The AM Forum

Attached is a .jpg slide that is a copy of a pair of traces I recorded from my oscilloscope. [The scope allows me to save 10,000 points for each of its two inputs / traces on a floppy disk... in the format of an Excel spreadsheet. The spreadsheet, once transferred to my computer, allows me to plot a graph with both traces... plus I added a scaled/offset version of one of the traces]

Transmitter: 4-FET  Class E / Class H transmitter... 200 watts at carrier

Trace 1 (red): the output of my off-air monitor (i.e., the output of the "pickup" that came with my REA modulation monitor)

Trace 2 (green): the audio input to the Class H modulator (when I look at this in real time, I usually scale and offset the audio trace to sit on top of the off-air monitor trace... but the spreadsheet that the scope gave me as an output file did not include the scaling and the offset)

Trace 3 (blue): Same as trace 2, except scaled and offset to sit on top of trace 1.

Note that trace 1 (the envelope of the rf output of my transmitter) tracks nearly perfectly with trace 3 (the audio input to the Class H modulator)... except at points around 2500 on the horizontal scale... where the negative peak limiter in the Class H modulator keeps the rf envelope from following the audio to negative modulation levels greater than 95%.

The horizontal scale (points numbered 2 to 10001) corresponds to 4 milliseconds of total sweep time.

The digital scope stores each sample in a memory... so what is displayed is whatever the two channels had on them since the most recent trigger. In this case, I was triggering on an upward level crossing... with the level set at about 70% positive modulation.

The display on the scope (which is still there since I decided to make this posting about an hour ago) is much cleaner than the attached .jpg

Best regards
Stu

Very nice and accurate!

To illustrate how a combination of an A/D converter (in effect, a DC-coupled sound card) and an associated software application can be used to perform analyses of a modulator's / transmitter's behavior... including functions typically performed by a modulation monitor:

Attached, below, is another scope trace (i.e. saved from the digital scope to a floppy disk as an excel file, then transferred to my computer for plotting a graph, and then saved as a .jpg)

This trace is of the output of an off-air monitor (an envelope detector), while modulating my homebrew, 6 meter, 6146 plate modulated AM transmitter.

I don't show the corresponding audio input to the modulator... but it overlays on the output of the off-air monitor nearly perfectly.

The horizontal axis is 100 milliseconds long (10ms per division x 10 divisions)... and corresponds to the last segment that triggered my scope.

I added two red lines:  carrier level and 100% modulation level

You can see the positive peaks going above 100%

Again, although the above was done using a modern digital oscilloscope... all of the A/D conversion and sample storage is taking place at audio sampling rates

Stu

That's alot of high tech instruments there. I guess the big advantage is being able to capture the data and save it for comparison. Keep up the good examples of how this is done!