Total posts : 45366
I don’t think your post was “problematic”. This site is to encourage part 15 operation and not “pirate radio”. Posting technical information should not be a problem if the major emphasis is not on illegal operation. Use your judgment on this.
The article you linked in your last post is a fair explanation of the trapezoidal display. I obtained the waveforms a bit differently than you or the article describe but I don’t see why yours didn’t work. Was there modulation applied? It is also quite possible that RF was being coupled into the audio scope probe, especially if an antenna rather than a dummy load is used. Make sure both probe grounds are connected to the circuit ground.
Here’s how I obtained the trapezoidal display. The scope was set for AC coupling and X-Y mode. With no signals center the dot using the X and Y position controls (keep the Intensity down so as not to burn the screen). The Y probe is connected across the dummy load at the output and the X probe is connected to the audio input of the AM-25. This X connection differs from the usual method of picking up the audio at the final amp but it works with the AM-25 since the audio circuit (U1, etc.) introduces no phase shift over most of the audio spectrum. It also lessens the chance of getting RF into the audio probe. With modulation (I used a 440 Hz sine wave from a signal generator) adjust the X and Y gains on the scope to get a usable display. If the transmitter is operating linearly the sides of the triangle will be straight as shown in the waveform I posted.
The modulation percentage can also be measured with a single channel connected to the RF output and works best with a sine wave audio input. Set the sweep at about 2 or 5 milliseconds per division or to display a few cycles of the modulation envelop and sync on the audio. Most scopes will sync to the envelop of the RF display but if not then use the external sync connected to the audio input and adjust for a stable display. Call the peak deflection of the envelop A and the minimum B. The percent modulation is M = ((A-B)/(A+B)) X 100%. This equation also works with the trapezoidal display.
Either method can be used to monitor the modulation with program material but the trapezoidal is easier to interpret. Watch for “pinching off” of the carrier on audio peaks since this indicates greater than 100% modulation.