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Plea for help with tracking transmitter... - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


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!







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