Modulation meter? - Page 1
Forum section: electronics / rf-microwave

author: vinlove
author_rank: Frequent Contributor
timestamp: October 25, 2019, 08:42:31 pm
content: I got this power / swr meter, but it has another meter in the box called Modulation.OK, I vaguely see it that it does read amount of modulation mixed in the SSB transmission.Would it be right? But is it any useful in real life?Why would you read how much modulation is in your ssb carrier?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: October 26, 2019, 07:24:53 am
content: I'm gonna keep an eye on this topic. Even though I work with modulation all day, a 'modulation meter' seems something from a different time when we didn't have the luxury of a scope with crazy sample rate and software to do measurements on the fly. Would be interesting to here how times have changed.

author: TheMG
author_rank: Frequent Contributor
timestamp: October 26, 2019, 04:34:20 pm
content: Must be for AM modulation, as there is no such thing as measuring modulation depth on SSB, with SSB you measure PEP (peak envelope power).

author: vinlove
author_rank: Frequent Contributor
timestamp: October 26, 2019, 07:46:47 pm
content: But is the PEP the true representation of the SSB modulation?When the Power Meter is reading 100 watts pep, does it mean it is all the modulated with the ssb audio?

author: TheMG
author_rank: Frequent Contributor
timestamp: October 26, 2019, 08:09:48 pm
content: Assuming a clean signal free of any significant spurious emissions... yes.That does not, however, mean that the SSB signal is being transmitted in the most effective manner, especially when it comes to phone (voice) operation. The audio frequency response can be manipulated to put that SSB RF energy where it will be most useful, and the use of sufficient (but not too much) audio compression can transmit a higher average power and therefore louder sounding signal even if the overall PEP is the same.Not something you can measure very well with just a standard power meter however. It will also not tell you if you are transmitting spurious (excessive IMD, harmonics, etc).Basically seeing 100W PEP on a power meter is a confirmation that your audio is in fact being transmitted (because with SSB, no audio = no power), and also that you are achieving the desired power output and not overdriving your amplifier.But to really be able to analyze the quality of the modulation for SSB the best tool is a spectrum analyzer, and a receiver to use to monitor the signal subjectively.Completely different story with full AM modulation, where you can use a modulation meter to determine the depth of modulation, and you can relatively easily also measure this on a scope and look for clipping, over-mod, excess distortion.

author: vinlove
author_rank: Frequent Contributor
timestamp: October 26, 2019, 08:42:40 pm
content: So if you use only SSB ever, then Modulation meter is redundant?

author: vk6zgo
author_rank: Super Contributor
timestamp: October 27, 2019, 12:56:18 am
content: Quote from: TheUnnamedNewbie on October 26, 2019, 07:24:53 amIf you go to an AM Broadcast Station, you will see them in regular service.Techs at such sites don't have time to fart around with fancy Oscilloscopes, as they are under time constraints.A standard "modulation monitor" will give you a good indication of modulation percentage on programme material, & usually has a red light which flashes on momentary peaks of overmodulation.They are only really repeatably accurate when you are modulating the transmitter with constant tones, in a test situation, which used to be performed regularly, but with the current domination of "suits" over technical people, is likely seldom done today.You can measure AM modulation with continuous tones with almost anyanalogoscilloscope, either by looking at the envelope, or more accurately, using a "trapezoid pattern".In the latter, the RF is fed to the vertical input & the modulating signal to the horizontal input of a 'scope.The trapezoid pattern is what we used to calibrate modulation monitors.If the results were within spec for such a MM, we passed it--- if not, it had to be investigated.It is not uncommon for workshop type "modulation meters" to include measurement of both FM deviation & AM modulation % in the same box.An AM "modulation meter", will not provide useful information when used with a SSB signal.

author: vinlove
author_rank: Frequent Contributor
timestamp: October 27, 2019, 05:46:53 pm
content: The Mod. meter reads when reading SSB transmission, so it is telling something. My speaking is being transmitted. And that is all.Yeah, I don't know what quality and if there is distortion from my transmission, but that I do need transmission monitor function in the rigs, or if not available, then a separate receiver tuned to the tx freq.