Conference vmszoo::rc

    I'm gonna stick my neck way out as radio theory is definitely _not_
    my strong-suit but I 'think' I know the answer to the half of yer'
    question regarding crystals.
    
    As I recall from my _very_ dated knowledge, attained back in the
    vacuum tube (what?) days, the crystal does'nt oscillate/produce
    the exact frequency printed on the can; it produces some frequency
    less than this value which, after being combined with the IF frequency,
    becomes the Tx's output frequency,that which is printed on the crystal's
    case.  If this is still true, a crystal for one make radio may very
    well _not_ work in another make as the two mfgr.'s may use different
    IF frequencies.  How 'bout it you radio experts out there, am I
    anywhere close to playing in the right ballpark? 
    
    As to crystal swapping; I, personally, wouldn't trust a radio (at
    least in an aircraft) which has undergone a frequency change _without_
    being gone through, checked-out, retuned by an experienced technician.
    You guys back there _must_ have a factory authorized service outfit
    somwhere nearby; we have 3-or-4 of them here in Phoenix.  They're
    usually just some private enthusiast, frequently an active modeler,
    who has the knowledge and equipment to do repairs and maintenance
    on most brands of radios...these guys usually are factory authorized
    by several mfgr.'s as well.  Check around; I can hardly believe
    at least one of these guys isn't established in yer' locality. 

      |
      | |      00	 Adios,      Al
    |_|_|      ( >o
      |    Z__(O_\_	(The Desert Rat)

RC crystals are generally of the overtone type. That is because the
fundamental frequency of a crystal is physically limited to
around 20 MHz.

An overtone crystal is one that produces an especially strong
multiple (overtone) of the base frequency, and it is this frequency
that is marked on the case.  However, this frequency is not high
enough for our applications, so the Tx contains a multiplier circuit
to pick off the 2nd or 3rd harmonic of this frequency. 

This is the main reason why you can't plug in a crystal from one
manufacturer into another's radio.  Because the multiplier circuit
may be different. A secondary reason is that the crystal oscillator
and multiplier circuits are designed to work with the specified
crystal, and the characteristics of another's crystal may be
different; this is a relatively minor concern, but a possible
factor. 

In receivers, its as Al said; the crystal frequency is chosen to
work with the receiver's designed oscillator and multiplier, which
can track the incoming signal either above or below its frequency to
produce the IF.  This, along with the secondary reason above, why
you should be cautious trading crystals between manufacturer's.

BTW-- because of all the overtones and multipliers, and because our
channels are spaced so close together, there is very little
difference in the marked crystal frequency between channels; check
the numbers carefully!

Its been a while since I've fooled with this end of the radio;
please correct me if I'm off base.

I was shocked when the hobby store that I was in mentioned buying crystals and 
swapping them on a regular basis. She deals mostly in cars and they do that 
regularly due to the "heats" that are being run. I was somewhat shocked when 
she said it to a plane person while I was there. I KNOW that the range is 
important in the air and I FELT that changing crystals would de-tune the radio 
to some significant amount.


We also had a ch20 Attack show up at our field. We're 100+ miles from Boston but
I stood down until this guy was done. I mentioned it at our afternoon meeting 
and nobody had any fears and several people flew successfully. Maybe I'll start 
grabbing the ch20 pin when I fly... I imprinted this guy in my memory so I know 
when he's AT the field.

Jim (the paranoid)

    RE-last ?
    
    Curious about the comments made on xtal swapping, I took my race
    car out to the local sand pit.  We *physically* walked the car to
    it's extreeme radio range, then swapped the *original* xtals into
    it---No difference.
    
    The original xtal is channel 90; the test xtal is channel 80.  I
    suppose I could try the extreem ends of the band, just to prove
    or disprove anything.
    
    Perhaps, if I'm understanding this correctly, the crystal *is* only
    the carrier, or center freq. in the case of an FM set, and thus
    not overly critical---The receiver is responsible for 'Detecting'
    (AM) or 'Converting' (FM) intelligence within a given bandwidth
    (whatever that bandwidth may be (1991)) which would be centered
    on the crystal frequency.  Given the inherant *feature* of removable
    crystals, one would think they would tune for the bandwidth and not
    the frequency, per se. 
    
    Any comments welcome. :-)
    
    Fred
    

    It took some time reading this notesfile to realise that you guys are
    serious about buying a new transmitter/receiver just to change
    channels. In the UK I have _never_ (in my admittedly relatively small
    knowledge of the hobby) heard of such a thing being done. My own JR kit
    has interchangeable crystals - both Tx and Rx have
    externally-accessible crystals for this very purpose - and I have never
    seen anything to suggest that this is not a perfectly acceptable
    practice. Certainly, come the day that I start flying other than in a
    field by myself and miles from anyone else flying, I will happily equip
    myself with another pair (or two) of crystals so that I can swap as
    necessary to coexist with other flyers.
    
    I know that the US channels have been given somewhere in this notesfile
    already, but I must confess that I don't remember the frequencies too
    well. I can't even remember the UK frequency allocations that well! But
    in the UK, we are talking about a band centred on around 35MHz that is
    less than 1MHz wide. In other words, if the radio gear is tuned to the
    nominal centre of the band, either edge is less than 1.5% off tune. 
    
    For the Tx, this is a gnat's whisker, compared with the capabilities of
    today's wide band amplifiers. I would expect 10% or greater
    frequency-changing abilities with little reduction in output. This sort
    of capability is widely available in amateur radio gear, for example,
    not just expensive commercial equipment.
    
    The situation is a little worse for the Rx. But even here, we are
    looking to the front-end signal filtering to only require the
    capability to reject image signals (and with a high enough IF this is
    easy). Adjacent channel interference is handled by the IF filtering,
    and that is independent of the actual RF signal frequency. 
    
    Even if the available bands in the US are wider, the above arguments
    suggest that you could at least shift channels across part of the band
    with negligible affect on radio performance (i.e. range!) and maybe
    make scheduling contests easier. 
    
    The other issue must be around the reliability of plug-in crystals. Up
    to a point, I would suggest that changing crystals in this case
    actually makes the connections _more_ reliable, as good sockets have a
    self-wiping effect on the crystal pins as they are inserted/removed. In
    other words, a setup with exchangeable crystals should have the
    crystals removed/inserted every so often, just to help keep the
    connections clean! A personal bias here - I fly a glider, so I'm not so
    concerned about vibration problems! Even in a power plane, though, I
    see no reason why the radio gear can't be so packed as to avoid any
    possibility of the crystal falling out altogether. An elastic band
    wrapped around the Rx would achieve that.
    
    I would be interested to hear any technical responses to these points,
    especially concerning the different RF environment that you are working
    in. You have the wide-open spaces - relatively speaking, we have the
    wide-open ether!
    
    - Brian
    
    (Electronic Engineering graduate, amateur radio operator, and
    self-teaching RC glider guider!)

re .-1   by Brian VANISH::Neale 

>>  [USA modelers buy] a new transmitter/receiver just to change channels. 

    I would not have said that; we can buy new crystals, and we can send in
    our systems for re-tuning at a reasonable price.  I have read that if
    the change is more than to a nearby channel then re-tuning is needed,
    but I don't have personal experience with frequency changes.

    Some radios, e.g. Airtronics Vision, have larger sub-assemblies to be
    changed, not just the crystals, but it is still less than the entirety.

>>  in the UK, ... a band centred on around 35MHz ... less than 1MHz wide. ..
>>  if the radio gear is tuned to the nominal centre of the band, either edge 
>>  is less than 1.5% off tune. 

    In the US, the aero frequencies are from 72.030 to 72.910, so the
    relative frequency range is half that of the UK.
---------------------------------------------------------------------------

Frequency changing is not a "1991" topic, so after this diversion settles
I'll move this string to a more appropriate place and keyword it accordingly.

Alton

From my limited knowledge of Radio I understood that the higher the frequency the 
greater the problems. ie: the US at 72Mhz have greater problems than the UK at 
35Mhz or Oz at 36Mhz and 29Mhz.

I have never liked the idea of crystal changing in the field, I strongly
opposed the European proposal in the F3D Pylon Subcommittee. It was proposed to
do that to enable final and semifinal racing. I will probably have a flame or
two saying you have done it for years without a problem, I also know people who
say they have jumped read lights for years and never hit anybody, that is not
the issue. Do you wash your hands or put on gloves before changing the
crystals?; do you have a clean work bench free of oil and dust to work on?; I
suspect the answer will be no so I won't change crystals on the field as 
without such care I believe the risk is too great.

John

    From rec.models.rc:
    
    Date: 24 Jan 91 20:09:27 GMT
    From: sawyer@twg.com (Bruce B. Sawyer)
    Subject: Re: Changing frequencies
    
    In article <1991Jan18.101513.17069@src.dec.com> dillon@src.dec.com
    (John Dillon) writes:
    >
      [Absolutely correct explanation of why more is involved in changing
      frequency than just swapping crystals.]
    >
    >Bottom line: I don't swap crystals because to do it safely I would
    >need factory service manuals and good test equipment.  I suggest that
    >others avoid the practice too.  I hope no one swaps crystals just to
    >avoid the modest service charges because such charges dwarf the cost
    >of your model.  Besides, your radio deserves a periodic factory tune-up.
    >
    
    John's explanation of this problem is well worded and technically
    correct. You can change Tx/Rx frequencies by just swapping crystals, 
    but then there are other circuits which need to be adjusted.  This 
    gives me a chance to put in a plug for why I have been so happy with
    Ace:  they give you the instructions on how to do the tuning.  All 
    that's required is a voltmeter, and you have to adjust several coil 
    slugs for a maximum reading.  Changing bands (e.g. from 72 MHz to 53
    MHz) would require changing a couple of passive components and then 
    retuning, but within the same band it's about a 5 minute procedure to 
    tune for a new set of crystals.  Parenthetically, I would add that the
    same holds true for any antenna changes.  Because the receiver
    antenna is not of a resonant length, there is a loading coil for the
    antenna. In other words, it's perfectly ok to lop off 6" of receiver 
    antenna wire, PROVIDED you then go in and retune the loading coil.  
    This is why the off-the-shelf units caution you against cutting anything 
    off the receiver antenna.
    
    [end of rec.model.rc article]
    
    In building two Heathkit radios and a World Engines Blue Max, this is
    the same simple procedure I used to tune them. The big issue is knowing
    the test points for the various can adjustments and the order of the
    cans to be adjusted.
    
    
    

    This slipped my mind until now:
    
    Last weekend there were two guys on chan. 20. One of them had a
    Kyosho 5 chan., the other was a Futaba, model ??
    
    The Kyosho guy has an Infinity on order, on chan. 20 also.
    While patiently waiting for the Infinity to arrive, he ordered
    a set of xtals on chan. 19, for the Infinity, from Tower.
    
    Since he had the chan. 19 xtals with him, it seemed logical to stick
    them in the Kyosho rather than wait for the other guy to finish
    flying.
    
    Guess what ? They worked perfectly, no loss of range, and, boding
    well for future odd/even questions, there was NO interaction with
    the chan. 20 Futaba in any xmtr. spacing or on/off combos that they
    tried.
    
    Thats the first time I've seen an odd channel in operation. So far,
    so good.
    
    Terry
                                  
    

    The Acton crew had a non-DECcie show up a couple of times with a ch33
    radio. We had ch22, ch26, and ch34 all operating at the same time with
    little/no problem. Lamar did have some glitches with his ch22 FM
    Aristocraft/Hitec but they seemed to clear up when he stopped pointing
    his antenna at the plane. He was pretty far up also.