FM transmitter [Oscillator Help]

[mvs sarma]

Most electret mics operate at 0.5mA then a 10k resistor from 9V is perfect.
I agree that the 10k resistor that feeds the mic should be decoupled from the 9V with a 1k resistor is series from 9V then a 47uF capacitor to ground. Then there will be no motorboating.

In fact, I was thinking about the 1st transistor's collector activity that needs the filtering. It is conventional to have a series drop resistor and a decoupling for powering the Electret MIC.

I see many kits in market with minimal components. The LC meter of electronicvs-diy is one such.It has no electrolytics at all on the power line even aftewr having 78x05 regulator.
with due regards to Colin, It is OK if one sells 100,000 kits. One could save that much on investment.

 

[colin55]

Eric Gribbs: You didn't even realise the RF stage was common base, so your comments don't hold much weight.

 

[dougy83]

The base of the 100MHz oscillator is held rigid by the 2n2 as it has a very low impedance at 100MHz.
I have designed over 25 RF transmitter circuits like the one shown (except all of mine have a self biasing oscillator stage) and sold over 100,000 kits.
An electrolytic is not needed on the supply rail as the audio is being injected into the RF stage and the current taken by the microphone and audio stage is very small in a well designed circuit. It is an unnecessary component.
However a 22n on the supply rail will improve the stability and increase the output of the RF stage as this stage is drawing heavy peaks of current.

The reason why the two stages should not be DC coupled is as follows: The oscillator stage needs to be self-biasing for stability. If is connected to the collector of the previous stage, this self-biasing is negated.

Hey collin,

Your not the guy that started the 'Talking Electronics' company are you?? If so, you still owe my a subscription to your magazine (I paid back in the mid 90's and you never came through with the goods)!

 

[colin55]

The 47u does not have any effect on performance, either when the battery is new or when it is old, so why include it?

I certainly do not agree with 10k load for the electret microphone. All my kits have sensitive microphones and we allow 22k to 47k for 3v operation and at least 47k for 9v operation. Otherwise the front-end will motorboat.
The 1u coupling is not needed as 22n is sufficient to produce absolute clarity. You cannot tell the difference between live performance and the output from an FM radio from these FM transmitters.

 

[colin55]

Send me your name and address and I will send you a CD of the whole site.

I have made this offer to anyone who bought a subscription when I was in the USA.

And anyone who bought a subscription to POPTRONICS just before it went under.

 

[ericgibbs]

Eric Gribbs: You didn't even realise the RF stage was common base, so your comments don't hold much weight.

Where is the audio signal injected into the oscillator.?
Its not into the emitter.!!! also the base isnt referenced to 0V.

 

[colin55]

The RF stage is common base as far as the 100MHz oscillator is concerned and the DC bias of the base is modified by the audio signal. That’s how the frequency is altered.

The base is actually “referenced to rail” via the 2n2 and via the 22n across the power rails.

 

[chicaman]

I already get it working but the transmitting range is very low

i need to get the phone radio handsfree close to the antenna t o get a clear reception. When i go far it is getting blur. How can I get it to transmit 20meter?

 

[colin55]

The circuit is just badly designed as I suggested at the beginning. My circuit gives 600 metre range on 9v.


The RF stage needs to be self-biased to make sure it is operating at the greatest gain.
The coupling to antenna can be increase by connecting it to the collector or it can be connected to the emitter to get medium output. Put 22n across the power rails.
How did you make the coil?
Here is the layout of a properly designed FM bug for 9v operation. Notice the coil and the battery snap. 5 surface-mount resistors are under the board.
**broken link removed**

 

[audioguru]

The 1u coupling is not needed as 22n is sufficient to produce absolute clarity. You cannot tell the difference between live performance and the output from an FM radio from these FM transmitters.

With only 22nF then all the low audio frequencies are missing in your kits.
Without pre-emphasis like FM radio stations have and like my FM transmitter has then all the high audio frequencies are missing.
Then your kits sound like a telephone.

My FM transmitter sounds exactly like an FM radio station, except in mono instead of in stereo.

 

[audioguru]

Hi Eric,
The RF oscillator is common-base. The 2.2nF capacitor at its base is less than 1 ohm at 100MHz. The current of the transistor is modulated by the audio.
When its current is modulated then its voltage between collector and emitter is modulated which modulates the capacitance of collector to emitter and collector to base that creates FM.
The transistor also creates a small amount of AM that is ignored in an FM radio.

 

[ericgibbs]

Hi Eric,
The RF oscillator is common-base. the 2.2nF capacitor at its base is less than 1 ohm at 100MHz. The current of the transistor is modulated by the audio.
When its current is modulated then its voltage between collecor and emitter is modulated which modulates the capacitance of collector to emitter and collector to base that creates FM.
The transistor also creates a small amount of AM that is ignored in an FM radio.

hi agu,
Many thanks for the clear, detailed explanation.

 

[audioguru]

Why am I getting funny e-mails of replies that have been deleted??

 

[ericgibbs]

Why am I getting funny e-mails of replies that have been deleted??

hi agu,
Its certainly not me.
Eric

 

[fishyho]

problem in oscillator ....argh...cant oscillate...
i used formula of the f(osc)
found that 0.25micro Henry inductor will have frequency about 92-96 MHz
but i tuned radio cant find my frequency,
used frequency spectrum to find also cant get.. =(
i changed a lot of inductor......
using a pair of 22 pF capasitor~
what wrong with it...?

in part 2 amplifier had test...
just normal ...no problem...
V out about 2 Vp~

 

[audioguru]

You cannot calculate the frequency of a VHF oscillator because the stray capacitance of the wiring might cause the frequency to be half.

You should make your own simple VHF inductors to be sure of all the spec's.

 

[colin55]

The problem with your circuit is the fact that two 27p in series produces 13p and the capacitance across the coil should be 33p to 47p for a 5 turn coil as shown in the photo I supplied. This will give the best "Q" for the tank circuit.
In other words, the two capacitors should be nearing 100p each.

 

[fishyho]

then should increase value pair of the series capacitors due stray capacitance of the wiring will cut half of frequency? that why calculation part not same as practical?

 

[colin55]

Put 47p across the inductor and place the 2p2 on the collector. Also the 390R is too small a value for 9v operation.
The circuit has lots of mistakes.

<mod edit: self promotion deleted. If you have content to share, please post directly to ETO. Do not link to your own personal website>

 

[Junker_eu]

Sorry,just a mark.I will read this thread tomorrow.