Understanding a well designed FM Transmitter - AudioGuru's MOD4

[Hero999]

Yes, you use something known as a resonant loop - pretty simple and not any larger than an HF mast.

 

[marcbarker]

Yes, you use something known as a resonant loop - pretty simple and not any larger than an HF mast.

He uses different approach, a plastic water-barrel as coil-former, which 'end-fires' a wire with high voltage. It's almost a Tesla Coil. The net radiation resistance of the 'twig' part of his antenna is milliOhms!

His RX antenna is a resonant loop frame though, on a rotator.

 

[KamalS]

I am not sure how stable JFET osc are compared to BJTs .. any info/links on that?

I sat down and calculated the length of the antenna for the 400Khz carrier and I stopped when the numerator went above 100 .. this is disappointing .. in that case I need a better/faster PLL .. any suggestions on them that would not break the bank?

Also am I wrong when I say a PLL can modulate AS WELL as demodulate?

Demodulation using a PLL should be intutive but I do recall FM modulation using a PLL too?

 

[audioguru]

A CD4046 PLL IC has a voltage-controlled-oscillator that can be used to make FM.
It can also demodulate FM.

 

[KamalS]

What would be the other parts (except BJTs) for a CD4046 based modulator?

 

[audioguru]

What would be the other parts (except BJTs) for a CD4046 based modulator?

You need to learn how to read the datasheet for you to be able to design a circuit.

There are many tutorials on the internet about the CD4046.

 

[Nigel Goodwin]

I have been noticing that you mention a 1970 version of the circuit. I am sure there were matured Colpitts by then, so can you trace out a variant of the circuit that closely resembles what you are talking about?

As with almost all circuit configurations, it's FAR older than that - and was first published (by Clapp) in 1948.

 

[Hero999]

The CD4046 is no good at 100MHz so you have to use various tricks such as heterodyning and harmonic filtering, even then that's of limited use when the maximum frequency is 400kHz.

 

[audioguru]

I made an FM transmitter in about 1962 that was not much different than the one I made in 2005.
It did not have a voltage regulator and did not have pre-emphasis.

 

[KamalS]

I have been doing a little bit of reading, from real books this time, and it seems this thread would be coming to an abrupt end because of that.

This circuit is probably "as good as it gets" for a simple circuit using a LC oscillator.

I still have questions though:

1. audioguru said that the stage 3 transistor Q3 is biased crudely.

Why not bias it properly then?

Or is there a reason why it should not be biased so?

2. Why not remove the stage 3 tank? I mean remove L2 and C13 completely.

Would it affect the transmission in any way (maybe send out more harmonics, but if we operate the unit completely from 5V, the power of the harmonics would be sufficiently reduced)

 

[audioguru]

1. audioguru said that the stage 3 transistor Q3 is biased crudely.

Why not bias it properly then?

Because it works fine with the C13/L2 tuned circuit.
If you bias it properly then its performance would be almost the same.

2. Why not remove the stage 3 tank? I mean remove L2 and C13 completely.

They pass the frequency that is wanted and attenuate harmonics.
They resonate so provide a higher output swing than is possible without a tuned tank.

if we operate the unit completely from 5V, the power of the harmonics would be sufficiently reduced)

The harmonics will not be reduced.
The desired output power would be reduced.

 

[KamalS]

audioguru, thanks for the fast answers

Any suggestions you would like to make to the circuit?

Would you do anything differently if you were to make something similar today (theoretically speaking)?

 

[audioguru]

I made my FM tramsmitter to confirm and fix problems of another FM transmitter project that somebody said didn't work.
Then I improved it in 4 modifications.

I can't use my FM transmitter because my FM dial is full of stations and my transmitter will cause interference to one of the stations.

Today I would make a stereo Micromitter (a project in Silicon Chip magazine and a kit in Australia).