Oscilloscope audio waveform & bias questions

[KK4JW]

I've been having a time trying to get my Connex CX33HPC-1 dialed in on my newly purchased oscilloscope. I don't have a proper signal generator, so I've been using a signal generator app on my smartphone at 1000hz, and sometimes a whistle or "auddiiioooo" into the microphone, and adjusting the trigger control to stabilize the pattern. I've been pulling an RF sample using the oscilloscope probe laying next to the RF output of the radio, but not hooked directly to it. Just kinda dropped down in there next to the little wire that feeds the SO239.

If I adjust the modulation control for symmetry on the scope without pinching off the carrier, it absolutely KILLS the PEP off. If I barely, just slightly pinch off the carrier (still not flat topping the peaks), I can get more PEP, right around 40-45 watts. Listening to myself on another radio, I can tell an increase in modulation, but am not hearing any distortion. It gets louder but isn't fuzzed up. In a situation like this, is it okay to pinch off the carrier a just little bit? I know it isn't perfect, but it's about as close as I can get it without sacrificing my audio.

Is what I'm seeing on the scope accurate? Obviously I can vary how much the waveform is pinched by simply changing how far I'm holding the microphone from the audio source. A 1000hz tone from my smartphone is pretty dang loud, compared to a normal speaking human voice.. I AM seeing "one box up and one box down" of carrier on the scope with a normal carrier.

Right now, I have the radio tuned at the following just because I plan to drive a Carl Built 200HD (2x2879) with it:

L Power - 2W carrier, 35W PEP (very pinched off carrier, not flat topped)
M Power - 4W carrier, 40W PEP (slightly pinched off carrier, not flat topped)
H Power - 10W carrier - 50W PEP (no pinched off carrier, not flat topped)

Sadly, I cannot run the radio on high power, but at this setting the waveform looked the best to me.

I also took the time to go in and properly set the driver/final bias voltages per alignment instructions. The voltages were a little high from whoever was in the radio before me. IIRC, I set the driver bias at 3.60V and each final's bias to 3.80V. These are MOSFET finals, do these voltages seem right?

 

[Shadetree Mechanic]

I used to set the modulation limiter by listening to another radio with no antenna and headphones. When I got a scope, I could finally see what I was doing. What I found is that there is a small area of adjustment where it starts to pinch the carrier, but still sounds good. I don't use a tone, I check by speaking into the mic in a normal voice with the microphone that I plan on using. And I will give it a whistle to confirm that speaking is right where I want it.

 

[KK4JW]

I used to set the modulation limiter by listening to another radio with no antenna and headphones. When I got a scope, I could finally see what I was doing. What I found is that there is a small area of adjustment where it starts to pinch the carrier, but still sounds good. I don't use a tone, I check by speaking into the mic in a normal voice with the microphone that I plan on using. And I will give it a whistle to confirm that speaking is right where I want it.

I like you, man. You seem to be a pretty knowledgeable guy! Sounds like you've done this a time or two..

I noticed the same. There's a very fine little area there where the carrier slightly pinches, and the audio is at it's maximum. Any more, and it starts getting fuzzy. I just wanted to be sure that having just a little bit of the carrier pinched was "okay". I know it's not optimal, but right now it's the best I can do, ha.

 

[Mustang 131]

I've been having a time trying to get my Connex CX33HPC-1 dialed in on my newly purchased oscilloscope. I don't have a proper signal generator, so I've been using a signal generator app on my smartphone at 1000hz, and sometimes a whistle or "auddiiioooo" into the microphone, and adjusting the trigger control to stabilize the pattern. I've been pulling an RF sample using the oscilloscope probe laying next to the RF output of the radio, but not hooked directly to it. Just kinda dropped down in there next to the little wire that feeds the SO239.

If I adjust the modulation control for symmetry on the scope without pinching off the carrier, it absolutely KILLS the PEP off. If I barely, just slightly pinch off the carrier (still not flat topping the peaks), I can get more PEP, right around 40-45 watts. Listening to myself on another radio, I can tell an increase in modulation, but am not hearing any distortion. It gets louder but isn't fuzzed up. In a situation like this, is it okay to pinch off the carrier a just little bit? I know it isn't perfect, but it's about as close as I can get it without sacrificing my audio.

Is what I'm seeing on the scope accurate? Obviously I can vary how much the waveform is pinched by simply changing how far I'm holding the microphone from the audio source. A 1000hz tone from my smartphone is pretty dang loud, compared to a normal speaking human voice.. I AM seeing "one box up and one box down" of carrier on the scope with a normal carrier.

Right now, I have the radio tuned at the following just because I plan to drive a Carl Built 200HD (2x2879) with it:

L Power - 2W carrier, 35W PEP (very pinched off carrier, not flat topped)
M Power - 4W carrier, 40W PEP (slightly pinched off carrier, not flat topped)
H Power - 10W carrier - 50W PEP (no pinched off carrier, not flat topped)

Sadly, I cannot run the radio on high power, but at this setting the waveform looked the best to me.

I also took the time to go in and properly set the driver/final bias voltages per alignment instructions. The voltages were a little high from whoever was in the radio before me. IIRC, I set the driver bias at 3.60V and each final's bias to 3.80V. These are MOSFET finals, do these voltages seem right?

We're not professional AM broadcasters. So a little carrier shutoff ok.

 

[KK4JW]

Thanks for your input, guys. It is much appreciated. I'm not going to stress over a small amount of pinch off, and just roll with it. It's cleaner than what it was, anyway. It was REALLY pinched off. Probably 1/2" between peaks on the scope before.. LOL

What about the driver/final bias? Does 3.60V for the driver and 3.80 for each final sound correct? (Measuring V on the gate of each transistor in TX).

Feeling of the heatsink after some long-winded audiooo-ing, it is warm, but not hot. Not as hot as the AM regulator anyway..

 

[Dr_DX]

There are several posts here regarding how to set the bias on Mosfets. My observation is that most techs feel that using current rather than voltage to set the bias is a better method.