For PEP measurement true, but I was measuring RMS. Anyway, I took my scope out from a box, will post measurement in a few minutes.
Mike
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Why do HF transceivers have such low level AM modulation?
- Thread starter SMILEX2692002
- Start date
Mike
There is a measurement of IC-7610 AM on the scope.
Looks like almost 100% AM modulation.
Mike
Looks like almost 100% AM modulation.
Mike
Last edited:
If that's a 25 watt carrier I'd say it's perfect. How does it look from 10 or 15 watts? That's about all anyone would need to get legal limit from a linear.
I'm Not an Expert of what a clean signal looks like on a Scope but that one does NOT look clean from other examples I have seen online?
SIX-SHOOTER
Single Tone I believe & a lot more uniform.That scope picture looks all out of shape like me. LOL Not Pretty & smooth at all.Like I said,I'm Not an Expert at what they should look like but it does not look like any I have seen demonstrated.I have never used or owned a scope & would not know how to use it if I had one.
SIX-SHOOTER
The Scope Carrier / 100% Modulation sp5it posted looks good to me, like out of a text book. The ARRL Hand Books use to show photo graphs of AM and SSB transmitted signals and such, picture perfect patterns.
Looking at the locals here a few years ago with a scope connected to a receivers IF before the detector circuit I was able to see every ones AM pattern/signal Modulation.
Not all looked good, but did sound good. A few distorted audio transmissions were able to be seen on the scope, but I am sure this is due to the golden screwdriver effect. And the mic gain control seems to get turned up too high.
Some AM Envelope flat topping was seen but still sounded good. They called it "D104 Audio". Listening to your audio and looking on a scope works for me.
Jay in the Great Mojave Desert
Looking at the locals here a few years ago with a scope connected to a receivers IF before the detector circuit I was able to see every ones AM pattern/signal Modulation.
Not all looked good, but did sound good. A few distorted audio transmissions were able to be seen on the scope, but I am sure this is due to the golden screwdriver effect. And the mic gain control seems to get turned up too high.
Some AM Envelope flat topping was seen but still sounded good. They called it "D104 Audio". Listening to your audio and looking on a scope works for me.
Jay in the Great Mojave Desert
Ok. I didn't want to try to explain something you already knew. That was me saying 4 and snapping a picture. If you modulate the carrier with a single tone (usually 1k) you will get the nice smooth sinewave that everyone likes to show.
The human voice is more complex than a 1k tone so the waveform looks much different. The tone of a person's voice and what they are saying will make a big difference as well.
What is important is to see the positive peaks are not squared off and the negatives are not pinched off. With a rig that uses a balanced modulator you will start to get wavelets along the baseline instead of the square wave negative and a flat baseline that happens with high level modulated rigs.
Keep in mind that the amplitude of that tone can be varied to show someone what they want to see. A radio with a hack tune can look great on the scope with the levels set right.
If a person has enough asymmetry in their voice and the transmitter has enough audio bandwidth to take advantage of it they can run higher than 100% positive peaks without the negatives cutting off. This is why you want some low end bass. You just have to limit the undesirable range that makes you sound boomy or muddy. Even if the guy on the other end has a narrow receiver and never hears that bass your higher percentage of modulation will still be beneficial.
Seems like a lot of HF radios have ALC that kicks in pretty early when operating AM. In other words when operating SSB the ALC allows the PA output to get to the rig's PEP rated output, typically 100 Watts. But in AM it seems like the carrier's presence "pre loads" the ALC loop so when the the two AM sidebands come up on voice peaks, the ALC detector already has some voltage developed due to the carrier. So the overall PEP level can't make it to 100 W PEP. Here's what I did with my Kenwood TS-590SG (notorious for low output, even on SSB). First of all I read about the sub menu settings and got in there and carefully cranked up the HF output power level so the rig was definitely hitting 100 Watts PEP output when running SSB as measured on a Bird Watt meter that's been modified to show PEP. Next, for AM operation I set the RF power setting to max which is indicated as "25 Watts"; meaning the carrier power. Then I set the MIC gain to max. Lastly and most importantly, I set the Carrier injection level by trial and error until I knew I had 100% forward swinging AM. You can see that in two ways. You can view your output power with a peak reading watt meter to see that your PEP output is four times your unmodulated carrier output power. And you can look at the modulation envelope on a scope to confirm you have the standard looking AM waveform with the carrier pinching off just to the point where it's fully pinched off to zero voltage. That should do it. I found with the TS-590SG you can set the carrier injection (carrier level) for about a 15 to 20 Watt unmodulated carrier. The swing on peaks will be to about 60 to 80 Watts peak, meaning 60 to 80 Watts PEP AM. That's 100% modulation. Don't worry about the fact that you're not getting a 25 Watt carrier swinging to 100 Watts PEP. Now run your exciter into the amp of your choice. My SB-220 will take that 20 Watt carrier/80 Watt PEP signal and turn it into a 250 Watt carrier/1000 Watt PEP AM signal. Now you can go hang out with the gang on 75 meters, or whatever band you like. Set up the exciter for nice clean 100% AM and use the work horse amp to provide the fire....