Building RF Noise Generator Circuit w/ LM324D SMD Op Amp

[hilbert2]

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:

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

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.

 

[berkeman]

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...

 

[hilbert2]

Ok, thanks.

 

[DaveE]

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.

 

[hilbert2]

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)?

 

[berkeman]

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...

 

[hilbert2]

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.

 

[DaveE]

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.

 

[DaveE]

If you do want to experiment with transmitters, stick to the ISM radio frequencies. You will attract fewer lawyers that way.

 

[DaveE]

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.

 

[berkeman]

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!

https://www.jameco.com/z/FMST-100-Electronic-Rainbow-FM-Stereo-Transmitter-Kit_151239.html

 

[hilbert2]

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?

 

[darth boozer]

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)