PC (laptop) power induces significant noise

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EastPier
Posts: 16
Joined: Sun Dec 04, 2016 11:17 am

Re: PC (laptop) power induces significant noise

Postby EastPier » Wed Oct 11, 2017 10:43 am

Thanks DerekR for starting this thread.

I have isolated my own noise issues by switching on and off the circuit breakers. There are two offenders:

1. the house alarm (I can switch off the breaker, as I'll be in the house when using the SDR - so no biggie there).
2. The lights in our kitchen extension; this was built two years ago and is all LED down-lighters. This is the worst offender, and makes no difference if the lights are on or off.

Has anyone any ideas about eliminating both these sources of noise. If there is another thread on this I would be happy to be redirected there.

73 Adrian

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DF1KZ
Posts: 22
Joined: Fri Nov 27, 2015 1:37 pm

Re: PC (laptop) power induces significant noise

Postby DF1KZ » Wed Oct 11, 2017 5:28 pm

Hello Adrian,

The problem you all have is nothing new - it is called EMC. Many devices especially those cheap ones coming from Asia do not fulfil the requirements of the EMC-directive (Europe) or FCC (USA). They produce to much noise on the power-lines, the power-lines radiate this noise and you hear the harmonics of the switching-frequency of your (or your neighbours) power-supply.
An other version of these noise-generator is called PLC (Power Line Communication). PLC misuses the power lines for data communication and because power lines are not made for data communication they radiate the PLC signal.

I would try to change the power supply for your kitchen LEDs or try to use line filters (expensive) near to the power supply. Ferrite cores may work but especially on low frequency their attenuation is not high enough.

Wolfgang (DF1KZ)

Reason: No reason
SDRplay (RSP1, RSP1A, RSP2 pro) with:
RFsystems DX-10 (14kHz to 50MHz), ICOM Discone AH8000 (10MHz to 2GHz)
12m above ground

EastPier
Posts: 16
Joined: Sun Dec 04, 2016 11:17 am

Re: PC (laptop) power induces significant noise

Postby EastPier » Wed Oct 11, 2017 6:09 pm

DF1KZ wrote:Hello Adrian,

The problem you all have is nothing new - it is called EMC. Many devices especially those cheap ones coming from Asia do not fulfil the requirements of the EMC-directive (Europe) or FCC (USA). They produce to much noise on the power-lines, the power-lines radiate this noise and you hear the harmonics of the switching-frequency of your (or your neighbours) power-supply.
An other version of these noise-generator is called PLC (Power Line Communication). PLC misuses the power lines for data communication and because power lines are not made for data communication they radiate the PLC signal.

I would try to change the power supply for your kitchen LEDs or try to use line filters (expensive) near to the power supply. Ferrite cores may work but especially on low frequency their attenuation is not high enough.

Wolfgang (DF1KZ)


Vielen dank Wolfgang

I am no electrician but the transformer, pictured below, was added when kitchen extension was built - I wonder if this is the offending item, have you any thoughts?

Thanks again
Adrian
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DF1KZ
Posts: 22
Joined: Fri Nov 27, 2015 1:37 pm

Re: PC (laptop) power induces significant noise

Postby DF1KZ » Thu Oct 12, 2017 2:18 pm

Hello Adrian,

For me this transformer looks like a transformer for your doorbell. It is rather unusual to use this kind of transformers for LEDs. Transformers for doorbells often are simple transformers without any active component, they don't produce noise on the power lines. You should look for a switch mode power supply (SMPS). These power supplies may be inside the LED bulbs or if the lamps are fed with 12V in a small box which is connected to the mains.
Maybe your "doorbell transformer" is this box then there is a SMPS inside which causes the noise.

Wolfgang

Reason: No reason
SDRplay (RSP1, RSP1A, RSP2 pro) with:
RFsystems DX-10 (14kHz to 50MHz), ICOM Discone AH8000 (10MHz to 2GHz)
12m above ground

K4ZOT
Posts: 8
Joined: Sat Oct 14, 2017 5:16 am

Re: PC (laptop) power induces significant noise

Postby K4ZOT » Wed Nov 15, 2017 7:30 pm

Thanks - it was a cheap replacement laptop PS. The SDRuno now works great.

Glenn

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vk7jj
Posts: 21
Joined: Thu Mar 02, 2017 9:56 am

Re: PC (laptop) power induces significant noise

Postby vk7jj » Thu Nov 16, 2017 11:27 am

Many thanks in advance for any ideas


Just a thought: Try disconnecting your antenna from the RSP's antenna connector, if the interference bars go away then the interference is being received by your antenna.

If its being received by your antenna then you should read all you can on the subject of eliminating common mode noise.

Here is a very simplistic overview:

The interference generated by the nasty devices in your house and your neighbour's houses appears as an RF signal impressed across a mains "feedline" consisting of mains neutral/ground (which are bonded together but certainly do not present a short circuit to RF) and mains active. Those wires with all that crappy RF on them are connected directly to your house using nice big fat mains wire conductors.

The RF on those wires goes a long way. Compare it with ADSL for example which is an HF signal up to 2MHz and works very nicely even beyond 4 kilometres using an underground twisted pair and a much lower signal level.

The key thing to note about this interference is that it's an RF voltage potential generated against GROUND/NEUTRAL.

If you use a balanced antenna, say a centre fed dipole or a quad loop for example, the RF is NOT a voltage potential against ground, it's the potential difference between the two poles of your dipole. Hence it is "quieter" than an end fed antenna which sees signals as an RF potential between their feed point and ground and the signal you are trying to receive is just another RF voltage mixed in with the crappy interfering RF signals.

Unfortunately even with a balanced antenna, if you use coax as a feed line it forms part of a chain that includes the ground of your RSP, your USB cable, your computer and your power supply and therefore is *directly connected* to MAINS/NEUTRAL/EARTH and thus to all those interfering devices.

So... all the crappy RF - because it exists as a potential difference variously against ground and neutral and mains active - uses your coax braid as an ANTENNA and your braid radiates crappy RF all the way up your mast to your feed point where it is received by your actual antenna.

A well designed common mode choke (or two or three) on your coax braid and/or at your feed point, presents a high impedance to RF on the braid that is at a potential against ground and thus eliminates or at least heavily reduces the interference.

The choke does not affect the "good" RF because the good RF exists as a potential difference between the inner conductor and the braid and therefore does not create an electromagnetic field and thus presents no impedance to your chokes.

Hope that's not too garbled or simplistic, it's meant to do no more than stimulate and stoke a desire to get the real info by searching for quality articles on the subject of common mode RF interference. It's fascinating.

Armed with the right information I was able to reduce my noise by well over 20dB using home made simple inexpensive chokes along with better grounding and feed line practices. I also now mostly use and prefer balanced feed lines, as does every rock and roll band with their audio systems, for exactly the same reason.

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