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

On Wed, May 10, 2017 at 3:31 AM, Matt Cooper <mattcoopernorton@gmail.com> wrote:

Hi guys!

I am trying to calculate the  Depth of Modulation
<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>  .

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.)