FM transmitter (mod4)

[mvs sarma]

I offer another artwork attached.

I have recently made another artwork for the FM Tx mod4. Of course I have used another resistor in series to power supply for Mic and a cap for decoupling it. They can be avoided while assembly, if not needed. the concerned schematic, solder layer and component overlay files are attached .

 

[audioguru]

Hi Sarma,
In my Mod4 FM transmitter, the emitter of the oscillator transistor (not its collector as you show) feeds the output RF amplifier for good frequency stability and so that the oscillator is not loaded down and actually oscillates.

I use a low dropout 5V regulator (not an ordinary 78L05) because a 9V battery quickly drops to the voltage where the ordinary regulator fails. My low dropout regulator needs a 100uF output capacitor.
Your C5 has an extremely high value that is one thousand times too high.
Your value for R4 will probably saturate the preamp transistor. I used 10k, not 47k.

 

[mvs sarma]

Yes AudioGURU,
I have now corrected the errors and a fresh set of files created and attached.

 

[cyberquest]

Wow! 483 post 33 pages. And still climbing. Amazing!

A simple FM Transmitter thats been designed an build and still not working. After 33 pages.
I'm a noob and could of build this transmitter in less time.

I get scolded for accumulating 8 pages. Do the math.
Because i didn't have the uints build before understand enough about the audio circuits that i was needing to go into my project.

I'm a noob who's been learning electronics on my own for the past 14 mouths. This is not a very long time to accumulate the full understanding to design every electronic circuit or audio circuits in a project. Reason for coming to this forum, Was for the Help!

I guess you need to work on a project the would get the most attention. Go figure.

No hard feelings... And im not being objective, rule or offensive towards the op or anyone on this Forum. Just an observation.

Best luck to the Op in getting your transmitter to work.

 

[sanjubluerock3]

fm transmitter

HEY is this fm tx circuit working????

 

[Birdman Adam]

I think AG's original circuit has always worked. Actually I built one a while ago and it worked perfectly.

mvs sarma has created new PCB artwork for those who want to make it on a PCB.

 

[audioguru]

I commented on why another simple FM transmitter did not work, then I made it to prove why it did not work.
I fixed it by modifying it in four steps. Each step improved it. My fourth modification works perfectly, sounds great and has a range of over 2km to a very sensitive home stereo tuner or car radio. It sounds bad and has a range of across the street on a cheap "radio" from the Dollar Store (it has a single IC similar to a lousy obsolete TDA7000).

 

[Birdman Adam]

It has certainly worked great for my uses:

Out on Jordan Lake there is a spot called 'the cove'. On weekends you can find everybody in the cove with their boats anchored. There's usually a big party and a huge grill going on shore. Naturally, everyone has their radios on, but many use different stations. Now when I come on my cabin-cruiser, I broadcast playlists of music from my laptop to the Mod4 Tx on the boat. Since many of the boaters there are part of one big yacht club, pretty much everyone knows to go to my station because it plays music non-stop, no commercials. Even throughout the whole lake, I will go by and hear my station! I will even plug in my mike and broadcast announcements, like if a storm is coming, or the foods ready.

 

[sanjubluerock3]

fm transmitter Mod4

hii audio guru even i have created a pcb schematic of this can you please verify it...I am very new to eagle so if there is any corrections please let me know i have used the schematic which was made by you

 

[mvs sarma]

I commented on why another simple FM transmitter did not work, then I made it to prove why it did not work.
I fixed it by modifying it in four steps. Each step improved it. My fourth modification works perfectly, sounds great and has a range of over 2km to a very sensitive home stereo tuner or car radio. It sounds bad and has a range of across the street on a cheap "radio" from the Dollar Store (it has a single IC similar to a lousy obsolete TDA7000).

Please consider whether the base bias resistor for final transistor could be connected to the regulated voltage,( instead of unregulated input) while keeping the collector on the unregulated supply.

 

[mneary]

hii audio guru even i have created a pcb schematic of this can you please verify it...I am very new to eagle so if there is any corrections please let me know i have used the schematic which was made by you

I recommend you replace some signals with real names like +9V, +5v, and GND etc. You should run design rule check, and correct the violations where S$14 (+9V) and S$6 (GND) try to cross on the same layer. You will also notice that S$2 is also GND and should be connected to S$6. The layout should be much more compact or it might be unstable (these are RF frequencies). I also recommend after you have done a quality layout that you should surround the design with a polygon named GND so all spaces are filled.

It should not be difficult to compress it into 100mm x 80mm which would work in the legal freeware version. (As it stands, we can open it but it's too big to modify.)

 

[audioguru]

I don't use Eagle.
I built my FM transmitter on Veroboard (stripboard). It is 2.0" (50.8mm) long and 1.3" (33mm) wide. It has spaces for two screws for mounting.

If the base resistor for the output transistor connects to the regulated 5V then the output transistor would become saturated when the battery voltage runs down.

 

[gauravkalra]

hello
by mistake in the circuit microphone gets shorted
will it damage it ?if so how can i check it

 

[gauravkalra]

also i measured inductance of the coil given by audioguru using inductance calculator .link is
Air Core Inductor Coil Inductance Calculator
but it gives inductance value of .19764 uH
will it be okay

 

[audioguru]

by mistake in the circuit microphone gets shorted
will it damage it?

No.
A microphone is not damaged if it is shorted.

i measured inductance of the coil given by audioguru using inductance calculator .link is
Air Core Inductor Coil Inductance Calculator but it gives inductance value of .19764 uH.
Will it be okay?

Why not simply make the coils like I show? I used 1mm enamelled copper wire tightly wrapped around a 3mm drill bit. use 8 to 10 turns of wire. space the coils away from the circuit board about 3mm.

 

[gauravkalra]

hii audioguru
i want to ask some silly questions about ur circuit as i am very weak in electronics especially in electronics
1 wat will output voltage range throu electret microphone when we speak into it?
2 ac current produced thru micrphone go towards c1 only or some of it go towards r1?
3 c1 is used to pass ac only but blocks dc thru microphone.am i tru?
4 r2 and r3 are used to bias transistor q1
5 r5 is used to stabilize q1
6 wy c2 c4 and r4 are used?
7 c3 is used as a bypass capacitor?
8 why c5 is used?
9 how L1 and c6 act as resonant circuit ?
i think lc pparallel circuit is used as resonance when apply ac voltage to them. But here we are giving dc voltage so how?

plz help me over it as i cannot go beyond without these reasons

 

[audioguru]

1 what will output voltage range throu electret microphone when we speak into it?

The output from the electret mic is about 10mV AC.

2 ac current produced thru micrphone go towards c1 only or some of it go towards r1?

Of course some of the mic's current goes into R1 but not much.

3 c1 is used to pass ac only but blocks dc thru microphone.am i tru?

No.
C1 blocks DC from R1 and the mic from upsetting the voltage at the base of Q1. C1 passes the AC signal.

4 r2 and r3 are used to bias transistor q1

R5 also biases Q1.

5 r5 is used to stabilize q1

Yes but it also sets the low frequency voltage gain.

6 wy c2 c4 and r4 are used?

C2 prevents Q1 from amplifying the radio signal.
C4 bypasses R4 for higher voltage gain at high audio frequencies for pre-emphasis (FM radio stations boost high frequencies then FM radios cut the high frequencies back down to normal which cuts hiss).
R4 is the collector load resistor for Q1.

7 c3 is used as a bypass capacitor?

No.
C3 passes the signal from the output of Q1 to the base of Q2.

8 why c5 is used?

C5 bypasses the base of Q2 to ground at radio frequencies so it can be a common-base oscillator.

9 how L1 and c6 act as resonant circuit ?

They are a parallel resonant circuit.

i think lc parallel circuit is used as resonance when apply ac voltage to them. But here we are giving dc voltage so how?

Q2 is a Colpitts oscillator where C7 applies positive feedback from collector to emitter to keep it oscillating. Its AC is the radio frequency of from 88MHz to 108MHz.

Do you understand how the audio signal from Q1 varies the frequency of the Q2 oscillator producing FM?

 

[gauravkalra]

hii audioguru
thanks for helping me
i want to ask more questions
1 what is low frequency voltage gain in this circuit ?how r5 sets it? pplz elaborate.
2 do transisotors act differently on low frequency and high frequency signals? why and how?
3 why q1 is prevented from amplifying radio signal? and so what is purpose of using q1?
4 when c4 bypasses r4,then ac will pass from c4 but not from r4?
5 but if we not use c4 then ac will pass from r4 as ac can flow through the resistors? so what is the need of c4?

 

[audioguru]

1 what is low frequency voltage gain in this circuit ?how r5 sets it? pplz elaborate.

When you learn about the details of transistors you will learn that the voltage gain of Q1 is (the collector resistance R6 in parallel with the load resistance)/(the effective emitter resistance of the transistor in series with the emitter resistor R5).

2 do transistors act differently on low frequency and high frequency signals? why and how?

No.
C4 bypasses R5 and causes the voltage gain of the transistor at high frequencies to be much higher.

3 why q1 is prevented from amplifying radio signal? and so what is purpose of using q1?

You want Q1 to amplify the tiny microphone signal, not amplify the large radio frequency.

4 when c4 bypasses r4, then ac will pass from c4 but not from r4?

No.
C4 bypasses R5, not R4. Then the effective resistance of R5 is reduced at high audio frequencies which increases the voltage gain of Q1.

5 but if we not use c4 then ac will pass from r4 as ac can flow through the resistors? so what is the need of c4?

C4 bypasses R5 which causes Q1 to boost high audio frequencies which produces pre-emphasis like FM radio stations have and is the opposite of the de-emphasis (high frequencies cut) that all FM radios have. Then audio sounds normal but hiss is reduced.
Cheap simple FM transmitters do not have pre-emphasis which causes audio to sound muffled with reduced high audio frequencies when heard on a normal FM radio.

 

[gauravkalra]

ok fine
1 why c4 is different ?
2 how signal from q1 modulates oscillations due to q2 producing fm?
3 why regulator is used?
4 why q3 is used as colpitts oscillator again?
5 can we use other mic in place of electret in this circuit?
6 which are best books for analog electronics,digital electronics,communication systems and network analysis and synthesis?