What's the function of transistors in this VCO circuit?

I found this circuit , and i want to know the function of the three transistors that conected to tuning voltage node , The owner of the circuit claimed that their function is as same as Varactor diodes , I want to understand how that is satisfied in Circuit , Can anyone explain to me ?

Here a Ccb of transistors is used instead varicap diode. This transistor capacitance is voltage dependent an can be used instead. Here a three transistors are in parallel conection to increase variable capacitance.

Do u mean that as the tuning voltage increase Cbc increase too , and if i increase the No. of parallel transistors the Capacitance will increase more and more ?

I have one more question , You will find a transistor in the bottom left corner sires with 1k , What's its function ?

Thanks

Like as at varicap diodes, when tuning voltage is increased diode capacitance decreases. Here are used three transistors to increase total variable capacitance.
Transistor you are asking for is for temperature compensation of oscillator (middle transistor).

Can i replace the mentioned 3 Transistors by Simply three Diodes (like 1N4002) or i have to replace with Diodes working in UHF Frequency , Can anyone give me Part numbers for Diodes working in UHF ?

One more Question , Is my Question itself is correct , Which means that UHF Diode not as UHF Varactor Diode, What is the Difference ? Although they both working as variable capacitance by Reverse Voltage applied on them

You can use BB105, BB205 or BB405 varicap diode instead transistors.
Diode 1N4002 has propper capacitance but I doubt it has good Q factor for use in UHF oscillator.

All diodes can be used as varicaps but they usually have small capacitances and small rate of change. Varicap diodes have wide range of capacitances with rate of change up to 20. They are produced for use in specific frequency band.

Ok, Now i want to develop this circuit to Transmitt FSK Signal by simply entering The Data Sequence 1's and 0's from the Tuning voltage terminal ,

Is it better to use the Varactor diode or the 3 Transistors or 3 Diodes to obtain the variable capacitance effect in this Case , Assuming that i want high data rate ( Say 100 Kbps ) , Can i increase that Data rate more in this Circuit Simply or i need some Modifications in Circuit ?!

My sugesstion is to use varicap diode. You will have less stray capacitance. Resistor 39k on VCO input should be replaced with 1uH inductor.

Why replacing the 39 K resistor with 1 uH Coil , I want to ask what is the function of the 39 K resistor in the Circuit

It is RF decoupling between oscillator and VCO input. But RC lowpass filter composed of 39kohm resistor and varicap diode (transistors) capacitance have cutoff frequency of only 200kHz and it would distort shape of modulation signal (square wave.). Using inductor (it can be even 10uH) makes RF decoupling efficient and at the same time moves cutoff frequency high enough.

Sorry for my too many Quesion , I designed this VCO but i have some Questions

1- I represent the Data with pulse source levels 1V , 0V with Data rate 100Kbps , and i tried to change (increase/decrease) the Data rate , Each time Data rate is changed the Mark and Space Frequency are slightly shifted , Why & How The Datarate affects the Mark and space frequency ?

O/p is taken on The 100 Ohm Resistor R6

2- The Spectrum of the Signal in Freq Domain Contains Three Small peak components (Undesired to me) bet. the largest 2 Peak components(Desired to me) So How Can i modify the Circuit to enhance the performance and get rid of the Undesired 3 Components ?

3- I tried to replace C1_x by Coil 10uH but the o/p was Distorted ? Why ?

Your circuit is FM oscillator. Centre frequency depends on average value of modulating signal. Changing the bit rate causes different side band componnents. Higher the bit rate greater is the distance between sidebands. It is normal behaviour for FM.
All spectrum components are desirable. This is wide band FM. If you preffer smaler number of spectral componnents, minimum three, reduce modulating voltage.
C1_x must not be replaced with inductor because varicap voltage will be then shortcircuited to ground.

Allright , But i designed this Circuit to be considered as FSK Transmitter , I know that FSK is Similar to FM , But my Question Shall my Circuit be considerd FSK TX , Or it can't be considerd FSK TX Because of the many No. of components

In the Fig. The i/p Signal is red Square Wave at bottom and o/p signal is Green

I'll intend in receiption of this Signal to use PLL, By adjusting the Center Frequency and the Loop Filter To Cover the Highest 2 peaks components , but there're 3 components in the middle , Will they Cause a problem in Receiption of PLL ?

I want to ask also about Why All the Spectrum components in Wide FM are Desired ?

Thanks

Your FM modulator can be considered as FSK. FSKs are build in different way.
Just reduce amplitude of square wave signal if you preffer less componnents.
You must use all spectrum componnents in demodulation to get square shaped output signal. Reducing number of componnents after modulation will produce distorted shape and illegibility will be poor. PLL loop filter must cover all spectrall componnents for the same reason.

If you want low THD in FM all spectral componnents up to 1% of amplitude must be used at receiver and demodulation.

Borber, Do u think that my Spectrum is accepted for FSK Transmission , Or it may need some improvments ?

I think that my Circuit need power Amplifier , I tried before on different Ct to make Class C tuned Amplifier "Trasistor used 2N2222A" but i failed to tune the freq. Practically after implementing the Ct on PCB , So Do u have any suggestions to how build GOOD Power Amplifier , My Goal is 100 meter Link ?

Hi, Your signal looks terrible. This is not a proper FSK signal at all. First it looks like you are NOT shaping your bit stream at all. second your not doing anything related to runlength encoding that is why you see the peaks between the two large peaks. and last your not only doing FM transmission but also some AM component. Use an varicap capacitor with a resistor of some 10K and a small capacitor of some 4.7pF parallel to your L of your osc. The 10K resistor you need to feed your bit stream with. Please use runlength encoding (removing large numbers of the same bit (zero or one). regards, Paul.

First , I don't know what's meant by runlength encoding .............
Second , I don't have large no. of same bits , it 's simply Square Wave (plz refer to the above uploaded graphs)........

Hi. Runlength is the length between different bits so. if 1 is high and o is low and they are bits then the runlength in the next stream is 2. 11001100. in the next stream the runlength is 5 01010101000001. You should use scrabling of bits to get rid of large runlengths in data streams you transmit otherwise you will have problems recovering them later. One simple way is using hamming encoding. runlength is constant and DC is 0. but hamming is veru inefficient since 50% of your bits are not needed. better use scrambling (look at google).

The problem with your circuit is that you also have AM modulation instead of only FM. Use a varicap BB149A in your circuit instead of the 2N2222A. The rest can stay the same. I also advice you to give a small DC offset to your varicap through the 25 K resistor and make this one 10K. use a capacitor to put your data on your varicap before the 10K.

Why do you want a transistor as FSK its not stable ?. Better use a ADF7011 or ADF 7012 from Analog is about 1.50 USD and has everything you need !..