I assumed anyone interested would have run the sim for themselves.

sorry for replying late
i was held up with assignments

I tried changing 470 to 1k and inductance to 50n and skip initial op soln. but I see no difference in the output of the first stage
check this out:

 

hi 2k4
I can get the oscillator to start up, providing I delay the modulation.
Any initial modulation seems to block the oscillator from starting up.
The simulation takes minutes to run,
E
View attachment 315870

i'm tryin to make a Frequency modulator, only the freq. should change right?
Can i assume it to be a wrong output or is there any reason for this?

 

See
View attachment 315867

Thanks for your response Bordodynov
I will surely try Qspice too!!

In frequncy modulation the amplitude shouldn't vary right ? why is this happening?
Can i assume it to be a wrong output?

 

hi 2k4.
In theory the RF amplitude should be constant and only the frequency change with FM modulation, but I would expect with such a simple transmitter the amplitude will also vary.
Have you seen this link.?
E
https://www.talkingelectronics.com/projects/Voyager/Voyager-P1.html

clip:
The stage is also said to be "self-biased" with the 1M base resistor turning the transistor on until the collector voltage drops to about half rail voltage. The value of the base resistor is chosen so that this occurs.
The value can be chosen by experimentation. If the value is too low, the voltage on the collector will be below half rail. If it is too high, the collector voltage will be too high. The AC gain of the stage is about 70 and the signal is amplified and passed to the oscillator stage via a 100n capacitor.
The signal is now typically 200mV to 2,000mV in amplitude and this is adequate for injection into the oscillator stage.

 

hi 2k4.
In theory the RF amplitude should be constant and only the frequency change with FM modulation, but I would expect with such a simple transmitter the amplitude will also vary.
Have you seen this link.?
E
https://www.talkingelectronics.com/projects/Voyager/Voyager-P1.html

clip:
The stage is also said to be "self-biased" with the 1M base resistor turning the transistor on until the collector voltage drops to about half rail voltage. The value of the base resistor is chosen so that this occurs.
The value can be chosen by experimentation. If the value is too low, the voltage on the collector will be below half rail. If it is too high, the collector voltage will be too high. The AC gain of the stage is about 70 and the signal is amplified and passed to the oscillator stage via a 100n capacitor.
The signal is now typically 200mV to 2,000mV in amplitude and this is adequate for injection into the oscillator stage.

View attachment 315876

I checked it out bro
and also found that the parasitic cap between base and collector changes with respect to the amplified message signal inducing a change in the oscillating frequncy
In an actual hardware implementation this happens but in simulation the parasitic capacitances are modelled constant.

but is there anyway to model such a small parameter?

 

check this out:

This is what I see:

Although the RF osc starts up, the audio seems to take over at ~2mS and the osc stops.

 

This is what I see:
View attachment 315881
Although the RF osc starts up, the audio seems to take over at ~2mS and the osc stops.

i too get the same thing like
both oscillation and amplific are not occuring simultaneously

 

Hi,
Check the voltages on Vc1 and Vb2, as Vc1 rises Vb2 falls, eventually cutting off Q2 transistor.
The problem is partly due to the long time constant of the 22N and 1Meg resistor.

E
Upgraded plot to show Vc and Va, Vo.

 

Try setting the signal amplitude to less than 1 mV. Then there will be no amplitude modulation and oscillation disruption.

 

Try setting the signal amplitude to less than 1 mV. Then there will be no amplitude modulation and oscillation disruption.

yeah it works
but the input from mic must be around 10-20mV

 

Hi,
Check the voltages on Vc1 and Vb2, as Vc1 rises Vb2 falls, eventually cutting off Q2 transistor.
The problem is partly due to the long time constant of the 22N and 1Meg resistor.

E
Upgraded plot to show Vc and Va, Vo.

View attachment 315886

yep!
is there any possiblities of changing values

 

Hi,
This is the plot for a 1mV input signal.
E

 

I checked it out bro
and also found that the parasitic cap between base and collector changes with respect to the amplified message signal inducing a change in the oscillating frequncy
In an actual hardware implementation this happens but in simulation the parasitic capacitances are modelled constant.

but is there anyway to model such a small parameter?

The LTspice model of the BC547B (and C) does indeed model the base-collector capacitance variation with the reverse bias voltage. Here's my test rig for that:

 

Do the calculation with a large final time until the operating point is established, as I did.

 

hi 2k4
I can get the oscillator to start up, providing I delay the modulation.
Any initial modulation seems to block the oscillator from starting up.
The simulation takes minutes to run,
E
View attachment 315870

Is there any way that i can simulate this circuit without any much change in the amplitude of the o/p waveform (without changing Input signal)?

Kindly help bro

 

Hi 2k4,
Are you not able to simulate the circuit, I am not sure what you are asking me to do?


E

 

Hi 2k4,
Are you not able to simulate the circuit, I am not sure what you are asking me to do?


E

I m able to simulate
Im getting amplitude variations in my output,which is not desired in FM
I showed simulation results to ,my facultybut they are not accepting
I am now doubting if it is really possible to make this circuit work using hardware components for my college project

 

Hi 2k$,
I have tried a number of circuit idea's but with such a basic oscillator circuit the way the modulation is applied, we will always get some amplitude modulation.

E

 

Hi 2k$,
I have tried a number of circuit idea's but with such a basic oscillator circuit the way the modulation is applied, we will always get some amplitude modulation.

E

I can understand
Is it guarenteed to work if i implement this ckt using hardware?
if not,Is there any simple alternative circuits that does the same( a FM transmitter with mic.).
I dont have much experience, Im not clear on this.

 

Is there any simple alternative circuits that does the same( a FM transmitter with mic.).

There must be plenty out there. Have you done some googling for them?