another FM transmitter project

[mvs sarma]

I dont know what software did you use for the PCB. the legend like RAJ's could be mirrored. so that the tomner transfer hobbyist wont confused. generally the name should ve readable as seen ferom solder side. Otherwise some one is likely to make pcb the wrong way.

Audio input is expected to have ground pin also. perhaps one can have a audio jack on board itself.
@Audioguru,
VC might be a Trimmer capacitor.

 

[audioguru]

VC might be a Trimmer capacitor.

Of course, VC= Variable Capacitor.

But C6 tunes the radio frequency and it is not variable. C13 is used to simply peak the output at the frequency tuned with C6.
Notice that C13 is marked wrongly as C14.

 

[Willen]

I did not check all the connections.
C14 looks odd. What is "VC"?

You should not use an ordinary 7805 with a 9V battery because when the battery voltage drops as it is used then the regulator "drops out" and does not regulate anymore. Instead use a Low-Dropout regulator like I did.

You mean- If 7805 gets 5V or less as an input, it won't work? I've corrected all mistakes found on my previous PCB layout.

 

[audioguru]

The typical "dropout voltage" for the 7805 is shown on its datasheet. Dropout is when its input is 7.0V or less and then it has already stopped regulating. It is in a large TO-220 power transistor case.

The typical dropout voltage for the low dropout LM2931AZ-5.0 regulator I use is 5.3V. It is in a small TO-92 transistor case.

With a load of 53mA a name-brand 9V alkaline battery voltage drops to 7.0V in about 2.5 hours. It drops to 5.3V in about 14 hours.

ALL semiconductor manufacturers make low dropout regulators.

 

[transistor495]

Actually I don't see anything special with our audioguru's fm transmitter thing. It is comparatively good but it doesn't use a varicap diode for a perfect fm modulation.

It is just nothing comparing to a professionally designed circuit for your info.

 

[audioguru]

Actually I don't see anything special with our audioguru's fm transmitter thing. It is comparatively good but it doesn't use a varicap diode for a perfect fm modulation.

It is just nothing comparing to a professionally designed circuit for your info.

The oscillator transistor is "a perfect varicap diode".
The frequency is fairly stable because the RF output transistor isolates the antenna from the tuning circuit.
It has a voltage regulator so the preamp continues to work and the frequency does not change as the battery runs down.

Try it. It sounds great because it has pre-emphasis and it has very low distortion.

If Rohm was still making their very high quality FM stereo transmitter ICs (BH1417 series) then you could use one instead.

 

[transistor495]

The oscillator transistor is "a perfect varicap diode".
The frequency is fairly stable because the RF output transistor isolates the antenna from the tuning circuit.
It has a voltage regulator so the preamp continues to work and the frequency does not change as the battery runs down.

Try it. It sounds great because it has pre-emphasis and it has very low distortion.

If Rohm was still making their very high quality FM stereo transmitter ICs (BH1417 series) then you could use one instead.

I have this stereo IC ready in my box, but didn't get much time to work on that. Also I don't have the darlington transistor and the varactor specified in the stereo transmitter circuit. I can try with alternate methods though. AlsoI have couple of BB109's in my box anyway.

 

[Willen]

Hello

Audioguru

Look this one Tx:- www.pira.cz/entx1.htm
And focus on T2 biasing. Already you have said that this kind of biasing won't be better. So I am trying to correct like this way:-
1. Adding one another inductor same as L1 instead of R9 and adding one Trimmer in parallel.
2. Connecting R8 from positive power line like R4.
DONE?

 

[audioguru]

R9 is 270 ohms so it can drive the very low input impedance of the output transistor and the very low value of R11.
Your idea has a tuned LC circuit at its collector which is a high impedance that will not be able to drive the low impedance.

 

[Willen]

R9 is 270 ohms so it can drive the very low input impedance of the output transistor and the very low value of R11.
Your idea has a tuned LC circuit at its collector which is a high impedance that will not be able to drive the low impedance.

-Here, this type of biasing won't increase the RF power, is it? If so, what may ba the purpose of Q2?
-Where is the exact oscillator? I think L1+Varactor+Q1 makes oscillation.
-Does it produce 400mW?

 

[audioguru]

T1 is a Colpitts oscillator with tuning and FM modulation provided by the old varicap diode. Its collector has a high impedance tuned circuit so it needs T2 as a buffer amplifier (high input impedance and low output impedance) to drive the low impedance output stage T3.

T2 amplifies voltage and current so it increases RF power.

The old circuit might have an output of 400mW if it uses the original high frequency transistors and is tuned carefully.

 

[Willen]

A person is saying that:- ''...connecting the transmitter to a large
amateur radio antenna (10 meter beam), I
was able to send signals from California to
Texas. To do that, you will want to get an
amateur radio license.''

How far (approx) between Californiya and Texas?
He used Morse code and sent Text Messages via RF using Morse Encoder-Decoder sortware.

 

[audioguru]

The 10 meters amateur radio band uses frequencies from 28MHz to 30MHz. Its antennas are huge and are usually up high.
I think these fairly low frequencies bounce off the ionosphere so can travel far with fairly low power.
A "beam" antenna might have lots of gain.

Look at a map of USA to see distances.

 

[Willen]

This is my FM transmitter circuit. I built it on stripboard and I cut the strips with a drill bit. The strips form half of a pcb.

Asking a general question please:-
What is the relationship between base basing resistor and gain/output on class A, B amplifier? I mean If I increase the value of R8 like 68K instead of 47K, the gain or output of Q3 wil increase or decrease?

 

[audioguru]

Asking a general question please:-
What is the relationship between base basing resistor and gain/output on class A, B amplifier? I mean If I increase the value of R8 like 68K instead of 47K, the gain or output of Q3 wil increase or decrease?

The output transistor operates in class-A because it is always conducting.
When R8 is 47k then the transistor has a maximum current that is higher than if R8 is 68k so the output level drops when the value of R8 is too high.

In a simulation the output power is a little higher when R8 is 22k but then the output transistor gets too hot and the battery does not last long. The new simulation shows with R8 at 47k then the output power into a 75 ohm whip antenna is 99mW.

 

[Willen]

The output transistor operates in class-A because it is always conducting.
When R8 is 47k then the transistor has a maximum current that is higher than if R8 is 68k so the output level drops when the value of R8 is too high.

In a simulation the output power is a little higher when R8 is 22k but then the output transistor gets too hot and the battery does not last long. The new simulation shows with R8 at 47k then the output power into a 75 ohm whip antenna is 99mW.

Get little confuse more than before seeing your attachment. Does this your Tx can produce 99mW? Hahaha confused in ''simulation'' term please

 

[audioguru]

Get little confuse more than before seeing your attachment. Does this your Tx can produce 99mW? Hahaha confused in ''simulation'' term please

I simulated my FM transmitter circuit using the free simulation software from Linear Technology. It shows the voltage swing at the 75 ohms antenna is 7.7V p-p. 7.7V/2.828= 2.73V RMS. 2.73V squared/75 ohms= 99mW.

 

[Willen]

The output transistor operates in class-A because it is always conducting.
When R8 is 47k then the transistor has a maximum current that is higher than if R8 is 68k so the output level drops when the value of R8 is too high.

In a simulation the output power is a little higher when R8 is 22k but then the output transistor gets too hot and the battery does not last long. The new simulation shows with R8 at 47k then the output power into a 75 ohm whip antenna is 99mW.

...and if I use PN2222 as a Q3 and applied 12V, can I get higher output? I think higher voltage gives higher output and higher input gives higher output.
And again if replace both Q2 and Q3 with PN2222 and applied 12V to both, I can get more power like 100mW, can't I?

 

[audioguru]

Q2 is the oscillator operating from a regulated 5V supply. If it operates from a regulated 12V (regulated so its frequency does not change when the battery voltage runs down) then it might get too hot. Try it to see.
I think if Q3 has a 12V supply then it will also get too hot.
Of course the output power will be higher with a 12V supply, maybe 200mW to 400mW.

 

[rodrigobad]

Hi all.
I'm from Brazil and need some help, i tried to make some fm transmitters, but the sound quality is bad/terrible.
I will use on my friend farm, i need only a 60meters, maybe 100meters range on open grass.
Anyone know a good circuit, some 'easy' to find components, maybe a bc547 transmitter?
Tkz all and sorry for my bad english =/

ps: audio signal from a cd-player, not a eletret mic, and 12v 1A from a 110vac transformer.
Its Possible?