I would start with source -> low pass filter(complex) - rational-resampler(complex) -> am demod -> number sink(float).
I have never used the AM Demod block and I have never calculated the Modulation Index for AM. But it does appear you have a plumbing problem. I would start with changing the AM Demod so the channel capacity is 192000 Hz with Audio Rate of 480000 Hz. Then set your re-sampler to decimate by 100000 Hz and interpolate by 192000 Hz. And replace your number sink with Qt time sink and see what it looks like. I haven't tried any of this - I'm making it up as I go along - based on experience with FM. -- Cinaed On 05/10/2017 07:07 AM, Kevin Reid wrote: > On Wed, May 10, 2017 at 3:31 AM, Matt Cooper <mattcoopernor...@gmail.com > <mailto:mattcoopernor...@gmail.com>> wrote: > > Hi guys! > > I am trying to calculate the Depth of Modulation > <https://en.wikipedia.org/wiki/Amplitude_modulation#Modulation_index > <https://en.wikipedia.org/wiki/Amplitude_modulation#Modulation_index>> > of > an AM signal using GNU Radio, but I just can't make it work. > ... > - In GNU Radio, I'm using this diagram > <http://i.imgur.com/YvYZtZ7.png <http://i.imgur.com/YvYZtZ7.png>> . > > I know the diagram is wrong (it's illustrative only), but it's > exactly here > where I need you help. What I'm trying to achieve is to obtain in > GNU Radio > the same depth of modulation as the one set in the signal generator. For > instance, if the generator is set to a depth of 56% (just like in > the first > image), I'd like to get that exact(ish) same number in a GNU Radio sink. > > > From one perspective, modulation index relates the strength of the > carrier to the strength of the sidebands. Therefore, you must be trying > to measure a /ratio/, but the flow graph you have there does not compute > one — the output level will be proportional to the RF input. > > If you insert an AGC block before the AM Demod (which does not contain > one; I just checked), then that will normalize the strength of the > carrier, and the output level should be proportional to the modulation > index. > > (Hm. That means that the AM Demod block does not do what its > documentation claims it does unless the input is already normalized. I > should file a bug about that.) > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
