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Author Topic: Connecting scope for trapezoid modulation pattern  (Read 4763 times)
K1YTG
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« on: March 30, 2010, 12:02:30 PM »

I want to connect the modulated audio to the horizontal input of a scope to get a trapezoidal pattern on the scope.  I have it set up for a sine display and that works fine.  I have a loop pick up for the RF.
But my scope for the horizontal has just a trigger function.  The trigger has internal or external.
Can I connect the horizontal signal from the modulated audio to the external trigger?
Seems like there should be an actual horizontal input, not a trigger.
I do have a circuit from the Handbook to put a capacitor and 5 megs of resistance between the modulated HV and the scope horizontal.
Do I need a different scope to get the trapezoidal display?
Thanks, Norm
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AA4PB
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« Reply #1 on: March 30, 2010, 12:08:50 PM »

No, the modulation has to feed the horizontal directly - not the trigger. Another issue is that the vertical input needs to be able to handle the RF frequency. In other words, if you have a 10MHz scope then you will only be able to view signals thru 40M. Some scopes have a direct input on the back that bypasses the scope vertical amplifier and works to a much higher frequency. It requires more signal amplitude but that's not usually a problem for a transmitter.
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WX7G
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« Reply #2 on: March 30, 2010, 02:57:32 PM »

The vertical bandwidth of the 'scope is not an issue. A 5 MHz scope will have enough gain at 14 MHz to show what you need to know.

But as you have found, your 'scope has no horizontal sweep input.

You can use the 'scope internal sweep and sync the signal source to this by carefully varying the source frequency. Troublesome but workable.
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KD0FAT
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« Reply #3 on: March 30, 2010, 04:03:37 PM »

Double check your 'scope manual. My HP 1740a is an older dual trace analog. To use an external signal for the horizontal sweep, I have to put one signal on channel A and the other on channel B, and then use an "A vs B" button that displays A channel vertically and B channel horizontally. A bit clunky, but it works.
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KF6QEX
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« Reply #4 on: March 30, 2010, 04:11:50 PM »

Quote
Do I need a different scope

What model 'scope do you have ?

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AA4PB
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« Reply #5 on: March 30, 2010, 05:05:23 PM »

"The vertical bandwidth of the 'scope is not an issue. A 5 MHz scope will have enough gain at 14 MHz to show what you need to know"

Enough gain perhaps, but is it going to be linear? If its not exactly linear then it will distort an otherwise good trapazoid pattern.

"You can use the 'scope internal sweep and sync the signal source to this by carefully varying the source frequency"

Not if he wants the trapazoid pattern that he asked for. The internal sweep will be linear even if you sync it to the audio. In order to get the trapazoid pattern the sweep needs to change in step with the amplitude of the audio waveform.
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W8JI
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« Reply #6 on: March 30, 2010, 05:59:18 PM »


Seems like there should be an actual horizontal input, not a trigger.
I do have a circuit from the Handbook to put a capacitor and 5 megs of resistance between the modulated HV and the scope horizontal.
Do I need a different scope to get the trapezoidal display?
Thanks, Norm

Norm,

The frequency response will not matter much because that is a gain vs. frequency problem. It is not a linearity problem.
As long as you get adequate vertical deflection, and as long as earlier scope stages do not overload from being overdriven, all will be well.

The problem is you can **NOT** use internal sweep for a trapezoid. The sweep has to come from the modulation source at a point where the modulation is very clean and not distorted. This would generally be in an early audio amplifier stage.

If you think of how the display works, you will understand why. If you just swept with a triggered sweep on the X axis, every RF cycle would show as a deflection related to RF amplitude.  You would see a standard waveform.

To see the trapezoid, the horizontal sweep has to vary with audio level and frequency. You would have a little dot at rest that would move at a slower rate left and right with audio positive and negative, and up and down with RF envelope at a very fast rate. If transmitter modulator and RF phase was in phase with the audio and you had negative left and positive right, the RF envelope would expand on right sweeps and contract on left sweeps. When you hit 100% zero on negative peaks, you would get a point. If the audio input continued negative but RF was already zero (100% negative peaks) it would trace a thin line left out of the point. The positive audio voltage peaks would drive the X axis sweep right, and the modulated envelope would increase. This would draw a wide vertical base to the right.

So you see, while you can and should trigger off the audio (to prevent burning the screen), you have to sample the undistorted audio for actual sweep. Then the RF becomes the other axis.

Jimmy Hendrix even sang about this axis, as he distorted his Marshall amplifiers.  :-)
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K1YTG
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« Reply #7 on: March 31, 2010, 01:32:09 PM »

The scope is a Gold star OS-9020p  that says 20 mhz on it.
It does have 2 channels.  So I think it will work.  It does look like it sets up for ch 1 being x and ch2 being X and ch 2 being Y.  It says the X and Y by each channel.
There is a mode switch that has 4 positions.  ch1,  ch2,  dual and add.
I do not have a manual so I am not sure which position sets it up for the x and y display.
I have planned on using the modulated HV as described in the Handbook.  Is it better to use that or to go off of the audio amp?  Where is a good place to pick up the audio from the amp.
My audio set up is a 50's amp with push pull 6L6's.  The 6L6 audio transformer has 8, 16, and 500 ohm taps.
The 500 ohm taps go to a 2nd line transformer that feeds the grids of the 100th tubes.  So there are lots of places to get the audio from.  
Thanks, Norm
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W8JI
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« Reply #8 on: March 31, 2010, 02:37:24 PM »

The scope is a Gold star OS-9020p  that says 20 mhz on it.
It does have 2 channels.  So I think it will work.  It does look like it sets up for ch 1 being x and ch2 being X and ch 2 being Y.  It says the X and Y by each channel.
There is a mode switch that has 4 positions.  ch1,  ch2,  dual and add.
I do not have a manual so I am not sure which position sets it up for the x and y display.
I have planned on using the modulated HV as described in the Handbook.  Is it better to use that or to go off of the audio amp?  Where is a good place to pick up the audio from the amp.
My audio set up is a 50's amp with push pull 6L6's.  The 6L6 audio transformer has 8, 16, and 500 ohm taps.
The 500 ohm taps go to a 2nd line transformer that feeds the grids of the 100th tubes.  So there are lots of places to get the audio from.  
Thanks, Norm

If you use the modulation transformer sample it will tell you the percentage of modulation. It will not show the real distortion. Just percentage. You also need to be very careful of voltage breakdown.

If  you tap off an early class A stage for the sweep, it will show distortion and percentage.

It will never tell you bandwidth.

The 16 or 500 ohm taps might be a good spot to tap off.


Usually the XY axis function is on the sweep time or speed knob of scopes. This is because to get X-Y displayed, the sweep has to be off.
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K1YTG
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« Reply #9 on: March 31, 2010, 03:49:40 PM »

Thank You for pointing out where to look.  It does have an X-Y setting on the sweep dial.
I will set it up for making the display and try both the HV and the audio output transformer and see what I can learn.
The point is to learn how to use the scope and create the pattern.  The transmitter sounds good so I am not sure if I will make changes unless I find something that is way off.
But there is always the urge to make adjustments.
Norm
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KL7AJ
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« Reply #10 on: April 01, 2010, 11:33:24 AM »

My Central Electronics 100V has a BUILT IN scope that will do zapetroids. Smiley  

Eric
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K4LED
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« Reply #11 on: July 05, 2013, 02:07:39 PM »

I have a Tek 7603 and the best way to get an X Y display is to simply remove the horizontal time base module and put a second vertical module in that (right most) slot.  Should work on most older scopes with plug in modules. I use this method with a CleanRF.com module to view an SSB trapezoidal display.  The trapezoid is good for adjusting and checking IMD but I prefer to not use it for every day general monitoring because you have a dot on the screen when not transmitting that could burn your CRT.  I just use the H time base on auto and it looks good for monitoring peak modulation, etc.
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