Modulation Monitor

[W4RFM]

This may have been touched upon before. Has anyone got a design for a diode detector AM mod monitor that can be calibrated with a scope?  Should not be a big deal, and I guessed it has already been done. That 6E5 thing in the ARRL books is not what I am looking for,

[W7TFO]

You don't need diodes, unless you wan to demodulate it for listening. 

Any good scope sampling the raw RF can give you a pattern, and if you go directly to your scopes' H&V plates, you can get a much easier to read trapezoid pattern.

RF sample to vertical, modulator output sample to horizontal.

73DG

[steve_qix]

This may have been touched upon before. Has anyone got a design for a diode detector AM mod monitor that can be calibrated with a scope?  Should not be a big deal, and I guessed it has already been done. That 6E5 thing in the ARRL books is not what I am looking for,

It is kind of a big deal    You first need a really good detector - then a very linear filter to get rid of the RF... then some kind of level shifting circuit to normalize the carrier, but not eliminate it because the monitor has to be DC coupled to be really accurate......after that, a couple of very good peak detectors.... then a display.  That's a lot of circuitry.

I have a new mod monitor under development, software based.  Price will be quite reasonable, and has high accuracy.  Includes a high fidelity output suitable for driving headphones.

[W4RFM]

The reason I stated it probably is not a big deal is that I have a big SWR/Mod meter I picked up at a ham fest for $40 that works fine, but I want my own, with an audio take off for head phones, and I want to be able to calibrate it against a scope for accuracy.  I really dont want to buy a used broadcast piece, although I have used one before and they work quite well on 160, 80 & sometimes 40 - if you have enough power to get the carrier level high eneough for the circuit.

[k4kyv]

I find the best modulation monitor of all is the envelope pattern displayed on an oscilloscope, with the rf sample directly feeding the vertical plates.  The ham radio appliance manufacturers used to carry monitor scopes that matched their popular transceiver lines, but apparently discontinued them sometime in the mid to late 80s. Sometimes good used ones can still be found at hamfests.  I use a Heathkit HO-10, highly modified to correct some of its many design flaws, including bypassing and disabling the built in vertical amp and feeding the CRT deflection plates directly with the rf sample from the transmitter.

The trapezoid pattern makes a good modulation percentage indicator and monitors modulation linearity of the final, but quickly burns a vertical line in the face of the scope tube.  The envelope pattern also burns a line, a horizontal one, unless some means is employed to blank out the screen when not transmitting. I use a relay connected to the T/R switching circuit to switch in a DC voltage to the horizontal plates, that deflects the pattern off- screen to one side.

[W4RFM]

I agree that is the best way for setting up a system, I did that for 35 years in the broadcast biz, but in my new ham shack I am going to have a panel with a Weston VU meter showing audio going in, and I want a Weston modulation meter showing the audio signal actually got there and at what level.

[steve_qix]

The reason I stated it probably is not a big deal is that I have a big SWR/Mod meter I picked up at a ham fest for $40 that works fine, but I want my own, with an audio take off for head phones, and I want to be able to calibrate it against a scope for accuracy.  I really dont want to buy a used broadcast piece, although I have used one before and they work quite well on 160, 80 & sometimes 40 - if you have enough power to get the carrier level high eneough for the circuit.

These types of meters are generally not peak reading, and are therefore inaccurate.  More of an demodulated audio "VU meter" than anything else.

[w1vtp]

My 2 cents:

Here's what I use for now although it is probably not as acccurate as it could be.  I have this neat scope that has two cursers - one reference and the other can be moved around the reference curser.  First thing I do is apply the carrier and adjust the carrier so that it's centered on the scope.  Then I put the reference cuser on the carrier.  I can then move the second curser to the peak and to the null noting each value.  Next I apply these values to a spreadsheet I have generated to do the math for me for the positive modulation (the negative is an "absolute" where it pinches off the carrier - that's considered 100% negative mod)  the positive is valued information in that you know the phase of your mod is correct and can see any changes - degradation due to adjustment or other problems

Picture 1 = positive mod value, curser is placed on positive peak
Picture 2 = negative mod value, curser is placed on negative null

In the spread sheet just apply the corresponding values to each yellow field and you will see your positive mod in percent. That will have to do until I get Steve's mod monitor.

Oh yeah, I fudged the symmetry so you could see some sort of assymmetry on the modulation

Al

[W7TFO]

A Dorrough model 40A meter will read both peak & average (VU) simultaneously.

All you gotta do is feed it an output sample, thru a good diode, heavily swamped so it be linear.

http://www.dorrough.com/

73DG

[w1vtp]

I find the best modulation monitor of all is the envelope pattern displayed on an oscilloscope, <snip comment 1>

The trapezoid pattern <snip comment 2>  The envelope pattern also burns a line, a horizontal one, unless some means is employed to blank out the screen when not transmitting<comment 3>. I use a relay connected to the T/R switching circuit to switch in a DC voltage to the horizontal plates, that deflects the pattern off- screen to one side.

Comment 1. Tektronix and other scopes that are present at most festers can use the "Normal" mode (not Auto) and the trace can be triggered so with no mod there is no trace,

Comment 2.  I agree this is the best one for linearity checks.

Comment 3.  Your application of DC on the plates is a great idea.  Any drawings on this hookup?

Stu, AB2EZ has a neat hookup where the audio is superimposed on the envelope. Any deviation shows as a departure of the envelope from the audio used to modulate the xmtr.  I haven't seen that hookup on his web page for some time.

I've had some 811A tubes go low emission on me - a scope alerted me of the problem and I knew it was time to replace them

[steve_qix]

A Dorrough model 40A meter will read both peak & average (VU) simultaneously.

All you gotta do is feed it an output sample, thru a good diode, heavily swamped so it be linear.

http://www.dorrough.com/

73DG

The 40A will definitely read peaks, however the usual modulation monitor problem is not solved.  Unless the monitor is DC coupled, it will not be accurate for certain types of waveforms and will not be accurate when there is a DC shift (which occurs rather frequently).  The inaccuracy can be significant, depending on the situation.  You can get an idea of your modulation percentage, but the accuracy of an AC coupled system cannot be relied upon.

[AB2EZ]

Here are some digital photos of the screen of my dual trace digital oscilloscope showing: the audio input and the rf output of my single-FET Class E transmitter (with a Class H modulator).

The yellow trace is the audio input waveform to the transmitter.

The blue trace is the rf output of the transmitter... picked up by a current transformer type coupler... with a 30:1 voltage step down between the transmitted rf signal and the output of the coupler. [The peak rf voltage at carrier is 1.13V/division (channel 2 scale) x 1.5 divisions x 30 volts/volt = 50.85V. Also, 50.85V peak rf into the 50 ohm antenna/load = 26 Watts of rf power at carrier]

The first photo has the horizontal sweep set at 400us/division
The second photo has the horizontal sweep set at 2ms/division

Note that the triggering level in the first photo is set to near the +80% modulation point. The triggering level in the second photo is set at around -80% modulation. In both cases, the trigering is off of the yellow (audio) waveform.

The vertical scale of the blue waveform has been adjusted to 1.13 volts/division to match the vertical amplitude of the yellow waveform. The yellow waveform has been offset, vertically, by 1.5 divisions to cause it to sit on top of the upper envelope of the blue waveform.

Note how faithfully the rf output envelope (blue) tracks the audio input waveform (yellow)

Since this is a digital scope, it continues to display the waveforms when I stop talking, because there are no new triggering events.

If you want, you can use the output of an envelope detector (looking at your transmitter's RF output), instead of the audio input to the transmitter, for the yellow trace. In that case, the scope will confirm that the envelope detector is accurately tracking the envelope of the transmitter's rf output. Furthermore, even low-end digital oscilloscopes will display the average value of the captured yellow waveform (i.e. the carrier level), and also the positive and negative peaks of the caputured yellow waveform. From those displayed values it is easy to calculate the positive and negative modulation index.

Stu