another FM transmitter project

[Willen]

Look at all the other higher power transmitter circuits. They use a tuned LC circuit to match the output impedance to the antenna impedance. Mine uses only a simple coupling capacitor.

They use much more average current than a little 2n3904 can handle. The maximum allowed current of a 2N3904 is 200mA and it works poorly above 50mA.

Oh then if i add a couple of trimmers to the output of your Tx (like others') to match impedance between Tx and antenn, Can I get more power or range?

 

[audioguru]

Oh then if i add a couple of trimmers to the output of your Tx (like others') to match impedance between Tx and antenn, Can I get more power or range?

My FM transmitter has a parallel tuned LC at the collector of the output transistor and has a fairly high input impedance.
Mine has a fairly low current in the output transistor.

But the higher power transmitters use a series tuned LC at the collector of the output transistor, extra trimmer capacitors and much more current in the output transistor. Their very low input impedance requires high input power so they need a buffer transistor in between the oscillator and the input of the power amplifier transistor.

 

[Willen]

Its output power might be 500mW into a 50 ohm or 75 ohm antenna if all the tuned circuits are carefully adjusted to one frequency.
The BC549 transistor is a bad choice for Q3 because its max peak collector current is 200mA and it performs poorly above 50mA.
A BC549 transistor can easily be destroyed in that circuit.

Transistors have a range of current gain. Some have low gain then the output power of the transmitter will be low. Some transistors have high current gain and they might be destroyed in that circuit.

The power meter uses a very small amount of output power to activate it because its input coupling capacitor value is almost nothing (0.9pF). The power meter will not work because its LED is upside down.

You were talking about:- www.electronics-diy.com/tx500.php

Nice project! May I use 2n3866 as a Q3? Then what will be the output power? If i start this project, sure I will add Pre-emphasis from your design- Mod-4.

 

[jamesdenoil]

thank you...nice post

 

[audioguru]

Nice project! May I use 2n3866 as a Q3? Then what will be the output power?

Then with a 12V supply its output power will be higher than with a BC549 transistor but maybe not much higher since the driver transistor Q2 also limits the output power.

The driver transistor and output transistor both operate in class-C that produces harmonics that might interfere with police, ambulance, fire department communications and TV.

 

[Willen]

Then with a 12V supply its output power will be higher than with a BC549 transistor but maybe not much higher since the driver transistor Q2 also limits the output power.

The driver transistor and output transistor both operate in class-C that produces harmonics that might interfere with police, ambulance, fire department communications and TV.

OK I want to save Q3 from destroy, and I have 2n3866 transistor, so I am using 2n3866.

- What is the chance of being hot this 2n3866 as a Q3? Need large or small heatsink?

-I will use filter to minimise harminics if i try it. If i connect this 500mW tx to 5 watt amp, on both output (Tx500 and 5 watt amp) should i have to connect filters? Or just on the end?

 

[audioguru]

Here are simulations of my FM transmitter.
One has the output in class-A and has an output of 118mW with not many harmonics.
The other has the output in class-C and has an output of only 20mW with a lot of harmonics. A filter will reduce the output.

 

[Willen]

Here are simulations of my FM transmitter.
One has the output in class-A and has an output of 118mW with not many harmonics.
The other has the output in class-C and has an output of only 20mW with a lot of harmonics. A filter will reduce the output.

I am sorry, I am little unfamilier to simulation software. Can this software predict the function of any particular circuit by using only schematic diagram like in picture? Is it to check its performance before making real circuit or device?

But I heared that class-C is better than Class A or B. And most of the powerful amplifier biased as a class-C for better performance and high output. But why there are such unwanted harmonics in your Tx?

I am again little confuse Your simulation shows 118mW out of your Tx at 75 ohm impedance but why your real Tx can produce ONLY 20mW? I think your Tx has high impedance than 75, is this reason?

 

[audioguru]

I am not an RF expert. Another member is an RF expert and said that my transmitter has an output of only 20mW.
Then I tried simulating it and got a much higher output. I don't know if the simulation is correct.

A class-C amplifier is cutoff for some of its cycle so it is clipping and produces harmonics. Because it is cutoff for some time then it is cooler and can have a higher output power (if it is driven harder) without getting too hot.

 

[Willen]

Asking again please:-
I am little unfamilier to
simulation software. Can this
software predict the function of any
particular circuit by using only
schematic diagram like in picture? Is
it to check its performance before
making real circuit or device?
I searched on google and I think my guess is Yes! Any additional?

 

[audioguru]

Some simulation software works very well for low frequencies up to about 1MHz. The inductances and stray capacitance of wiring is not included which changes response to higher frequencies.
The details of parts in its library might not include accurate high frequency spec's.

I use LTspiceIV simulation software which is free from Linear Technology. It has models of many transistors and opamps but not all. There is a forum where members have posted models for many other transistors and opamps.

 

[Willen]

Hi audioguru,
MIC and Pre-emphasis of your Tx Mod-4 is unable to catch my voice from more than 1 or 2 meter. Is it normal in your Tx?
It can catch my voice within 1 meter far from MIC. So I added a another audio amplifier before pre-emphasis. Now MIC can catch voice arround 10 meter.

 

[audioguru]

My FM transmitter picks-up my voice about 3m away to be as loud as a radio station.
Your pickup is 10m then your gain is too high so when you speak near the transmitter then the level is far too high and will cause severe distortion. At 10m indoors then the sound will be very reverberant (have many echoes).

 

[Willen]

I am trying to make Tx500 too from www.electronics-diy.com/tx500.php . But I am using your oscillator and pre-emphasis of Mod-4 because of its best performance. Then....in this replacemant, which one is correct and suitable for Tx500:-
- RF out from collector of oscillator (like Tx500's)
-RF out from emitter of oscillator (like your)?

 

[audioguru]

I am trying to make Tx500 too from www.electronics-diy.com/tx500.php . But I am using your oscillator and pre-emphasis of Mod-4 because of its best performance. Then....in this replacemant, which one is correct and suitable for Tx500:-
- RF out from collector of oscillator (like Tx500's)
-RF out from emitter of oscillator (like your)?

The collector is a high level at a high impedance and the emitter is a lower level at a much lower impedance.
The circuit has an LC impedance converter to allow the collector of the oscillator to feed it.
Try it both ways.

 

[Willen]

audioguru,
-Can I use smaller inductors than your Oscillator's with same inductance? Like:- small inductor but lots of turns that provides same inductance (0.1 nH). Or smaller/tiny inductors used ferrite and same inductance 0.1 nH. (Smaller inductor but same inductance) What happens if I used it instead of your big inductor? Does it critical in output power?
- And what happens if I used such smaller inductors in your amplifier but same inductance 0.1nH?

 

[audioguru]

audioguru,
-Can I use smaller inductors than your Oscillator's with same inductance? Like:- small inductor but lots of turns that provides same inductance (0.1 nH). Or smaller/tiny inductors used ferrite and same inductance 0.1 nH. (Smaller inductor but same inductance) What happens if I used it instead of your big inductor? Does it critical in output power?
- And what happens if I used such smaller inductors in your amplifier but same inductance 0.1nH?

The inductors in my FM transmitter are about 0.1uH (100nH), not 0.1nH.
The inductance and the tuning capacitance (including stray capacitance of the wiring) determine the resonant frequency.
You can use coils with a smaller diameter but the number of turns must be changed to have the same inductance as a larger coil.
You can use a ferrite core then the number of turns in the coil can be less.

 

[Willen]

-In Mod-4, Can I adjust R2 or R3 to increase gain of Q1? I think- if I replace R2 with 100K, then I will increase audio amplification. If I adjust the resistance, will this pre-emphasis lost its 'Pre-emphasis accurate friquency'?
I am making your Tx again as small as I can.

 

[audioguru]

R2 and R3 bias the preamp transistor and have nothing to do with AC voltage gain. If you decrease R2 from 160k to 100k then the transistor will be saturated with severe clipping distortion.
The AC voltage gain is determined by R4/R5 but R5 has the transistor's internal emitter resistance in series and C4 in parallel.

Pre-emphasis boosts the gain of high audio frequencies. Then medium and low frequencies must have less gain.
The AC gain at 15kHz is almost as high as is possible.
The frequency where the pre-emphasis begins is determined by the values of R5 and C4. The gain increases at 6dB (double the level) per octave.

Here is a simulation of my FM tx preamp transistor and with the transistor turned on too hard by decreasing the value of R2.
In my simulation the parts do not have the same numbers as in the transmitter.

 

[Willen]

-And.....what happens to output when I increase the supply voltage at oscillator?

If you decrease R2 from 160k to 100k then the transistor will be saturated with severe clipping distortion.

- But I found a audio pre amp of BC547 (opereted at +6V) which has 2.2 Mega ohm base biasing resistor from positive and 270K base biasing resistor from ground. Why such large resistance? To get high gain? (Amp has 2.7K resistor at collector.)