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Crystal FM tracking transmitter

Will777

Sep 2, 2010
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Hi,

I have built a few FM Transmitter kits - all using coils. These tend to not be stable in terms of frequency.

I'm interested in building a crystal-locked FM tracking transmitter using one of the more readily available crystals - e.g. a 10Mhz in a third overtone mode to get to the normal FM band.

With a tracking transmitter I mean a circuit that will produce a beep or continuous tone at a certain frequency.

Has anyone built something like this and can someone tell me which approaches works well?
 

Will777

Sep 2, 2010
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I found this circuit (from http://www.talkingelectronics.com/):

XtalLockedBugCct.gif


Can someone please have a look and tell me if the design seems sound?

It uses a 10MHz crystal.

This is not a circuit for a tracking transmitter - it's a normal FM transmitter - but I thought it's a good place to start.

Then there is this circuit which is a tracking transmitter (from http://www.gw1tdv.co.cc/)

9010132.17e43417.gif
 
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(*steve*)

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Can you give us a direct link to the explanation of this circuit.
 

Will777

Sep 2, 2010
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Can you give us a direct link to the explanation of this circuit.

Hi Steve,

The first circuit diagram is from this page:
http://www.talkingelectronics.com/projects/CrystalLockedBug/CrystalLockedBug-1.html

The second circuit diagram I cannot link to directly - the site has some kind of frame detection that disallows direct links - its at http://www.gw1tdv.co.cc/, under the menu item "AJP Circuits", labelled "300mW VHF Tracking Transmitter"

This is the text that accompanied the diagram:

I originally designed this circuit for use on the 144MHz band to track a vehicle, but it could be used to track anything or be used as the fox for a Amateur Radio Fox Hunt. Ranges of over 3KM have been acheived using 58cm wire antenna, receiving the signal on a REXON RL102 144MHz Handheld Tranceiver with a 3 element Yagi and the transmitter being powered of the 9Volt. This narrowband crystal controlled VHF transmitter is modulated with a 700Hz Tone outputting approximately 300mW of RF power. The circuit is crudely phase modulated hense can be detected by an AM or FM receiver, An AM receiver will receive the signal slightly better. The original circuit was designed around a siren audio generator IC, UM3561, but as this is no longer available I have re-disigned the circuit around a LM555 flip/flop IC. The LM555 generates a 700Hz tone and modulates the output stage. If no more than 9Volts is being used, a fixed 27K resistor can be used in the O/P biasing circuit instead of a preset and the modulation fed directly to the Base. The emitter circuit of the oscillator/trippler stage is tuned to the Xtal frequency filtering the fundalmental frequency. The collector circuit is tuned to 144MHz and outputs the required modulated frequency signal to the amplifier stage. 12pf trims the O/P power into the antenna for maximum power transfer and the 22pF trims the Xtal for spot frequency. This circuit will operate between 5.6Volts and 10Volts without any circuit alterations, but the range will be a lot less at lower voltages. 12Volts can be used by inserting a 82Ohm resistor in series with the positve lead. When using above 9Volts, a heatsink should be used on the 2N2222 as it will get very hot. It is Ideal to tune up this circuit using a wave detector meter placed 10-20cm away from the transmitter. I have a circuit on this site.
 

(*steve*)

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The second one won't work for a normal FM transmitter because it's not actually frequency modulating the signal. I can't actually get to the site, I am basing this on the explanation you have copied.

The first one seems to be a better option.
 

Will777

Sep 2, 2010
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The second one won't work for a normal FM transmitter because it's not actually frequency modulating the signal. I can't actually get to the site, I am basing this on the explanation you have copied.

The first one seems to be a better option.

Thanks Steve,

I will attempt the first design.
 

(*steve*)

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RF isn't my thing. Others may have different opinions.
 

davenn

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Sep 5, 2009
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With a tracking transmitter I mean a circuit that will produce a beep or continuous tone at a certain frequency.

what you really mean is a Beacon transmitter... tracking transmitter.... strange termimology

the second cct as steve said isnt Freq modulated, in fact applying modulation to the final stage is more similar to Audio Modulation

the tone output from the 555 cct needs to be applied to the crystal osc via a varicap diode

remove the 22pF variable cap and replace with a varicap a BB102 or similar and apply the tone (via a 10k trimpot for level adj) to the junction of the diode and the crystal.
then you have a FM TX :)
see modified cct below ....
Dave

please dont transmit on 144MHz without an appropriate licence :)
 

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Will777

Sep 2, 2010
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you really mean is a Beacon transmitter... tracking transmitter.... strange termimology

the second cct as steve said isnt Freq modulated, in fact applying modulation to the final stage is more similar to Audio Modulation

the tone output from the 555 cct needs to be applied to the crystal osc via a varicap diode

remove the 22pF variable cap and replace with a varicap a BB102 or similar and apply the tone (via a 10k trimpot for level adj) to the junction of the diode and the crystal.
then you have a FM TX :)

Dave

Hey Dave,

Tracking transmitter - must've picked that up from some of the sites I've been to trying to get more info.

Let me try and understand - I remove the 22pF variable cap - put the diode in there (polarity? anode towards the crystal?).

Which pin from the 555 produces the tone output?
 

Will777

Sep 2, 2010
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please dont transmit on 144MHz without an appropriate licence :)

Dave,

Thank you for your help.

I understand the warning - will not do that.

In terms of the first circuit then - it's supposed to be a low-power FM transmitter (not a beacon transmitter) in the normal FM band at 90MHz.

Do you think the design looks sound?
 

(*steve*)

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Of course, transmitting in the FM band may not be legal either...
 

Will777

Sep 2, 2010
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Of course, transmitting in the FM band may not be legal either...

Point taken - not sure of the legislation regarding this in South Africa.

We can, however, legally buy FM transmitter kits in this country - small hobby kits that do not have a great range.
 
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