1000 meters per the specifications in manual). I know there are less sensitive lower quality receivers but i do not know of any better ones!.
Let me add to your comment on range. There are a LOT of receivers on the market with ranges of a mile or more, which does not contradict what you said, just adding this point. Hitec has both single and dual conversion receivers with published ranges of 1 mile + in the air. Ground range may be less.

Yes, there are receivers with shorter ranges, some with ranges of 500 feet or less.

Quote:
Originally Posted by moster
4)I will never run PCM again!! The problem is that if a PCM receiver does not get the expected signal, it ignores everthing and goes into failsafe mode . There is a very low chance your plane will come back safely...it is completely out of control. When running PPM, a glitch (which is extremely rare) does not mean you are out of control. .
I am not aware of any PCM receiver that go into a failsafe state and stay there, as you suggest. Rather, if they lose signal or get corrupted information, they will go failsafe until such time as signal is recovered. To suggest that this in some way would cause you to lose a plane makes no sense.

BTW, there are PPM receivers that also have failsafe modes. You can also buy failsafe add-ons for your receiver which create a failsafe state for the receiver.

What it does mean is that a glitch will not likely cause a random hold in the last position, or surface giration, as can happen with many PPM recievers. Failsafe normally sets all controls to a preset position. In my case, I have all surfaces neutral and throttle at zero. It will hold this position till signal is restored. In most cases that will be till a clean frame is received, which may only be a few hundredths of a second. Then I am back in control.
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Old 02-11-2007, 09:04 PM
moster moster is offline
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More Radio info

First off, I want to point out that i am not trying to offend or try to "prove" someone right or wrong.

1) My statement about dual conversion applies only to FM radios on 72 Mhz (the ones used in the states) and not AM. How RC Fm works is it takes a 72 Mhz signal, mixes the signal with another frequency which make the new signal less noise prone and easier to amplify. Once amplified, strip the data off to give you control. If you look at FM radio theory, 10.7Mhz and 455Khz have been the standard IF frequencies since the beginning of time. I am aware of single conversion FM on 35Mhz (europe) in which there is only a single IF stage.
Here is the problem: If I was on 72.030 (channel 12) and had a IF frequency of 455kHz, when mixed, I would have 2 new frequencies one at 72.030Mhz-455Mhz, and a second at 72.030Mhz + .455Mhz = 72.485Mhz. This is real close to 72.490Mhz (channel 35) and could cause problems. So . they would have to create a new, nonstandard IF frequency to accomadate a low cost, undesireable receiver...wouldnt make sense.
Check out AARL for more FM radio theory.

2) I dont like the receiver manufacturer giving you a range...can I get a mile out of 1 milliwatt??..dont think so. Futaba premium receivers are speced out at .6 miles at max allowable FCC output 500MW. Can someone else make a receiver that has a 80% better range than the top of the line futaba receiver?? .. They should be giving a sensitivity number!

3) I guess my point is in my personal opinion, i would rather get a noisy signal than no signal at all. Either way a 1/4 sec signal interference would probably not change the final outcome, PCM of PPM.
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Old 02-11-2007, 10:18 PM
aeajr aeajr is offline
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OK, I understand. In relations to dual conversion, you are talking about the transmitter. I was talking about the receiver. You can get single and dual conversion receivers.

I won't buy a receiver that doesn't have a published working range, which is what I take as the "reliable" range. In most cases the receiver will work at far greater ranges.

I fly 3M+ sailplanes and we can get them pretty high and pretty far out so I am very aware of range.

Hitec Receivers - published as 1Mile plus+
http://www.hitecrcd.com/Receivers/supremeairRX.htm

http://www.hitecrcd.com/Receivers/supremeairSSRX.htm

http://www.hitecrcd.com/Receivers/HFS04MG.htm
http://www.hitecrcd.com/Receivers/HFS05Ms.htm

I have had the Supreme 8s out at least 3/4 mile and 800 feet up in my 3M sailplanes.


There have been published users tests where Hitec receivers have still be working at 2 miles and JR receivers were working at 3 miles. I would not suggest those are reliable ranges, just what these guys posted as what they tested.
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