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Plea for help with tracking transmitter... - RF Cafe Forums
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SnakesAndStuff
Post subject: Plea for help with tracking transmitter...
Posted: Fri Jul 28, 2006 6:14 pm
Captain
Joined: Fri
Jul 28, 2006 5:52 pm
Posts: 6
I need some help with tracking
transmitter design/explanation. Here is some background information.
I'm currently working on my M.S. in Biology at Arkansas State University.
I've run into a unique situation involving pitvipers and previously
undocumented behavior. Radio telemetry is an important part of this
project, but currently I'm working on all borrowed equipment as I have
no funding. I've rebuilt a few old transmitters that were given to me
(the last time they were used were in the early 1980's) and have refurbished
transmitters for others, including transmitters with a mass of .48g
or less for use in bat research (mass including battery and antenna).
New transmitters that are commercially available are cost prohibitive
as they usually are $120-150. Working with small electronics under a
disection scope is not anything new to me as I've been working under
disection scopes for years for various reasons including tissue prep
for transmission electron microscopy. I have just enough background
in electronics to make me dangerous (although I am the only person that
you'll find that has a mamba cage that is thermostatically controlled
by a linux box using a network of 1-wire devices that also control the
light cycle).
This is the simplest single stage transmitter
design I have run across. It is still a common design that is in use
today. In fact most of the variance I've found in different transmitters
are in the timing circuits for the pulse (be it controlled by a simple
resistor/cap network, controlled by an astable multivibrator, or even
cmos controlled). The timing circuits I understand. The oscillating
circuits I'm completely clueless about. From what research I have done
on transmitters in the last year it seems to me that radio frequency
electronics are more voodoo than science (but, admittedly so, I am from
a biology background, not electronics)
Following the above design,
I have one specific schematic that lists the parts used as a 57.730-57.778
MHz third overtone xtal, and for the LC circuit a 8.2 pF cap and a for
the inductor 12 turns of 40 s.w.g. enamelled wire around 2.5mm. For
the coil, I get an inductance to the order of around 67-70 microH, which
puts the LC circuit at around 6 MHz. The schematic calls for this transmitter
to be in the 160-174 MHz range. The only difference between this schematic
and the one I have pictured above is the fact that they have added a
trimmer between the xtal and the battery side of the choke coil, which
is listed as being there for tuning purposes. If I could have this explained
to me how the frequency is affected by the xtal and the LC circuit it
would be GREATLY appreciated.
If I could get a working model
of a 150 MHz transmitter for demonstrative purposes that would be great,
but my ultimate goal is to be able to build these for my animals, and
I'm currently working in the 218 MHz band (218.000-218.999 MHz). I realize
that these are very narrow tunings and that very low ppm xtals are crutial,
so I'm also needing to find a source for such parts. I've had conflicting
reports about 218 being achieved through 3rd overtone xtals, and some
reports that say you have to use a higher overtone xtal. If that is
the case, how does that affect the oscillator circuit?
If you
think that you can help me, or know someone that can, please reply to
this message, or reply to me at robert.neal@smail.astate.edu
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SnakesAndStuff
Post subject: clarification...Posted:
Fri Jul 28, 2006 6:25 pm
Captain
Joined: Fri Jul 28,
2006 5:52 pm
Posts: 6
what I mean by my last paragraph is I want
to understand it enough to be able to order parts so that I can construct
a working model... I'm not looking for handouts of equipment
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fred47
Post subject: TransmitterPosted: Mon
Jul 31, 2006 11:15 am
General
Joined: Wed Feb 22,
2006 3:51 pm
Posts: 104
Hi!
First, just to help your understanding
- the value you calculated for the inductor is way off. Are you sure
you didn't use a formula with the results in nanoHenries?
Trimmer
capacitors are there to let you set the frequency exactly - even though
the original tolerance may be 0.005% (50 ppm), you can adjust the frequency
to be precisely correct - IF (and it's a big if) the crystal has a wide-enough
"pulling" range. Overtone crystals can't be pulled as far as fundamental-mode
crystals, however.
Technically, the description of the oscillator
you have is a Pierce circuit. You should be able to get some specific
assistance by calling the Applications Engineering department of one
of the big crystal manufacturers:
JAN Crystals, Fort Myers, FL
ICM (International Crystal Manufacturing), Oklahoma City, OK www.icmfg.com
California Crystal Labs (Cal Crystal) www.calcrystal.com
There's
a larger list (with web sites & phone numbers) at
http://www.bobbamont.com/crystal.html
One thing that's not clear to me - it sounds like this circuit is
supposed to operate as a low-frequency blocking oscillator, so that
it puts out a short pulse periodically. I don't off the top of my head
see how that's going to happen as drawn. I'll do a more rigorous analysis
when I have time, but I hope the material above helps.
Good Luck!
Fred
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SnakesAndStuff
Post subject: Posted:
Mon Jul 31, 2006 1:30 pm
Captain
Joined: Fri Jul 28,
2006 5:52 pm
Posts: 6
Thank you VERY much... I'll post the circuit
description that I have a little later and hopefully we can go from
there and we can get this thing figured out.
MANY THANKS for
all help.
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nubbage
Post subject: Plea for
help with tracking transmitter...Posted: Tue Aug 01, 2006 9:24 am
General
Joined: Fri Feb 17, 2006 12:07 pm
Posts:
218
Location: London UK
Hi Robert
If you do not need the precise
frequency accuracy of a crystal oscillator, which is likely if you are
dealing with very short range, where the signal level is high, then
you might consider a tunnel diode oscillator. The diodes are now very
cheap, and the circuit is even simpler than a transistor crystal oscillator.
The oscillators only generate a few hundred microwatts, but that is
enough at UHF to reach tens of meters.
By suitable choice of bias
circuit I believe the oscillations can be made to squedge on and off
like a blocking oscillator. All my circuit notebooks are back home,
and I am away from base for a couple of weeks.
There are now some
good web sites that specialise in grouping electronic circuits as to
function, like "oscillators - low power". Can anyone out there suggest
a good one? Is www.allaboutcircuits.com one of these? I will need to
log off and re-check.
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SnakesAndStuff
Post
subject: Posted: Tue Aug 01, 2006 4:13 pm
Captain
Joined:
Fri Jul 28, 2006 5:52 pm
Posts: 6
Hey Fred,
Regarding
the pulsing of the circuit, from what I've gathered is that the timing
circuit is basically the battery, the 4 microF cap, and the 200 kOhm
resistor.
The resistor regulates how fast the capacitor can charge
at, and once the cap is charged enough it turns on the oscillation circuit
(I'm guessing the point of charge at which it can turn the circuit on
is regulated by the 1.8 kOhm bias resistor) until the cap is discharged
and this starts over again...
Sorry if I'm stating the obvious
to anyone here. I'm on my way back out into the field and I'll check
my messages and everything tomorrow when I get back in.
Thanks
again people, you've been very helpful.
Bobby Neal
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yendori
Post subject: Posted: Tue Aug 01, 2006 6:29
pm
General
Joined: Thu Sep 25, 2003 1:19 am
Posts:
50
Location: texarcana
It seems you have a pretty good understanding
of the circuit.
Just for the record , the circuit isn't necessarily
simple
or difficult. I have several years of experience and
whenever somebody puts a new circuit in front of face
my eyes always
go crossed for a couple of minutes.
Don't forget your leather
boots!
Rod
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SnakesAndStuff
Post subject:
Posted: Sat Aug 05, 2006 4:24 pm
Captain
Joined: Fri
Jul 28, 2006 5:52 pm
Posts: 6
I'm back, been busy doing field
work the last few days.
If anyone here can help me out more on
the oscillating circuit it'd be greatly appreciated (off board help
is welcome also).
As far as using non-crystal oscillators, I'm
not sure if it would work. I'm using a receiver that works from 218.000-218.999.
If a transmitter drifts more than .005 or so I'm likely to think that
the animal is not in the area (100 meters or so) and lose the animal
and the transmitter. Stability is very important.
If any of you
are in the Springfiled, MO area KY3 is is airing a story soon about
my research (they actually came out and watched us process snakes, release
them, and capture others, fun stuff).
Once again, thanks and
I look forward to any future help.
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nubbage
Post subject: Help with a tracking transmitterPosted: Tue Aug 08,
2006 7:56 am
General
Joined: Fri Feb 17, 2006 12:07
pm
Posts: 218
Location: London UK
Hi
Not sure if this
helps, but I found that AVX Corp produce miniature quartz oscillators
for around $1 in quantity. This is a quote from a press handout and
note the dimensions, unbelievable 2.5mmx2.0mm:
AVX Corp is now
offering an ultrasmall crystal oscillator designed for telecommunications
systems. Designated the CX-2520SB, the surface mount crystal oscillator
contains a ceramic package that offers high reliability with repeatable
performance in a compact package. Resonating at a frequency range of
26.000-60.000MHz, the CX-2520SB oscillator provides reflow compatibility
for mobile communications, Bluetooth and wireless local area network
applications.
The lead-free, RoHS-conforming oscillator has motional
series resistances of 100ohm at 26.000-34.999MHz and 50ohm at 35.000-60.000MHz
with a load capacitance of 8pF.
'We developed this oscillator
with the design engineer in mind', said Kio Ariumi, AVX Product Manager.
'Designers continue to be pushed to miniaturise their products with
increased functionality and reliability'.
'With the CX-2520SB
oscillator, AVX supports their design efforts in meeting these difficult
design requirements'.
Measuring 2.5 x 2.0mm (+/-0.1mm), the 0.45mm
(+/-0.1mm) thick crystal oscillator has a frequency tolerance of+/-10ppm
at 25C and an operating temperature range of -30 to +85C.
Typical
pricing for the CX-2520 crystal oscillator starts at $1.00 each for
1000 pieces
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SnakesAndStuff
Post subject:
Posted: Tue Aug 08, 2006 3:16 pm
Captain
Joined: Fri
Jul 28, 2006 5:52 pm
Posts: 6
Wow, I'm going to have to look
into those. From looking at them really quickly it seems that they meet
the specs that I'm looking for, but I'll be sure to check and let you
know.
Thanks a ton once again!
Posted 11/12/2012
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