few-MHz empty frequency bands

You might want to make it a moveable feast.

There isn't much allocatable bandwidth that hasn't been used somewhere except in the scientific astronomical observing frequencies and even then some met offices do not honour the "KEEP OUT" signs as they should.

In some places maybe. Amateur radio has sort of been out evolved by VoIP.

--
Regards, 
Martin Brown

Can't you shield things? A Danish-Butter-Cookie-can level of shielding should get the noise level down below thermal, and you get to eat the cookies, too.

That frequency range does sound good. Maybe pick the frequency of some huge shortwave transmitter (do they still exist?) in some distant part of the world, which would discourage any local competitors.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation

80m is OK, since its the wild west there - plenty of spurious signals. Of course, you don't plan to radiate, but use seat belts etc. anyway...

From a ham standpoint, there is in the US already plenty of interference on 3.58 MHz (NTSC subcarrier).

Does your system get upset if a neighbor comes on the frequency with a 1 kW signal?

--

Tauno Voipio

Maybe this frequency?

It's mainly set aside for emergencies, and twice each hour even the licensed operators are encouraged to stop talking.

Yes, I'll be doing that (DDS), so I can move things around a bit, maybe to avoid harmonics of an local AM radio station, but there are always going to be analog filters and such like that can't easily be changed.

Actual noise levels are insanely low, and there's effectively a detector diode that has wide bandwidth so it's not so easy even with pretty good shielding and fully differential cabling.

Strategic thinking (or is that tactical). Anyway, I like it.

Will you be doing narrowband synchronous detection or an equivalent that gets you down below normal thermal levels? That would make it a probability game, a chance that your measurament bandwidth winds up on top of a big spectral line in the EMI background. You could maybe sense that and hop frequencies, or go spread-spectrum a bit to average things out.

Or shield like hell, cans and hardlines and such. If you're running distances between boxes - you mentioned cabling - maybe use fiberoptics?

Radio North Korea or something, a place where your gear is unlikely to be sited.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation

On a sunny day (Fri, 16 May 2014 10:07:24 -0400) it happened Spehro Pefhany wrote in :

Those bands are extremely noisy, from local RF sources like switch mode supplies, and in the evening from world wide located stations.

Above about 16 MHz or maybe 20 things become more quiet here. There are quiet spots in the broadcast FM band here, and low power FM modulators are allowed in Europe, like for mp3 players in cars, I have one. It is tuned a a spot where I only hear clear noise (106.7 MHz now). Maybe in the US where there are more stations that is full too, dunno. Short wave bands are almost never guaranteed quiet, due to propagation.

I subscribed to that philosophy once upon a time. Now that I got back into mountain biking I realized how difficult the aftermath is. Almost half a year of high-power biking and only a mere three pounds down (but I do feel a lot better). So no more cookies for me :-(

Got to filter that. If it's pulse-echo stuff make the system frequency agile as Martin suggested. On an upcoming design I'll constantly run an FFT even though Goertzel or bandpass would suffice. The core reason being to constantly watch for noise coming up and dodging. It means using a 100MHz+ 32-bit uC instead of a 16MHz/16-bit but, as John Wayne said once upon a time, man has got to do what man has got to do.

Think about thunderstorms as well. Even distant ones can produce quite the crackling up there.

--
Regards, Joerg 

http://www.analogconsultants.com/

Sitting and clicking a mouse isn't good for your body. I like to draw, not click, so I move a lot between my PC and my drafting table, and over to my work bench to fiddle with circuits [1].

Daily exercize keeps your body and brain in shape. I climb stairs and streets on my way home; the views help distract me from how much I hate exercize.

There's nothing wrong with the occasional cookie. Lunch today will be a banana and an oatmeal cookie, with some dark chocolate as an afternoon booster. Lunch makes you fat.

[1] Currently testing some Fujitsu tiny DPDT relays for coil-power-induced thermal offsets on the contacts. The choice is to use regular or latching relays, to keep the thermals down. I don't see a full microvolt of offset on the non-latching relays, pretty impressive.

We recently got a batch of Omron relays, our (formerly) qualified second-source for the Fujitsus. Bad, bad news.

We rarely see lightning here, less than once a year maybe. Our power lines go not have the upper ground wire like you see in most places. Humidity is usually high, so we don't see a lot of ESD. As you know, the RF fields are horrendous.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I have to walk two miles/day because we have dogs. Then the occacional prototyping effort in the garage. But I found that it ain't enough. So since I am self-employed I watch out for every opportunities where I can sneak out of the office for 2-4h to hit the trails. Shortcut to the grocery store but with a serious workout:

Almost in our backyard:

You just can't crash there, nobody would find you. Some trails contain steep and rocky descents. I remember when I told an experienced local mountain biker about these trails. He couldn't believe it until we both went. On the first one I heard him yell something like "Oh man!" and riding really low, stomach over the seat.

This is another one which I can reach by bike, no need to use the car to get there:

Lunch is where we splurge. Today it'll be homemade bread and also Italian farmer's bread, well-aged brie cheese, olive loaf, hard Italian salami, Emmentaler, chipotle cheddar. No large quantities though.

As long as the gold won't wear off small-signal relays can be great. But eventually it does wear.

Probably a good idea to cycle the Fujitsu relays over the weekend and see if they are still as good on Monday. Since that was often a bit disappointing I try to avoid relays whenever I can.

Well ...

Sutro Tower must get a lot of hits as well. But that spews tons of RF anyhow.

--
Regards, Joerg 

http://www.analogconsultants.com/

Yikes, that's like the Rubicon Trail, but for bikes instead of jeeps.

I mostly walk streets and stairs for exercize

Well, you work from home, and we don't. Around here we have the taco truck, the sushi place, Zuni, and Zeitgeist, and some others. A tiny expensive urban McDonalds. But I mostly eat at my desk.

We did go to The Boxing Room yesterday, cajun/creole. I had a fried oyster sandwich and some Scrimshaw pilsner. One of the ladies had fried alligator, and I swapped some for half of my oyster sandwich. Frankly, the fried alligator tasted a lot like the fried oysters.

These relays get cycled infrequently.

We use them for BIST, to switch analog channels off the input connector an onto a cal bus. The Fujitisu relays have been perfect so far.

I got bored with measuring the thermals at the rated 5v coil voltage, so I cranked it up to 10. Looks like close to a full microvolt now.

Beautiful, and rare.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

How about an international standard-checking frequency? At least that gets you clear of broadcasts and hams. What about 465 Kc/s, that is kept clear because it is the I.F. of most domestic A.M. radios.

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk

On the western sid of the Pond, the AM IF is usually 455 kHz.

--

-TV

I used to do a lot of shortwave listening, and the lower frequency part of the spectrum was FILLED with trash. Dirty insulators on medium-tension distribution feeders (we have some 45 KV lines a few blocks away) can turn into huge spark transmitters. That pretty much wiped out any hope of listening to anything below 5 MHz in our area. 3.579545 MHz used to be massively noisy due to all the color TV receivers driving their color burst crystals. I'm guessing that has quieted down a lot.

Jon

I need something in the few MHz. 500kHz is too low. But I like the spread-spectrum idea JL mentioned.. just have to make sure that nothing goes bad with a front-end bandwidth that wide. As Jan says, it's a noisy band and there are probably 50-100 switching power supplies withing a meter or two, and some VHF transmitters, hundreds of amperes of noisy DC etc. etc. .. I think the noise temperature of the preamp is a few K.

Best regards, Spehro Pefhany

I want to do the Rubicon trail with a client some day. He said it's fairly easy via mountain bike (but not with a Jeep). We have some local trails that scare the s..t out of visitors. On the western side east of the Salmon Falls Bridge they put up signs to walk the bikes because people have died there. On the eastern side I was muscling up a steep incline and then in one harpin turn saw paint scrapes on rocks and small chunks from a shattered helmet.

Looks like a kid crashed through the left rail.

Ah, mountain bike turf! But probably not allowed.

When I did the TI on a small corporate building I made sure there was a concrete pad where we could safely place a Weber kettle and barbecue. Just had to watch out for rattlesnakes. But the same goes on my mountyain bike trips. Like this fellah that turned around and took a hike when I finally got my backpack off and the camera out:

I've cruised past two of them with maybe 5-10" between my foot and the snake's head. The last one was really pretty, dark brown with shiny rings, and not shy. Instead of rattling it wagged the rattle-tail noiselessly, as if happy to see me. I never knew they could do that.

Sounds yummy.

If they are hermetically sealed it should be ok then. I found that anything non-hermetic, when deployed to places such as Houston, will corrode or become funky. No matter what the materials.

For BIST it should be fine, provided a fuzzy contact causes an alarm.

[...]

--
Regards, Joerg 

http://www.analogconsultants.com/

How about the range between 30MHz and 70MHz? Since most UHF-band transmitters have vanished from there it's very quiet in most places. I'll never understand how they could give that up without a fight and accept the assignment of a measly UHF channel as a "replacement".

Even where VHF-I is still in use the spectrum just above 30MHz is very quiet. That's where I put my stuff if I have a choice.

--
Regards, Joerg 

http://www.analogconsultants.com/