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Frequency Deviaition

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Yaa:
Hi i got a question regarding to Frequency deviation.

Carrier frequency = 150Mhz , channel spacing = 12.5kHz. When audio frequency varies from 300 - 3kHz, the modulation analyzer will show deviation readings around 2~2.5kHz.

What is the deviation reading at the modulation analyzer, if the audio frequency changes to 5kHz?


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per my understanding is the maximum frequency deviation is -+2.5kHz.
1) is the any possibilities that audio frequency affect the deviation?
2) can you guys explain more on this?

fourfathom:
Basic FM deviation is not determined by the modulation frequency, just the modulation amplitude.

But in practice, in a communications or FM broadcast transmitter there is audio pre-emphasis (a high-pass filter of sorts) between the audio input and the modulator.  This will cause the modulation amplitude (and thus the deviation) to increase as the frequency increases.

PM (Phase Modulation) is a different story though.  There are different types of phase modulators, but the result is essentially FM with a particular pre-emphasis filter.

MrAl:

--- Quote from: Yaa on March 01, 2024, 03:32:32 am ---Hi i got a question regarding to Frequency deviation.

Carrier frequency = 150Mhz , channel spacing = 12.5kHz. When audio frequency varies from 300 - 3kHz, the modulation analyzer will show deviation readings around 2~2.5kHz.

What is the deviation reading at the modulation analyzer, if the audio frequency changes to 5kHz?


----------------------------------------
per my understanding is the maximum frequency deviation is -+2.5kHz.
1) is the any possibilities that audio frequency affect the deviation?
2) can you guys explain more on this?

--- End quote ---

Hi,

It might help if you look up some methods for these kinds of circuits.
For example, in AM we can look at:
y=sin(t*w1)*sin(t*w2)*A*B

where w1 is the carrrier frequency and w2 is the modulation frequency and A and B are the amplitudes.  This can be resolved into:
y=(cos(t*w2-t*w1)*A*B)/2-(cos(t*w2+t*w1)*A*B)/2

and that is the same as above yet it involves just a subtraction, and the sum and difference of the two frequencies.

CaptDon:
It is a difficult thing to picture mentally. As the modulating index changes so will the number and amplitude of the sidebands. So strangely enough you can have a 1KHz modulating signal with only a measured deviation of 100Hz at low modulation levels. But on an analyzer you will still see the sidebands at 1KHz! Carsons rule states that total bandwidth of an FM signal which will include 98% of the total signal power can be represented by (Tone + Dev) X 2. An example would be (1KHz Tone + 5KHz Dev) = 6KHz X 2 = 12KHZ occupied bandwidth. As you increase the modulation index (ratio of deviation to tone) sidebands and even the carrier will come and go. With a good quality spectrum analyzer a handy way to set 5KHz deviation is to use a 2.079KHz tone and slowly increase the deviation until the first instance of the carrier disappearing. This magic carrier disappearance occurs at a modulation index of 2.405, 5.52, 8.654 and so forth. So by Carson's rule occupied bandwidth will be bigger than the 'deviation'. Deviation alone does not describe bandwidth, the modulating tone must also be considered. It is amazing to look at an FM commercial broadcast station who has the 67KHz and the 91KHz subcarriers turned on. Even stranger to consider a 91KHz subcarrier on an FM station using +/- 75KHz deviation? These are my observations only. I am sure there is some Einstein math that describes my observations.

newbrain:
Had started answering when I was you post. Some effort spared, thanks!


--- Quote from: CaptDon, edits by me on March 01, 2024, 02:25:12 pm ---I am sure there is some Einstein Bessel math that describes my observations.

--- End quote ---

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