Unknown 50 Hz noise in CW radar output - Need help removing it

Is your line frequency 50 Hz? If so, that’s probably where it’s coming from. Now, is it coming in from a poorly filtered DC supply, or is it being picked up by your antenna (or somewhere else along the signal path)? Is there any filtering on your input path? what is your input bandwidth? You don’t even show a hint of what your hardware is.

barry said:

Is your line frequency 50 Hz? If so, that’s probably where it’s coming from. Now, is it coming in from a poorly filtered DC supply, or is it being picked up by your antenna (or somewhere else along the signal path)? Is there any filtering on your input path? what is your input bandwidth? You don’t even show a hint of what your hardware is.

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Hi Barry,

Sorry . I'm using LiPo drone batteries and have used ferrites to reduce the potential noise from the battery. It's basically an antenna transmitting at 5Ghz and another antenna 30cm away receiving the reflected signal.

The received signal is mixed using an IQ mixer and I've got two down converted intermediate frequency outputs. My signal of interest is from 20Hz really to 400Hz and the 50Hz noise is ruining the results and drowning my signal.

I have a active bandpass filter to amplify the signal which operates from 5Hz to 450Hz

It's probably coming from an external source by magnetic coupling. Are there any linear power supplies or transformers ?

Hi,

a sketch about your signal flow, a couple of photos will help us to help you.

Klaus

added:
don´t know how your equippment looks like.
Can the 50Hz be caused by mechanical vibration? (PC, air condition, refrigerator...)

Another potential source could be fluorescent lighting; signal reflecting off the ionized gas, I've come across this myself with a 10GHz cw radar. Not much I could do about it except not point at the tube.
I've also had light getting into varactor diodes causing FM.

G4BCH said:

Another potential source could be fluorescent lighting; signal reflecting off the ionized gas, I've come across this myself with a 10GHz cw radar.

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Good point, modulation of fluorescent lamps is however dominantly 100/120 Hz.

BigBoss said:

It's probably coming from an external source by magnetic coupling. Are there any linear power supplies or transformers ?

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Hi Big Boss! Thanks for pitching in as always

It's powered by a drone battery. I've used quite a few DC coupling capacitors before feeding it to the circuitry. I've also used a ferrite bead prior to that. The drone battery is then down regulated using switch mode power supplies. to power the RF boards. The power was also passed through ferrite beads and DC coupling capacitors.

There are a lot of wires though.. and I fear that there is not enough EMI shielding in the wires and since everything is placed so close together its causing this interference...

KlausST said:

Hi,

a sketch about your signal flow, a couple of photos will help us to help you.

Klaus

added:
don´t know how your equippment looks like.
Can the 50Hz be caused by mechanical vibration? (PC, air condition, refrigerator...)

Click to expand...

Hi KlausST

I made sure I turned the power off of any devices nearby. There is a laptop running, which has a power cable and a transformer. Also there is a refrigerator in the next room . I'll try turning that off next time I attempt to do some testing (RIP Ice cream)

Also the antenna spacing is about 10 cm. Think that's causing it ?

G4BCH said:

Another potential source could be fluorescent lighting; signal reflecting off the ionized gas, I've come across this myself with a 10GHz cw radar. Not much I could do about it except not point at the tube.
I've also had light getting into varactor diodes causing FM.

Click to expand...

Hi @G4BCH

Thanks for helping out

That's a good point. I'll try to avoid that!

Hi,

please do one test by running the laptop from batteries. (power supply disconnected)

I see one loop: IQmixer --> IF1 --> ADC --> IF2 --> IQmixer
Be sure the cables are fed parallel in close proximity. Use cable ties. The enclosed area by the loop should be minimal.

Klaus

drone battery? like as if this was mounted on a DRONE?

the first thing i would check would be oscillator phase noise caused by motor/blade rotation.

you label your signal source as VCO.
but what is it? does it stay at one CW frequency? If so it needs to be phase locked, and likely vibration isolation mounted.

is it sweeping, as in an FMCW radar?

1- Is your mixer active or passive? If it's an active mixer, supply can be the problem.
2- Ac coupling capacitor at the output of the mixer can cause problems if you are using a Zero-IF conversion scheme. You may need to change the capacitor.

drone motors, mostly, are BRUSHLESS. that means there are some pretty strong low RF frequencies near them. could be an issue