1.5 Volt Tracking Transmitter

[andrebc]

Hi Colin and all,

I just have a question about how your FM transmitter circuit works. What does the 18 p and 1 nf capacitors do? is the 1 nf a decoupling capacitor?

Thanks

 

[audioguru]

I just have a question about how your FM transmitter circuit works. What does the 18 p and 1 nf capacitors do? is the 1 nf a decoupling capacitor?

You should provide a link to which of the many FM transmitters in this thread or attach the one you are talking about. It might be this one.

The Rf oscillator is a common base type. The 1nF capacitor grounds the base at RF frequencies (yes it decouples the base) and the 18pF capacitor provides RF postive feedback from collector to emitter for oscillation to occur.

 

[colin55]

The Rf oscillator is a common base type.

Now Audioguru admits the circuit is a common base configuration, after arguing it was a common emitter configuration. What a little education will do.

 

[Roff]

Now Audioguru admits the circuit is a common base configuration, after arguing it was a common emitter configuration. What a little education will do.

I searched this entire thread for the word "common", but couldn't find another instance. Where did he say that?

 

[audioguru]

Now Audioguru admits the circuit is a common base configuration, after arguing it was a common emitter configuration. What a little education will do.

Of course the RF oscillator is a common-base type. I never said it was a common-emitter type.

 

[colin55]

Sorry, It was Eric Gribbs that made the statement that the oscillator stage was common emitter.

 

[Bogidar]

Thanks for answering my questions regarding the circuit colin55 posted using a flashing LED. I‘ve got a few more if you don’t mind.

Does this circuit call for 1% tolerances and ¼ watt resistors and do the capacitors have to be NPO type? I've noticed It is recomended for similar circuits. I’m having trouble finding the 470R with these specs.

Also, If I want to replace the 47p capacitor with a 39p ceramic disk capacitor and 4-11p trimmer, do I just place them side by side in the circuit? Would a 2-30p trimmer do the job?

Thanks. I appreciate the help. I’m new to this.

 

[colin55]

Any resistors of any value (one above or below) will work perfectly.

39p and any small value (up to about 25% of 40p) will work.

NPO means the capacitance will remain stable as the temp changes. Most small ceramic caps are NPO

 

[Roff]

Sorry, It was Eric Gribbs that made the statement that the oscillator stage was common emitter.

When you make an allegation like this, I think it would be reasonable to post a link to the relevant thread.

 

[audioguru]

Does this circuit call for 1% tolerances and ¼ watt resistors and do the capacitors have to be NPO type? I've noticed It is recomended for similar circuits. I’m having trouble finding the 470R with these specs.

The circuit is extremely simple so it has terrible spec's (It is a toy). Its frequency drifts all over the place. 5% resistors are fine, inexpensive and available everywhere. The low value capacitor from collector to emitter should be NPO. 470R is an ordinary 470 ohm resistor.

Also, If I want to replace the 47p capacitor with a 39p ceramic disk capacitor and 4-11p trimmer, do I just place them side by side in the circuit? Would a 2-30p trimmer do the job?

My FM transmitter does not have a fixed ceramic capacitor in parallel with the coil. It has just a 5pF to 35pF trimmer capacitor in parallel with the coil for tuning the frequency.

 

[ericgibbs]

When you make an allegation like this, I think it would be reasonable to post a link to the relevant thread.

hi Ron,
I did mistake the configuration on this occasion, 'agu' was good enough to explain the osc and where I had misread it.

I appreciate being corrected by a professional/experienced member on this forum anytime I make a mistake.

The difference between me and that supposed engineer is that I have the knowledge and experience to know that I have made a mistake, he never will.!

If you look thru his posts he is constantly back tracking and editing out his previous errors in response to others posts, he works on a trial/error/copy basis.

Regards

 

[mvs sarma]

hi Ron,
I did mistake the configuration on this occasion, 'agu' was good enough to explain the osc and where I had misread it.

I appreciate being corrected by a professional/experienced member on this forum anytime I make a mistake.

The difference between me and that supposed engineer is that I have the knowledge and experience to know that I have made a mistake, he never will.!

If you look thru his posts he is constantly back tracking and editing out his previous errors in response to others posts, he works on a trial/error/copy basis.

Regards

Is it in another thread? as I never found any other post from you in this thread.

 

[ericgibbs]

Is it in another thread? as I never found any other post from you in this thread.

hi Sarma,
No its another thread, about 2 weeks ago.

 

[bryan1]

Sorry, It was Eric Gribbs that made the statement that the oscillator stage was common emitter.

C55 or whatever you call yourself, the information you have spammed on this site is getting too much. Now to Flame one of our most respected members in a thread he hasn't taken part in is what I say too much. Maybe your getting senile and dont realise it but please DONT FLAME SERIOUS MEMBERS OF THIS FORUM.

Enough is enough

 

[andrebc]

Thanks Colin and all for replying my question. I was looking at the second FM transmitter actually and I am trying to understand how these circuits work using what I have learned so far in my first Microelectronics class. The first one is an oscillator, but I just don’t know how you are going to transmit a signal, I mean if I want to transmit a signal I need an output needs an input? like the second circuit that gets an input from the microphone.

And also regarding the second circuit, it looks the designer here is using a 4.7 pF feedback to the collector, is that feedback needed to maintain the gain or for some other reasons? The 0.01 UF seems it will short at certain frequencies and therefore block all those frequencies from being coming out to the antenna. Am I getting this right?. But what does happen if some noise is coming from the DC power supply +9V, would that affect the performance at all? My last question is to the designer of the last circuit: how much did you calculate your gain for the transmitter. I found it to be around 2 for a 100 Ohms antenna

 

[colin55]

The tracking transmitter turns the 100MHz oscillator on and off via the pulsing of the flashing LED. When the transmitter is off, the receiver picks up the "snow" or "background noise" and when it is on, the background noise is "silent." That's the trick to the circuit.
As you increase the supply voltage, you can decrease the feedback capacitor (between the collector and emitter) as it is simply passing a signal from the tank circuit and injecting it into the emitter to maintain oscillation. That’s why you can use 4.7p at 9v. The 4.7p provides enough energy to do this when the supply is 9v - mainly because the amplitude of the waveform of the tank circuit is slightly higher at 9v and the transistor has a higher gain when on a 9v supply.
I don’t know what you mean by the "0.1UF . . . . "
It is very difficult to determine the gain of the transistor as it is operating at the highest limits of its range and it could be very low.

 

[Sceadwian]

Colin it's called 'modulating' not 'a trick' this is an electronics forum, try to use normal electronics terms, your constant use of 'quotation' marks needlessly without proper terminology is also out of place for a forum that is blatantly about electronics, anyone coming here either knows what the words mean will look them up, or will ask, so just use the right words in the proper context and wait for questions don't try to dumb it down or no one can learn anything.

 

[Sceadwian]

While I'm here =) I almost forgot to ask. Since this is a thread about tracking transmitter's where is the thread about making a receiver and antenna to actually track them? 100mhz is in the neighborhood that hams use for fox hunts but I've not seen any practical receivers or antennas that a hobbyist could build and get to work.
Not much point to building one if you can't receive the signal from a directional antenna.

 

[forumlicker007]

I don't think the tracking x'mitter is a worthy gadget. It has a limited range of operation maximum 50meters. The pot plant thief will take it and run away from the receiver's visibility. Now the total loss is pot plant+x'mitter!!

 

[duffy]

Heh. Is that what this is for?