[RFI] Birdies every 8.5 kHz

[Hare, Ed W1RFI]

"Birdies" can mean any of a number of things. They could be rock-steady carriers, they could be the modulation or modulation sidebands of a digital signal or they could simply be broad peaks in the noise from a switching power supply. Even switchers can range from being almost carriers to just being board peaks of noise every 5 to 100 kHz spacing or so.

It is generally pretty easy to tell switching supplies from digital devices. In a digital device, the signals will generally be rather stable in frequency.  In a switcher, as the device warm up, those carriers or noise peaks will drift.  Also, switchers tend to change frequency with slight power glitches, so you will occasionally hear it burble just a bit when there is voltage peak or dip.

No one that I know of has a data base that could indicate what particular model numbers of devices might have undescribed birdies at certain intervals.  With switching power supplies, the exact free-running frequency of the switching signals does not need to be accurately controlled, so one device could have noise ever 8.5 kHz, while the next might be as low as 8 kHz, for example.   Even if you knew that a Sheetzu Model 123 had noise every 8 kHz, so might an Acme Model 234, and "knowing" the wrong model might be somewhat confusing.

In reality,  even if you did know that it was an Acme 234, if you didn't own one, that is not a very good place to start. What count more is not, "What is it?," but "Where is it?"  Once you know where it is, you can use better techniques than hoping that someone else's information is pointing to the right model number in a neighbor's house.

Most of the time, the problem is in YOUR house, unless you are in an apartment building.  Sometimes, a device in a home connected to the same secondary wiring as your home can be a noise problem. Rarely, it is a device farther away.  

Start with your own house. Using a battery operated receiver, turn off the main breaker to your house. If the noise goes away, turn on your breakers one at a time, and when the noise comes back, look to see that is plugged in.  Almost every device sold today uses a switching supply and some can be noisy. 

I can't emphasize enough how important this step is.  ARRL has dozens of cases where a ham refused to do this test, "knowing" that it could not be in his home, spending hours on the phone, hours asking friends to help and hours scouring the neighborhood, only to discover a battery charger left plugged in somewhere in his garage.

If it is a neighboring device, it may take a bit of detective work to find it.  First, ask yourself what you will do if you find it.  If you would choose to not confront a neighbor to correct the problem, you may choose to just live with the noise, so why find it at all?  If it's not in your house, and you don't want to spend hours tracking it down, or will not approach the neighbor, you can consider other solutions. Sometimes, the noise cancellers such as sold by MFJ can significantly reduce the noise from a single source.  They are trick to set up, and it's a bit painful to have retune them every 10 kHz of so, but that may be a solution for some people.

On HF, DFing loops can sometimes pinpoint the house. I have found houses with 3.5 MHz devices in them, so it is possible to find noises that come from houses that way. Loops don't work well for power-line noises, because the noise comes from the entire line.

If you have a portable receiver with an S meter, you may be able to find the house using it.  You may need an attenuator to reduce the signal a bit.  You want to place a receiver and antenna at a specific short distance from the electrical wiring in any suspect building.  In some neighborhoods, you may be able to get close to an external light or the AC power meter.  If you are 3-feet away from the building wiring, it will be much stronger in the offending house than other houses.   You can now approach the house occupant to see what may be the problem.

Understand that these people will NOT know RF and will not understand the rules. You are telling them that the device they just bought brand new at Wally Depot is being operated in violation of a federal law they don't understand. Diplomacy and empathy for their not knowing the rules is critical.  If you do approach a neighbor, I suggest that you initially describe the problem as an "interesting" problem. They may have radio noise in their own home, and very rarely, a noisy device can represent a failure in progress and may pose a safety hazard. 

The "neighbor" discussion can be the most challenging part of resolving the problem. I can tell you that making statements about your rights, and how they are breaking the law and other similar approaches will NOT result in a neighbor that is willing to talk to you. What is said in those first few seconds sets the tone for all that follows, so make your opening a good one, and be neighborly.

Ed Hare, W1RFI


-----Original Message-----
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Steve
Sent: Monday, July 27, 2015 2:35 PM
To: rfi at contesting.com
Subject: [RFI] Birdies every 8.5 kHz


I have birdies appearing every 8.5 kHz (or so) on 40 through 10 meters on my scope (P3). Anyone have any idea what it might be?
Steve AA4V


Sent from my I-Phone
_______________________________________________
RFI mailing list
RFI at contesting.com
http://lists.contesting.com/mailman/listinfo/rfi