FM transmitter (mod4)

[bananasiong]

I didn't know about this fuse, I've just googled it. Is it added between the stepped down voltage and the rectifier? Does polyfuse have any value?

 

[Hero999]

Yes it does, they have a current rating, if this is excedded it will blow but this will take a long time and it's temperature dependant but all fuses are to some extent.

 

[bananasiong]

If it is shorted for 1 second, it won't blow right? Then will I blow the LM317 or the transformer?
So what current rating should be used?

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.

This sounds like AM? Not frequency modulation right?
Thanks

 

[audioguru]

If it is shorted for 1 second, it won't blow right? Then will I blow the LM317 or the transformer?
So what current rating should be used?

If the output is shorted then the LM317 limits the current to about 2.2A. If it gets too hot then it shuts down to protect itself.
The transformer needs a few minutes to overheat and burn out.
The polyfuse heats quickly then increases its resistance to limit the current. Use a 1A transformer and polyfuse.

This sounds like AM? Not frequency modulation right?
Thanks

These simple FM oscillators produce a little amount of AM and a lot of FM at the same time. A good radio's FM detector rejects the AM.

 

[bananasiong]

I'm using a 3VA transformer, I don't know whether I can get the polyfuse or not, I've never heard it before. Any heatsink required fot the polyfuse?

There is a LM386 audio amplifier in my super regenerative receiver, if I don't want to control the volume, can I just connect the input signal to the non inverting instead of a variable resistor? Or I still need a 10khm: resistor to pull down?
Its gain is determined by pin 1 and pin 8 right?

Thanks

 

[audioguru]

The inputs of an LM386 have 50k resistors to ground in the IC.
The gain of an LM386 is 20 without a capacitor and the gain is 200 with a 10uF capacitor between pin 1 and pin 8.

 

[Hero999]

I'm using a 3VA transformer, I don't know whether I can get the polyfuse or not, I've never heard it before. Any heatsink required fot the polyfuse?

No, that's the whole point. A Polyfuse is a type of PTC (positive temperature coefficient) resistor, the hotter it gets the higher the resistance gets. At the normal working current the I²R losses are low so it it doesn't get warm enough to increase the resistance to a significant amount. When the current increases beyond the rated value I²R losses increase causing it to heat up more, this increases the resistance to a significant amount, a higher resistance means higher I²R losses which causes more heating increasing the resistance further. This "thermal runaway" process continiues until the resistance is so high only avery small current flows, the fuse if now said to be tripped.

 

[bananasiong]

PTC, is it found in some temperature control circuit? I didn't know that it is applicable in this function also. I thought it is just burned and increase the resistance, then beeping comes from the speaker

How do you make the square (2) ?

The tank circuit at the output side is to peak the output power, is it the same for all frequency? Or I need to tune it to the peak for different frequency using oscilloscope to see the amplitude?

Thanks

 

[Hero999]

PTC, is it found in some temperature control circuit? I didn't know that it is applicable in this function also. I thought it is just burned and increase the resistance, then beeping comes from the speaker

https://en.wikipedia.org/wiki/Polyfuse

How do you make the square (2) ?

Use a character selector.

I use KCharSelect under Linux and Character Map under Winwoes.

The tank circuit at the output side is to peak the output power, is it the same for all frequency? Or I need to tune it to the peak for different frequency using oscilloscope to see the amplitude?

Yes, for optimum performance the LC tank on the output needs to be tuned to exactly the same frequency as the oscillator tank.

 

[audioguru]

I made a simple field strength meter with an antenna and a 1k resistor to ground, a germanium diode, a 1000pF filter capacitor then a 100k output resistor to ground. I connected its output to my multimeter and it showed up to a few hundred mVDC.
I peaked the output tank at 98MHz and at 88MHz and at 108MHz the ouput power was down only a little. So just tune it to 98MHz and it works pretty well over the entire FM broadcast band.

 

[bananasiong]

If I use oscilloscope to view the output waveform, and tune until the highest amplitude, can I get the same result? Since it is quite difficult to count the frequency at around 100MHz.

Is coil needed in the field strength meter, isn't it?

Thanks

 

[Nigel Goodwin]

If I use oscilloscope to view the output waveform, and tune until the highest amplitude, can I get the same result? Since it is quite difficult to count the frequency at around 100MHz.

Do you have a 200+MHz scope?.

 

[audioguru]

Don't connect your 'scope to the transmitter's antenna. The input capacitance of the 'scope will change the frequency of the tuned circuit.

 

[Hero999]

Not if you use a 5ohm piece of coaxial cable and connect a 50hm: resistor across the scope input to mach the impedance of the cable.

 

[bananasiong]

Do you have a 200+MHz scope?.

No, only 100MHz maximum CRO is available for me.

Don't connect your 'scope to the transmitter's antenna. The input capacitance of the 'scope will change the frequency of the tuned circuit.

Yes, you told me to add another small wire next to the antenna for the probe or else the transmission will be affected.

Not if you use a 5ohm piece of coaxial cable and connect a 50hm: resistor across the scope input to mach the impedance of the cable.

A 5hm: coaxial cable? Do you mean the resistance of the probe? Touch a 50hm: from the antenna to ground when measuring by CRO?

Thanks

 

[audioguru]

We don't know the impedance of the antenna since its length and the frequency are variable.
Use a simple field strength meter that doesn't touch and upset the circuit.

 

[Hero999]

A 5hm: coaxial cable? Do you mean the resistance of the probe?

That was a typo, I meant 50hm: adn I was talking about the characteristic impedance of the coaxial cable.

Touch a 50hm: from the antenna to ground when measuring by CRO?

Most scopes have a 1Mhm: input impedance, this will cause reflections and cause the cable to behave as a capacitor, connecting a 50hm: resistor at the scope end will match the impedance of the scope to the cable. Some oscilloscopes have a switch to change to 50hm: and I think you might be able to buy inline 50hm: resistors to connect to your scope for this particular purpose.

 

[bananasiong]

No supply is needed for some simple field strength meter, and is it needed to be tuned to the right frequency?

I'm not able to buy online since I'm only 20, can't apply for credit card yet, and I can get a 56hm: easily from the lab .

Thanks

 

[audioguru]

My field strength meter had an antenna, a germanium diode, a capacitor and 2 resistors. It wasn't tuned. It drove my DC meter. A 1N4148 silicon diode will probably work.
Some FM transmitter projects have it driving a transistor that makes an LED glow brighter when the transmitter is at max power.

 

[bananasiong]

1N34 can be used also right?
How come yours doesn't have coil? The simplest I've seen is the antenna is tapped between the coil and the diode, a resistor and a capacitor in parallel at the output. You use a 1khm: instead of the coil?

Thanks