Oscilloscope doubles as radio station! - Page 1
Forum section: electronics / beginners

author: testtube44
author_rank: Regular Contributor
timestamp: February 07, 2019, 12:33:19 am
content: Last year I got the DSO5202P 200Mhz digital oscilloscope as a present for finishing the school year. I like it alot, and am excited to learn how to use it. But recently I discovered a strange issue. I have an old alarm clock (90s) that has an AM and FM radio built in, today I was doing some experiments with it and discovered that the radio background goes dead silent at somewhere around 108Mhz when I turn it on. When I tuned slightly up or down from there, a very loud buzzing is heard emitting from the scope. This is a big deal and the field extends across my entire room, the damn thing is a radio station! Do I have a defective, is this normal? Is there anything I can do to reduce this noise?

author: james_s
author_rank: Super Contributor
timestamp: February 07, 2019, 01:40:03 am
content: It's not uncommon with cheap generic Chinese gadgets, but I'm surprised a branded unit like that is spewing so much RFI.

author: Rerouter
author_rank: Super Contributor
timestamp: February 07, 2019, 01:44:59 am
content: Best bet, Contact the manufacturer, if its not normal, you may be able to get it replaced.If no luck there, well 200MHz scope, 108MHz signal, And I suspect either Power supply, or PLL

author: testtube44
author_rank: Regular Contributor
timestamp: February 07, 2019, 03:25:43 am
content: What should I do now? If I can't get it replaced or its normal is there anyway to stop it radiating?

author: OwO
author_rank: Super Contributor
timestamp: February 07, 2019, 03:46:57 am
content: It's normal. A single microstrip transmission line of a few cm will leak enough RFI to be picked up by FM receivers in half of the house. A FCC part 15 radio transmitter will go farther than this.

author: james_s
author_rank: Super Contributor
timestamp: February 07, 2019, 04:52:50 pm
content: This sort of thing should be shielded though, none of my gear causes interference on radios across the room.

author: wolfp
author_rank: Contributor
timestamp: February 07, 2019, 05:46:23 pm
content: Many electronic devices have radiated or conducted emissions. These are caused e.g. by switch mode power supplies (mostly conducted emission) or clock frequencies of uP (radiated). A conducted emission can become a radiated one and vice versa. Maybe your oscilloscope does not fullfill the EMC-requirements.

author: testtube44
author_rank: Regular Contributor
timestamp: February 07, 2019, 07:48:34 pm
content: I have no idea what I can do with it, or if there IS anything that can be done. I don't think any of my power supplies for anything else in the room creates noise quite like that. I'm also not sure it would be safe for me to crack it open and try to shield it myself, even if directed by a professional; I'm totally new to electronics and don't want to end up dead. So I'm in a pickle, it's not good to have an oscilloscope that interferes with itself or any prototype electronics I may want to work on.

author: max_torque
author_rank: Super Contributor
timestamp: February 07, 2019, 07:55:09 pm
content: When i'm bored i occasionally listen to the radio on my Spectrum Analyser (using the real time FM demodulator)

author: Ian.M
author_rank: Super Contributor
timestamp: February 07, 2019, 08:13:11 pm
content: Start looking for a second hand pure analog CRO in good condition. 50 or 100 MHz bandwidth, dual channel, delay timebase. If you can find one with a linear PSU, and no digital readouts, it will emit very little EMI. Don't get one with a storage tube as they've nearly all aged to the point the storage mode is unusable. Ask here for advice on specific models before buying to avoid known lemons.You'll still want to keep and use your DSO because its far more versatile and convenient, but if you need low EMI an old-skool CRO is the way to go if you don't have a five figure budget. However once you start chasing the elusive low EMI environment, you'll find just about everything in modern life is problematic, from the LED or CFL lightbulbs overhead to the SMPSU wallwart down on the skirting board, including the phone in your pocket. If you live in a dense urban area, its pretty much hopeless due to all your neighbours gadgets.

author: viperidae
author_rank: Frequent Contributor
timestamp: February 07, 2019, 09:46:23 pm
content: It doesn't take much transmission power to pick up a signal on an FM radio in the same room.Here's a circuit that consumes ~200uW and transmits "up to 50m"https://makezine.com/projects/make-tiny-fm-transmitter-spying/

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 07, 2019, 10:11:20 pm
content: Does it change if you change sample rates, memory depth, time base, number of channels, etc.?

author: Rerouter
author_rank: Super Contributor
timestamp: February 08, 2019, 11:23:58 am
content: What Tomorokoshi Is trying to pin down is if its the power supply or not, Another thing could be sticking a phone or something on the front USB jack to pull some current.If its the power supply you may be able to kill it by fitting a clip on ferrite bead near where the power cord exits the device,If its the PLL or some sampling clock that changes when you change your time scales, see if there are any firmware updates, they may have corrected it.

author: xavier60
author_rank: Super Contributor
timestamp: February 08, 2019, 11:43:22 am
content: I had a troublesome amount of hash getting into measurements with my HP 54645.When I eventually suspected the caused being its own SMPS, I confirmed it by varying the main voltage to it and noticed a corresponding shift in time between the busts of damped oscillation that were making up the hash.