Building RF Noise Generator Circuit w/ LM324D SMD Op Amp

In summary, the author is trying to create a circuit that produces RF random noise. He found a webpage with information about it and seems to be unsure about how to proceed. He recommends using a transistor-based RF amplifier and connecting it to a zener diode for noise generation. He cautions against transmitting interfering noise and suggests instead looking into Ham radio.
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hilbert2
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TL;DR Summary
How to choose capacitors, zener diodes and resistors for a circuit that generates radio waves.
I haven't been building electrical circuits for quite a while, but I'm now trying to create a circuit that produces RF random noise. I found this webpage with some information about it.

http://www.n5ese.com/noise.htm

There is this diagram there:

rf-generator.jpg


I have an LM324D SMD op amp with pins arranged to this kind of order:

op-amp-pins.jpg


Can I really expect the circuit described on that web page to do what it's supposed to do? And how to choose the capacitances, resistances and zener voltages in the circuit to produce as much RF energy as possible with the given voltage source? It's also a bit unclear to me what I'm supposed to connect to the other input of the amplifier when only one is shown in the diagram. And the length of the conductor in the position termed "RF out" probably affects the effectiveness too.

Thanks in advance for any replies.
 
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  • #2
hilbert2 said:
Summary:: How to choose capacitors, zener diodes and resistors for a circuit that generates radio waves.

I have an LM324D SMD op amp with pins arranged to this kind of order:
They use discrete transistors to build their circuit, it looks like. That will give you a lot better bandwidth than that dog-slow LM324 opamp. Also, the opamp requires more biasing than is shown in the simplified amplifier block diagram that you posted.

Here is the transistor-based version from their web page, including using an LED instead of a Zener diode for the noise source...

1590001624865.png
 
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Ok, thanks.
 
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The zener diode is a good source of broad-band noise. But, @berkeman said, the circuit you connect to it can act as a filter to modify that noise. If you want an RF noise source, you will need a circuit that can respond to the RF noise generated by the Zener. This, of course, extends to the connectors and cables that you use to connect it.

There are RF amplifiers that are similar to the LM324 op-amp. However, they are expensive and probably no easier to use than the transistor amplifiers, like the one shown above. There are also many low cost versions you can buy online (eBay, and such), typically PCB assemblies.
 
  • #5
DaveE said:
The zener diode is a good source of broad-band noise. But, @berkeman said, the circuit you connect to it can act as a filter to modify that noise. If you want an RF noise source, you will need a circuit that can respond to the RF noise generated by the Zener. This, of course, extends to the connectors and cables that you use to connect it.

There are RF amplifiers that are similar to the LM324 op-amp. However, they are expensive and probably no easier to use than the transistor amplifiers, like the one shown above. There are also many low cost versions you can buy online (eBay, and such), typically PCB assemblies.

What should it be connected to if I want to produce EM waves that can disturb a radio receiver a couple of meters away (or mess up the van Eck radiation that can be used to monitor computer screens from distance)?
 
  • #6
hilbert2 said:
What should it be connected to if I want to produce EM waves that can disturb a radio receiver a couple of meters away (or mess up the van Eck radiation that can be used to monitor computer screens from distance)?
Be careful transmitting interfering noise -- it is illegal in most places.

What is the overall goal? Most radio receivers do a pretty good job of rejecting broadband nose. If you want to interfere with them (which is illegal, as I said), you would typically use a targeted narrow band signal...
 
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berkeman said:
Be careful transmitting interfering noise -- it is illegal in most places.

What is the overall goal? Most radio receivers do a pretty good job of rejecting broadband nose. If you want to interfere with them (which is illegal, as I said), you would typically use a targeted narrow band signal...

I'm not thinking of disturbing neighbor's radio with that. This is just as an experiment with no real goal.
 
  • #8
hilbert2 said:
I'm not thinking of disturbing neighbor's radio with that. This is just as an experiment with no real goal.
Yes, but whether you are intending it or not, you probably will. I would suggest that you stay away from transmitting RF signals. Or, alternatively, look into Ham radio. Much of that hobby is related to doing what you want (EM wave-wise) without screwing up other radios. Making a transmitter that will target only a specific radio receiver isn't easy.
 
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If you do want to experiment with transmitters, stick to the ISM radio frequencies. You will attract fewer lawyers that way.
 
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Since you mentioned eavesdropping, you might want to looking the NSA Tempest program. The DOD's response to electronic security.

I am many decades out of date, But they way it was in the 1980's, the NSA determined that you couldn't adequately secure electronics once they have emitted EM waves of any sort. There will always be a better spy technology to defeat reasonable countermeasures. The solution was to completely shield sensitive electronic devices, often in really well shielded rooms, with no outside connections except really well filtered power supplies. We used to call these tanks, they look like bank vaults (sometimes) with a guard at the door and lots of badges, passwords, etc. The alternative, like electronics in an airplane, was big clunky expensive over-engineered stuff (again, lots of shielding).

Of course, the other choice is cryptography, when you do have to talk to the outside world.
 
  • #11
hilbert2 said:
I'm not thinking of disturbing neighbor's radio with that. This is just as an experiment with no real goal.
You can legally transmit in the licensed bands (usually) as long as your Tx power is low enough to only be received within a few 10s of meters, and you do not generate harmonic interference. I've used simple kits like the one linked below to prototype various FM radio product ideas, including stuff like a Real Estate roadside transmitter to help househunters shop for new homes by listening to their car radio while driving around neighborhoods.

The key is that these circuits use very weak coupling to short antennas to transmit weak signals that do not violate FCC limits.

Have fun! :smile:

https://www.jameco.com/z/FMST-100-Electronic-Rainbow-FM-Stereo-Transmitter-Kit_151239.html
 
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Looks like the circuits in the linked web page and some others produce radio waves with audio frequencies, i.e. less than about 20000 Hz. Putting smaller capacitances in the circuit would probably increase the frequency.

A theoretical way to produce radio noise I thought of would be to amplify an external radio signal with some method that causes really bad 2nd and higher harmonic generation and then retransmit it, but that's obviously something you wouldn't do near any equipment that it could disturb.

Edit: I was now able to build a working circuit by modifying the one shown on this page to have no capacitors or inductors and to have smaller resistors. By putting the antenna wire from my phone right next to the circuit and turning on a radio channel that already had some static noise on the background, the circuit had paradoxically the opposite effect than I expected. When I connected it to a 9 V battery, the noise on the radio channel became smaller than before, and came back right away when I opened the circuit. This happened several times. Can the receiver filter the background noise better when I artificially add more of it?
 
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  • #13
DaveE said:
Of course, the other choice is cryptography, when you do have to talk to the outside world.
Cryptography is defeated if the "enemy" can pick up a radiated signal related to the original plaintext message or the key.
https://en.wikipedia.org/wiki/Tempest_(codename)
 

Related to Building RF Noise Generator Circuit w/ LM324D SMD Op Amp

1. What is an RF noise generator circuit?

An RF noise generator circuit is an electronic circuit that produces random noise signals in the radio frequency (RF) range. This circuit is commonly used in testing and troubleshooting RF equipment, as well as in scientific experiments and research.

2. What is an LM324D SMD op amp?

The LM324D is a type of operational amplifier (op amp) that comes in a surface mount device (SMD) package. It is a quad op amp, meaning it contains four separate amplifiers in one package. The LM324D is commonly used in low-voltage applications and is known for its low power consumption and high gain bandwidth product.

3. How does the LM324D SMD op amp work in an RF noise generator circuit?

The LM324D op amp is used as a voltage-controlled oscillator (VCO) in the RF noise generator circuit. It amplifies the input signal from the noise source and feeds it back to the input in a closed loop, creating a self-sustaining oscillation. The frequency of the output noise signal can be adjusted by changing the value of the feedback resistor.

4. What components are needed to build an RF noise generator circuit with an LM324D SMD op amp?

In addition to the LM324D op amp, you will need a noise source (such as a reverse-biased diode or a zener diode), a feedback resistor, a power supply, and some passive components like capacitors and resistors. You may also need a breadboard or a printed circuit board (PCB) to assemble the circuit.

5. Are there any precautions to take when building an RF noise generator circuit with an LM324D SMD op amp?

Yes, there are a few precautions to keep in mind. First, make sure to use appropriate safety measures when working with high-frequency signals and electricity. Also, be careful not to exceed the maximum ratings of the LM324D op amp, as this can damage the component. Finally, pay attention to the layout and grounding of the circuit to minimize unwanted noise and interference.

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