Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Uhhh... audioguru... read this

Status
Not open for further replies.
Next we have an FM transmitter that looks to be well-designed. Apart from the complex circuit, there are a number of fundamentally incorrect features that make the circuit unreliable.
The first item we will look at is the "Q" of the tank circuit.
This is a factor known as "Quality" and comes from the fact that an inductor will produce a voltage (of opposite polarity) that can be many times higher than the voltage applied to it.
And that's what a circuit like this FM transmitter does. The voltage produced by the capacitor and parallel inductor on the collector, will produce a voltage many times higher than the 5v on the rail.
These two components are called a TANK CIRCUIT and to get them to produce a high voltage, the energy stored (and released) by the capacitor must be equal to that of the inductor. The two work like tipping water from one jug to another of the same size and back again. If one jug is smaller, we only get the energy from the smaller jug.
In this case the 5-35p air trimmer will be set at about 20p for 90MHz while the energy stored in the 10 turn coil will be twice that needed. The 10 turn coil should be reduced to 5 turns and the capacitor should be increased to 39p - 47p. This will give the circuit a higher "Q."
With a low Q, the energy through C7 (4p7) will be very small.
We don't know how or where the tracks are cut on the "strip-board" but you can see some of the track will connect to the end of the 4p7 that goes to the emitter.
This track acts like a "transmission line" and since it is very wide, it will have a high value of radiation.
This means a certain amount of the energy delivered by the 4p7 will be lost to the surroundings and any handling of the project will cause drifting or it could come to a point where the oscillator fails when handled.
The circuit may be successful as the oscillator transistor is being heavily driven via the 220R in the emitter. This is overcoming the short-falls in the other design-concepts.
High frequency circuits like this need to be designed so the power rails are "tight." This not only means electrical and electronic "tightness" but also physical tightness.
The 1n across the power rails for the oscillator is insufficient to give good tightness (it should be 22n) and the placement of the components on the board is far too spread-out.
This makes the project very susceptible to handling and drifting.
Since the output transistor is a buffer, the 22p on the antenna is not needed and simply reduces the range.
The 10k resistor on the electret mic is too low for our high-sensitivity microphones. It should be 47k for 5v rail.
Overall, I consider the circuit is taking 2 - 3 times more current than it needs.
Our 9v Voyager circuit consumes 7-10mA for 800metre range. This circuit will consumes more than 25mA.
One final point. The air trimmer should be in parallel with a capacitor (39p) so the trimmer is only adjusting a small amount of the total capacitance. This makes it easier to tune across the band and set the frequency.

**broken link removed** i haven't looked at the circuit closely enough to know if he's right or not. he seems to be overly critical of every single circuit that he posts.

IE:Here is a misleading pin-out:

Pin 14 is not a clock line but when it is HIGH, the chip does not respond to the clock pulses on pin 14.
Pin 12 is not ENABLE CARRY but simply a "carry out" pin.

-overly critical, the only mistake is only in the wording, look it up if you are confused. I've attached the data sheet for the 4017. (I'm not implying that you're stupid, I'm just being through.) don't yell at me for posting this, i just thought I'd point it out.
 

Attachments

  • Audioguru-FM-Tx.jpg
    Audioguru-FM-Tx.jpg
    86.9 KB · Views: 128
  • 4017Inputs.gif
    4017Inputs.gif
    3.1 KB · Views: 137
  • 4017.pdf
    317.1 KB · Views: 138
Last edited:
My FM transmitter is the only one I ever designed. It works perfectly. It was another circuit that did not work and I fixed all its problems.
 
IE:Here is a misleading pin-out:
Pin 14 is not a clock line but when it is HIGH, the chip does not respond to the clock pulses on pin 14.
Pin 12 is not ENABLE CARRY but simply a "carry out" pin.
He is wrong about pin 14. It can be used as a clock line if pin 13 is held low. This allows you to have either positive edged clocking or negative edged clocking. Pins 14 & 13 are defined in the datasheet as an clock and a clock enable line, and maybe that is what he means by "misleading".
 
Last edited:
He apparently doesn't like anything that he didn't design himself. I found three serious errors in my first 5 minutes on his "spot the mistake" page. A lot of the "corrections" that he made to other peoples circuits would not "fix" them and in many cases would make them worse.

He says don't believe everything you read on the web. That goes double for his site.
 
Last edited:
He apparently doesn't like anything that he didn't design himself. I found three serious errors in my first 5 minutes on his "spot the mistake" page. A lot of the "corrections" that he made to other peoples circuits would not "fix" them and in many cases would make them worse.

He says don't believe everything you read on the web. That goes double for his site.

LOL i agree
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top