Author Topic: I see 99MHz FM radio on my scopes  (Read 1391 times)

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Offline lordvader88

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I see 99MHz FM radio on my scopes
« on: February 26, 2018, 07:42:28 pm »
While making a regenerative receiver and using a tek2215, I realized that what I thought was just noise on the scope at 50ns was actually the strong local 99MHz FM station.

I'm pretty new to scopes, And just looking at my DSO tek2430A on 20ns, I see the 99MHz FM there too. I always just thought that was the scope itself, noise/jitter and and lack of accuracy as I zoom in towards 20mV and 5ns.

So right now on it I have about 80-100mVpp @99MHz with a cheap probe in the air, is there any easy way I'm supposed to block that out ? Maybe a knob inside for blocking strong radio stations ?????

Or do u just ignore it ?
 

Offline CNe7532294

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Re: I see 99MHz FM radio on my scopes
« Reply #1 on: February 26, 2018, 09:19:35 pm »
A picture helps greatly to see exactly what you are doing. |O As for your issue, what are your probing techniques? "cheap probe in the air" If you're using long leads, of course they'll pick up signals like an antenna. Try probing with shorter leads and watch the signal pick up minimize. See picture here (short lead to use). Hope this helps.

« Last Edit: February 26, 2018, 09:23:02 pm by CNe7532294 »
 

Online chrisl

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Re: I see 99MHz FM radio on my scopes
« Reply #2 on: February 26, 2018, 09:43:11 pm »
If you do not need the full BW you can turn on and limit the BANDWIDTH to 20MHz to block the interference.
 

Online T3sl4co1l

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Re: I see 99MHz FM radio on my scopes
« Reply #3 on: February 26, 2018, 10:06:14 pm »
Good probing helps, and shielding helps.

There isn't much you can do to eliminate a strong local station, heroic actions aside (shielded chamber).

What is it, local college station?  Commercial transmitters aren't usually nearby.  I remember about 50mV worth of the college station picked up by the banana jack "probes" in the EE lab, that was a 100W transmitter (I think) and 1 city block distance.

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Need engineering assistance? Drop me a message!
 

Offline donmr

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Re: I see 99MHz FM radio on my scopes
« Reply #4 on: February 27, 2018, 02:48:35 am »
Certainly proper probing and perhaps chokes on all incoming power cables is the first step.

Someone I know had a lab very close to a major FM and TV antenna.  It was associated with an institution that couldn't move.  So they had to build a shielded room in the basement.
 

Offline lordvader88

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Re: I see 99MHz FM radio on my scopes
« Reply #5 on: February 27, 2018, 03:48:52 am »
Well it's about a 0.5m coax with 3-4" leads/ alligator clips on the end.

OK with the probes unhooked tho it goes away, or no signal is displayed at all, just checked that.

I'm in a little city and the tower is within 5-10km I think.
 

Offline w2aew

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Re: I see 99MHz FM radio on my scopes
« Reply #6 on: February 27, 2018, 11:40:24 am »
Interesting story - the 20MHz BW limit that is present on nearly all oscilloscopes today was actually originally designed into the 7A11 vertical plug-in for the 7000 series scopes.  The designer of that plug-in told me that he put the BW limit circuit in there because during the development he was getting significant interference at 54MHz from the local VHF TV Channel 2 broadcast transmitter.  It's been part of nearly all analog scopes since then.
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Online edpalmer42

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Re: I see 99MHz FM radio on my scopes
« Reply #7 on: February 27, 2018, 12:09:49 pm »
I've got three FM stations within nine miles of me that are each putting out 100KW of power.  It doesn't matter what probing technique you use.  If you're looking for low level signals, you have to use the 20 MHz filter.  Otherwise, every circuit you work on has 100 MHz noise!  I also make a point of using double-shielded coax for everything - preferably foil and braid rather than two braids.

If you want to find out what your local situation is, go to fmfool.com and put in your postal code.  They say zip code, but it works for us, too.  If you want to check the local situation for tv stations, go to tvfool.com.

Ed
 

Online rhb

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Re: I see 99MHz FM radio on my scopes
« Reply #8 on: February 27, 2018, 12:42:59 pm »
I once measured something over 1 V of RF on the input to my scope.  Measurement *very* suspect as it was a 5 MHz Heathkit.  But the nearby FM station was so strong that if I held my finger 1/2" from the inputs of a 120dB DC coupled amplifier I built from an ARRL schematic I could listen to the FM station.

Ever since I have had a lust for a screen room which I still hope to satisfy.
 

Offline Audioguru

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Re: I see 99MHz FM radio on my scopes
« Reply #9 on: March 02, 2018, 11:21:55 am »
CKFM on 99.9MHz in Toronto is very powerful and its antenna is up high. They blast out a lot of power because they are paid by advertisers by the number of listeners they have.
All the other FM stations in Toronto are also very powerful.
 

Offline Kalvin

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Re: I see 99MHz FM radio on my scopes
« Reply #10 on: March 02, 2018, 10:51:52 pm »
I once measured something over 1 V of RF on the input to my scope.  Measurement *very* suspect as it was a 5 MHz Heathkit.  But the nearby FM station was so strong that if I held my finger 1/2" from the inputs of a 120dB DC coupled amplifier I built from an ARRL schematic I could listen to the FM station.

Ever since I have had a lust for a screen room which I still hope to satisfy.

Nice, black conductive paint job?
https://www.lessemf.com/paint.html
https://emfacademy.com/yshield-emf-paint-review-does-it-work/
 

Offline CNe7532294

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Re: I see 99MHz FM radio on my scopes
« Reply #11 on: March 04, 2018, 02:39:06 am »
I once measured something over 1 V of RF on the input to my scope.  Measurement *very* suspect as it was a 5 MHz Heathkit.  But the nearby FM station was so strong that if I held my finger 1/2" from the inputs of a 120dB DC coupled amplifier I built from an ARRL schematic I could listen to the FM station.

Ever since I have had a lust for a screen room which I still hope to satisfy.

Nice, black conductive paint job?
https://www.lessemf.com/paint.html
https://emfacademy.com/yshield-emf-paint-review-does-it-work/

Interesting. Now I've just realized why a broadcast room I was in (a decade ago) was painted all black. Every single crack. I still don't know what they did with the floors of any upper floor rooms though (screen/foil used here instead?).
 

Offline james_s

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Re: I see 99MHz FM radio on my scopes
« Reply #12 on: March 09, 2018, 01:45:00 pm »
I once lived across the valley from a powerful FM station. On most FM radios the crappy dance music would come through at 5 or 6 positions across the dial and it also created noise on the scope. I've heard of people using a metal cookie tin as a shielded enclosure for the device under test. You could also build a Faraday cage around your whole bench I suppose.
 

Offline Gary350z

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Re: I see 99MHz FM radio on my scopes
« Reply #13 on: March 09, 2018, 09:52:36 pm »
 

Offline FrankE

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Re: I see 99MHz FM radio on my scopes
« Reply #14 on: March 10, 2018, 08:21:30 am »
When I was doing a DTMF project at university I tried probing the DTMF chip on my home telephone and it played Radio Scotland. I can't recall what frequency the station was on then. I assume on AM..
 


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