RF noise on ground


Dave Peterson
 

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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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


Bob Albert
 

Looks like a local broadcast station.

On Friday, April 9, 2021, 09:34:41 PM PDT, Dave Peterson via groups.io <davidpinsf=yahoo.com@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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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


cmjones01
 

Yes, it's very likely that the noise is from the Sutro tower. It has FM
transmitters on it.

I used to have an office a similar distance (about 2500ft according to
Google Maps) from the Palace of Culture and Science in Warsaw (
https://en.m.wikipedia.org/wiki/Palace_of_Culture_and_Science) which also
has big FM transmitters on the top. I could see the antennas out of the
window. The RF got in to everything. Indeed, if I connected an amplifier
and speaker to the "video out" terminal on my 7L12 spectrum analyser, I
could listen to the radio stations. It was a massive pain when doing
high-bandwidth work.

The reason the frequency is so specific is that the stations are all in the
FM broadcast band, 88-108MHz. Looking at the Wikipedia page for the Sutro
tower, its transmitters seem to be concentrated in the range 95-100MHz.

I think no amount of grounding, short of building an EMC-shielded chamber,
will help. The signals are there in the air at high field strength and will
induce currents in every loop of wire they can find.

Chris

On Sat, 10 Apr 2021, 06:34 Dave Peterson via groups.io, <davidpinsf=
yahoo.com@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:

https://groups.io/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:
https://en.wikipedia.org/wiki/Sutro_Tower.

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






Tom Lee
 

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
http://www-smirc.stanford.edu

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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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




Dave Peterson
 

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@ee.stanford.edu> 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
http://www-smirc.stanford.edu

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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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





Bob Albert
 

Borrow a spectrum analyzer and you will see each station.

On Friday, April 9, 2021, 10:55:04 PM PDT, Dave Peterson via groups.io <davidpinsf=yahoo.com@groups.io> 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@ee.stanford.edu> 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
http://www-smirc.stanford.edu

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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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





Dave Peterson
 

I have a TinySA on the way in the mail. That's exactly what I'm going to do!
Dave

On Friday, April 9, 2021, 10:59:31 PM PDT, Bob Albert via groups.io <bob91343=yahoo.com@groups.io> wrote:

Borrow a spectrum analyzer and you will see each station.
    On Friday, April 9, 2021, 10:55:04 PM PDT, Dave Peterson via groups.io <davidpinsf=yahoo.com@groups.io> 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@ee.stanford.edu> 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
http://www-smirc.stanford.edu

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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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





 

First, I see something very similar on my 475s here on the opposite coast. I do live just north of Washington DC with a clear view of "Broadcast Hill" in Tenleytown to the south, and (until they were demoed a year or two ago) the WTOP towers five miles to the north west in Wheaton, MD. I haven't done nearly enough work to figure out what is causing the HF interference on my scopes now, but years ago (mid-1980s) when I was fooling around with an audio synthesizer kit from Radio Shack, adding RC networks to various pins to get different frequencies or envelopes to the synthesized audio. At some point, to my surprise, the circuit started to speak to me! It wasn't until it gave a station identification that I realized what was happening: it doesn't take much to receive a local, megawatt AM station.

Similarly, a few years later, in community college, I worked at the student run radio station. Our transmitter had broken a year before I joined the station, and we "transmitted" through the campus AC power. You could listen to the station in your car at 650 KHz AM in the parking lots (the light poles made good antennas), and in the student union through the PA system. Each time we tried to bring the real transmitter back on line, however, we were swamped by a transmission tower just across the street from campus.

I wasn't really involved in fixing the transmitter, other than manning the studio during a couple of attempts, so I've got no idea what was actually wrong, or what was tried to fix it, but the power of a local transmission tower made a lasting impression on me.

-- Jeff Dutky


Arne Buck
 

Dave,
As others have replied, no doubt FM broadcast from the Sutro Tower. Unreliable sources (Wikipaedia) indicate there are at least 4 FM broadcast stations 96.5 to 104.5 MHz transmitting from there. Most everything else is television. (I remember the Chronicle columnist, Herb Caen, also complaining about the Sutro Tower. He proposed a sci-fi story whereby the Tower marches down Mt. Sutro and mates with the Transamerica Pyramid, another of his architectural objections, along with the Jack Tar Hotel (R.I.P.), "The box Disneyland came in." So you're in San Francisco, eh? Could be worse; at least you don't shovel snow!)

Anyway, I've had similar RFI problems, with limited results as to their solution. I worked in Downtown Boston MA, within sight of Emerson College's WERS (88.9MHz) Ansin Building and broadcast antenna, one mile away. Best solution to attenuate the ~90 MHz blip was to move to a lab on the other side of our building. Otherwise I could tone it down by looping the 'scope probe cable a few turns around a ferrite torroid. Not perfect, but good enough for most of what needed doing. The equipment I was measuring was decently shielded and tests indicated the radio station (confirmed with a R&S spectrum analyzer with FM demodulator) was getting in via the probe, not the 'scope vertical amplifier.

A tougher problem was in Waltham MA. We were not 500 feet from a clear-channel (50 kW) AM station antenna at 1010 kHz. That signal was so strong I didn't need a spectrum analyzer to figure out the source. All I needed to do was tune my ancient Viscount 6-Transistor radio to the station and watch the amplitude of the noise on the CRT. The louder Glenn Beck shouted, the modulation grew in amplitude accordingly. Ultimately (there was no "other side of the building" I could use), I build a little Faraday cage for my test setup, incorporated the ferrite, fiddled around with the geometry of the 'scope probe on the bench, used my TEK 7854 with the Option 3 EMC modification, assuming that worked both ways. Again not perfect (50 kW!), but it sufficed.


Leon Robinson
 

Dave

Tell your kids that the wifi will be off at a certain time and be off for xx minutes,  they will not die, you are the boss.


Leon Robinson  K5JLR

-------- Original message --------
From: "Dave Peterson via groups.io" <davidpinsf=yahoo.com@groups.io>
Date: 04/09/2021 11:34 PM (GMT-06:00)
To: TekScopes@groups.io
Subject: [TekScopes] RF noise on ground

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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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


Roy Thistle
 

On Fri, Apr 9, 2021 at 10:59 PM, Bob Albert wrote:


Borrow a spectrum analyzer and you will see each station.
I was able to use an FM radio, probe, and Internet streaming to identify FM stations on noise. No need for a spectrum analyser... even though we have them.

--
Roy Thistle


Richard Loken
 

In my teens I live 10km from the CBK transmitter: 540KHz 50KW clear channel omnidirecional... I had an end fed antenna in the back yard of no more
than 30m in length.

I built a cheap Heathkit GR64 shortwave receiver but it didn't work so I started touching the end of the antenna on various points moving backward from the detector until the CBK audio went away. The problem was a shorted
trimmer capacitor in the front end RF amp/mixer stage.

I had no interest in what CBK broadcast but it was so powerful that I could pick it up on anything so I developed a taste for opera inspite of my best efforts.
--
Richard Loken VE6BSV : "...underneath those tuques we wear,
Athabasca, Alberta Canada : our heads are naked!"
** rlloken@telus.net ** : - Arthur Black


Roy Thistle
 

On Sat, Apr 10, 2021 at 07:55 AM, Arne Buck wrote:


I build a little Faraday cage for my test setup
Just generally (perhaps not specifically for calibrating verticals) you can move to smaller but cheaper (if not as accurate, and as low a noise floor) like the TinySA, NanoVNA, et. al.
They also generate data that can be processed. They are small, so smaller cages can be used... and they generate data, so processing can be used.
Paint cans, bread boxes, even cardboard boxes with Al foil, or conductive polymer films (even used large anti-static bags) can bodge up an approximation to a Faraday cage. (No one can afford to put up a copper screen Faraday cage... and the copper hounds got all the ones here, in the colonies, long ago.)
Or... you could move to West Virginia into the NRQZ.

--
Roy Thistle


Leon Robinson
 

Back in my early teens we lived on s slight hill and could see the State Police tower red lights on another hill 2-3 miles and could hear them on my homemade crystal radio and they were on 30 megacycles, yes early 1950s.

Leon Robinson  K5JLR

-------- Original message --------
From: Richard Loken <rlloken@telus.net>
Date: 04/10/2021 11:15 AM (GMT-06:00)
To: TekScopes@groups.io
Subject: Re: [TekScopes] RF noise on ground

In my teens I live 10km from the CBK transmitter:  540KHz 50KW clear channel
omnidirecional...  I had an end fed antenna in the back yard of no more
than 30m in length.

I built a cheap Heathkit GR64 shortwave receiver but it didn't work so I
started touching the end of the antenna on various points moving backward
from the detector until the CBK audio went away.  The problem was a shorted
trimmer capacitor in the front end RF amp/mixer stage.

I had no interest in what CBK broadcast but it was so powerful that I could
pick it up on anything so I developed a taste for opera inspite of my best
efforts.
--
   Richard Loken VE6BSV           : "...underneath those tuques we wear,
   Athabasca, Alberta Canada       : our heads are naked!"
   ** rlloken@telus.net **         :    - Arthur Black


Dave Peterson
 

Thanks everyone for their inputs.

Keep the stories coming. They're great!

I was pretty punchy last night and probably wasn't saying what I was trying to very well. I think what was throwing me was the apparent modulation that can be seen. After folks mentioned I was seeing all the stations it dawned on me what that is. It's merely the sum of all the carriers. These are FM stations, so any actual modulation on the carriers is going to be imperceptible frequency shifting in time domain, not amplitude. I'll have to study the tinySA instructions and specs more, and play with it when it gets here, to see how much frequency resolution I'll be able to make out. Anyway, the amplitude modulation is the "beating" of the combined slightly different carrier frequencies. Like instruments out of tune. And thus the stability of the pattern.

Makes me realize how selective even cheap mass production radio receivers are.

Sutro Tower is one of those things that can be seen in good and bad light. I find its history a fairly classic example of powers that be having their way with things. There's an interesting story of news folks going to check out the goings on at Sutro's old mansion on the hill in '72, only to be chassed away and chastised by their management. This was _not_ supposed to be a news story.

It is something of an iconic landmark. I've used it as an entertaining photography subject: https://www.flickr.com/photos/davidpinsf/

My neighbor is a retired CBS cameraman and producer. He got my family a tour of the facility by the general manager. We didn't go up the tower, but did to get to see a lot of the massive RF equipment feeding it. There's one room that you can go into that is the feed to all the waveguides going up the tower. Kind of creepy to be standing in such a small space with that much power. The primary sources are feeding those three, now digital, television antennae at the top. The radio stuff is small potatoes.

I find it something of a nuisance for all the noise always coming from it. It's constantly being worked on. They recently removed all the panels covering the legs. Months and months of power tools cutting and rattling on it. I count myself lucky. I don't have a $5M view at the top of the hill with that thing right overheard. Paint chips, welding slag, all manner of crap still falls despite all the assurances that it doesn't happen. And on the other hand I always find it fascinating to study. I look at it every night when taking the dog out for a piddle. It's often a sight to behold against a starry backdrop, watching ISS float by it on occasion (haven't managed to get a good picture of that yet), and sometimes playing peak-a-boo through the fog.

It's a sight to behold, but this noise on the scopes is yet another annoyance. I always suspected it was the source, but thought good shielded cabling would minimize it better than it is. It's even quite apparent with a good 10x probe with a short ground lead. I wish there were more mitigation methods available. Another aspect that's funny is that the human body makes a good antenna. It's rather frustrating to try to trigger a weak signal and have my hand being on/near the trigger level change the triggering of whatever signal. Sometimes I do have to hold my hand by the trigger knob while observing what I'm trying to test.

But better to know the cause. I'll just learn ways to work around it.

Dave


J Hunt
 

Dave,
I visited Smith-Kettlewell Eye Research Institute in SF around 1974 for some collaborative work.  Google maps shows them about 2.5 miles from the Sutro Tower that is your RF ground noise nemesis.  One of the EEs there showed me a small hand-wired board he built that consisted of maybe ten 4000 series CMOS logic ICs.  He was puzzled that it ran (though eratically) without the battery connected.  It turns out that the input protection diodes on the gates along with the short and tidy wiring were acting as crystal sets and providing enough Vcc and current to run the circuit (depending on the level of modulation from the nearby radio stations).  Mounting the board in an aluminum project box solved the problem, but he did need to install a battery.

John HuntPortland, OR


Eric Schumacher
 

Hello Dave

I have been skipping over this thread, assuming that it concerned dealing with "ground bounce" on MSI logic chips on a buss. It’s a synchronous logic thing from the days of 54/74 TTL logic. On closer reading it turns out that your noise issue might have more to do with kissing my GF while standing under high tension transmission lines. We were camped in the desert on a knoll along a DWP service road for the power line. When you stand under a transmission line your head is a tap on a voltage divider that exists between the HT line overhead and the ground you are standing on. What does this have to do with scopes you are asking? At home I lived a mile or so from KGIL 1260 and KMPC 1130 khz and 50 kw days. My scope probe tips (witches hats) obviously were a voltage divider tap between KMPC and my cement slab. Good for several hundred millivolts on my 465M. My mitigation technique was to cover the bottom of my bench top with aluminum foil connected to a ground post next to where the 465 sat. Jumper the ground to 465 ground and waala that tap moved a lot closer to ground. Audio circuits on perf-board looked much cleaner. Your luck may vary but freezer foil is pretty cheap.

Eric WB6KCN

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dave Peterson via groups.io
Sent: Friday, April 9, 2021 9:34 PM
To: TekScopes@groups.io
Subject: [TekScopes] RF noise on ground

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:

https://groups.io/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: https://en.wikipedia.org/wiki/Sutro_Tower.

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


Leon Robinson
 

When I worked at Curtis Mathes
TV plant in Benton AR in late 1963 early 1964 they had a homemade screen room. 

8 ft on all sides.  Each section was 2x4 framework covered on both sides with 1/2 inch hardware cloth, all joints soldered, the floor section had plywood.  The door same as the walls.  For a door gasket they used brass width control seleves from the TV line folded double and stapeled around the opening.  Light was a fixture laid on top.

Power was brought in through a large CorCom filter.  Bench was more plywood.  It was all bolted together from the outside. 

Worked well


Leon Robinson  K5JLR

-------- Original message --------
From: "Dave Peterson via groups.io" <davidpinsf=yahoo.com@groups.io>
Date: 04/10/2021 12:05 PM (GMT-06:00)
To: TekScopes@groups.io
Subject: Re: [TekScopes] RF noise on ground

Thanks everyone for their inputs.

Keep the stories coming. They're great!

I was pretty punchy last night and probably wasn't saying what I was trying to very well. I think what was throwing me was the apparent modulation that can be seen. After folks mentioned I was seeing all the stations it dawned on me what that is. It's merely the sum of all the carriers. These are FM stations, so any actual modulation on the carriers is going to be imperceptible frequency shifting in time domain, not amplitude. I'll have to study the tinySA instructions and specs more, and play with it when it gets here, to see how much frequency resolution I'll be able to make out. Anyway, the amplitude modulation is the "beating" of the combined slightly different carrier frequencies. Like instruments out of tune. And thus the stability of the pattern.

Makes me realize how selective even cheap mass production radio receivers are.

Sutro Tower is one of those things that can be seen in good and bad light. I find its history a fairly classic example of powers that be having their way with things. There's an interesting story of news folks going to check out the goings on at Sutro's old mansion on the hill in '72, only to be chassed away and chastised by their management. This was _not_ supposed to be a news story.

It is something of an iconic landmark. I've used it as an entertaining photography subject: https://www.flickr.com/photos/davidpinsf/

My neighbor is a retired CBS cameraman and producer. He got my family a tour of the facility by the general manager. We didn't go up the tower, but did to get to see a lot of the massive RF equipment feeding it. There's one room that you can go into that is the feed to all the waveguides going up the tower. Kind of creepy to be standing in such a small space with that much power. The primary sources are feeding those three, now digital, television antennae at the top. The radio stuff is small potatoes.

I find it something of a nuisance for all the noise always coming from it. It's constantly being worked on. They recently removed all the panels covering the legs. Months and months of power tools cutting and rattling on it. I count myself lucky. I don't have a $5M view at the top of the hill with that thing right overheard. Paint chips, welding slag, all manner of crap still falls despite all the assurances that it doesn't happen. And on the other hand I always find it fascinating to study. I look at it every night when taking the dog out for a piddle. It's often a sight to behold against a starry backdrop, watching ISS float by it on occasion (haven't managed to get a good picture of that yet), and sometimes playing peak-a-boo through the fog.

It's a sight to behold, but this noise on the scopes is yet another annoyance. I always suspected it was the source, but thought good shielded cabling would minimize it better than it is. It's even quite apparent with a good 10x probe with a short ground lead. I wish there were more mitigation methods available. Another aspect that's funny is that the human body makes a good antenna. It's rather frustrating to try to trigger a weak signal and have my hand being on/near the trigger level change the triggering of whatever signal. Sometimes I do have to hold my hand by the trigger knob while observing what I'm trying to test.

But better to know the cause. I'll just learn ways to work around it.

Dave


Tim Phillips
 

Wasn't that Nikola Tesla's idea of broadcasting power, rather than sending
it through cables ?
Saw a TV prog on Wardenclyffe, Shoreham NY. I bet if that thing had worked
it would have zapped a few front-ends !!

Tim



On Sat, 10 Apr 2021 at 18:37, J Hunt via groups.io <jmailhunt=
yahoo.com@groups.io> wrote:

Dave,
I visited Smith-Kettlewell Eye Research Institute in SF around 1974 for
some collaborative work. Google maps shows them about 2.5 miles from the
Sutro Tower that is your RF ground noise nemesis. One of the EEs there
showed me a small hand-wired board he built that consisted of maybe ten
4000 series CMOS logic ICs. He was puzzled that it ran (though eratically)
without the battery connected. It turns out that the input protection
diodes on the gates along with the short and tidy wiring were acting as
crystal sets and providing enough Vcc and current to run the circuit
(depending on the level of modulation from the nearby radio stations).
Mounting the board in an aluminum project box solved the problem, but he
did need to install a battery.

John HuntPortland, OR






Jim Adney
 

On Fri, Apr 9, 2021 at 11:34 PM, Dave Peterson wrote:

https://groups.io/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
When I bought my 465B, I had it shipped to me at work, and, of course, I checked it out there and found it to be perfect. When I brought it home, I discovered noise on the most sensitive setting that hadn't been there at work. I tried different outlets around the house, but that made little difference. This was puzzling, until I noticed a correlation with the station my radio happened to be playing. Turning the radio off didn't help. Took the scope back to work and there it was fine.

Conclusion: There's a fundamental difference between a wood frame house and a steel building.