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How to test a transmitter circuit?

Teh

Mar 25, 2017
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Mar 25, 2017
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Hello, I have made a simple FM VHF transmitter circuit. But how do I test to see what frequency it is transmitting at? I've tried playing a song at the mic of the transmitter and slowly tuning a radio to look for the song. I've also set the receiver at an unused frequency and slowly tuning the variable capacitor on the transmitter circuit. But so far I've not been able to see any results that show the transmitter is transmitting. So is there a way to show that the transmitter is transmitting? And at what frequency?
 

Teh

Mar 25, 2017
7
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Mar 25, 2017
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upload_2017-3-28_0-18-38.png
This is the circuit we built.WhatsApp Image 2017-03-27 at 23.49.55.jpeg And this is how we connected the circuit, we pulled out the microphone and variable capacitor to the surface of the box. We checked and the circuit connection was according to the circuit diagram.
 

73's de Edd

Aug 21, 2015
3,365
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Aug 21, 2015
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Sir Teh . . . . .

Is your monitoring receiver old school, that lets you tune to any frequency and thereby be able to receive white noise / hiss when tuned off of a station, to a dead spot on the FM band ?.
Or is it digitized and will step to and stop on ONLY active received stations ?
Old style is better in your particular signal seeking situation here.
AND a small portable unit, so that it can be almost kissing the transmitter board.

You mention tuning condenser, but it must be hidden on the other side of the board ? . . . across its air wound companion resonationg coil.

Its now just a matter of setting the receiver atop the unit and off station at the high end of the FM band and slowly sweeping the TC thru its full range to see if you hear QUIETING of the white noise by your incoming transmittted signal.
Next tune the FM receiver to mid band and try the slow transmitter sweep frequency again.
Lastly, place the FM receiver at its lowest received frequenct and do the slow transmitter sweep again.
I'm fully expecting the signal to have been found now . . .if it IS transmitting .
If not, you have the capability to alter the coils effective inductance by compressing its turns to extend your lower frequenct range or by stretching the coil to extend its high frequency range.
Before doing that,. also know that you can alter the low frequency tuning range by the different degree of progressively grasping the center of the coil with thumb and 2 fingers. Try that at high nid and low end of the FM band tests. You will hear the quieting change as you tune past frequency, and can make it sweep up and down by playing with that critical grip level..
Your problem would only be related to the osc xstr, and you did not sub and end up with another xstr nmber with a case lead arrangement that swaps E-B-C connections ? Did you ?
Don't even worry about it NOW, but if the finally working unit will end up not reaching the high end of the FM band, that wound inductance seems one turn too many. Either needing a coil stretching or a turn removal.

73's de Edd
 
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GhostLoveScore

Nov 27, 2016
71
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Nov 27, 2016
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I would recommend to get this RTL SDR http://www.ebay.com/itm/RTL-SDR-Blo...458376?hash=item3f6cfcfb48:g:3HYAAOSwxKtX~vBo

You have to install one program for it and you can see entire spectrum on your computer in a waterfall display. Place the antenna near your transmitter and look around on your computer. 50MHz - 150MHz, it should be somewhere if you made your inductor correctly.
When you find some signal, try turning off your transmitter and see if the signal disappears. If it does, that's your transmitter. You can even listen to it on your computer.
 

duke37

Jan 9, 2011
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You do not have a power supply or a RF bypass capacitor.
You need to measure if the oscillator is working, a 1N4148 diode in series with an analog meter may do this but it may stop a working oscillator.
I would use a grid dip oscillator to find the resonant frequency of the tank circuit but you are unlikely to have one of these.
You could also measure the voltage across R7 and see if it changes when the coil is shorted.

I think the inductance looks to be too high. Have you measured it?
 

Audioguru

Sep 24, 2016
3,640
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Sep 24, 2016
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I chatted about that circuit a few minutes ago, maybe on another website and maybe by someone else in your school. It was also posted 12 years ago and did not work. I fixed it with 4 improvements:
1) Its audio transistor was not biased correctly saturated when the battery is new and was cutoff when the battery is old so I biased it properly and powered it with a low dropout 5V regulator.
2) Its radio frequency changed as the battery ran down so I powered the oscillator with the 5V regulator.
3) Its radio frequency changed when something moved towards or away from the antenna so I added an RF amplifier to isolate the oscillator from the antenna.
4) Its sound on an FM radio was muffled with no high audio frequencies so I added pre-emphasis like all FM radio stations use and the pre-emphasis matches the de-emphasis that all FM radios have. There are two different pre and de emphasis used in different parts of the world.

Your coil is spread out too much, mine has the same 9 turns but is with the (enamel insulated wire) turns close together. Here is my circuit built compactly on stripboard:
 

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