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Author Topic: STATION MONITOR SCOPE  (Read 969 times)
W4PBU
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Posts: 86




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« on: December 19, 2007, 07:30:22 PM »

Never owned a scope.
Would like to monitor modulation, linearity and accurate signal bandwidth.(especially bandwidth)
Budget is pretty limited.
Have no idea what to look for.
Help!
Merry Christmas & Happy New Year & Great Hope for #24
Dave
W4PBU
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KZ1X
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Posts: 3228




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« Reply #1 on: December 19, 2007, 07:52:01 PM »

... bandwidth is frequency domain (vs. power) like a spectrum analyzer; a 'station monitor' is (usually) time domain (vs. voltage) like an oscilloscope ... so, that's two instruments ... Do you need both?

How deep a dive do you want to do here, and what do you mean by "limited budget" ?  

What sort of equipment will you be monitoring?  What do you do that would make you want to monitor your station, as opposed to just setting it up?
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AC5E
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Posts: 3585




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« Reply #2 on: December 20, 2007, 03:39:07 AM »

Well, that's asking a lot of one instrument. But what mode's bandwidth do you want to know about. AM, SSB, FM, or other - and how many of your children do you want to mortgage for the information? Or would you rather do it on the cheap?

For AM/SSB the secret is keeping the audio below the clipping point and restricting the audio bandwidth to that necessary - or desirable - for good communications. No particular monitor is needed, just good operating practice. And some honest friends who will tell you when you are "running the mike gain hot."

There are FM bandwidth meters; I own a couple; and they have their uses. But not around my ham station! If my deviation has crept up a bit I will start dropping out of the local repeaters. The first comment means it's time to dig out the manual and cut the audio gain to the reactance modulator until that does not happen.

Hope that gives you something to think about.

73  Pete Allen  AC5E

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KA5N
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Posts: 4380




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« Reply #3 on: December 20, 2007, 08:34:36 AM »

A monitor scope is one of those devices that sound like they should be great guns for usefulness.  Once you have one you find that it is rarely used.  This is the reason that you will find few new ones on the market.  Fix up an old Heathkit and see what it's like and get it out of your system without spending a lot of money.
Allen
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W4PBU
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Posts: 86




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« Reply #4 on: December 20, 2007, 09:05:24 AM »

Thank you for those that have responded to my question reg the monitor scope.
I have been asked why I was looking for one and what was the use going to be.
Here is the scenario
On several occasions I have been informed, on the air, that "my signal was extremely wide" or " you are splattering all over the band" and almost immediately other operators listening will jump in and tell me that my bandwidth and signal is just fine(very clean), one guy imparticular said that my signal was just 2.4 khz up and down. All of these responder have told me not to pay any attention to the complainer who seems to be the same guy every time and maybe just a trouble maker.
Several of the helpful guys have said that they were looking at my signal with a scope.
Anyway, I thought it would be nice to have a way to ck my own signal just to be sure that everything is o.k. and thought a scope would be the way to do that.
I have a Heathkit SB-610 monitor scope which seems to work fine, linearity with trapazoid pattern looks perfect but there does not seem to be any way to ck bandwidth just to be sure that I am under 3 khz both sides.
The Heath manual does not contain any info on the subject of bandwidth.
With regard to this subject, I know just enough to be dangerous, hi hi.
Any more help would be appreciated.
Thanks
Dave
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K1BXI
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Posts: 812




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« Reply #5 on: December 20, 2007, 10:19:17 AM »

Dave... Using the 610 monitor scope as you are in the trapezoid setting will only tell you when the linear amplifier is loaded correctly. At that point it will faithfully reproduce the signal that is fed into it's input. Feed in garbage and you will get garbage out.

As others have said, you cannot check bandwidth with a monitor scope. I use an old Heath HO-10 the same as you use the 610. Just for easy tuning of a linear amp. If the transceiver is over driven and puts out a distorted signal so will the amplifier. But it still will have a nice looking trapezoid on the scope!

One other thing, ask this one person that is complaining about your bandwidth if he has his noise blanker on. If he has, ask him to check your signal with it off.

John.....K1BXI  
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WB2WIK
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Posts: 20611




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« Reply #6 on: December 20, 2007, 11:39:02 AM »

Guys who tell you "you're 2.4 kHz wide" and such can be doing this many ways:

-Guessing

-Tuning across your signal with their receiver and watching where your signal falls off, then just subtracting one frequency (low side fall off) from the other (high side fall off), which is just slightly better than guessing

-Using their built-in "spectrum scope" set on a narrow mode to "watch" your signal as you modulate it, and then read the width it appears to be; this is slightly better than the first two above, but still not very definitive since none of those scopes are accurately calibrated in the vertical (Y) axis, or amplitude plane

-Or possibly using software and a sound card, such as "Hamalyzer," which actually displays frequency vs. amplitude and can do so with accuracy but unfortunately still depends on the characteristics of the receiver it's connected to and the linearity and dynamic range of the sound card used; this is still far less than perfect, but probably better than the three above

*Real* "measurement" of transmitted signal bandwidth, including both intended and unintended products like IMD, is not so easy to do and generally requires two signal sources and a lab-quality spectrum analyzer.   The latter can be purchased used/surplus for a couple of thousand dollars, but the former is still a problem.  To accurately make this measurement using an "over the air" (received) signal is nearly impossible.

Which brings us back to the bullet points above.

I agree that in most cases, if you get a report of being "very wide" followed by multiple reports of *NOT* being wide from other users who receive you just as strongly as the first station (complainant), you're probably okay.  People will almost all tend to complain if you've got a serious problem -- the complaint shouldn't be isolated to one or two individuals.

Of course it is possible that you're only *occasionally* "very wide," based on the occasional overdrive of a stage that normally isn't, or the occasional "underloading" of a linear amplifier, created by load changes and such.  An amplifier (or even a barefoot transceiver, which has a critically matched power amplifier stage in it, as they almost all do) can go from "clean" to "dirty" in milliseconds when an antenna fault occurs.

WB2WIK/6


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N3OX
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Posts: 8847


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« Reply #7 on: December 21, 2007, 07:07:10 AM »

If you can get a good signal source and have a good sound card, you can do this:

http://www.astromag.co.uk/ssa/

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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