Borrow a spectrum analyzer and you will see each station.
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On Friday, April 9, 2021, 10:55:04 PM PDT, Dave Peterson via groups.io <davidpinsf@...> wrote:
I guess I expected the different stations to be more distinct, but it is hard to nail down the exact frequency when looking closely. Is it the mix that's causing the consistent beat pattern? Not really modulation (Frequency). I guess that makes more sense. Like out of tune musical instruments.
Dave
? ? On Friday, April 9, 2021, 10:29:34 PM PDT, Tom Lee <tomlee@...> wrote:?
Hi Dave,
You gave the answer yourself: Sutro Tower, and "100MHz". That's FM
broadcast. No point in turning off WiFi, etc. in search of something
else. You already found it!
--Cheers,
Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
On 4/9/2021 21:34, Dave Peterson via groups.io wrote:
Could use some help here:
I've always had persistent high frequency noise on my scopes. So far I've been focused on lower frequency signals with reasonable signal strength. Between BW Limit turned on, and HF Rej on the trigger coupling I've been able to live with it. But now I need to calibrate some vertical assemblies, and it's become untenable.
In exasperation trying to find the source I've tied the scope input to ground, only to find an even stronger noise signal! See the pictures in:
/g/TekScopes/album?id=262825
This is about a 100MHz signal. I also have to say I happen to live about 2000ft. away from Sutro Tower in San Francisco: .
I've turned off all manner of potential sources, short of the household Wi-Fi. Can't do that with teenagers in the house! Could the wireless router really be the source of ground noise? I've also tried relocating - the pictures are taken from my living room. It's the same no matter where I go, or what equipment is on or near. This only comes from the scope inputs being tied to ground, or a floating probe, or on a DUT. It's very intermittent, variable, and dynamic when not grounded. Selecting GND on the input does suppress the noise, but clipping a 10x probe to the chassis yields the noise again. It's consistent across scopes, both 2236's and 465s.
I have a pretty well grounded house. When we had our service upgraded the electrician drove a grounding post at the service entrance, and we have good ground straps throughout. I've also run a 4 gauge (0.2") ground line to my work bench and it doesn't much make any difference if I ground the scope to the bench or not. The supply ground and bench ground measure 0 ohms as well as my DMM can tell.
Is this really RF energy coming from Sutro Tower? And why so specific a frequency? I see no other signals. The modulation is pretty obvious, but it's not dynamic. But who knows what all is coming out of that collection of antennae.
And is there anything I can do to mitigate it?
Thanks for whatever help the collective can provide.
Dave
