Issue with an oscillator circuit

Hi, i am a new user, my name is David and i'm from Italy.

I'm writing here to get help about a circuit which i made, for instructional purposes.
The following pic is the schematic:

**broken link removed**

The IC is a 7404 and the capacitor should be a variable tuning capacitor between 3 to 20 pF. But at the local store i have only found a variable capacitor between 5 - 65 pF. And i also found an 7404N IC.

As i've said the circuit is just for instructional purpose, to understand how an oscillator works.

How I could understand if the circuit correctly works? When is turned on near an FM radio or a TV with antenna, it should disturb images on TV and the audio on the FM radio. But in my case it does nothing. I suspect that the culprit could be the variable capacitor, due his excessive range. Or could be the IC? The original project mention to use a simple 7404 or a 74N04, but not 74LS04 or 74L04. The IC which i've found is a 7404N, is equal to the plain 7404?

Someone could give some hints?

Disclaimer: i know that these kind of oscillators, in some countries, aren't allowed, and i don't have the intention to use this circuit for illegal purpose; is however a low power oscillator, and doesn't make use of a power transistor.

Thank you.

David, The "N" simply indicates the type of package-DIP, in your case.

Your circuit may very well be oscillating, but not a frequency that upsets your TV or radio. And I think you might want to connect your antenna (whatever it is) to pin 2 rather than where you have it, as it will affect your frequency since it's connected to the capacitor.

Hi Barry,

The fact is that on internet (also on youtube) the circuit claims to work near vhf frequencies.
What determine the working (or range) frequencies of the circuit?

And why this circuit relies on a single IC instead of a classic oscillator with transistor?

Thank you.

If you put an odd numbers, let's say "n", of logic gates in series with the ouput of the last connected to the input of the first (like your configuration without capacitor) this will oscillate. The frequency will be given by:

f≈1/(2*n*Td)

where Td is the propagation delay of each gate.

in your case the capacitor should add a propagation delay, then the maximum frequency should be reached with no capacitor. Inthis case, from data-sheet Td is on the order of 15ns (typical), the the maximum frequency should be roughly

fmax≈1/(2*3*15ns)≈11 MHz

However this is a very rough estimate and it can change with part to part variation, temperature, etc. Adding the capacitor you will reduce it. It could be that trying many parts you can find one having a lower propagation delay with respect to the typical 15 ns.
This kind of oscillator is not used in practice because you cannot design it to have a precise frequency of operation.
To have a reliable oscillator you have to add a resistor from pin 6 to capacitor (side not connected to pins 4-5) and another resistor from pin 1 to the capacitor (same side as before). The output will be on pin 6. If the two resistor have the same value the f≈0.56/(R*C). In any case you cannot exceed the fmax calculated before.
Using a 74AC04 you will have a 4.5 ns (typ.) propagation delay, then fmax≈37 MHz.

Take into accont that VHF frequency means taht also the layout plays a not negligible role.

Many thanks also to you for the reply, albbg.

I found this argument very interesting, but we have to assume the fact that i am a newbie about RF.
I work as computer technician and my interest for electronic started some years ago. Since some months i also have interest for the RF part of electronics (i also made a simple FM transmitter).

Looking on internet i have also found that exist oscillators which doesn't rely on a variable capacitor for tuning, like for example complex circuits which are able to transmit on the whole band/channels.
In my schematic, there is a way to achieve this, to get rid of the variable capacitor?
I have also read that sometimes we can tune these oscillators using an adjustable oscillator coil, or act on the power supply.

For example i have found this circuit: **broken link removed**

From what i can understand, we can choose a frequency between 1 and 30 Mhz, but i'm not sure how this circuit works (about the crystal oscillator) and the presence of the variable capacitor. Someone can explain?

Many thanks.

The first oscillator using an SN7404 operates at tens of MHz because a 7404 does not work at very high frequencies. Its frequency is determined by the variable capacitor value but yours is much too high so its frequency is too low. Stray capacitance of your wiring also affects the oscillation frequency. If you built it on a solderless breadboard then it probably will not work because it has too much capacitance between its rows of contacts and its wires that are all over the place.

The output of the oscillator is a square wave with many odd harmonics. If its oscillation frequency is 15MHz then its odd harmonics are at 45MHz, 75MHz, 105MHz in the FM broadcast band and on some old analog TV channels. It might interfere with airplane navigation and communication and police, ambulance and fire department communications.

Hi Audioguru,

I've grabbed a variable capacitor from an FM radio (this capacitor should be more appropriate), but is provided with six pins (three above and three below).
Which i have to use? The four in each corner are named from C1 to C4; i have tried to connect C1 and his middle pin, also C3 and his middle pin but the circuit still doesn't work.

In anyway i didn't imagine that this circuit would be dangerous. Obviously i will just use this circuit inside my home, to avoid potential troubles.

- - - Updated - - -

P.S: i didn't used a breadboard, but an appropriate pcb.

It isn't matter of variable capacitor. You should check the output of your circuit by means of an oscilloscope, otherwise you can't understand what really happens.

FM radios do not use a 40 years old 7404 TTL logic IC as their local oscillator. They use a high frequency transistor and the tuning capacitors are resonant with inductors. The frequency of the oscillator in an FM radio is above or below the radio station frequency by 10.7MHz.

Your home probably does not block your oscillator from causing radio and TV interference to your entire neighbourhood and to airplanes overhead.

On youtube there is a video of a guy which made the same circuit and he's able to disturb a modern FM radio.
Unfortunately i don't own an oscilloscope.

In anyway i have made a classic oscillator with capacitors, coil and transistor, and this new circuit works like a charm. About the circuit: i have tried to go on the ground floor and this circuit is not able to interfere with radio nor with tv on the upstairs. So maybe i can't interfere with the outside, assuming however the fact that the circuit is not always turned on: as i've said is only for instructional purpose.

You told me about interferences because sometimes these circuits are powerful enough to cause interferences with the entire neighbourhood and to airplanes overhead?

Interference depends on the ratio of the signal strengths.

If an old neighbour is listening to a weak distant radio station or watching a weak distant TV station then your oscillator can cause interference if it or one of its harmonics is on the same frequency and possibly cause your neighbour to have a heart attack.

If an airplane overhead is listening to weak distant instructions from an air traffic controller then your oscillator can cause interference and possibly cause the airplane to crash into another airplane.

Audioguru said:

Interference depends on the ratio of the signal strengths.

If an old neighbour is listening to a weak distant radio station or watching a weak distant TV station then your oscillator can cause interference if it or one of its harmonics is on the same frequency and possibly cause your neighbour to have a heart attack.

If an airplane overhead is listening to weak distant instructions from an air traffic controller then your oscillator can cause interference and possibly cause the airplane to crash into another airplane.

Click to expand...

If the signal from a lousy 7404 oscillator can cause an "airplane to crash into another airplane", better find a different airline.

I watch the TV show called Mayday where they are investigating many airplane accidents. Most accidents are caused by the pilot or by a mechanical failure.

When I was living in an apartment when I went to university I was visited by the RF Cops. My do-it-yourself fan caused radio and TV interference. We had a nice discussion about electronics then they took away my "fan". The neighbour who called them bought me a real fan.