Conference vmszoo::rc

I'm not a radio expert, and I don't want to be. What I do have is significant
good experience with dual conversion radios. I own 3 Airtronics Vanguard
FM sets. Two on channel 24 and one on channel 16. 

Our club has, over the last several months, undertaken a very detailed study
of the RC radio frequencies in our area. One result is that channel 24 should
be considered unsafe due to fairly regular interference. I fly it all the time.
I've flown it when the frequency checking was done. I usually get in three to
five flights a day on every good weekend, and on several weekdays to boot. I 
have NEVER had a hit! Well actually there was one, but I found when I got down
that I had failed to fully extend my Xmitter antenna so it hardly counts.

What does this mean? Well, nothing really, except that I fully believe that dual
conversion is an absolute necessity in the 1991 narrowband environment. There
have been several excellent notes about the details from folks who know far
more than I on the subject. If you haven't already I suggest you search them
out for the technical details.

Oh yeah, all three radios have, as far as I can tell, only one crystal. Thats the
normal changeable one found in all xmitters and receivers. There may well be a
non-swappable one buired inside, but I've not noticed it on my brief excursions
inside the boxes.

You're milage may vary,
Randy

    I'm not an RF expert by any stretch of the imagination, so I won't
    debate fine points of theory with anyone. But based on my, and others,
    empirical evidence and experiences I can only say that I have flown
    a single conversion AM receiver for the last time, at least in any
    airplane that I care about. My experiences last weekend that brought
    about this decision was outlined in 771.507 which has since
    mysteriously disappered and I won't bother to repeat it here.
    
    Our local problems with single conversion rcvrs centers around the
    interference experienced between radios of this type when a number
    of them are flying in close proximity, on different channels of
    course. This problem is never experienced with FM dual conversion
    types.
    
    We fly within 22Km, and line-of-sight, of a max power TV channel
    4 xmtr. which can cause problems with RC chan. 20, causing it to
    hit other rc channels. Also one of our fields is 200 meters from
    a police sub-station with a large short wave antenna on it's roof
    and constantly broadcasting. Neither of these conditions has caused
    the slightest problems with RC flying to my knowledge.
    
    If JR builds Graupners' radios I'm surprised that Graupner hasn't
    used dual conversion rcvrs before now. All of JRs' are dual conversion
    at least the higher end models.
    
    By the way, thanks very much for the information in 387. My electric
    flight experimentation is on hold for a while as I'm trying to finish
    a new glider, debug another, and get ready for a contest next month.
    
    Terry
    
    

Thanks for your inputs so far. No, I did not scan the notes for detailed info
on dual conversion yet. I just set up a batch job to do this, but it always
gets postponed for one more hour. Not so easy from this side of the pond when
the net is busy.

I forgot to mention that we practically only have FM systems over here. There
are some entry level AM systems on the market, but they are practically not
used for planes. Maybe some beginner planes, but not for people who want to 
stay in this hobby longer.

FM became popular when the Deutsche Bundespost (german FCC?) went to 
narrowband RC for the whole 35 MHz and part of the 27 and 40 MHz bands 
during the end of the 1970s.

Your experiences convince me more than the (only) theoretical discussions.
Thanks again.

Regards,
         Hartmut

It has been explained in other notes, but I'll try to cover it
briefly here - it will take less time than digging up the note
reference.

My knowledge is for U.S., I don't know if any of this differs
for Europe. 

Single conversion receivers use an I.F. (intermediate frequency)
of 455kHz. They have a crystal whose frequency is the desired
frequency plus or minus 455kHz. So if the desired frequency is
10.000 MHz, the crystal is either 9.545 MHz, or 10.455 MHz. When
a signal at 10.000 MHz is heard, it beats with the crystal, and
produces the sum and difference as additional signals. If you
use a 9.545 Mhz, you get two more signals at .455 Mhz (455 kHz)
and 19.545 Mhz. You then filter out everything but the 455 kHz
signal, and detect that.

However, another signal at 9.090 MHz will also beat with the crystal,
providing two more signals at 18.635 Mhz, and at .455 Mhz! Thus,
a signal at 9.090 MHz will also be detected. The only way to stop
that is to put a very good filter on the antenna input (front end)
that will pass 10.000 MHz and block 9.090 MHz. However, a strong
9.090 Mhz signal will still overload the receiver front end and
cause interference. The 9.090 Mhz is known as an "image" frequency.

Dual conversion uses two intermediate frequencies. The first intermediate 
frequency that is much higher than 455 kHz. If you use a 4.000 Mhz IF, 
then your would use a crystal that is 14 Mhz, and the difference between
that and the desired 10 Mhz signal gives you 4.000 Mhz. The interfering
frequency would be 18 Mhz. This is far enough away from 10 Mhz that it
can be easily filtered.  The 10 Mhz first I.F. is then converted down
to 455 kHz, thus the "dual conversion."

So the issue is the quality of filters. Yes, it is possible to design
a single conversion receiver that is as good as a dual conversion.
It is easier, and probably cheaper to use dual conversion.

Note this discussion is independent of AM or FM or PCM. Those are
additional factors that all count in the overall quality and image
rejection of a receiver.

Dave Hughes
(21 years since I got my ham licence, but I still remember a bit of it!)

    Thanks a lot so far. Especially your explanation, Dave, in .-1 was very
    helpful. In the meantime I managed to get the notes regarding dual
    conversion extracted. It will take some time going through and
    understanding them. From the low interest during the last day or two:
    Am I right to assume that noone in notes$land prefers single conversion
    over dual conversion receivers as Mr Robitschko does? Very interesting,
    for the supplier of my 10 year old radio (MULTIPLEX) - which I am very
    happy with - doesn't even offer a single conversion receiver for their 
    PCM radios. I always _thought_ dual conversion was better, but never 
    could really explain why. Thanks again! If there are any more (differing?) 
    opinions or experiences, keep them coming!
    
    Regards,
             Hartmut
                

The safest way to buy a radio is to look for the specifications.
The buzz words can mean anything. It is easy to design a "dual conversion"
receiver that is terrible. Since the vast majority of RC'ers are not
knowledgeable about radios, but insist on having cheap radios, and
latch on to buzz words like "dual conversion" and "1991", the manu-
facturers are obliging us by making cheap 1991 dual conversion radios.
Some of them may be inferior designs, and it's sure hard to tell the
difference. The best bet is to look for good technical data or articles
that give unbiased test information.

This applies to broadcast receivers too. I was looking at the fine print
specs in a catalog for car radio/tape decks. There is a 2 to 1 difference
in FM sensitivity between some of the radios, and it usually had nothing
to do with price! Other specs were widely varied too. 

The specifications to look for in RC receivers are things like bandwidth,
image rejection, and sensitivity. Can somebody add to this? Those of
you who are looking into this stuff - please share your data here so
the rest of us can benefit from your hard work!

Dave

(p.s. My old wide band AM 6 meter radio works fine, and I hope to use it
for years to come!)

According to George Meyers in MA, the key to 1991 is both dual
conversion and filtering.  The filtering is not the common
ceramic or crystal filter, its another type that slips my mind at
the moment.

The only way to guarantee this is to have the Rx go through some
complex laboratory tests.  This is what the AMA requires for
their certification.

1991 is enforced (the letter of the law by the FCC) in the Tx,
but implemented in the Rx (by the AMA guidelines).  

                Something that  hasn't  come  up  in these discussions is
        the difference in  cleanness  of  the airwaves between the US and
        Europe.  In Europe  the  PTTs have enforced very rigorous control
        of  the airwaves requiring licensing  and  certification  of  ANY
        radio transmitter and operator.  As a result they don't have 1000
        Watt CBs and all the incredible garbage we have to deal with over
        here.
        
        Anker

    The 'normal' control of our airwaves may be better - there is the
    mentioned rigorous control, at least on paper - but we have jerks, too.
    People operating radios (not RC) with way too much output, and,
    sometimes, military radar operations which are stronger than any other
    signal. And the circumstance that licenses are required does not
    necessarily mean that everybody has one...
    
    Granted, since I went to FM I never had a problem that was for sure
    related to interference. But since this GRAUPNER demo I'm dreaming a
    bit of buying a computer radio some day (probably not this year), and I
    want to make a good decision. That's why I asked.
    
    Bill, nice to hear that Bernd managed to talk to you. We are looking
    forward to have you here. Are you planning to take a plane along?
    
    Regards,
             Hartmut