# Diode laser FM/AM modulation

Started by February 9, 2017
```On 10.2.17 16:24, George Herold wrote:
> On Friday, February 10, 2017 at 8:05:03 AM UTC-5, Tauno Voipio wrote:
>> On 9.2.17 19:39, George Herold wrote:
>>>
>>> Hmm can I ask a silly radio question.  If I've got
>>> (say) 50% AM, modulation index = 0.5, how big are the side bands compared
>>> to the central carrier?  (I guess I should be able to go look that up somewhere.)
>>> (Or measure it myself)
>>>
>>> I could then go measure the AM MI for the laser and get some number.
>>>
>>> George H.
>>
>>
>> There's no such thing as modulation index in AM. MI belongs to
>> angle modulation (PM, FM).
>>
>> An AM signal modulated with a single sine wave is
>>
>>   AM(t) = A(1 + M cos(omega_m t)) cos(omega_c t),
>>
>> where
>>
>>   A = carrier amplitude
>>   M = modulation depth (0 .. 1)
>>   omega_m = modulation signal angle frequency
>>   omega_c = carrier signal angle frequency
>>   t = time.
>>
>> The expansion of the expression and cranking out the sideband
>> signals from the cos(omega_m t) cos(omega_c t) is left as homework.
>>
>> --
>>
>> -TV
>
> Right, Thanks Tauno.  (I scribbled down the math...(I missed the 1+ term..
> so thanks for that.)
>
> I should let you all know that when I post questions like this, I'll often
> send a link to whomever asked me the question... saves me having to repeat
> what others say here.
>
> A quote from my last customer.
>
> "cheers George - impressive backup you have there! "
>
> I just wanted to second that!
>
> George h.
>

If you leave the 1+ away, you'll get what is called DSB-SC or,
with square wave modulation, BPSK.

--

-TV

```