NetNews Usenet Archive 1992 #18/NN_1992_18.iso/spool/rec/models/rc/3356

Internet Message Format | 1992-08-18 | 11.4 KB

Path: sparky!uunet!olivea!samsung!transfer!necis!rbono
From: [e-mail] ( NM1D)
Newsgroups: rec.models.rc
Subject: Re: re:AM vs FM AGAIN? - Have Mercy
Message-ID: <[e-mail]>
Date: 18 Aug 92 18:05:15 GMT
References: <Bt4suE.Jpx@paulsun>
Organization: NEC Technologies, Inc., Boxborough, MA
Lines: 191

In article <Bt4suE.Jpx@paulsun>, [e-mail] (Paul Moreau) wrote:
>
> In article <[e-mail]>, [e-mail]
> |>
> |> No - AM by itself is not a problem.  The problem is you want a dual conversion
> |> receiver.  The inexpensive Futaba AM systems are NOT dual conversion.
> |> If you want a radio RIGHT AWAY - call Tower and get the Futaba FM (Conquest).
> |> I know they are in stock because I ordered one this Tuesday.  But for
> |> my purposes it is just a flight pack - I have no use for the transmitter.
> |> It will cost you a bit more than the AM but it is a dual conversion receiver
> |> and it will be safe against 2IM interference.
> |>
> |> Of course if you really want a radio RIGHT AWAY - you should go to your
> |> local hobby shop.  You'd be surprised - most shops now match Tower prices.
> |> If they won't ask them to.
> |>
>
>    Dual conversion on an AM?   I thought that dual conversion was impossible/
>    useless on an AM receiver because of the method of detecting an AM signal.
>    Besides,   I've yet to see an AM receiver that had 'Dual Conversion' on it.
>

 Maybe I can help (I will try).

   AM/FM - This is simply the modlulation method (PCM actually uses FM to
send the PCM information so it can be considered FM for this).  I don't know if
I can explain this without pictures or using my hands (I am of Italian decent
:-) ), but I will try......

  I use the following analogy in classes in Amateur radio that I teach, some
seem to understand it, others.....

  Assume that I want to send you some information.  I am going to use light
to get the information to you (light with be the transmission method, I chose
light because we humans can see light.  We can't see radio waves.  Light is
similar to radio waves except they are at a higher frequency).

  We are discussing R/C models here, so lets further assume that the
information that I want to send you will be the position information for
a servo.  We need some basic rules to follow, lets first agree on the
modulation method to use.   We will start by varying the brightness of
the light to indicate the position of the servo.  We could just turn the light
on and off, but this could only (easily) indicate two servo positions. Since
most R/C modlers want porportional control, we will indicate servo position by
the brightness of the light, full brightness will be full servo deflection in
one direction.  Minimum brightness will be full servo deflection in the other
direction (minimum doesn't mean to turn off the light completly).

 Now we have our transmitter... It will vary the light's amplitude (brightness)
to indicate the position of the servo.  Our receiver will decode this
information and send a signal to the servo to indicate the position of
the servo output arm.  This is fairly simple.  We can even visualise this
without too much difficulty.  Now imagine what would happen if someone turned
on another light source.  The intensity of light that our receiver sees would
now be different.  This other light source would interfere with our light
source, and the two light sources would mix in the receiver to produce
something different than we wanted!!  Of course our servo would now get an
unintended command and would move its output arm to a different position
than we wanted!!

 This analogy is (roughly) similar to that in an AM system.  AM stands
for Amplitude Modulation.  That is the output power of the radio frequency (RF)
transmitter is varied (or modulated) by the intended signal (the servo position
information).  The disadvantage of AM systems is that most NOISE (both natural
noise, and man made noise) has an AM component to it.  This means that most all
interference (such as electrical noise from power lines, or passing automobile
engines) is generating a signal very similar to the signal that your AM receiver
is trying to recover.  This is complicated by the fact that most AM receivers
will mix incoming signals together even if the two signals significantly
different in signal strength as compared to one another (in other words a
strong signal and a weak signal will mix togther and produce an undesired
mix of the two signals).

Ok... if you followed me so far, lets try and FM system:

 To continue with the analogy using light, lets change some of the rules of
the game.  Instead of varying the light intensity, lets change the color of
the light!  We will shift the color of the light from red to blue.  When the
light is blue, we want full servo deflection in one direction.  When the light
is red, we want full servo deflection in the other direction.  If the light is
halfway between red and blue (magenta?), we want the servo output arm to be in
the middle of its throw.  Because we are varying the color of the light and
not its output level (or output power) the light can be left turned on at full
brightness.  This will insure that our receiver always has the strongest signal
to decode.  In our receiver we would need a special type of decoder that would
respond to these changes in color.  To be sure that the reciever only responded
to changes in color and not in intensity, we will include a circuit that will
filter out any changes in intensity and will only allow changes in color to
be seen (this circuit is called the limiter).  This works great and has a
nice side effect.  This receiver will only see the strongest signal.  This is
due to the limiter circuit.  So weak signals (weaker than the signal currently
being received) will be ignored!

 This color changing modulation method is very similar to FM.  FM stands for
frequency modulation.  In fact, different colors of light just happen to be
at different frequencies (red is a lower frequency of light than blue), so
this is a fairly good analogy (if you can follow it). In and FM transmitter, the
servo position information actually changes the output frequency of the RF
transmitter slightly, the amplitude (or output power) stays constant.
FM systems have several advantages over AM systems:

    1) Most noise is of an AM nature, so noise is natually ignored by FM
       systems.  Note: If the noise is strong enough it CAN and will degrade
       an FM system, but it will have to be very strong as compared to the
       level that will cause interference in an AM system.

       In our R/C models, we tend to see a lot of AM noise caused by metal-
       to-metal contact.  FM systems will tend to ignore this noise.  BUT
       you SHOULD still strive to remove this type of interference, because
       it can/will degrade the range of your FM system!  FM systems will
       simply be more tolerant of this type of noise than AM systems!

    2) Due to the use of the limiter circuit, FM receivers will ignore
       weaker signals (this is called the FM capture effect).  This is
       a very nice advantage of FM systems.  However,  if two interfering
       signals are very close in strength to one another, then there will
       be interference.  Also, if the interfering signal becomes stronger
       than the desired signal, the the interfering signal will completely
       obliterate the desired signal.

       During R/C modeling, this capture effect could help in several ways.
       If another modeler was down the street and using the same channel as
       you were, I would expect an FM system to be more immune to interference
       than an AM system because your transmitter should be closer to your
       receiver (and therefor have a stronger signal) than the offending
       transmitter down the street.  This is not to say that there would not
       be any interference, but that an FM system would maintain control for
       a longer period of time than an AM system would.

 What is the disadvantage of FM?  The circutry is more complex to design and
build than an AM system, so FM systems will usually be slightly more expensive.
I believe that FM systems can have more trouble decoding a signal when the
transmitted signal gets VERY weak (I could be wrong on this).  AM systems
sometimes can do better with very weak signals if there is no additional noise
or interfering signals. R/C modelers would completely lose sight of their
models well before this weak signal point was reached so this should not
be an issue to worry about (unless you are doing distance record work and
are tracking your model by radar or something!).

Does all this make any sense??? I hope it helps....  Remember: AM or FM is the
method of modulation.....  What about single conversion as compared to double
conversion???   Well, I have taken up enough net bandwith already (and I
don't know if any of this will make sense to anyone), so I will be REAL terse
on this:

   The RF (radio frequency) of the transmitter (which is a fairly high
frequency of 72 or 75 MHz, MHz = Millions of Cycles per second) must be
converted to a lower frequency that the receiver can decode.  To do this
the receiver uses a series of mixers.  Mixers take two input frequencies and
produce four output frequencies....  WAIT!!! I said that I would keep this
short!!.....  Ok... lets skip ahead......
 .... The number of conversions used is usually referred to as:

          dual conversion receivers convert the incoming RF twice before
           presenting the signal to the decoder circuits.  For example:
            72 MHz --> 10.7 MHz ---> 455 KHz ----> decoder.

          signal conversion receivers convert the incoming RF once before
           presenting the signal to the decoder circuits. For example:
             72 MHz ---> 455 KHz ----> decoder.

    Yes, there are such things as direct conversion receivers (which directly
use the incoming RF frequency without converting it to a lower frequency);
72 MHz ---> decoder.  And there are triple conversion (and probably quad
conversion) receivers too!

 In short, double (or triple) conversion reveivers can reject images better
(another subject) than single conversion systems if they are designed correctly.
Of course having more components makes double conversion circuits more
expensive (and larger/heavier too).  This is NOT to say that simply having
more stages of conversion is better...  it all depends if the engineer did
his homework properly (and if marketing didn't cause him to cut too many
corners to cut costs).

  Both AM and FM receivers can use either single or double conversion designs.
I think that most AM systems are designed to be inexpensive, so manufacturers
might make their AM equipment single conversion to try and keep costs down to a
minimum (this is not to say that all AM or inexpensive systems are of a
single conversion design).

  I hope I havn't fanned the old (FM/AM) flames... AM systems will work fine,
as will FM systems.  If properly designed, in my opinion, FM systems tend to
have some advantages over AM systems.  Be aware that I have tried to OVER
simplify things here.... RF engineers, please be gentle with me if I screwed
up too many of the details!!!

--
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 * Rich Bono (NM1D)              IMRICH              [e-mail] *
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