another FM transmitter project

[Willen]

Wow! Then you were the best modifier from begining . I cannot imagine these starting days. You have a "Wide-band experience" really!

Um...hard to find fairchild's datasheet for 2N4401(TO-92), what is the gain of it at 100MHz? Is it better than 2N3904? (I know it has higher collector current but lower hFE at the level current needed for mod 4. I cannot find "gain" at ordinary datasheet.)

 

[audioguru]

I get datasheets from www.datasheetarchive.com where I can select the manufacturer.
Fairchild's 2N4401 datasheet does not show its high frequency current gain.
Its gain-bandwidth product is 250MHz minimum byt the 2N3904 is 300MHz minimum. So it has a little less high frequency current gain than a 2N3904.
Its higher max output current allows it to get hotter and/or provide more output power when its input power is high enough.

 

[Willen]

And now I designed perfect smaller Layout for Mod-4. Perfect for VHF circuit i think . There was 100th of 90 degree angles on my previous design which might be very bad for VHF band because they may act as a capacitor. Now there are no 90 degree angle. I kept enough ground plane which is good (I think) for VHF board.

I included both input option- mp3 audio from 3.5mm jack socket and mic audio from mic. And used Two-way switch for these two input. I added a LED as power indicator. I used fixed capacitor as a 1st tank bacause here most of frequencies are blank so lots of space I can use by changing fix capacitor so fine tune from trimmer is not necessary .

I almost finished by making the board by toner transfer method and also finished by soldering too. When totally completed, I will post the photo. Wow my work!
Designed from ExpressPCB.

 

[audioguru]

Your pcb looks good.

I think it is dangerous to have 9V on a plug because if it easily gets shorted when it is not used then the battery might explode or be wasted. A battery or power supply should have a jack, not a plug. The circuit should have a plug, not a jack.

 

[Willen]

Yes, I understand! In electronics, smaller mistake leads to dangerous (huge) problem. I used Plugs for audio and battery (both) on my previous design but due to these wires it looks like hard to carry in pocket So in this design I used both jack sockets to make best portable and good to see. There is no power switch so I will unplug the battery while not transmitting .

What is the main purpose of Metal Case for transmitters?

 

[audioguru]

A metal case connected to 0V for the transmitter will prevent the frequency from changing when your hand or something moves toward or away from the oscillator parts.
Frequently the audio amplifier is shielded with the metal case and a metal blocking part inside the case.

 

[Willen]

Confused!
I think it is not "BC547B". I think it is BC547 only, isn't it?

 

[audioguru]

An American or European BC547B is marked BC547B. Your BC547 was made by a company called T (Thomson of France?).

 

[Willen]

what happens Zener diode in series? If I use 5.1V two zener in series, will it work as a 10.2V zener?

 

[audioguru]

A 10.2V zener diode's voltage increases a little when it gets warmer but a 5.1V zener diode's voltage barely changes.
A 10.2V zener diode regulates better than two 5.1V zener diodes in series.
Nobody makes a 10.2V zener diode but 10V is available.

 

[Willen]

Oh! Then I can make poor man's 10.2V zener by series connection of two 5.1V zener!

Thinking another question (may be foolish) -
Wave cycles completed on a second is called ...Hz (KHz, MHz...). But if a wave cycle completed on 5 seconds, how electronics defines it? Is it 0.2 Hz ?

 

[audioguru]

But if a wave cycle completed on 5 seconds, how electronics defines it? Is it 0.2 Hz ?

Yes of course.

 

[Willen]

I know some littlr thing about RMS. I think it is an AC voltage which is quivalent of DC. And I know

RMS= 0.707 X peak
peak= 1.41 X value measured by voltmeter

thus, peak of 230V is 325V

But can't understand overall.

So please describe the RMS in simple and who to use RMS to calculate the output of MOD 4? Basic calculation please!

 

[audioguru]

I tweaked a few capacitor values for maximum output power in the simulation.
The output power is explained.

 

[Willen]

Oh yes, I think it means- peak to peak 9.5V AC is equal to 3.36V (RMS) DC.

But i got failed on calculation... What is 2.828 and how it came please?

 

[audioguru]

The peak voltage of a sinewave is 1.414 times the RMS voltage.
A distorted sinewave sometimes has half of it larger than the other half (mine has -4.4V and +5.1V) so the peak-to-peak voltage is more accurate than one peak voltage.
Then the peak-to-peak voltage is 2 x 1.414= 2.828 times the RMS voltage.
Then the RMS voltage is the peak-to-peak voltage divided by 2.828.

 

[Nepaliman]

Hm...... I want to ask about two type of calculations which is confusing me please audioguru:-

A)

The formula is _dB = 10 log (p1/p2)

where p1 and p2 = power 1 and power 2 respectively.
For example if p1 = 5mW and p2 = 10mW
5/10 = 0.5
LOG(0.5) = 0.3
10 x 0.3 = 3dB (gain)

if we reverse the values.
10/5 = 2
LOG(2) = -0.3
10 x 0.3 = -3dB (loss)

To convert dB back to gain.
16db / 10 = 1.6
10^(1.6) = 39.8 (close enough to 40 times)

These examples are for power.
If voltage (or current) is used, then the formula is dBV = 20 log (v1/v2)

Will you elaborate please how "LOG(0.5) = 0.3" happened? May be "log" has a certain value.



B) You calculated the Power using like this-

- Power in resistor of an attenuator is:
0.01V squared/22 ohms=0.0000045W.

AND

- The RMS voltage is 9.5/2.828=3.36. The output Power is 3.36V squared/75 ohms= 150mW

In both place you squared the 1st voltage to calculate, why this squared value? Is it a formula? Because I know only Power= Current x Voltage but don't know about square.
learning basics...

 

[audioguru]

Nepaliman,
Look in Google for Electrical formulas.

Power= voltage x current.
Power= (voltage squared)/resistance.
Power= (current squared) x resistance.

Try it: For example use 10V and 2A in a 5 ohm resistor.
P= 2 x 10= 20W.
P= (10 squared)/5= 20W.
P= (2 squared) x 5= 20W.

 

[Nepaliman]

wow! How simple it is!
Ok will search about LOG. Thank you!

 

[Nepaliman]

The peak voltage of a sinewave is 1.414 times the RMS voltage.
A distorted sinewave sometimes has half of it larger than the other half (mine has -4.4V and 5.1V) so the peak-to-peak voltage is more accurate than one peak voltage.
Then the peak-to-peak voltage is 2 x 1.414= 2.828 times the RMS voltage.
Then the RMS voltage is the peak-to-peak voltage divided by 2.828.

Did you mean that your sinewave has 2 peaks- Positive 5.1V and Negative -4.4V? So due to this two peak voltages you multiplied 1.414 x 2? But all AC has negative and positive peaks......um....
Can you say in basic please!