Can software defined radio be used as noise generator?

Question

I'm working on solving an issue in a metal detector that generates false detects when subject to electrical interference. The electrical interference usually stems from devices such as shortwave radio (walkie talkies) when being used in proximity, HF fluorescent lighting, Variable Frequency Drives etc.

I want to design/buy a system that can produce these frequencies with low energy. The idea being that I can repeatedly subject the metal detector to noise to understand the behaviour of the metal detector and in turn design a solution that mitigates the effects of the environmental noise.

• Could I achieve this with a software defined radio?
• If not, could I achieve what I want with a signal generator, a power amplifier and an antenna?

I understand that there may be concerns about intentionally generating radio frequencies but I'll ask that in an other question.

Answer 1

Could I achieve this with a software defined radio?

You can produce any band-limited signal, given its bandwidth is within the capabilities of the SDR. So, yes.

You'll find that "shortwave radios used in proximity" is often incredibly high power density compared to what modern SDRs do, so not sure whether you can induce the same problems when doing that at a lower power. But that's a different question!

Answer 2

I had the same problem with a Motorola walkie talkie that operated at around 500 MHz. I just keyed the walkie talkie now and then and tried "this and that" on and around the search head coils and front-end circuits until the problem was solved without detriment to the performance of the metal detector.

Usually you'll find that there is one weak spot in the design and once you hang capacitors to ground (for instance) on that vulnerable EMI point, you solve the problem for other frequencies as well.

could I achieve what I want with a signal generator, a power amplifier and an antenna?

Cut-out the "middle man" and use the thing that generates the problem i.e. use the walkie talkie to find a solution. Using the walkie talkie means you can see how robust your potential solution is at different distances just by moving nearer or further away. That's what I did anyway and it worked.

Answer 3

Actually for a budget option, you could totally buy two of the 433MHz "Ali Baba quality" radio modules that are popular with the hobbyists (Google "433MHz duplex radio module") - we always get questions here about how to get those working. Well - the aim here is the opposite, get it not working at all, which is something these cheap modules excel at!

You'll want a semi-duplex pair since those as a rule of thumb toggle their power amps in evil ways. These are excellent noise generators! Tune it up to max power, set it to OOK or some madness modulation like that, maybe also use an antenna with gain. Then watch it spew wide band noise all over ~400MHz plus harmonics. Most definitely the cheapest noise generator on the market and roughly the same frequencies as used by walkie-talkies.

Are they legal? Not at all, but that's Ali's problem since he's the one who put it on the market.

Answer 4

You basically want an EMI test lab, where they will put your device into a shielded room and just throw a sine wave with rising frequency at it until your device signals an error. For example when testing a computer system, you'd have it blink a LED, and the test runs until the LED stops blinking.

The frequency where the error occurred is likely to correspond to the geometry of the unintentional antenna, so there is a good chance that they can tell you "the problem is four centimeters long", and also try various mitigation techniques like shielding certain parts and testing if that solves the problem.

Basically, this is already available as a service from people who have a lot of experience zeroing in on the problem.