SOLVED: Identifying interference with oscilloscopes and software-defined radio

I run station #1500 and am trying to reduce local RF interference so that my receiver performs better. My station statistics suggest that I am receiving lots of interference, so I have been trying to identify and hopefully eliminate some of them. I live in a row of apartment buildings, so I have many neighbors with lots of  electrical equipment. Finding and fixing everything will be nearly impossible.

There has been much written on interference, particularly this thread:

I previously used my System Blue's web interface for trying to diagnose interference, it was poorly suited for the task. The time-domain signals that it displayed could not give me any frequency information, and also were not useful for tracking the interference over long time periods. Therefore, I am documenting two methods I found to be much more powerful and insightful.

Oscilloscope Method
Earlier in the summer, I was using an oscilloscope to examine the signals that my System Blue was receiving. I soldered the four optional SMA connectors to the PCB (1). These connectors provide access each of the four amplified antenna signals. I connected these signals to my oscilloscope (2) and used the scope's FFT mode to view their frequency spectrum. From this, I was able to identity several spurious frequencies.

After a lengthy process of elimination, I found that one very significant source of interference was the network switch that my receiver was connected to. It was a large 24 port TP-Link Gigabit Ethernet switch, and it was actually conducting the interference through the Ethernet cable into my receiver. I solved this problem by instead connecting my receiver to a port on one of my WiFi routers. Apparently, that router was less noisy than the network switch and did not tend to conduct noise through its Ethernet jacks.

After fixing this, I still had a few sources of interference. I still received a constant signal at 25 kHz and a flickering one at 60 kHz. I tried turning every piece of electrical equipment in my apartment off, but the interference was still there..

Software Defined Radio Method
After much experimentation, I stumbled upon an even better way to troubleshoot the interference. Basically, it involves connecting an inexpensive software-defined radio (SDR) dongle to my receiver, and viewing the received signals on a PC. Similar to the oscilloscope method above, you must install the optional SMA jacks on your System Blue board and connect the dongle to them (3).

I am using an R820T2 RTL-SDR dongle (approx $20). This particular model is capable of receiving between 24 – 1766 MHz, which is obviously not the correct frequency range.

To receive the low-frequency signals that I am interested in, I use the Ham It Up Upconverter. This is connected between the System Blue and the RTL-SDR dongle. It shifts its input signals up in frequency by 125 MHz, where they can then be received by the RTL-SDR dongle. (4)

The final thing required is software to view the signals. I am using GQRX (5). It has an instantaneous frequency display, similar to the FFT plot on my oscilloscope. More importantly, it has a nice waterfall display that allows me to view the received frequencies as they change over time. The time scale on the waterfall can be adjusted from a few seconds to many hours, which is extremely useful.

LED Lighting Causing Interference
Here is a short capture that I took while turning on and off a fixture with six LED light bulbs. The timescale of the waterfall is about 30 seconds. The area circled in red, just above 70 kHz, shows emissions that appeared each time I switched the bulbs on. Perhaps I should find different bulbs, or switch to incandescent.
[Image: HrWa0it.png]

Long Time Spans
Here is a very long capture that I made starting late at night. In this case, the time scale on the waterfall is 10 hours! Notice the increased emissions near the top of the waterfall. These appear at about 8:00 and 9:20 AM. One might suspect that they might be caused by my neighbors switching on electrical devices after getting up in the morning. I was still sleeping at the time, so I could not have caused them.
[Image: 9xCkKwH.png]
Also note that there are significant emissions at 25 kHz and 60 kHz. These are the signals that I originally identified using my oscilloscope! They seem to be present all the time.

I still have not found the source of my two main interfering signals, but in the process of searching for them I think I developed some useful and powerful methods for investigating interference. Hopefully others will find this information useful.

If anybody has questions or wants clarifications on anything I discussed, please ask. I'll try to post more screenshots if I find any other interesting interfering signals.


(1): Mentioned in the "Digital filter option" section of the assembly instructions
(2): Be sure to switch your station to fixed gains, not "automatic mode" if you want to make measurements, otherwise you will see sudden changes in signal amplitude whenever the gain is automatically adjusted by the receiver's microcontroller.
(3): I drilled an extra hole into my receiver's case, then installed a small SMA cable from the connector on the PCB out to the outside of the case. This gives me a convenient connection for my experimentation without the need to open the case.
(4): Software-defined radio receivers have become extremely popular in the past several years, and many different models are available.  Other models should certainly be suitable, and some are able to receive signals below 1 MHz without the need for a separate up-converter.
(5): Similar programs exist for Windows, for example PothosSDR

Messages In This Thread
SOLVED: Identifying interference with oscilloscopes and software-defined radio - by djhuft - 2017-08-06, 20:02

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