Re: [Discuss-gnuradio] Help needed to obtain AM Depth of Modulation

[Cinaed Simson]

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 <address@hidden
> <mailto:address@hidden>> 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.)
> 
> 
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