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setting up an oscope for xmit monitoring




 
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Author Topic: setting up an oscope for xmit monitoring  (Read 9226 times)
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KB3RRX
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« on: December 27, 2009, 10:52:14 AM »

I have an old precise model 308 o scope and was wondering how i would go about setting it up to monitor my xmit audio. If that is possible
I dont know much about scopes so any info is appreciated.

Wayne
KB3RRX
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W2VW
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« Reply #1 on: December 27, 2009, 12:47:59 PM »

I found this using the search feature. Hope it helps.

http://amfone.net/Amforum/index.php?topic=21983.0
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KB2WIG
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« Reply #2 on: December 27, 2009, 12:54:51 PM »

W,

Click on this link

http://www.pmillett.com/tecnical_books_online.htm

Look for " The Radio Handbook 15th Edition ". Chapter 9 has a nice section on the Oscope and its uses..

klc
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KB3RRX
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« Reply #3 on: December 29, 2009, 08:37:38 AM »

thanks for the links guys!!  ok so according to one of the posts in the one link i can pick up my xmit signal by making a sort of inductive coupler with a bnc
by putting a coil around the feed line.

guess im gonna be using my other oscope though the old 308 wont sweep for some reason  wish i had a manual for this old beast time to check the tubes i guess.

I have a much more modern kenwood 40 mhz dual trace scope i know it works fine.

Wayne
KB3RRX
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N2DTS
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« Reply #4 on: December 29, 2009, 09:00:23 AM »

I much prefer to hook the scope up to the receiver IF, and look at my signal at 455 KHz, as well as other signals on the air.
It sometimes takes so fooling around to get the level close, depending on the receiver, but is well worth it.

The r390a I had was very good, during mute, the rx still worked to the IF takeoff, the level was close to the rx level of most signals (good shielding).

Brett
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k4kyv
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Don
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« Reply #5 on: December 29, 2009, 11:50:30 AM »

I much prefer to hook the scope up to the receiver IF, and look at my signal at 455 KHz, as well as other signals on the air.
It sometimes takes so fooling around to get the level close, depending on the receiver, but is well worth it.


The problem with that is you don't get a true picture of your signal.  The i.f. transformer/filter selectivity, phase shifts and non-linearities in all the stages in the receiver from the first r.f. stage to the i.f. output, accumulate to distort the pattern.

The best way to  get a  true picture of the signal is to feed an r.f. sample from the transmitter output directly to the vertical deflection plates of the scope tube, by-passing the vertical amplifier in the scope.  Many bench-type test scopes have a set of terminals that offers a direct connection to the plates. One of the mods I did to my HO-10 was to eliminate the built-in vertical amplifier altogether.

The pattern you see on other stations' signals is even less a true image, since ionospheric propagation can radically distort the signal waveform.
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Don, K4KYV                                       AMI#5
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KB3RRX
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« Reply #6 on: December 29, 2009, 01:27:37 PM »

ok here is my newer scope
here's the back bnc ports

Here's the front and controls of the scope


now if I make an inductive pickup coil where should I run the bnc to to get a good reading?

I'm learning about scopes The newer one is a 40 mhz kenwood cs 1044 dual trace

Thanks again for all the help (got a book coming to help me learn) Wish i had a manual for this scope.

Wayne
KB3RRX



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K1ZJH
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« Reply #7 on: December 29, 2009, 02:31:12 PM »

You would connect the sampling loop, via a length of interconnection coax, to the front panel Channel 1 input BNC. Be very careful not to overdrive the scope input, or you can damage it.  Set the sweep speed, and the Channel 1 attenuator, for the best pattern.

If the sweep speed is too fast, you won't be able to clearly see the voice peaks and valleys. The vertical gain is set for the maximum display height that you want to see on the screen.

If you want to do a trapazoid, the setup will be completely different.

Pete
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flintstone mop
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« Reply #8 on: December 29, 2009, 03:06:31 PM »

Very nice 'scope. Can someone find the link from the AM Window for adjusting the 'scope?
Rough memory recall is to get 2 graticules worth of a thick green trace. That's the carrier no modulation. This is set by the sensitivity of the 'Scope.

Here's some goodies for a nnice RF pick-up
http://amfone.net/ECSound/RFsampler.htm
More:
http://amfone.net/ECSound/K1JJ8.htm

I agree with the other posters about using the I.F. out for the 'scope....it will not show overmod of the negative cycle accurately. It will display "baselining" but that actually occurred much sooner.

The other adjustments for the scope would be for the time base and that would be to see the thick green trace and when modulated with 1000hz tone you would actually see the audio representation of the tone on the screen.

This link has a commercially made pick-up BUT the main purpose is to let you see the traces.
http://www.icycolors.com/nu9n/scope_your_audio.html

Fred
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Fred KC4MOP
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« Reply #9 on: December 29, 2009, 07:06:20 PM »

I much prefer to hook the scope up to the receiver IF, and look at my signal at 455 KHz, as well as other signals on the air.
It sometimes takes so fooling around to get the level close, depending on the receiver, but is well worth it.


The problem with that is you don't get a true picture of your signal.  The i.f. transformer/filter selectivity, phase shifts and non-linearities in all the stages in the receiver from the first r.f. stage to the i.f. output, accumulate to distort the pattern.

The best way to  get a  true picture of the signal is to feed an r.f. sample from the transmitter output directly to the vertical deflection plates of the scope tube, by-passing the vertical amplifier in the scope.  Many bench-type test scopes have a set of terminals that offers a direct connection to the plates. One of the mods I did to my HO-10 was to eliminate the built-in vertical amplifier altogether.

The pattern you see on other stations' signals is even less a true image, since ionospheric propagation can radically distort the signal waveform.

The above is right but decent scopes do OK with a sample using the vertical amp. One common problem is R.F can ride along the scope's line cord and create a funky baseline. Use an R.F. choke on the line cord if this happens to U.

The receiver I.F. scope thing is very useful to learn envelope patterns watching someone else's pattern if one has just been exposed to this for the first time. Tune to a medium wave broadcast station, shut off the AGC, turn down the R.F. gain and watch. Receiver limitations will be learned simultaneously.
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N2DTS
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« Reply #10 on: December 29, 2009, 09:01:14 PM »

I have never had any problem seeing my own modulation, its quite clear and tracks the mod monitor.
I dont think every receiver is good (without work) but the r390a was fine.
My homebrews also work ok, but #2 is over sensitive and the gain needs to be turned down.

On rx, you cant always see the modulation clearly, but most times you get a good indication of the percent of modulation, or if they are overmodulating.
Most times, you get a quite clear indication, or at least I do....
It seems easy (for me) to see if someone has lots of audio, or little.

Brett


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ke7trp
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« Reply #11 on: December 29, 2009, 10:07:36 PM »

I love my IF scope. It really shows you want you are listening too.    I just wish I could get this SP600 to interface to the Yo100. 

C
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flintstone mop
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« Reply #12 on: December 30, 2009, 08:35:20 AM »

I agree with the I.F. 'scope application. Mine is hooked to my R390A and it's handy to tell another operator about any flat topping of his/her peak audio AND if there is a phasing problem with the mircophone.
Instead of nice peaks, you'll see severe negative overmodulation and very little positive peaks. The pos peaks will be rounded off.
Adjust your TX with the RF pickup and then switch to the I.F. to see other stations and yours as a monitoring tool.
Fred
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Fred KC4MOP
K9ACT
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« Reply #13 on: December 30, 2009, 10:36:22 AM »

I use a Heath SB610 because it was cheap and could be dedicated to the transmitter end of the shack.

I didn't realize until I had it for awhile that it also displayed the trapezoid with the flip of a switch.

I don't really understand how it does this with a simple RF pickup and a friend is convinced that it is a hoax that proves nothing other than possibly indicating when 100% is reached.

When I hook up a real scope with audio and RF, I get a trap that seems to respond to all the usual parameters.  If I leave the transmitter set up with distortion displayed and then switch to the 610, there is no distortion, just a perfect triangle.

The only thing that changes on the 610 is the length of the trap and when 100% is exceeded, it stays at 100%.

This leads me to believe that my friend is right, it's a hoax.

Any other opinions?

js
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W1RKW
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« Reply #14 on: December 30, 2009, 10:57:19 AM »

When in the Trap position RF from the transmitter is rectified or demodulated to provide the audio sample to the scope.  Indeed, not a good way to determine exactly what's occurring with the transmitted signal but provides a pretty picture.
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KB3RRX
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« Reply #15 on: December 30, 2009, 11:24:15 AM »

I like the capacitive pickup in the article

Thanks for the link Fred!!

and it will work when i finally get my amp also

and thanks for the help everyone. guess i will have to make a trip to rat shack.

Wayne
KB3RRX
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k4kyv
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Don
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« Reply #16 on: December 30, 2009, 12:26:26 PM »

When in the Trap position RF from the transmitter is rectified or demodulated to provide the audio sample to the scope.  Indeed, not a good way to determine exactly what's occurring with the transmitted signal but provides a pretty picture.

I agree.  That's a totally bogus method to get a trapezoid pattern. It won't indicate overmodulation and it won't indicate modulation non-linearity in the final amplifier stage.  The result is meaningless, so I don't see the point of even putting that feature on the scope.  You need to sample the audio directly off the modulator.  The usual method is to use a resistive voltage divider connected to the +HV line to the final, and you may have to use one or more capacitors to compensate for phase shift.  On mine, the HV plate voltage meter multiplier resistor does double duty to provide the audio sample, and I added a couple of mica caps to correct for phase shift, using trial and error.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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http://www.mwbrooks.com/dvorak
KC2IFR
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« Reply #17 on: December 30, 2009, 02:52:52 PM »

Setting up the scope is one thing.........knowing what u are looking at is another.......
No disrespect meant....

Bill
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