FM transmitter (mod4)

[Hero999]

Correct. .

 

[bananasiong]

Hi,
My transmitter is working, but the distance, I can't even hear it after 5 feets! I've tried both auto and manual tuning radios.
Previously (One track wasn't cut) it worked with the super regenerative radio, up to 10 feets. After that, it didn't work as I didn't cut one track. I also don't know why it was working previously.

Now my antenna is around 70cm, I've tried both lab PSU and the PSU I made, same problem. I have added also the resistor and bypass capacitor at the emitter of the output transistor to reduce the power dissipation by the transistor.

Thanks..

 

[audioguru]

Post voltage measurements of its power supply and transistors to see what is wrong.

 

[Hero999]

I have added also the resistor and bypass capacitor at the emitter of the output transistor to reduce the power dissipation by the transistor.

What value resistor and capacitor are you using?

 

[bananasiong]

Post voltage measurements of its power supply and transistors to see what is wrong.

I'm using 9 volts for the circuit. The collector voltage is 9 volts, base voltage it around 7 volts and emitter voltage is around 3 volts.

What value resistor and capacitor are you using?

56hm: and 1 nF

I've connected a short wire next to the antenna, it can transmit further with the probe touching it.

Thanks

 

[audioguru]

The base and emitter of a transistor is a low current diode. A low current diode never has a forward voltage more than about 1V. Yours has 4V and is probably broken.

 

[bananasiong]

Oh, thanks. Maybe be junction is opened circuit, the 4 volts maybe from somewhere else? But there shouldn't have any current at the emitter right? I'll replace the transistor and check all the connection again. Maybe there is one or more wrong connection.

Thanks

 

[audioguru]

But there shouldn't have any current at the emitter right?

Of course there should be emitter current, the transistor should be conducting about 28mA from collector to emitter. The emitter should be about +1.57V and the base should be +2.33V. If you didn't have your 56 ohm emitter resistor then the collector to emitter current would be about 35mA and the base would be about +0.77V.

 

[bananasiong]

Yes, the voltage rating is almost correct after changing the transistor, now I'm testing the range.

Thanks

 

[bananasiong]

Hi,
It is now really working with my super regenerative 'radio'! Even through wall. Previously I thought the problem is on the receiver, but since it can receive 2 channels, it should be able to receive signal from my transmitter which is placed so near.

Thanks.. Hooray

 

[audioguru]

Good, you fixed it.

 

[bananasiong]

Hehe.. you helped a lot, Hero999 as well. Thanks
But one thing is weird to the super regenerative receiver. It works without an antenna, when I connected antenna onto it, I can't receive anything, no matter long or short antenna.

 

[audioguru]

A super-regen isn't a radio. It has only a single transistor so it overloads and stops working with a strong signal and is not sensitive enough for a weak signal.

A real radio has many transistors and an IC for very high gain and sensitivity and many tuned circuits to select one station and reject the others. It has the proper kind of FM detector. A good radio has automatic gain control to avoid overloading.

 

[Hero999]

Good for you, I'm glad it works.

Does it work with a real radio, when the antenna is connected?

 

[bananasiong]

I mean, the antenna of the transmitter is always there but the super regenerative receiver has to be without an antenna in order to receiver signals.
The transmitter works with both manual tuning and auto tuning radio.

 

[bananasiong]

Hi,
I'm not sure whether I'm right or not.

Q1 is used for boosting up higher frequency? Or to amplify input signal from the mic?

Q2 is the oscillator part of the circuit, modulation happens here?

Q3 is to amplify modulated signal?


If I want to make a variable power supply (variable from 1 to 20 volts is okay) using LM317 with the load current not more than 1A, then the dropout voltage is around 2.3 volts, so I just need to choose the right variable resistor and transformer right? Do I need other components or heatsink? One of the application is for my PIC programmer.

Thanks

 

[audioguru]

Q1 is used for boosting up higher frequency? Or to amplify input signal from the mic?

Both.
At lower audio frequencies the reactance of C4 is too high to have an effect. So the gain of Q1 is R4/(R5 + Re)= about 18.
At 10kHz the reactance of C4 (150nf) is 107 ohms so the gain is R4/([107 x 1.414] + Re)= 51. At 15kHz the gain is higher.

Q2 is the oscillator part of the circuit, modulation happens here?

Yes. The audio signal at its base amplitude modulates it which causes its collector voltage to increase and decrease with the audio. The changing of the collector to emitter voltage changes its capacitance so changes its frequency.

Q3 is to amplify modulated signal?

Yes.

If I want to make a variable power supply (variable from 1 to 20 volts is okay) using LM317 with the load current not more than 1A, then the dropout voltage is around 2.3 volts, so I just need to choose the right variable resistor and transformer right? Do I need other components or heatsink?

The power supply needs a rectifier bridge and a big filter capacitor.
Of course the regulator needs a heatsink. If it has a 5V output at 1A it will create 18W of heat. It needs a 0.1uF input capacitor and a 10uF output capacitor. The ratio of a resistor and the pot sets its output voltage. Use a 120 ohm resistor and a 2k pot for 22V max output. A 2k pot will be 1.6k to 2.4k so the 22V could be higher or lower.

 

[bananasiong]

Oh, that's why the range is so short when Q3 is not working.
If I increase R1, the sensitivity of the mic is reduced right? I can hear people talking from my receiver even they are talking far from the transmitter.

A 2k pot will be 1.6k to 2.4k so the 22V could be higher or lower.

Is this tolerance affected by temperature? I can add an analogue voltmeter parallel with the output right?
What happen if +ve terminal and -ve terminal are shorted? I mean the output of the LM317 is shorted to ground.

Thanks

 

[audioguru]

If I increase R1, the sensitivity of the mic is reduced right?

Try it. The gain of the jFET in the mic might increase a little, since it has a high resistance load from Q1.

Is this tolerance affected by temperature?

The resistance of a pot isn't affected much by temperature. They have a resistance tolerance of about 20% so a 2k pot could be from 1.6k to 2.4k.

I can add an analogue voltmeter parallel with the output right?

Yes. Analog or digital voltmeter.

What happen if +ve terminal and -ve terminal are shorted? I mean the output of the LM317 is shorted to ground.

It is smart. It will limit the current to from 1.5A to 3.4A and if the transformer doesn't burn out then the LM317 will get hot and shutdown. It will cool and start working again, shutdown again and work again over and over. The manufacturer says the hot and cold over and over might cause thermal fatigue which breaks it. Use a slow blow fuse.

 

[Hero999]

A PTC resistor aka polyfuse is probably the most convenient solution, replacing fuses is too much of a pain especially for lab power supplies.