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   Home   Help Search  
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Author Topic: How to hook oscilloscope to radio?  (Read 3523 times)
K9JH
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« on: October 28, 2008, 05:36:45 PM »

I am interested in seeing how my signal looks, mostly for curiosity, but also to play with compression, mike gain, alc, etc...  How do I hook up an oscilloscope to the radio?  Does it go inline?  Can the oscilloscope take 100 watts?  Do I somehow just sample the signal?  I do not own a scope yet and any suggestions for setup and also a basic scope model would be appreciated.  My main radio is an IC-718.

K9JH
James
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KE4DRN
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« Reply #1 on: October 28, 2008, 06:06:30 PM »

hi James,

take a look at this thread

http://www.eham.net/forums/Elmers/197158

regards,

james
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W7KJ
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Posts: 5




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« Reply #2 on: October 28, 2008, 06:06:44 PM »

http://www.cleanrf.com/
This site has some info as well as a list of scopes that will work.
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K1BXI
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Posts: 821




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« Reply #3 on: October 28, 2008, 06:51:25 PM »

For what you want a service bench scope would be an over kill. See if you can find a station monitor scope They just go in-line and will show you on SSB the "sideways xmas tree pattern".

If you use it with a tunable linear amp it becomes a bit more useful. You feed the transceiver into the horizontal amplifier and the output of the linear feeds the vertical plates. The resulting pattern is a trapezoid on the screen. Once the linear is loaded correctly the top and bottom corners will come to a point and it will be in it's linear state and will reproduce a faithful reproduction of the driving signal. Of course if you feed garbage in, you will get garbage out!

I still use an old Heathkit HO-10 monitor scope with my Alpha 77 just for that purpose.

All station monitor scopes have all the things built in to handle any power that is legal on the amateur service. Can I say they are "plug and play"?

John
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K1BXI
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« Reply #4 on: October 28, 2008, 07:00:42 PM »

To correct myself, It samples a very small portion of the signals, you don't actually feed the whole signal into the scope. And yes, you can do that with a service scope but you have to build the attenuation circuits your self.

John
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K9JH
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« Reply #5 on: October 28, 2008, 07:17:35 PM »

Does anyone still make station scopes?  
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K1BXI
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Posts: 821




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« Reply #6 on: October 28, 2008, 07:38:45 PM »

"Does anyone still make station scopes? "

Good question. I suspect the demand would be low, as this is the age of solid state pre tuned everything. Guess you would have to find one used and hope for the best. Kenwood and Yaesu were the last that I remember and a quick search only brought up used ones.

God, I'm behind times............

John
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N4CR
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« Reply #7 on: October 28, 2008, 08:01:18 PM »

You can get a probe close to your transmission line and probably pick up all the signal you need to see what is going on.

I clip the probe around the insulation of my 450 ladder line, not contacting the wire and I can see my signal very clearly.

If your coax is perfect and your connections are perfect, this won't work, but that's seldom the case.

Otherwise, you'll have to build a tap and have it attenuate the signal way down for the scope connection. Since you are experimenting, you might consider using an old Heathkit Cantenna, which has a scope connection built right into it and you won't be on the air while you are testing. What you want to see doesn't require using a real antenna.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
K0BG
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« Reply #8 on: October 29, 2008, 06:24:13 AM »

One thing to keep in mind. An O-Scope is a very useful device, but it makes a lousy monitor scope, no matter how you connect it.

What's more, you can't rely on what you see to adjust the level of compression, or to look for distortion in the transmitted signal. For that, you need a really good, laboratory quality, very fast, storage scope, and they aren't cheap! Even then, you need the rest of the stuff to go along with it.

Alan, KØBG
www.k0bg.com
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WB2WIK
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« Reply #9 on: October 29, 2008, 08:22:07 AM »

I kind of agree with Alan.

Although I had fun building my Heath SB-610 station monitor scope back in 1968 or so, its usefulness in making station adjustments, especially concerning "modulation," is very limited.

The CRT screen is too small, for one thing.  You can't even hold a straightedge up to the trapezoid (assuming you can create a trapezoid -- which you cannot do by "speaking") because the screen's too small to effectively do that.  Observing the "Christmas tree" modulation pattern that SSB provides, you can't tell if you have 0% or 10% distortion -- there's no observable difference.

It is more useful for RTTY and AM (full-carrier AM, not SSB) work.  At least with an AM signal you can "measure" % modulation by looking at the scope and jotting down some readings.  No such thing as % modulation on SSB...

I have three lab-grade bench scopes and none of them are connected in any way to my transmitters.  They're in the shop.  To measure PEP output power, I use PEP wattmeters, and to monitor "how I sound" I use a second receiver and a pair of good headphones.

WB2WIK/6
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WB2WIK
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« Reply #10 on: October 29, 2008, 08:28:03 AM »

Further tidbits relating to the actual question (!):

1. Station Monitorscopes such as those from Heath, Kenwood and Yaesu (and none are on the market today) have a provision for "transmitting directly through" the scope.  That is, they have a pair of SO-239 coax receptacles right on the rear panel of the scope, and the sampling network that drives the vertical plates in the scope CRT is located there.  So, to "sample the RF" to make measurements, you don't need to do anything except splice the scope into your transmission line with a coax jumper, and adjust a "sensitivity" control to select the amount of coupling necessary for full-screen vertical deflection.  Basically, "plug & play."

2.  Using any other kind of scope involves you doing the sampling yourself, and providing that to the scope.  In such a case, I'd always recommend making a resistive divider, rather than a capacitive or inductive one, because a resistive divider will be "flat" across a broad frequency range and provide the same level signal to the scope (assuming the same transmitter power) at all operating frequencies.  Reactive dividers don't do that.  Another "plus" for using a resistive divider is that you can make its termination a 51 Ohm or 47 Ohm carbon resistor, which will eliminate the effects of any coax between the sampler and the scope input.  If you use reactive sampling and it's fairly high impedance, then the coax between the coupler and the scope will form a frequency shaping network and this isn't good.

WB2WIK/6
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K1BXI
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« Reply #11 on: October 29, 2008, 09:34:14 AM »

James, you said "I am interested in seeing how my signal looks, mostly for curiosity, but also to play with compression, mike gain, alc, etc..."

Don't expect a monitor scope to tell you much about the quality of your signal on SSB. Today's transceivers are hard pressed to drive to a flat top condition so as to be seen on the scope, unlike the early days of SSB.

You could sound awful to the other person and still look perfect on your scope. So for curiosity's sake go ahead and play with one so long as you understand what you are seeing may not be what the other guy is hearing.

John
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N5EG
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« Reply #12 on: October 30, 2008, 11:58:39 AM »


A capacitive voltage divider works rather well as a RF sampling tap and is fairly broadband. It's commonly used as an attenuating sampler with the advantage that it is not DC-coupled.

A SPICE model of a 40 dB tap, using a 50pF series cap, and a 5000pF shunt cap shows flatness within +/-1 dB from about 900 Khz to 40 Mhz, assuming a 50 ohm load and 50-ohm resistive generator impedance. The high-frequency roll-off is due to the 50pF capacitor loading the transmitter's 50-ohm resistive output.

With a tuned transmitter adjusted to compensate for the 50pF of capacitance, the bandwidth is much greater on the high end. With a high-impedance load (like a 1M scope) the bandwidth is also a lot wider at the lower frequency end.

The capacitors don't dissipate much power compared to a resistive divider so it's a simple way to make a good tap. A 50pF cap with about 500V capability should handle maximum legal power with margin. That requires higher power resistors than you'd initially imagine if using a resistive divider instead.

-- Tom, N5EG


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N4NYY
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Posts: 5062




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« Reply #13 on: October 30, 2008, 05:10:12 PM »

I built this RF Sampler:

http://www.qsl.net/k6ls/rfsampler.html

Works like a charm
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