noise floor

4 Articles

Putting The Magic Smoke Back Into A Dodgy Spectrum Analyzer

September 12, 2023 by 3 Comments

The trouble with fixing electronics is that most devices are just black boxes — literally. Tear it down, look inside, but it usually doesn’t matter — all you see are black epoxy blobs, taunting you with the fact that one or more of them are dead with no external indication of the culprit.

Sometimes, though, you get lucky, as [FeedbackLoop] did with this Rigol spectrum analyzer fix. The instrument powered up and sort of worked, but the noise floor was unacceptably high. Even before opening it up, there was clearly a problem; in general, spectrum analyzers shouldn’t rattle. Upon teardown, it was clear that someone had been inside before and got reassembly wrong, with a loose fastener and some obviously shorted components to show for it. But while the scorched remains of components made a great place to start diagnosis, it doesn’t mean the fix was going to be easy.

Figuring out the values of the nuked components required a little detective work. The blast zone seemed to once hold a couple of resistors, a capacitor, a set of PIN diodes, and a couple of tiny inductors. Also nearby were a pair of chips, sadly with the markings lasered off. With some online snooping and a little bit of common sense, [FeedbackLoop] was able to come up with plausible values for most of these — even the chips, which turned out to be HMC221 RF switches.

Cleaning up the board was a bit of a chore — the shorted components left quite a crater in the board, which was filled with CA glue, and a bunch of missing pads. This called for some SMD soldering heroics, which sadly didn’t fix the noise problem. Replacing the two RF switches and the PIN diodes seemed to fix the problem, albeit at the cost of some loss. Sometimes, good enough is good enough.

This isn’t the first time [FeedbackLoop] has gotten lucky with choice test equipment in need of repairs — this memory module transplant on a scopemeter comes to mind.

Continue reading “Putting The Magic Smoke Back Into A Dodgy Spectrum Analyzer” →

Tracking Down Radio Frequency Noise Source, With Help From Mother Nature

September 25, 2020 by 18 Comments

Amateur radio operators and shortwave listeners have a common enemy: QRM, which is ham-speak for radio frequency interference caused by man-made sources. Indiscriminate, often broadband in nature, and annoying as hell, QRM spews forth from all kinds of sources, and can be difficult to locate and fix.

But [Emilio Ruiz], an operator from Mexico, got a little help from Mother Nature recently in his quest to lower his noise floor. Having suffered from a really annoying blast of RFI across wide swaths of the radio spectrum for months, a summer thunderstorm delivered a blessing in disguise: a power outage. Hooking his rig up to a battery — all good operators are ready to switch to battery power at a moment’s notice — he was greeted by blessed relief from all that noise. Whatever had caused the problem was obviously now offline.

Rather than waste the quiet time on searching down the culprit, [Emilio] worked the bands until the power returned, and with it the noise. He killed the main breaker in the house and found that the noise abated, leading him on a search of the premises with a portable shortwave receiver. The culprit? Unsurprisingly, it was a cheap laptop power supply. [Emilio] found that the switch-mode brick was spewing RFI over a 200-meter radius; a dissection revealed that the “ferrite beads” intended to suppress RFI emissions were in fact just molded plastic fakes, and that the cord they supposedly protected was completely unshielded.

We applaud [Emilio]’s sleuthing for the inspiration it gives to hunt down our own noise-floor raising sources. It kind of reminds us of a similar effort by [Josh (KI6NAZ)] a while back.

The RFI Hunter: Looking For Noise In All The Wrong Places

January 15, 2020 by 21 Comments

Next time you get a new device and excitedly unwrap its little poly-wrapped power supply, remember this: for every switch-mode power supply you plug in, an amateur radio operator sheds a tear. A noisy, broadband, harmonic-laden tear.

The degree to which this fact disturbs you very much depends upon which side of the mic you’re on, but radio-frequency interference, or RFI, is something we should all at least be aware of. [Josh (KI6NAZ)] is keenly aware of RFI in his ham shack, but rather than curse the ever-rising noise floor he’s come up with some helpful tips for hunting down and eliminating it – or at least reducing its impact.

Attacking the problem begins with locating the sources of RFI, for which [Josh] used the classic “one-circuit-at-a-time” approach – kill every breaker in the panel and monitor the noise floor while flipping each breaker back on. This should at least give you a rough idea of where the offending devices are in your house. From there, [Josh] used a small shortwave receiver to locate problem areas, like the refrigerator, the clothes dryer, and his shack PC. The family flat-screen TV proved to be quite noisy too. Remediation techniques include wrapping every power cord and cable around toroids or clamping ferrite cores around them, both on the offending devices and in the shack. He even went so far as to add a line filter to the dryer to clamp down on its unwanted interference.

Judging by his waterfall displays, [Josh]’s efforts paid off, bringing his noise floor down from S5 to S1 or so. It’s too bad he had to take matters into his own hands – it’s not like the FCC and other spectrum watchdogs don’t know there’s a problem, after all.

Continue reading “The RFI Hunter: Looking For Noise In All The Wrong Places” →

RF Noise Floor Concerns From Both Sides Of The Atlantic

July 20, 2017 by 47 Comments

Our feed is full of stories about the RF noise floor today, and with good reason. The ARRL reports on the International Amateur Radio Union Region 1 president, [Don Beattie, G3BJ] warning that in densely populated parts of Europe there is a danger that parts of the RF spectrum have become so swamped with noise as to be rendered unusable, while on the other side of the Atlantic we have RadioWorld reporting on similar problems facing AM broadcasting in the USA.

At issue are the usual suspects, interference from poorly shielded or suppressed domestic electronic devices, VDSL broadband, power-over-Ethernet, solar and wind power systems, and a host of other RF-spewing electronics. The combined emissions from all these sources have raised the noise level at some frequencies to the point at which it conceals all but the strongest signals. Any radio amateur will tell you that a station in a rural location will be electrically much quieter than one in a city, it seems that this effect has now reached a crescendo.

In the RadioWorld article, the author [Tom F. King] and his collaborator [Jack Sellmeyer] detail a series of tests they performed on a selection of lighting products from a quality brand, bought at a local Home Depot store. They were gathering data for a submission to the FCC enquiry on the noise floor issue we reported on last year. What they found was unsurprising, significant emissions from all the products they tested. They make some stiff recommendations to the FCC and other bodies concerned with radio spectrum to get tough with offending devices, to stay on top of future developments, and for operators of AM stations to pursue sources of interference.

It could be that there is so much equipment contributing to the noise floor that this battle is lost, but it doesn’t have to be this way. Anyone who has had to prepare a product to pass a properly carried out EMC test will tell you that the requirements are stringent, and it is thus obvious that many manufacturers are shipping products unworthy of the certification they display. It is to be hoped that the authorities will begin to take it seriously before it becomes an order of magnitude worse.

Compliance label image, Moppet65535 [CC BY-SA 3.0].