another FM transmitter project

[Willen]

Ups! Mistakly pressed 'Vote No'.
Why they use pre-emphasis and again de-emphasis? What is the purpose of boosting high frequencies and reducing low frequency and de-emphasis again? To reduce low frequency noise?

 

[audioguru]

When the transmitter boosts high frequencies (pre-emphasis) and the receiver cuts the high frequencies back down to normal (de-emphasis) then high frequency hiss is reduced.

 

[Willen]

Do you know any replacement of TL072 Dual OpAmp for around 100KHz audio? As usual I am having trouble to get it

 

[audioguru]

There are hundreds of good audio opamps. The TL072 is made by many semiconductor manufacturers.
An NE5532 is another fairly old but excellent dual audio opamp made by many semiconductor manufacturers.

Go to www.farnell.com and click on the flag of your country. They can get these opamps for you.

 

[Willen]

audioguru,
What is the relation between coupling capacitor C12 (30pf) and output power? Talking about Mod4. What happens if i use 2pf and what happens if i use 1nf as a C12? Does it related to impedance matching? In Harry Lythall's 'bug5' fm tx, i found 1nf as a coupling capacitor. Where it should be high and low? Please

 

[audioguru]

On a simulation, the output voltage into a 75 ohm load is maximum when C12 is between 150pF and 270pF.
When C12 is 30pF the output voltage is about half of when it is 220pF.
When C12 is 18pF the output voltage is half of when it is 30pF.
When C12 is 2pF or 1000pF the output voltage is very low.

 

[Willen]

OK then i will try 220pf as a coupling capacitor for higher output and for higher range. I think higher voltage output will cause higher radiation at antenna. Can i think like this way?

(i tried LTSpice IV but cant simulate. I made schematic but failed on last stage of 'simulate'. I have no guide about it.)

 

[audioguru]

OK then i will try 220pf as a coupling capacitor for higher output and for higher range. I think higher voltage output will cause higher radiation at antenna. Can i think like this way?

Yes.

(i tried LTSpice IV but cant simulate. I made schematic but failed on last stage of 'simulate'. I have no guide about it.)

Here is my file:

EDIT: my file does not run because the associated files can not be uploaded.

 

[Willen]

Check this FM Tx . It has a preamp without pre-emphasis. Can I simply add 470 ohm bypassed resistor with 100nf at emitter and 100pf from base to emitter (like on mod 4) to add pre-emphasis?

 

[audioguru]

Check this FM Tx . It has a preamp without pre-emphasis. Can I simply add 470 ohm bypassed resistor with 100nf at emitter and 100pf from base to emitter (like on mod 4) to add pre-emphasis?

It was NOT designed properly. The designer tried it with only one transistor (it had low hFE) then maybe it worked. I simulated it with a "typical" transistor that has "typical" hFE and it distorts very badly because it is almost always saturated. If the hFE is even higher then it is worse.

I biased it correctly but it still distorts if the hFE is lower or higher than "typical". The pre-emphasis will work fine. I forgot to add the 100pf but it is needed in a radio transmitter.

 

[Willen]

- Actually what is mean by '50 microsecond' in pre-emphasis?
- What might be the difference between (470x100nf)=47uS and (47Kx1nf)=47uS? I think it has difference in voltage pass, Not in audio pass.

 

[audioguru]

- Actually what is mean by '50 microsecond' in pre-emphasis?

An RC circuit with a time-constant of 50us has a cutoff frequency (+3dB for pre-emphasis) of 1/(2 x pi x 50us)= 3200Hz.

What might be the difference between (470x100nf)=47uS and (47Kx1nf)=47uS? I think it has difference in voltage pass, Not in audio pass.

A 47k emitter resistor probably will not work with a transistor but it is fine with an opamp.

 

[Willen]

Every time I am suffering from lack of desired components.
If i add a class C RF amplifier on Mod-4 FM tx (23mW) using 2n2219 at 12 volt DC for 10 dB gain (230mW), does the transistor 2n2219 need heatsink at 230mW dissipating? Sorry for repeating almost same type of question. Actually i never found this type of round heatsink for TO-39 case.

 

[audioguru]

The datasheet for the old 2N2219 says 800mW at an ambient of 25 degrees C when its chip will be at its max allowed temperature of 200 degrees C. If a perfect heatsink holds the case at no more than 25 degrees C then it can dissipate 3W.
These very old heatsinks are still made and sold:

 

[Willen]

Once, I tried 9V for all 3 stages of Mod-4, but got noise and noise and noise.... almost at all frequencies 88 to 108 MHz in receiver
I want to use 8 or 9 v regulated supply for little high output if i can. Probably the freq. might be out of FM band at that voltage due to high voltage at oscillator and also there might be bias factor for preamp too. Which few value should i have to change? If it is complicated, just leave it.

 

[audioguru]

Once, I tried 9V for all 3 stages of Mod-4, but got noise and noise and noise.... almost at all frequencies 88 to 108 MHz in receiver

Probably your receiver is cheap and it was overloaded by the very strong signal. My high quality home stereo FM tuner and high quality car radio do not get overloaded.

I want to use 8 or 9 v regulated supply for little high output if i can. Probably the freq. might be out of FM band at that voltage due to high voltage at oscillator and also there might be bias factor for preamp too. Which few value should i have to change? If it is complicated, just leave it.

You must not have the transmitter operate at the aircraft communications band which is directly above the FM broacast band.
The preamp has fairly high gain (a low value emitter resistor) so its biasing must be accurate. Its biasing depends on a certain supply voltage (5.0V now).

 

[Willen]

Yes.


Here is my file:

EDIT: my file does not run because the associated files can not be uploaded.

You show me lot of images of simulation of Mod 4. Please can you email me these files by attaching at rajforbest(at)gmail.com ? [no '@' to escape from auto spam] please
I want to learn LTSpice my own by running your files

 

[Willen]

Where is audioguru?

 

[audioguru]

I am trying to figure out why my simulations do not work when posted here.

 

[Willen]

Ok, if your files are not working, can you write me few steps to simulate correctly? ( I want to simulate like preamp or oscillator+amplifier and want to see its performance by changing different values of components. And also want to see its maximum power output by changing components from amplifier) I can draw a schematic well but I don't know what to do to simulate it. I donz't know where and how much value I should have to write while running simulation. I did by learning few ideas but got failed.

I have read few helpful guide but these are not VHF related so I got faild.

Help me at email or here, anywhere plz